plasma-assisted ignition and combustion: nanosecond discharges and development of kinetic mechanisms

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Plasma-assisted ignition and combustion: nanosecond discharges and development of kinetic

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2014 J. Phys. D: Appl. Phys. 47 353001

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Journal of Physics D: Applied Physics

J. Phys. D: Appl. Phys. 47 (2014) 353001 (34pp) doi:10.1088/0022-3727/47/35/353001

Topical Review

Plasma-assisted ignition and combustion:nanosecond discharges and developmentof kinetic mechanismsS M Starikovskaia

Laboratory of Plasma Physics (CNRS, Ecole Polytechnique, Sorbonne Universities, University of Pierreand Marie Curie-Paris 6, University Paris-Sud), Ecole Polytechnique, route de Saclay, 91128 Palaiseau,France

E-mail: [email protected]

Received 12 May 2014, revised 23 June 2014Accepted for publication 30 June 2014Published 15 August 2014

AbstractThis review covers the results obtained in the period 2006–2014 in the field of plasma-assistedcombustion, and in particular the results on ignition and combustion triggered or sustained bypulsed nanosecond discharges in different geometries. Some benefits of pulsed high voltagedischarges for kinetic study and for applications are demonstrated. The necessity of and thepossibility of building a particular kinetic mechanism of plasma-assisted ignition andcombustion are discussed. The most sensitive regions of parameters for plasma–combustionkinetic mechanisms are selected. A map of the pressure and temperature parameters (P –T

diagram) is suggested, to unify the available data on ignition delay times, ignition lengths anddensities of intermediate species reported by different authors.

Keywords: kinetics, nanosecond plasma, combustion

(Some figures may appear in colour only in the online journal)

1. The history of the field

Combustion of hydrocarbon fuels is the world’s major sourceof energy. At present, about 80% of worldwide energysupport (transport, electric power generation, heating and soon) is provided by combustion [1]; therefore it is very muchworthwhile studying and optimizing this process. Accordingto combustion experts [2], new technologies for internalcombustion engines are dictated rather by regulations thanby market. Modern international initiatives [3, 4] are veryactive in increasing the demands of greenhouse gas regulationsand it is argued that by achieving greater fuel economy, CO2

emissions can be halved by 2050.Low temperature nonequilibrium plasma is an efficient

tool for influencing combustion-related chemistry. This canbe via acceleration of ignition (plasma-assisted ignition, PAI)or sustaining/intensification of combustion (plasma-assistedcombustion, PAC). In the first review on PAC [5], it was

mentioned that the pioneer experiments on PAC were publishedin the 1930s [6], when, following the discovery of chainreactions [7], large-scale research efforts on chain chemicalreactions were organized in Russia, at the Chemical PhysicsInstitute. The extension of explosion limits for H2 : O2

mixtures in the temperature range 670–870 K and pressurerange 10–150 Torr produced by a pulsed high current dischargewas observed. A section in the textbook [8] published in 1958is entitled ‘Spreading of boundaries of H2 +O2 mixture ignitionunder the action of short wavelength radiation or under theadmixture of O-atoms’. It gives a theoretical considerationof the shift of the ignition limits under artificial addition ofradicals.

Equilibrium plasma from spark discharge has been usedfor the ignition of combustible mixtures since 1858, whenJean J Lenoir, a Belgian engineer, developed the first internalcombustion engine. Internal combustion engines have beenused in industry for more than 150 years, but it is the progress

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J. Phys. D: Appl. Phys. 47 (2014) 353001 Topical Review

in supersonic aviation at the end of the 20th century whichtriggered interest in using nonequilibrium plasma for ignitionand combustion. For high velocities of gas flow, it is achallenge to initiate combustion at the earliest possible timeinstant. Use of low temperature plasmas, creating activespecies and heating at the same time, may be among thepossible solutions as regards how to get fast ignition andto sustain the combustion process under the conditions ofhypersonic flight.

The principles of operation of ramjet and scramjet enginesare rather similar: both contain no moving parts and use oxygenfrom the incoming air flux as an oxidizer. Both need a highspeed of incoming air to start the operation, that is they canbe used as cruise engines only, and need a supplementarysystem—engine or host aircraft—to get an initial velocity. Thedifference is that in the ramjet, operating at Mach numbers 1 <

M < 5, the flux slows down significantly, while in the scramjet,operating at higher M , it slows only partially. Burning occurssubsonically in the ramjet and supersonically in the scramjet.Ramjets are developed and used in aviation. Scramjets havebeen under development during recent decades [9–12].

A pioneering conception for a hypersonic airframe AJAX[13] was suggested in the 1980s by Vladimir Freighstadtfrom the Hypersonic Systems Research Institute in Leningrad.He suggested using the heat of the incoming high speedflow. According to the AJAX concept, a hypersonic vehicleis a non-isolated aerothermodynamic system in which at allstages of atmospheric flight part of the kinetic energy of thehypersonic airflow is assimilated by the onboard subsystems,increasing the vehicle’s total resource and transforming itselfinto chemical and electrical energy. A few innovative solutionshave been suggested, namely MHD control of the ionizedair flow in the engine, plasma control of the aerodynamicsand an active thermal protection system based on partial fuelconversion at high temperatures.

At the end of the 1990s, AFOSR (the US Air Force Officeof Scientific Research) [14] launched, under the guidance ofDr Julian Tishkoff, a broad international research program inthe field of plasma-assisted aerodynamics (PAA) and PAC.The key idea was that plasma-based technologies may providea cutting-edge approach to high speed air vehicle flight andcombustion control. Although the program was launchedmainly in the USA and in Russia, in more than 15 years ofcollaboration some tens of laboratories throughout the worldhave participated in the research on PAA and PAC. Startingfrom this idea, the scientific community achieved deep insightinto the physics and chemistry of combustion triggered orsustained by nonequilibrium plasma.

Among the recent activities influenced by research inthe field, the author would like to particularly mention theMulti-University Research Initiative (MURI) ‘FundamentalMechanisms, Predictive Modeling, and Novel AerospaceApplications of Plasma Assisted Combustion’ (ProgramManager Professor Chiping Li), which has been a seriousdriving force in the domain during the past few years, unitingscientists from leading universities in research on the problemof the fundamentals of PAI/PAC.

The present paper gives a review of recent, 2006—2014, fundamental PAI/PAC experiments for a wide range

of gas number densities: from low gas number densityexperiments for hypersonic research to high gas numberdensity experiments for engine research. They are mainlylaboratory-scale experiments for different fuel-containingmixtures—from hydrogen to heavy hydrocarbons—oftenaccompanied with kinetic numerical modelling.

2. The review papers available and the aim of thepresent review

A few review papers are available in the field of PAI.The first review [5] summarizing non-numerical—at thattime—studies of applications of nonequilibrium plasmafor ignition/combustion enhancement and comparing theefficiency of thermal initiation of ignition and ignition bynonequilibrium plasma was published in 2006. The mainprinciples of PAC are discussed in [5], together with analysis ofdifferent gas discharges used for PAI/PAC. Three experimentalapproaches are considered: ignition by microwave discharge insupersonic flows; ignition by different discharges in low speedflows in laboratory facilities; and ignition by a nanoseconddischarge in shock tube (ShT) experiments. It is concludedthat the experimental data on gas temperatures and densities ofintermediate species during the induction period will providea basis for kinetic models of PAI/PAC. It was suggestedthat we should study the class-by-class impact of differentkinds of particles produced in the discharge on the subsequentchemistry.

Nanosecond discharge was suggested as a tool for PAIin 1996 [15] by the Physics of Nonequilibrium SystemsLaboratory in Moscow Institute of Physics and Technology.A review of the experimental work of this group in theperiod between 1996 and 2009 is presented in [16]. Startingfrom the numerical modelling of the kinetics of combustiblemixtures in ShTs, the authors suggested using a ShT as ameans for getting a given pressure and temperature of thegas mixture. The nanosecond discharge was initiated behindthe reflected shock wave, allowing direct comparison of theautoignition (or thermal) delay time with the delay timecaused by the impact of the highly nonequilibrium plasma ofa nanosecond discharge. More than ten gas mixtures withdifferent combustible agents, from hydrogen to pentane, havebeen studied at initial temperatures close to the autoignitionthreshold. A corresponding kinetic mechanism was suggested,and it was confirmed by numerical modelling that, at highinitial temperatures, a decrease of ignition delay time by a feworders of magnitude is explained by efficient dissociation in thedischarge and early afterglow, mainly caused by the productionof atomic oxygen.

The review [17], published in 2009, analyses non-thermaland thermal effects in PAI and high speed flow control. Thisis mainly a review of the experimental and numerical work ofthe Michael A Chaszeyka Nonequilibrium ThermodynamicsLaboratories, in the Gas Dynamics and Turbulence Laboratoryof Ohio State University, for the period 2005–2009. Theauthors suggested experiments in a low pressure, P = 60 Torr,test section with a slow (a few m s−1) flow of an air–fuelmixture. The discharge initiated as a barrier electrodeless

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discharge, and the ignition length has been measured fordifferent parameters of the discharge and gas flow. The pulsegenerator was operated in a burst mode, producing sequencesof up to 1000 pulses 20 kV in amplitude at repetition ratesvarying from 20 to 50 kHz. Comparing the ignition delay timepredicted by the model for PAI and for ignition by equilibriumheating, the authors concluded that chain reactions of radicalsgenerated by the plasma reduce the ignition time by up totwo orders of magnitude and the ignition temperature by upto 300 K.

A few original papers [18–20] should also be notedhere. Presenting kinetic models of discharge action ondifferent gases, the papers give a detailed analysis of kineticdata available in the literature and the results of numericalmodelling of the experimental work of other authors. Forthese reasons, they can be considered as review papers ofa kind. A model describing the influence of a nanoseconddischarge on H2–air mixtures is developed in [18]. The modelincludes processes of: ionization, dissociation, and excitationby electron impact; ion–molecule reactions and processeswith electronically excited atoms and molecules; reactions ofneutral species, describing the ignition of hydrogen–oxygenmixtures. The calculations are done for a wide range ofreduced electric fields, E/N ; here E is an electric field andN is a gas number density. The model is tested on autoignitionexperiments and used finally to analyse the ignition of an H2–air mixture by a pulsed discharge. The author concludes that,for discharges at high E/N values and at high temperatures,the ignition delay can be significantly reduced by the artificialaddition of atoms produced by the discharge, while at lowinitial temperatures the dominating process responsible for thedecrease of the ignition delay time is a gas self-heating due tothe recombination of atoms.

The paper [19] studies the effect of singlet oxygenO2(a 1�g) molecules produced in a gas discharge on theignition of H2 : O2 mixtures. The excitation energy of singletoxygen is 0.98 eV [21]. Analysis of available informationand numerical modelling corresponding to the conditions ofa few experimental papers allowed the author to concludethat quenching of molecular singlet oxygen is more efficientthan chain initiation or branching with O2(a 1�g). The authorclaims that even small initial densities of atomic oxygen, about10−4 at relatively high pressure, lead to a dominant role of Oatoms in comparison with singlet oxygen, and so discharges athigh electric fields optimal for dissociation are more efficientfor PAC than the discharges at low electric fields.

Although the paper [20] describes the kinetics in N2 : O2

mixtures, the results presented are crucial for PAC. Fastrelaxation, at a time scale less than that of VT relaxation,of the discharge energy in a wide range of reduced electricfields, E/N < 1000 Td, is considered. According to themodel, the dissociations of O2 by electron impact, quenchingof N2(B 3�g, C 3�u, a′ 1�−

u ) by oxygen, and quenchingof excited O(1D) atoms by nitrogen provide the main inputto a fast gas heating. At E/N > 400 Td, the dominantreactions are the quenching of excited molecular nitrogenby O2 and the processes involving charged particles. Theauthor suggests a simple estimate of the energy ηE converted

to fast gas heating: for a wide range of parameters, ηE canbe estimated, for air, as 30% of the energy expended on theexcitation of electronic degrees of freedom, ionization anddissociation of molecules. Finally, a review of experimentaland theoretical papers concerning the effect of atoms andelectronically excited O2(a 1�g) molecules on the ignitiondelay time and on the shift of the ignition temperature thresholdof hydrogen–oxygen and hydrogen–air mixtures is presentedin [22].

The Handbook of Combustion, published in 2010,contains two chapters devoted to the interaction of lowtemperature plasma with the combustion process. Oneof them gives a brief review of—available in 2010—experimental data and theoretical approaches [23] and thesecond chapter [24] reviews available diagnostic tools in thefield of combustion enhancement and flame stabilization bynonequilibrium plasmas.

The paper [25] presents an invited review written around15 different ‘key topics’ in the physics of plasmas. One of theconclusions is that further understanding of PAI/PAC physicsand chemistry at low gas temperatures, low equivalent ratiosand/or high pressures needs detailed chemical mechanismsto be developed, taking into account joint discharge andcombustion chemistry.

A series of review papers [26, 27] provide the mostcomplete observation of the state of the art in the fieldof plasma-assisted ignition and combustion (PAI/PAC). Inparticular, the review [27] presents experimental resultsobtained recently under different conditions—going fromsupersonic flows to quiescent gases. Detailed analysis ofthe action of plasma on combustible mixtures is given. Theauthors discuss the formation of an electron energy distributionfunction (EEDF) and energy branching in different discharges.A hierarchy of energy relaxation is traced with examplestaken from the literature. Rotational, vibrational relaxation,quenchings of low energy electronic states, like O2(a 1�g),are considered. Special attention is given to the analysis ofthe dissociative quenching of high energy electronic levels,and in particular to the dissociation of O2 via excitation ofN2(C 3�u) states of molecular nitrogen. The model of fast gasheating suggested in [27] is different from the model of [20],providing more heating at high electric fields. The role of VUVemission, ionization and recombination processes is analysed.Plasma action on combustible mixtures for different initialtemperatures (room temperature, and temperatures below andabove the self-ignition threshold) is discussed. Differentaspects of plasma action on flames, like chemo-ionization, flowturbulization and radical generation ahead of the flame front,are reviewed.

An idea of the complexity of the air flow, plasma andcombustion interaction can be obtained from papers where theignition of supersonic and fast subsonic gas flows by plasmais studied [28–30]. Hydrodynamic effects are also extremelyimportant for high voltage pulsed discharges in quiescent gasmixtures, when the energy is released during a short timescale [31].

Advanced numerical modelling, such as the 2D modellingof discharge [32], with plasma chemical kinetics, and

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plasma-initiated combustion [33], linked or not linkedwith the discharge, has recently become available in theliterature. This approach with multiple scales in time andspace, although neglecting the detailed analysis of chemicalpathways, allows the most general analysis of PAC as acomplex physical phenomenon including the physics of gasdischarge, hydrodynamics and chemical kinetics. Althoughthe interaction of these fields can be very important, in thepresent paper we will, when possible, restrict the description tothe consideration of the chemistry initiated by nonequilibriumplasma in combustible systems.

In spite of the fact that different discharges, such as RFones [34] and gliding arcs [35], are used for PAI/PAC, highvoltage nanosecond discharge, at low and at elevated pressures,has been the most popular tool, during the past decade,for studying kinetic effects connected to PAI/PAC problems.Specific reasons for this are that the nanosecond discharges:(i) are uniform at low and moderate gas densities; (ii) arenaturally synchronized in time in the case of a multi-streamerconfiguration at high gas densities; (iii) provide efficientexcitation and dissociation of the gas at a time scale shorterthan the typical time scale of combustion kinetics. Anotherimportant issue is the recent progress in solid-state highpower electronics: modern companies suggest high voltagenanosecond generators (see, for example, [36]) for a broadrange of parameters of the pulse, allowing operation bothin laboratories and in the extreme conditions of industrialapplications. Here, we will indicate the second restrictionof the present review: the analysis will be limited mainly toexperimental works on PAI/PAC triggered by short pulseddischarges. Analysis of numerical modelling will be givenonly in connection with the experimental results.

While quantitative insight into the coupling betweenlow temperature plasma kinetics and combustion chemistryremains a challenge, the aim of the present review is to analysethe necessity of and the possibilities for building a particularkinetic mechanism of PAI/PAC, to select the most sensitivefields of parameters for PAI/PAC mechanisms, and to reviewthe available experimental kinetic data on ignition or sustainingof combustion by pulsed nanosecond single-shot or repetitivehigh voltage discharges.

3. Principles of PAI/PAC

The questions to be answered when developing a PAI/PACkinetic mechanism are numerous. How important is it to builda separate mechanism or mechanisms for ignition/combustioncaused or assisted by low temperature nonequilibrium plasma?Are there any particular fields of parameters for which thismechanism can be particularly simplified, or, on the other hand,for which this mechanism is complicated but very informative?Is the coupling between the discharge and the combustionchemistry important? For any discussion of this kind, it isimportant to know how both the chemistry of combustion andthe discharge chemistry in the fuel-containing mixtures can bedescribed, and what the achievements and problems are. In thissection the background, or available kinetic information, fromthe ‘combustion’ and ‘plasma’ communities will be brieflyreviewed.

3.1. Combustion as a chemical process

Combustion is the sequence of exothermic chemical reactionsbetween a fuel and an oxidant, accompanied by the productionof heat and conversion of chemical species. In combustionprocesses, fuel and oxidizer are mixed and burned. It isuseful to identify several combustion categories [1] based uponwhether the fuel and oxidizer are mixed and then burned(premixed combustion) or whether combustion and mixingoccur simultaneously (non-premixed combustion). Each ofthese categories is further subdivided on the basis of whetherthe fluid flow is laminar or turbulent.

Generally speaking, combustion is a chain chemicalreaction with the following main steps [7]:

• initiation, where at least one free radical is produced;• propagation, which can vary depending upon the

conditions: chain branching where there are more radicalsafter the reaction than before, and chain transfer where thenumbers of radicals before and after the reaction are thesame;

• termination, where radicals recombine or transform tovery slowly reacting radicals.

In the experiments, the induction delay time or length ofinitiation of combustion is recorded. This is the time (length)between the beginning of the chemical process (mixing ofthe components, setting them to a given initial temperatureand pressure by compression, etc) and the sharp increase ofpressure and temperature corresponding to the start of thecombustion. The delay is explained by the slow accumulationof radicals due to thermal dissociation and the followingdevelopment of the chain reactions.

In reality, the ‘combustion reaction’ is a complex kineticmechanism. Any kinetic mechanism is a set of elementarysteps describing a particular process or a set of processes andallowing one to make predictions. If the reaction mechanismis composed of all possible elementary reactions in the system,if the rate constants are well known and verified and if, finally,all of the mechanism is validated on available experimentaldata, it is possible to say that this mechanism is complete andvalid for all tested conditions. Complete mechanisms are rarelyavailable.

3.2. Combustion of complex fuels; kinetic mechanisms

Even the simplest combustion mechanism, namely H2 : O2

oxidation at high temperatures, must include, in the opinionof various authors, at least 9–19 elementary steps [37].They are reactions of O2 and H2 dissociation followed bya set of reactions containing O, H, OH, HO2, and, at lowertemperatures, H2O2.

Building a combustion mechanism is a large-scale,thorough work including the analysis of available experimentaland theoretical data, constructing the mechanism itself,performing detailed sensitivity analysis and validating themechanism on the basis of the experimental results.Depending upon the purposes, the mechanism can be moreor less detailed. For example, a mechanism describingthermal ignition of a combustible mixture and targeted

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to calculate the ignition delay time is not necessarilybest for describing the detailed behaviour of low densityintermediates. Typically, combustion mechanisms arevalidated on experimental data obtained for jet-stirred reactors[38], counterflow burners [39], ShTs [40, 41] or rapidcompression machines (RCMs) [42]. The data obtained can berather general in character, like flame speed/autoignition delaytimes. They can also be the kinetic curves of intermediatechemical components. For these particular measurements,comprehensive spectroscopy techniques have been developedfor combustion. Between these are laser-induced fluorescence(LIF) and two-photon-absorption laser-induced fluorescence(TALIF), cavity ringdown spectroscopy (CRDS), and coherentanti-Stokes Raman scattering (CARS) [43]. A particularclass of picosecond laser techniques allowing quenching-free measurements are especially important for chemicallyactive combustion mixtures changing in the process ofmeasurements. In spite of the long history, the diagnosis ofcombustion intermediates is an intensively developing field.For example, the measurement of cross-sections of absorptionof formaldehyde (CH2O) and acetaldehyde (CH3CHO) athigh temperatures after the reflection of a shock wave bylaser infrared absorption [40] might be mentioned. Anotherparticular example is photo-ionization mass spectrometryusing tunable vacuum ultraviolet synchrotron radiation in lowpressure burners used to identify the intermediates in complexisomeric mixtures [44].

The lower alkanes, from methane to pentane, are the mostextensively studied. The paper [45] presents one of the alreadyclassical examples of validation of the well-known GRI-Mech 1.2 mechanism [46] developed with the support of theGar Research Institute. This mechanism describes the ignitionof CH4 at high temperatures. It includes 175 elementaryreactions and 32 species. The validation covers a wide rangeof experimental conditions for the ShT measurements: T =1505–2043 K, P = 9.4–86.8 atm, equivalence ratio (amountof fuel relative to stoichiometry conditions) ER = 0.5–4.0.It should be noted that in ShT experiments, the combustiblemixtures are highly diluted, and typically Ar or N2 is usedas an inert diluent. In the aforementioned experiments, theconcentration of Ar or N2 in the mixture varied from 90%to 99.61%.

Although the GRI-Mech 1.2 mechanism (and the latermodified version GRI-Mech 3.0) described fairly well theignition delay time and flame velocity for a wide range ofexperimental parameters, it significantly overestimated theignition delay time for rich mixtures at high pressures (P >

40 atm) and low temperatures (T < 1300 K). This is illustratedin figure 1.

To avoid this discrepancy, RAMEC (from ‘RAMaccelerator mechanism’) has been developed [47]. In all,13 reactions and 6 species were added to the original GRI-Mech 1.2 model. Most of the new reactions in RAMECwere taken from the lower temperature methane oxidationmechanisms. It is important to note that, in spite of themore than 30 species involved, both GRI-Mech and RAMECdescribe high temperature oxidation of methane and were notdeveloped and tested for describing the chemistry of higherhydrocarbons.

Figure 1. Comparison between GRI-Mech 1.2 calculations and theexperimental data [46] for a 0.2CH4 + 0.13O2 + 0.67X mixture(X = N2, Ar) at two pressures: 40 and 115 atm. Reproduced withpermission from [47], Copyright 1999 Elsevier.

A few other mechanisms for lower hydrocarbons havebeen developed. The well-known Konnov mechanism [49]describes the combustion of small hydrocarbons (C1–C3) withspecial attention paid to NOx chemistry. Although the website[49] has not been supported since 2009, a recent paper [50]describes the basic mechanism together with a new promptNO sub-mechanism. It should be noted that the Konnovmechanism consists of 127 components and 1207 reactions—that is, detailed chemistry needs a significant increase of thenumber of reactions.

In total, there are about ten methane combustionmechanisms. Comparative analysis of these mechanisms canbe found in the review [51]. The review contains the analysisof available mechanisms of oxidation of alkanes from methaneto decane, with remarks concerning higher hydrocarbons.Combustion of alkenes, dienes and aromatic molecules is alsodiscussed.

A series of more recent papers [48, 52, 53] from theCombustion Chemistry Center [54] develops combustionmechanisms for complex hydrocarbons and fuel blends.The work [48] represents the set of methane/ethane/propaneignition delay time measurements available in a single study,which extends the composition envelope over an industriallyrelevant pressure range. It is the first study to presentthe ignition delay times, under significantly overlappingconditions, from both a ShT and a RCM. Typically, ShTsprovide high temperatures—thousands of K—and relativelylow pressures—tens of atm—while RCMs can operate at aslow initial temperatures as 700–1000 K and at initial pressuresof tens to hundreds of atm. The data were simulated usinga detailed chemical kinetic model comprised of 289 speciesand 1580 reactions. An example showing experimental datawithin the 1–30 atm range together with results from numericalmodelling [48] is presented in figure 2. On the web-pageof the Combustion Chemistry Center, recently developedmechanisms of combustion of alkanes from C1 to C5, acetone,ethers, methylcyclohexane and other quite complex fuels andtheir mixtures can be found. The number of species andnumber of reactions are increased significantly even for the

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Figure 2. Effect of pressure on ignition delay times for 70%CH4/15% C2H6/15% C3H8 oxidation, ER = 0.5, in air. Blacksquares, ShT 1.0 atm; red circles, ShT 6.5 atm; green hollowtriangles, RCM 10 atm; blue triangles, ShT 20 atm; blue hollowtriangles, RCM 20 atm; magenta hollow diamonds, RCM 30 atm.Lines are model predictions at the different pressures. Reproducedwith permission from [48], Copyright 2008 Elsevier.

0

500

1000

1500

2000

C3

Ko

nn

ov

Number of reactionsNumber of species

GR

I

C3

Ko

nn

ov

GR

I

Figure 3. Comparison of number of species and elementary steps indifferent mechanisms. GRI-Mech for CH4, the Konnov mechanismfor CH4–C3H8 and the mechanism of the Combustion ChemistryCenter for CH4–C5H12 are compared; see the text for details.

set of the simplest alkanes. An illustration is given in figure 3,comparing these values for GRI-Mech, the Konnov mechanismfor C1–C3 and the Combustion Chemistry Center Mechanismfor C1–C5 alkanes.

It should be noted that for propane and higher alkanes, aspecific behaviour of the ignition delay time at relatively lowtemperatures is well known to the combustion community:the change of slope of the curve for the dependence of theignition delay time on the temperature. The region wherethe ignition delay increases with the temperature is called anegative temperature coefficient (NTC) region. Typically, anNTC is observed for large alkanes, starting from propane orbutane [1, 53], but some papers discuss NTC conditions forlower hydrocarbons, even for methane [55, 56].

The origin of the NTC region has, in general terms,been described already by N N Semenov [7]. This isa low temperature interval where the chain branching isgoverned by a density of relatively stable intermediate radicals(so-called degenerate chain branching). The decrease indensity of the radicals with temperature leads to an eventualdecrease of the overall rate of the reaction. Although themodern consideration of degenerate chain branching considersa large number of governing reactions [53], the principle isvery similar. One of the main actions of nonequilibriumplasma on a combustible mixture is dissociation of themolecules by electron impact. So, plasma action on stableintermediate species can significantly change the kinetics in alow temperature region.

3.3. Principles of acceleration of ignition and modification ofcombustion by nonequilibrium plasma

The main physical principle of spark discharge, used for morethan a century in the automotive industry, is local gas heatingand the following propagation of the combustion wave fromthe point of ignition. In contrast, in the case of nonequilibriumplasma the gas is not initially heated by discharge: electronenergy goes mainly to excitation of the internal degrees offreedom, that is, to increase of the internal energy of the atomsand molecules. Use of internal energy for the initiation of achemical transformation can be a more efficient method thanthat involving transformations initiated by thermal energy only.

The simplest example of such an energy transformation isa dissociation of molecular oxygen by gas discharge. For anoxygen molecule, the dissociation takes place [57] via A 3�+

uand B 3�−

u electronic states, with the thresholds of 5.4 eV and8.4 eV respectively [21]. The mean electron energy in thedischarge is determined by the reduced electric field E/N [58].At a reduced electric field of E/N = 100–300 Td (Td (thetownsend unit) is used commonly in the gas discharge physics;and 1 Td = 0.33 V cm−1 Torr−1 = 10−17 V cm2 at 20 ◦C),the dissociation rate is within the range 10−10–10−8 cm3 s−1,and thus is at least a few orders of magnitude higher thanthe constant rate for the reaction of thermal dissociation atgas temperatures typical for combustion. The big differencebetween the constant rates means that the artificial initiation ofa chain reaction must be more efficient that the thermal one.

At high initial temperatures, close to the ignitionthreshold, additional injection of O atoms initiates chemicalchains within a classical combustion mechanism, leading toefficient ignition of the mixture. This can be a useful methodfor starting, with a low deposited energy, the combustion offuels with a high energy threshold of dissociation. If underthese conditions the energy is deposited uniformly in space,non-detonative uniform combustion initiates in the system.In [59], the kinetics of ignition by nanosecond high voltagedischarge was experimentally and numerically studied for astoichiometric CH4 : O2 mixture (10%) diluted with 90% ofargon at initial pressures of 0.5–2 atm and initial temperaturesof 1200–2000 K. It was shown that, at a discharge energydensity of 10–30 mJ cm−3, the ignition delay time can be 3–4orders of magnitude shorter than in the case of autoignition.

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(a) (b)

Figure 4. Analysis of artificial addition of radicals to a stoichiometric H2 : O2 mixture at P = 1 atm: (a) calculated temperature with time fordifferent conditions: 1: 1200 K, autoignition; 2: 930 K, autoignition; 3: 900 K, autoignition; 4: 900 K, 0.2% of O atoms added; 5: 690 K, 0.2%of O atoms added; 6: 680 K, 0.2% of O atoms added; 7: 690 K, 0.7% of O atoms and 0.4% of H atoms added; (b) calculated ratio of H2O andradical densities: 1: 1200 K, autoignition; 2: 900 K, 0.2% of O atoms added; 3: 690 K, 0.7% of O atoms and 0.4% of H atoms added [60].

If the initial gas temperature is low, the developmentof chemical chains is blocked. For example, the rate of aclassical reaction of chain development, O + H2 = OH + H,drops by more than by four orders of magnitude when thegas temperature decreases from 2000 to 500 K. An exampleanalysis of the efficiency of radicals at low temperatures, takenfrom [60], is reproduced in figure 4. Pulsed discharge in anH2 : O2 stoichiometric mixture at P = 1 atm was modelled, asa first approximation, by artificial injections of radicals. TheKonnov mechanism [49] was used to model the combustionkinetics. A summary of the results for different initialtemperatures and densities of O atoms is given in figure 4(a).The ignition delay strongly depends on the initial density ofthe O atoms. For relatively high temperatures, T = 900 K,a significant shift of the ignition delay is obtained on adding0.2% of O atoms. There is some temperature limit for ignitionwhich depends upon the density of the O atoms. It is notpossible to ignite an H2 : O2 mixture at 680 K and 0.2% of Oatoms, while ignition at 690 K is initiated and the delay time isclose to the autoignition delay time at 930 K. Artificial ignitionis accompanied by a relatively smooth temperature increasebefore the main peak. The behaviours of radicals differ forthe autoignition and artificial ignition. Figure 4(b) gives animpression of the ratio of the final products (the H2O densitywas taken) and the total density of the radicals, dependingupon time.

As a simple and robust example, the dissociation of theoxidizer and fuel in a discharge is important, but it is not theonly process responsible for the PAI/PAC. In plasma chemistry,species with increased internal energy are considered as ‘activespecies’ for a simple reason: this energy can potentially be usedto overcome the energetic threshold of a chemical reaction [61].The review [27] gives a detailed analysis of the hierarchy ofthe discharge processes relative to PAI/PAC. Figure 5 illustratesenergy branching as a function of E/N for typical values of thereduced electric field realized in the discharges. It is clearlyseen that for both air and a methane : air stoichiometric mixture,at low electric fields—tens of Td, molecular vibrations are

mainly excited by collisions with electrons. The probabilityof excitation of the electronic levels is small—except for thelowest metastable electronic levels, such as the O2(a 1�g)level of molecular oxygen. Vibrational relaxation can be animportant factor in nonequilibrium chemical reactions [61],but the excitation and subsequent relaxation of electroniclevels at higher electric fields, 100 Td and higher, providesa more significant effect on the initiation and sustaining ofcombustion. Figure 6 provides the rate coefficients for thedissociation of oxygen and the excitation of nitrogen by directelectron impact in air, as a function of E/N . Calculationswere made by using the BOLSIG+ code [62, 63]; cross-sections for nitrogen and oxygen were taken from [64] and[65] respectively. It is clearly seen that the most efficientchannel is the dissociation with the production of O(3P) inthe ground state and of excited O(1D) atoms. Energy storedin the electronic excitation can be efficiently used to achievefurther chemical transformation. In the problem of combustioninitiated or assisted by nonequilibrium plasma, the question ofenergy relaxation to heat [20] on a time scale less than that ofthe combustion processes is important, determining the initialconditions for the combustion chemistry. It should be notedthat no ‘standard’ set of chemical reactions for describingthe discharge and afterglow kinetics in complex chemicallyactive mixtures exists. For air, the most frequently used set ofreactions can be found in [66].

Before going further into the discussion of theexperimental data available for PAC/PAI kinetics, let us pauseat the description of the initial excitation of gas mixtures byelectron impact. The first step of solving any problem ofdischarge action on a chemically active mixture is to calculatethe rates of production of ions, excited species and dissociatedspecies. Even when the 0D stationary case can be considered,solving the Boltzmann equation [58] to obtain the EEDF canbe a challenge. For complex mixtures of molecular gases,EEDFs are significantly different from Maxwellian functions.Cross-sections of electron collision with atoms and moleculesare the fundamental data necessary for calculating electron

7


Figure 5. Fractional power dissipated by electrons into the different molecular degrees of freedom as a function of E/N : (a) air; (b)CH4 : air stoichiometric mixture. Reproduced with permission from [27], Copyright 2013 Elsevier.

energy distributions, energy branchings between the differentprocesses in the discharge, and rate constants.

For the simplest atomic and molecular gases participatingin the problem of combustion as diluters or oxidizers, theproblem is solved. Cross-sections of N2 and O2 collisionswith electrons are well known and can be found elsewhere(see, for example, [64, 65]); cross-sections for rare gases arealso well known (see [67] for Ar). The next section reviewsthe more delicate question of the availability of cross-sectionsfor electron collisions with hydrocarbons.

3.4. Available data on electron interactions with hydrocarbons

A review of the current status of the physics of charged particleswarms is given in [68]. According to [68], there are a largenumber of review papers giving tabulations of cross-sectionsets for electron scattering and those may be categorized intothree groups.

The first group contains different compilations of binarycollision data. These sets of cross-sections are not complete,in the sense that no validation on the swarm parameterswas carried out. Swarm analysis demands verification of acomplete set of cross-sections and their normalization, suchthat the final calculated electron drift and diffusion coefficientscorrespond to those measured experimentally.

The second group includes reviews with a criticalanalysis of both binary collision data and swarm parameters;normalization of the sets based on swarm analysis is presented.The following papers are cited in the review [68] as providingexamples of sets of cross-sections for individual gasescompletely verified on swarm parameters: [64, 69–71].

The authors of [68] consider the databases supplied withopen access codes for solving the Boltzmann equation as thethird group of available cross-section data. They argue for thisselection on the basis of the fact that even if the cross-sectiondata are not complete, the user can always easily validate thedata because most of the available open codes allow directchecking of the swarm parameters.

Traditionally, cross-sections of electron interaction withhydrocarbons come from nuclear physics. The interaction of

200 400 600 800 1000

10-10

10-9

10-8e+O

2= e + O + O (1D)

e+O2

= e + O + O

Rat

eco

nst

ant,

cm3 /s

E/N, Td

e+N2

= e + N2(C3Π

u)

Figure 6. Rate coefficients of dissociation of oxygen and ofexcitation of nitrogen by direct electron impact in air as a functionof E/N .

a hydrogen plasma with the carbon-containing first wall orwith the highly exposed divertor plate segments of the vacuumchamber in a magnetic fusion device leads to the generationof hydrocarbon molecules CxHy that are released into theplasma [72]. During the past 3–4 decades, the most importantreasons for looking for the electron scattering cross-sectionsfor complex gases and gas mixtures were connected to thedevelopment of microelectronics. A few books in the field ofgaseous electronics have been published recently, with the aimof a systematization of the data obtained for a set of complexmolecules, including hydrocarbons. The book [73] gives adetailed review of collision fundamentals, and experimentalmethods for getting total, differential cross-sections, and cross-sections of ionization, excitation and attachment. A review ofthe data available for rare gases, nitrogen, oxygen, H2, NO,CO2, N2O, H2O and other gases is given. Special discussionis initiated concerning ionization in non-uniform fields and inhigh frequency discharges. Only five pages are devoted tocross-sections available for hydrocarbons.

8


Table 1. Range of energies (eV) where the cross-sections and swarm parameters for alkanes are available, based on [74].

Qi QT Qdiff Qel Qa Qvib Qdiss Vdr αi

CH4 0.01–104 0.1–4000 0–1700 10–5000 0–15 0–3.6 10–700 0–3000 0–3000C2H6 3–1.2 × 104 0.01–500 2–100 0.4–1300 0–14 0–25 15–100 0.02–300 10–4500C3H8 11–1.2 × 104 0.01–600 2–100 0.8–600 0–15 0.05–20 E∗) 0.03–30 110–6000C4H10 200–1.2 × 104 0.8–400 — — 0–15 — E∗) 0.01–21 125–5500C5H12 600–1200 — — — — — — 0.01–30 —

Note: E∗)—excitation cross-sections are not available; estimates are present in the literature.

More detailed data analysis is presented in the recentlypublished handbook on gaseous electronics by the sameauthor [74]. Cross-sections available for acetylene and foralkanes from methane to pentane from the beginning ofthe 20th century to, mainly, 2000–2007 are structured andreviewed. Special attention is given to the presence/absenceof available swarm parameters, to cross-sections of ionizationand attachment, and to comparison of the available data withthose for the other hydrocarbons. Besides these cross-sections,there are sets of cross-sections for CO2, NO2, N2O, O3, H2O,NH3, CH2O2, C2H4, CH3OH, C3H4, C2H4O, CH3NH2, C3H4,C3H6, c-C3H6, C3H6O, C4H6, 1,3-C4H6, 2-C4H6, C6H6, C4H8,1-C3H8O, 2-C3H8O, C5H10, C6H14, C6H12, i-C4H10, i-C8H18,C8H18, C4H8O, C5H10O2, C7H8.

Table 1, composed on the basis of the information pre-sented in the handbook [74], gives an impression of the avail-ability of cross-sections and swarm parameters for alkanesfrom CH4 to C5H12. It can be concluded from the tablethat, for all hydrocarbons considered, first of all, experimen-tally measured ionization cross-sections Qi are available for awide range of energies. The first ionization coefficient αi isknown for most of the gases up to high energies, ε ∼ 3000–6000 eV. Attachment cross-sections Qa are known for theenergies below 15 eV. Drift velocities Vdr are mainly availablewithin the energy ranges close to the thresholds of inelasticprocesses, ε < 30 eV, although methane drift velocities areknown up to 3000 eV. Total scattering cross-sections QT, dif-ferential scattering cross-sections Qdiff , and elastic scatteringcross-sections Qel are reported. There is very limited infor-mation about the vibrational cross-sections Qvib for energiesless than 20 eV; these cross-sections are available for C2H2,methane and ethane. Information about the excitation of elec-tronic levels and dissociation via excitation of the electroniclevels Qdiss is even more limited. Some cross-sections areavailable for CH4 and C2H6. Starting from C3H8, only exci-tation potentials and photon absorption cross-sections can beused to supply approximate values.

Special attention should be paid to the paper [72],where electron impact ionization and dissociation of CxHy

components, dissociative excitation and ionization of CxH+y

and recombination with electrons, and both charge transfer andatom exchange in proton channels are considered for x = 2, 3.The cross-sections collected in [72] are based upon a criticalassessment of available experimental data and upon extensiveuse of a number of semi-empirical cross-section scalingrelationships. Information is also provided for the energeticsof each individual reaction channel. The cross-sections andrate coefficients are presented in compact analytic forms.

10-1 100 101 10210-2

10-1

100

101

5

4

32

1

Energy, eV

Cro

ssse

ctio

n,1

0-16

cm2

Figure 7. Electron collision cross-sections for CH4 [75] as afunction of electron energy, taken with permission from [59],Copyright 2008 Elsevier. (1) Momentum transfer in elasticcollisions, (2) and (3) vibrational excitation, (4) electronicexcitation, and (5) ionization.

Among recent studies, a paper [84] may be cited, whereswarm parameters were measured for C2H2 and 0.517% and5.06% C2H2 : Ar mixtures measured over wide E/N ranges,from 5 to 200 Td. The swarm parameters were determineddifferentially, from arrival time distributions of electron pulsesobserved at several drift distances.

For extreme conditions of high densities, electronmobilities and electron–ion recombination, rate constants havebeen measured for solid, liquid and gaseous methane by an x-ray pulse conductivity technique over the density region from2 × 1020 to 1.9 × 1022 molecules cm−3, including the criticalregion and the liquid–solid phase change [85]. It was foundthat the observed electron–ion recombination rate constant isin good agreement with the value calculated using the reducedDebye equation for solid methane but not those for liquidand gaseous methane. The deviation from the reduced Debyeequation is larger for the gas phase than for the liquid phase,and the authors [85] explain this in terms of the differencein efficiency of the excess-energy loss of electrons within thereaction radius of the electron–ion recombination.

To give an idea of the most well-known sets of cross-sections for hydrocarbons, a set of cross-sections for methanetaken from [75] are presented in figure 7. Other well-cited setsof cross-sections for methane are presented in [76, 77]. Thepaper [78] discusses the modification of swarm parametersin Ar : CH4 mixtures at different densities of methane. Thetime-dependent behaviour of electron transport coefficients isdiscussed and compared against the experimental results for

9


Figure 8. Electron drift velocity calculated as a function of E/N in50% and 75% mixtures of methane with argon and in 100% puremethane [78]. Experimental data are represented by the followingsymbols: 1: [79] 2: [80], 3: [81], 4: [82], 5: [83].

reduced electric fields within the range of reduced electricfields, equal to 0.1–1000 Td. The authors report that allcalculated steady-state swarm parameters agree well withavailable experimental data. An example of such a comparisonis given by figure 8. Similar checking of swarm parametersis important in implementing the cross-sections of electroncollisions with hydrocarbons in kinetic schemes.

4. PAC in different temperature and pressure ranges

Unlike combustion process, which are very sensitive to theinitial gas mixture temperature, the discharge developmentis determined, primarily, by the gas number density. Thisis dictated by the fact that the mean energy of electrons isa function of the reduced electric field, E/N , where E isthe electric field and N is the number density of neutralparticles responsible for the loss of electron energy in inelasticcollisions. So, the initial pressure in the problem of PACmust be considered together with the initial temperature, andgas number densities must be analysed when discussing thedischarge efficiency.

The efficiency of PAC is determined: (i) by the spatial–temporal distribution of the energy deposited in the discharge;

(ii) by the values of the reduced electric fields during the stageof the main energy release, and, thus, by the composition ofactive species in discharge and near afterglow. The electricfields are limited by different factors, such as the voltageapplied to the system, the geometry of the electrodes and thedensity of charged species produced in the plasma.

The transient plasma of short pulsed discharges providesconditions that are nearly ideal for studying the physicsand chemistry of combustion initiated or sustained bynonequilibrium plasma. Indeed, it is known that nanoseconddischarges initiated at ambient gas temperature by pulses10–100 kV in amplitude, a few nanoseconds in rise time and afew tens of nanoseconds in duration produce uniform plasmaover a wide range of pressures, from a few torr to fractionsof atm [87]. At short distances between the electrodes, thepressure region where the glow-like discharge is observed canbe extended to a few atm [88]. Electric fields in the front ofthe discharge can be as high as a few kTd for a very short time,less than a few nanoseconds. After this, the fields decreaseto values optimal for the excitation of electronic degrees offreedom and dissociation [89]. As the gas density increases,the discharge loses its uniformity.

One important advantage of the nanosecond discharge isthat the typical times for discharge gap closing and for theproduction of active species do not exceed units and hundredsof nanoseconds, and the main processes of recombinationand energy relaxation from electronically excited states aretypically achieved in times much shorter than that typicallynecessary for the initiation of combustion.

4.1. Low pressure experiments

The uniformity of the initial energy distribution in the dischargeis a very critical issue for further kinetic analysis. Thepresence of filaments and hot spots changes the kinetic effectssignificantly. In the threshold region, due to the exponentialdependence of the combustion upon the temperature, it isenough to heat the gas by a few tens of K to initiate thecombustion. This heating can be easily provided within thefilaments, and in this case calculations or estimates based onthe uniform energy distribution will not be correct. Althoughno strict criterion for discharge homogeneity is available,the author is convinced that a recording of the map ofoptical emission of short-lived electronically excited states inthe discharge, and then in combustion, with an appropriatetemporal resolution (at least about 1 ns in the discharge andduring a time comparable to the temperature and pressure risetime in combustion) gives a general idea of the distributionof energy in the discharge. The advantage of low pressureexperiments is that it is possible to produce a homogeneousvolume of plasma.

ShT experiments. The key point of the present discussionis how the combustion chemistry is modified by nonequilib-rium plasma, but papers where a direct comparison of igni-tion/combustion with and without plasma is performed are notnumerous. The idea of using a high voltage nanosecond dis-charge to study the kinetics of PAI with direct reference toautoignition has been suggested in [15] and experimentally

10


Figure 9. Calculated ignition delay time versus energy input forthermal and nonequilibrium plasma excitation.H2 : O2 : N2 = 29.6 : 14.8 : 55.6 mixture. P = 1 atm; T = 1000 K; 1:autoignition (corresponds to zero deposited energy); 2: equilibriumexcitation; 3: nonequilibrium excitation (E/N = 300 Td,t = 40 ns). Reproduced with permission from [90], Copyright 2003Elsevier.

realized in [90], where a ShT served to prepare a gas mixtureat a given pressure and temperature. ShTs have been usedfor combustion studies for decades, serving as reactors whichprovide a given initial pressure and temperature after the com-pression [40]. The nanosecond discharge is initiated after thereflected shock wave. The ignition delay for PAC has beencompared, experimentally and by means of numerical calcula-tions, with the autoignition delay time for H2 : O2 and CH4 : O2

mixtures diluted with He, Ar or N2.An important conclusion drawn in [90] is that, near the

autoignition threshold, and at high deposited energies, the equi-librium thermal heat decreases the ignition delay time in a moreefficient way than the nonequilibrium plasma (see figure 9; forenergies higher than 0.1 J cm−3). The nonequilibrium plasmabenefit is evident at low energies: decrease of the ignition delayfrom 200 to 100 µs demands 3×10−2 J cm−3 of thermal energyor 10−4 J cm−3 of energy deposited in a gas discharge at areduced electric field equal to 300 Td.

A cycle of experimental measurements accompanied bynumerical calculations [59, 90–92], where the shortening ofthe ignition delay time by the action of nonequilibrium plasmawith deposited energy at the level of 10−3 J cm−3 was observedafter the reflected shock wave, led to an explanation of thekinetics of ignition of combustible mixtures by nonequilibriumnanosecond plasma near the ignition threshold. The shockwave velocity, gas temperature, and OH or CH emission weremeasured on a microsecond time scale, while the dischargeelectrical current, voltage drop and deposited energy wererecorded with nanosecond resolution. Figure 10 compares, onthe basis of data from [92], experimentally obtained valuesfor the ignition delay time with the values calculated ondifferent assumptions concerning the degree of equilibriumof the deposited energy. For the same conditions and forexperimentally measured energy deposited in a gas mixture,(i) the autoignition delay time, τa, (ii) the ignition delay time,

Figure 10. Measured delays (symbols) for autoignition and ignitionof a C2H6 : O2 : Ar mixture by nanosecond discharge at a typicalenergy density in plasma of about 10−3 J cm−3, and calculatedignition delay time (lines) under various assumptions on the gasheating (see detailed explanations in the text). Reproduced withpermission from [92], Copyright 2009 Elsevier.

τ�T −diss, corresponding to the assumption that the depositedenergy minus the energy expended in producing radicals isquickly transformed into gas heating, and neglecting the effectof radicals, (iii) the ignition delay time, τ�T , assuming thatall energy deposited in the discharge is expended only inheating, and neglecting the effect of radicals, (iv) the delaytime, τd, neglecting gas heating by the discharge, and takinginto account the effect of radicals, and (v) the delay time,τd+�T −diss, taking into account gas heating in the dischargeby analogy with (ii) and the nonequilibrium effect of radicalsby analogy with (iv), were calculated. It is clearly seen fromthe figure that the experimental observations correspond to anignition delay decrease when the action of radicals is taken intoaccount.

The authors of [92] conclude that at temperatures nearthe ignition threshold for electric fields in which most of theelectron energy is expended on the excitation and ionizationof neutral particles, the electron impact dissociation andexcitation of molecules in the discharge phase are the mainmechanisms driving the effect of gas discharge on the ignitionof stoichiometric fuel : oxygen mixtures. This process leadsto an essential increase in the densities of O and H atomsat the beginning of ignition. As a result, the chemicalchain reactions become more efficient, and a partial chemicaltransformation, initiated by the initial density of radicals, isalready started during the ignition delay period. The mainconclusion is that decrease of ignition delay time is guidedby nonequilibrium chemistry initiated by excess density ofradicals at the beginning of the ignition delay period. Oneof the important observations in this group of papers is thatthe ignition by nonequilibrium plasma smooths, due to thepartial conversion of a gas mixture, the difference between thefuels: the ignition of methane by nanosecond pulsed dischargeis similar to the ignition of C2H6–C5H12.

Further experiments with a ShT/nanosecond dischargereactor were presented recently by the papers from MIPT

11


for C2H2 [93] and from Princeton University for lean C2H6-containing mixtures [94]. The experimental conception ofa ShT–plasma reactor can be a powerful tool for studyingPAC under conditions of advanced diagnostics for intermediatespecies during the induction delay period.

The advantages of the ShT–nanosecond dischargeexperiments are the following: (i) both discharge andcombustion parameters are studied in a single-shot mode; (ii)discharge and combustion are separated in time, so the cross-links between the discharge excitation and the ignition can beminimized, (iii) a wide range of pressures and temperaturestypical for combustion can be reached with a high accuracy;(iv) the results can be easily compared against those fromautoignition experiments. The drawback of the same approachis that the measurement of the detailed kinetics of a fewdifferent species with sub-microsecond resolution in thereactor combined with a nanosecond discharge for the ShTis a challenge. The alternative suggestion is to record thedetailed nonequilibrium chemistry of radicals in the regime ofaccumulation of the signal in a permanent gas flow below theautoignition threshold, to avoid any plasma–flame interaction.

4.1.1. Flow reactor experiments. The paper [95] reportstemperature and OH density measurements for conditionsof barrier discharge in preheated fuel–air mixtures at lowpressures. The experiments were conducted in lean, slowlyflowing H2 : air, CH4 : air, C2H4 : air, and C3H8 : air mixturespreheated to T0 = 500 K in a quartz tube furnace, at a pressureof P = 100 Torr. The discharge cell/plasma flow reactorconsisted of a 280 mm long, 22 mm×10 mm rectangular innercross-section quartz channel with two plane quartz windowsat the ends. The electrodes were placed on the externalsides of the channel. An FID GmbH FPG 60-100MC4 pulsegenerator (peak voltage up to 30 kV, pulse duration of 5 ns,repetition rate up to 100 kHz) was used to initiate the dischargein the repetitive burst mode. Bursts of 50 pulses at a pulserepetition rate of 10 kHz and burst repetition rate of 5 Hzinitiated the discharge. The flow rate was about 40 cm s−1,to provide the gas change between the bursts. Specialexperiments were carried out to prove the uniformity of thedischarge after different numbers of pulses for all gas mixturesinvestigated. LIF and picosecond (ps), broadband CARS wereused for measurements with time-resolved temperature andtime-resolved, absolute OH number density after the end of theburst (at the time instant 5 ms from the start of the discharge).

The experimental results for temperature and OHdensity were compared with kinetic modelling calculationsusing a plasma/fuel chemistry model employing severalH2 : air [18, 96] and hydrocarbon : air chemistry mechanisms[49, 97, 98]. The discharge was calculated as a series of pulsesof an electric field of Gaussian shape, with the peak reducedelectric field (E/N)peak = 600 Td and the coupled pulseenergy of 1.25 mJ/pulse, or 4.6 × 10−4 eV/molecule per pulse.Both the reduced electric field and the energy were estimated onthe basis of the analytical model, and the authors [95] claim thatthe OH density after the discharge is not very sensitive to theelectric field value. The authors indicate that the uncertaintyof the predicted coupled energy value is ±50%, and the final

value of 1.25 mJ/pulse provides the best agreement with theexperimental data. It should be noted that a value of thespecific deposited energy of 10−4 eV/molecule is typical forthe experiments with fast ionization waves (FIWs) [99].

Examples of comparisons of calculations and experimentsare given in figure 11 for H2 : air and C3H8 : air mixtures.The authors conclude, on the basis of calculations forH2:air, CH4 : air, and C2H4 : air mixtures using a few kineticmechanisms, that the kinetic mechanism developed byKonnov [49, 96] provides the best overall agreement with OHmeasurements. For the C3H8 : air mixture, all the mechanismsused predict a much faster drop of the OH density than isobtained in the experiments, and this fact proves the necessityof detailed sensitivity analysis and the development of aplasma-assisted mechanism for higher hydrocarbons.

A few other papers from the same group should bementioned here. In [100] the concept of a low pressure(25 Torr) burner equipped with a high voltage mesh (open area90%) electrode installed 40 mm away from the burner surfaceto produce nanosecond discharge that is uniform in space issuggested. The first experimental results, published recently[101], demonstrate a solid experimental base for further kineticmodelling, including 2D maps of CH* chemiluminescence,LIF-measured OH profiles at different heights above the burnerwith and without plasma, and comparison of the data for twonanosecond pulses of different shape and duration (12 kV/7 nsand 3 kV/170 ns).

Another setup demonstrating a significant potential forquantitative, time-resolved and spatially resolved studies ofcoupled radical reaction kinetics and diffusion over a widerange of pressures, temperatures and gas mixture compositionsis presented in [102]. The nanosecond discharge is ignitedin a closed volume with a slow gas flow, about 6 cm s−1,between two spherical copper electrodes 7.5 mm in diameter,where the distance between the electrodes is equal to 12 mm.Under these conditions, a diffuse discharge along the axisconnecting the two electrodes is observed. Combined OHLIF/H TALIF/Rayleigh scattering data are presented for anAr : O2 : H2 = 80 : 20 : 2 mixture at P = 40 Torr and the initialtemperature 300 K. The important feature of the last two setupsis that they are systems with open electrodes; the current ismuch higher than that in [100] and, hence, direct time-resolvedmeasurements of the deposited energy are possible. The dataare supported by quasi-one-dimensional numerical modellingbased on the code of [103]. It was shown that the temperaturedistribution along the radius of the channel of the dischargeguides the chemistry of the chain reactions, changing the mainreaction of OH production from H + O2 → OH + O in thecentral zone of the channel to H + HO2 → OH + OH at theperiphery of the discharge, at r > 2 mm. The experimentalinstallations described, combined at the same laboratory andequipped with high level laser diagnostics, represent a uniqueplatform for the study of PAC mechanisms at low gas densities.

It should be noted that there are some other papersdiscussing the roles of intermediates and comparing differentcombustion mechanisms for the same initial ‘discharge’conditions. The paper [104] is devoted to the numericalanalysis of ethylene–air mixtures under conditions typical for

12


Figure 11. Comparison of experimental (y-axis on the left) and predicted (y-axis on the right) time-resolved, absolute OH number densitiesafter an f = 10 kHz, 50-pulse discharge burst at T0 = 500 K, P = 100 Torr, and different equivalence ratios. Data points for temperaturesmeasured 2 µs after the burst are circled. The Konnov mechanism is used for afterglow modelling. (a) H2 : air mixtures; (b) C3H8 : airmixtures. Reproduced with permission from [95], Copyright 2013 Elsevier.

scramjet combustion chambers, P = 1 atm and T = 700 K.The authors conclude that the kinetics of small intermediates,such as HO2, H2O2 and CH2O, governs the ignition delaytime when using GRI-Mech 3.0 and the Konnov mechanism.They refer to the Kintech mechanism [105], specially designedfor low temperature plasma-assisted chemistry, indicating thatadditional components, such as C2H4O, can be important there.The authors believe that the difference in calculated ignitiondelays arising when using different combustion mechanismsis a convincing argument for the necessity of the developmentof a mechanism linking plasma and combustion chemistry.

A mechanism of methane oxidation for a wide range ofpressures (0.1–100 atm) and low temperatures (600–1000 K)was presented recently [106]. That paper does not givea complete reaction set, but describes the main classes ofreactions important for plasma and for combustion conditions,provides an analysis of available combustion mechanisms formethane and the parameters for which the mechanisms arevalid, presents a scheme for the initial stages of methaneoxidation and compares calculations made in accordance withthe suggested mechanism with available experimental datafrom other authors.

4.1.2. Combustion burner experiments. Let us come back tothe idea illustrated by figure 9. The action of nonequilibriumplasma is the most distinctive and the most efficient in the‘boundary’ regions, where the combustion reactions are notefficient yet. In these regions, the equilibrium can hardly beshifted by applying a small amount of energy in an equilibriumway, that is by heating: because the reaction rate remains low,the gains are less than the losses. When the same amount ofenergy is applied in pulsed discharge at high—hundreds ofTd—electric fields, two dominant classes of active species areproduced: radicals and electronically excited species. At leastthree main processes should be considered when describing theaction of the nonequilibrium plasma of a nanosecond dischargeon a combustible mixture: (i) a fast start for short chemicalchains resulting in a partial gas conversion; (ii) gas heatingdue to recombination; (iii) gas heating due to the relaxation ofelectronically excited species in collisions. Processes (ii) and(iii) lead to increase of the length of the chains and, finally,combustion occurs.

Actually, this means that the experiments on ‘boundary’regions, where the combustion processes are nearly suppressedby losses, will provide the most exciting kinetic data for

13


ignition and combustion assisted by nonequilibrium plasma.These regions are well known in combustion physics andchemistry. Between them are the regions of NTC withpossible modification of cool flames under the plasma action,and the ignition–extinction region in a premixed gas flow.The modification of the NTC region by plasma will beconsidered later, in the section corresponding to high pressureexperiments.

The recent paper [107], based on direct comparison withclassical combustion experiments, presents—experimentallymeasured and obtained by numerical modelling—themodification of the low pressure combustion kinetics in theignition–extinction region for a flow containing dimethyl ether(DME; CH3OCH3).

In combustion chemistry, to describe the combustion in aperfectly stirred reactor (PSR), the concept of the S-curve isimplemented: the numbers of points where combustion occursare typically plotted for the coordinates ‘reaction temperature’or ‘burning rate’ versus Damkohler number [108]. TheDamkohler number, D, can be calculated by dividing acharacteristic mass transport time, or time of flow in thewell-stirred reactor, by a characteristic chemical or collisiontime. In this case, for small D numbers, the impact of theflow is high as compared to that of the chemistry, and thesituation of a chemically ‘frozen’ flow limit is realized. Withincrease of the D number, the system follows a weakly reactivebranch corresponding to low temperatures and low intensityof chemical reactions. At some DI number, correspondingto the ignition limit, the system ‘jumps’ to a state with highreactivity—to the high temperature intensely burning branch.As regards the progressive motion from D = ∞ to lower D

numbers, the transition from a reactive mixture to a slowlyreacting mixture happens at DE < DI : physically, this meansthat the heat loss from the flame becomes too excessive forsustaining steady burning, and flame extinction is observed.

The experiments of [107] were carried out on aDME : O2 : He mixture at P = 72 Torr. A uniform dischargewas generated between the burner nozzles by placing porousmetal electrodes at the nozzle exits. The temperature at theposition of the discharge was estimated to be about 600 K.The densities of the OH and CH2O were measured using aplanar laser-induced fluorescence (PLIF) technique. It wasshown that application of nanosecond discharge significantlychanges the ignition and extinction characteristics of DME.The authors observed experimentally a significant differencein CH2O mole fraction for DME and methane [109] plasma-modified chemistry.

The effect of low temperature plasma was modelled usingthe plasma mechanism suggested in [110] and the DMEcombustion mechanism from [111]. To model the collisionsof DME with electrons, the authors postulated that the cross-sections for the reaction of dissociation of DME by electronimpact, e + CH3OCH3 → CH3OCH2 + H + e and e +CH3OCH3 → CH3O + CH3 + e are similar to the cross-sections for ethane dissociation. They concluded that the mostimportant agent in the experimental conditions is the atomicoxygen, and performed parametric calculations of the S-curvefor different additions of atomic oxygen. The results are

Figure 12. PSR calculation for DME/O2/He (3%/9%/88%) atT0 = 650 K with atomic O addition at different levels, atP = 72 Torr. Reproduced with permission from [107], Copyright2014 Elsevier.

represented in figure 12. It is clearly seen that with increase ofthe initial O atom density, a direct transition for the ignition andextinction diagram without hysteresis was observed, changingthe conventional S-curve to a fully stretched one.

4.1.3. Experiments verifying the role of O2(a 1�g) and ozonein PAI/PAC. Finally, a few low pressure experiments directlyrelated to the subject of PAC should be mentioned here. Theyare based on artificial additions of O2(a 1�g) or O3 moleculesto combustible mixtures.

The idea of the intensification of the ignition process inH2 : O2 mixtures by singlet molecular oxygen produced by aglow discharge in molecular oxygen was first discussed manyyears ago [112, 113]. Later, this approach was developedfor hydrogen-containing and methane-containing mixtures bymeans of detailed numerical modelling [114–117]. Theauthors demonstrate that the presence of singlet oxygenmolecules in the H2 : air and CH4 : air mixtures, in amountsof about 5% to 10% of the total concentration of the molecularoxygen, can considerably influence the ignition delay timeand/or the speed of laminar flame propagation (by 50% to 70%for lean mixtures).

An experimental study of the influence of singletmolecular oxygen on the ignition delay time in ahydrogen : oxygen mixture is presented in [118]. The initiationof combustion in a gas flow in a H2 : O2 = 2.5 : 1 mixture atlow—10 Torr—pressure, and temperature equal to 780 K, wasstudied. Excited O2(a 1�g) and O2(b 1�+

g ) molecules wereproduced in a gas discharge before the gas mixing and deliveredto a preheated flow chemical reactor. Oxygen atoms wereremoved from the gas flow by the catalytic reactions O + HgO→ O2 + Hg and O + Hg → HgO on the walls of the dischargecell and those of the drift tube coated with HgO. Singlet oxygenmolecules, in concentrations of about ∼0.04 in the oxygenflow, significantly shortened the induction zone length, from 51to 17 cm. The authors concluded that the efficiency of singlet

14


oxygen molecules in the initiation of combustion is higher thanthe efficiency of equilibrium thermal heating.

The conclusion as regards the efficiency of singlet oxygenin combustion is argued against in [19], where, on the basisof analysis of the kinetics, the author concludes that, forH2 : O2 mixtures, the presence of even a small (10−4) initialconcentration of atomic oxygen reduces the effect of O2(a1�g)admixtures on the processes of ignition to just additionalheating of the mixture in the process of O2(a 1�g) deactivation.

Although controversial for practical applications, thequestion of a role for singlet O2(a 1�g) and O2(b 1�+

g )molecules in combustion kinetics holds significant fundamen-tal interest. Two recent papers can be mentioned here. Theexperiments of [119] study the isolated effect of O2(a 1�g) onthe propagation of C2H4 lifted flame at low pressures (3.61 and6.73 kPa). The singlet oxygen was produced in a microwavedischarge and then isolated from O and O3 by NO addition. Thedensities of O2(a 1�g) and O3 were measured quantitativelythrough absorption, via integrated-cavity-output spectroscopyand one-pass absorption, respectively. The O2(a 1�g) was pro-duced in concentrations of over 5000 ppm, and enhanced theflame propagation by several percent. Comparison with nu-merical modelling using the existing kinetic schemes demon-strated a significant—at least 5–10 times—overprediction ofthe flame increase velocity by the model. The lack of tem-perature dependent data for the rate of quenching of singletmolecular oxygen by hydrocarbon species was suspected to bethe main cause for the discrepancy.

The temperature dependences for quenching of O2(a 1�g)by CH3 radicals and the potential energy surface of thereaction CH3 + O2(a 1�g) were investigated in [120]. Threereaction channels were studied, namely: CH3 + O2(a 1�g)↔ CH3O2(A′); CH2O + OH; CH3O + O. The reaction rateswere calculated and used in a refined kinetic scheme of lowtemperature (T < 1000 K) methane oxidation. The finalconclusion was that the last two channels are responsible forthe acceleration of methane oxidation.

Two recent papers should be mentioned when speakingabout the role of singlet oxygen in the combustion: the paperdescribing 2D modelling of the influence of O2(a 1�g) onthe ignition of hydrogen : oxygen mixtures [121] and thepaper presenting quantum chemical calculations of the mostimportant constants of elementary processes involving singletoxygen [122]. In [121], the comprehensive analysis of theavailable experimental data and, in particular, the analysis ofthe densities of residual atomic oxygen, allowed the obtaining,for the first time, of the value (10%) for the probability ofthe branching channel H + O2(a 1�g) = O + OH at hightemperatures.

The intensification of combustion by the addition ofozone has also been discussed for decades. For example,the paper [123] compares, by means of numerical modelling,direct laser initiation of ignition with the initiation of ignitionvia laser excitation of the asymmetric vibrations of the O3

molecule at the wavelength of 9.7 µm. The paper [124] studies,experimentally and numerically, the influence of the controlledO3 density on the propagation speeds of C3H8 lifted flames.It is demonstrated that the addition of 1260 ppm of ozone to

O2 : N2 flow leads to 8% enhancement in flame propagation.The author showed that the kinetic effect on flame propagationwas much greater than the thermal effect from the energycontained within O3. It was also shown that, in this case,ozone molecules serve mainly as a source of O atoms and, inreactions with H early in the pre-heating zone, as a source ofOH radicals. The subsequent reactions of O and OH with fueland fuel fragments provide a chemical heat release, increasingthe propagation speed.

The results are consistent with the results from the recentpaper [125], where laminar burning velocities of methane–airmixtures with and without O3 were determined experimentallyand analysed numerically. Around 16% enhancement wasobserved on the lean side (ER = 0.65) but only 9.8% and 9.0%on the rich side (ER = 1.4, ER = 1.45) with 7000 ppm ozonepresent in the oxidizer. With 3730 ppm seeding, experimentaldata demonstrated ∼8% increase in fuel-rich mixtures and∼3.5% in the stoichiometric mixture. A linear relationshipwas found between the enhancement and increase of the O3

concentration. Numerical simulations and detailed sensitivityanalysis were carried out, and it was concluded that the extraO radicals contributed by O3 were produced in the pre-heatingzone, initiating the chain propagating reactions. Radical chain-branching reactions were found to be the main contributor tothe burning velocity increase, rather than thermal effects.

To study the peculiarities of the kinetics and energyexchange in fuel-containing mixtures in the discharge, oxygen-free mixtures are used. The paper [126] presents a detailedkinetic scheme for propane conversion in N2/C3H8 mixtureswith a concentration of hydrocarbon molecules up to 5500 ppmunder the action of a photo-triggered discharge at P =460 mbar and ambient temperature. The homogeneous plasmavolume of 50 cm−3 between two electrodes, 50 cm long witha spacing d = 1 cm and a flat profile over 1 cm in width, wasdirectly connected to an energy storage unit of capacitanceC = 17.44 nF, charged up to a voltage V0 in a few hundrednanoseconds. At the moment when the V0 voltage is applied tothe electrodes, the gas breakdown is triggered by an additionalphoto-ionization of the mixture by UV photons which areproduced by an auxiliary corona discharge located at thebottom of the main discharge. The discharge frequency was1.25 Hz, and a slow gas flow was used to change the gas mixturebetween the discharges. A self-consistent 0D dischargeand kinetic model was used to interpret chromatographicmeasurements of propane, hydrogen and hydrocarbons withtwo or three carbon atoms. It was suggested, on the basisof measurements and model predictions, that quenching ofnitrogen metastable states by C3H8 leads to the dissociationof the hydrocarbon molecule, mainly to H2 and C3H6.This idea was further developed in [127], where similarexperiments were performed for C3H8 : N2 : O2 mixtures belowthe flammability limit. The relative roles of reactions ofoxidation of propane, as well as reactions with electronicallyexcited molecular nitrogen and electronically excited oxygenatoms, O(1D), were discussed. It was shown that, under theexperimental conditions of [127], the OH radical is one of themajor components of the conversion of propane.

Although low pressure PAI/PAC experiments providekinetic information, atmospheric pressure experiments allow

15


direct comparison with standard laboratory burners. Theparameters for comparison are typically the flame propagationspeed and the concentrations of intermediates.

4.2. Atmospheric pressure experiments

Achieving discharge spatial uniformity at high gas numberdensity is a challenge. This is one reason why, at atmosphericpressure and higher, experiments are mainly directed tochecking the possibilities of industrial applications or todemonstrating the foremost features and peculiarities of PAC.

4.2.1. Pin-to-pin repetitive discharge. It has been shown[128] that nanosecond repetitively pulsed plasma (NRPP)can be used for the stabilization of a 25 kW lean turbulentpremixed propane/air flame (Re = 30 000). Positive polarityelectric pulses 10 kV in amplitude and 10 ns in duration at arepetition frequency of up to 30 kHz initiated the dischargebetween a refractory steel wire, 2 mm in diameter (anode),and an aluminum bluff body, 10 mm in diameter. Dischargepositioned in the recirculation zone of the flow significantlyincreased the heat release and the combustion efficiency,stabilizing the flame under lean conditions where it would notexist without plasma. Stabilization was obtained with a plasmapower of about 75 W, or 0.3% of the maximum power of theflame. The authors separated a few zones as a function of flowrate and equivalence ratio, namely a lean flammability limitwithout plasma, transition from the V-shaped to an intermittentflame, then transition to a pilot flame, and transition from a pilotflame to extinction. The lean flammability limit decreased bya factor of 3, from ER = 0.9 to ER = 0.3 at air flow rates ofabout 0.3 m3 h−1.

In the most general way, use of pin-to-pin geometryfor a nanosecond repetitively pulsed discharge (NRPD) atatmospheric pressure needs a joint analysis of the kineticsand hydrodynamics at both discharge and combustion stages.The most detailed description of the NRPD in air can befound in [129]. The discharge was initiated by pulses witha duration of 10 ns and an amplitude of 5.7 kV at a repetitionfrequency of 10 kHz in air preheated to 1000 K flowing with avelocity of 2.6 m s−1. The pin-to-pin electrodes were separatedby 4 mm. Temporally synchronized measurements of thecurrent, voltage, gas temperature, and absolute densities ofO(3P), N2(A 3�+

u ), N2(B 3�g), N2(C 3�u) and electrons arepresented in the paper. The diameter of the discharge obtainedfrom the optical emission of the first and second positivesystems of nitrogen using an Abel inversion was found tobe 450–500 µm. A temperature increase of about 900 Kin 20 ns was observed, corresponding to a heating rate of4.5 × 1010 K s−1. So a sharp energy release in a concentratedzone creates a shock wave propagating in a radial direction[31]. The fraction of dissociated oxygen in the channelbetween the electrodes reached about 50% at 20 ns. Both theoxygen density and the gas temperature evolution are in goodagreement with the fast gas heating mechanism suggested in[20]. It is shown experimentally that: (i) a significant part of thedissociation proceeds via collisions of O2 with excited nitrogenmolecules; (ii) the observed rate of heat release is due to fastcollisional relaxation of the energy stored in electronically

excited atoms and molecules. The fraction of energy expendedon dissociation of oxygen was found to be about 35%, and thefraction of energy expended on gas heating was about 21% ofthe total energy deposited in the plasma.

A set of consistent experiments and calculations, wherethe NRPP is used as a tool of modification of lean fuel–air flows, is presented by [130–134]. Repetitive nanosecondpulsed plasma discharge was used to stabilize a lifted methanejet diffusion flame in ambient (300 K) and elevated temperature(855–975 K) vitiated co-flow. Pulses of 10 kV peak voltage,with 15 ns pulsewidths, at a 50 kHz pulse repetition rate, wereused to initiate the discharge. The interelectrode distancewas adjusted, from 0.8 mm at 300 K to 2.6 mm at 975 K.The flame stability was greatly improved by the applicationof the discharge, extending the stability limit to temperatureswell below 940 K, where there was no flame observed in theabsence of a discharge. At high temperatures, the flame liftoffheight increased with increasing temperature, in contrast towhat was observed above 940 K in the absence of a discharge.The conclusions of the authors were that: (i) the dischargeultimately serves as a reformer of the fuel, converting a fractionof the fuel to H2 and CO; (ii) at high temperatures, the rapiddepletion of the molecular hydrogen and carbon monoxide isthe reason for the diminishing effect of the discharge.

The idea that the discharge produces a region similar, inkinetic behaviour, to a cool flame, that is a premixed flowof sustained low temperature partial oxidation and reformingreactions, is suggested in [131] for a laminar premixedmethane : air flame, and further developed in [132, 133] onthe basis of experiments and numerical modelling. Theexperiments are performed for interelectrode distances ofabout 1 mm, discharge voltage amplitudes of 6–8 kV, and pulseduration of about 10 ns at a 50 kHz pulse repetition rate. Thegas flows are about 1 m s−1, and the equivalence ratio is equalto ER = 0.45–0.55. Results from emission spectroscopy andgas chromatography are compared to ones from 2D numericalmodelling combining plasma and combustion codes. Thepaper [132] discriminates between the plasma produced regionof radicals (‘preflame’ in [132]) and cool flame (typicalfor low temperature combustion), presenting their detailedcomparison. Both the plasma-excited region (‘preflame’) andthe cool flame provide a partial conversion of fuel, supplyingH2 and CO for the following combustion, but their physicalnatures are different. In pulsed discharge at high electric fields,the energy for breaking chemical bonds is delivered mainlyvia excitation of electronic levels of molecules or atoms,while the cool flame is governed by temperature increase andfollows low temperature chemistry. So, the optical emissionof the discharge must be dominated by simple molecules andradicals, like excited N2, or produced—mainly as a resultof oxygen dissociation in the discharge—OH radicals, whileoptical emission from the cool flame is dominated by therelaxation of formaldehyde (HCHO* → HCHO).

A detailed 2D numerical model for plasma-enhancedCH4 : air combustion, developed in [133], takes into accountthe gas kinetics and heating in the nanosecond discharge, thechemistry, the heating and the diffusion at the combustionstage. The plasma reaction set used by the authors is given

16


Figure 13. Comparison of (a) methane consumption, (b) CO, (c) H2 mole fractions and (d) gas temperature at discharge downstreamlocations between the simulation and the experimental results for methane : air at ER = 0.45 and 0.55. Reproduced with permissionfrom [133], Copyright 2012 Elesvier.

in the paper. It was combined with the DRM19-developedreduced methane : air combustion mechanism [135]. Figure 13illustrates the comparison of the experimental data and thenumerical modelling for a few principal components andfor gas temperature downstream from the discharge. It isclearly seen that main trends correlate well for CH4, H2 andCO, although the quantitative comparison for H2 and COis questionable. The temperature behaves differently in theexperiments and calculations; the authors attribute this tothe possible intrusive nature of the thermocouple probe. Itshould also be noted that calculations of mole fractions ofactive species produced by the discharge result in significantlylower atomic oxygen density than for a similar dischargein air [129]: the paper [133] reports about 2% of O atomsproduced in the discharge. The conclusion following that ispresented in [133, 134] is that for equivalence ratio less thanthat associated with the lean flammability limit (ER = 0.45),the nonequilibrium plasma was able to sustain ignition, but theextent of the combustion was diminished with a downstreamposition. At an equivalence ratio just above the lean limit(ER = 0.55), the combustion propagated beyond the discharge

into the surrounding flow. The localized discharge was foundto be a source of radicals and heat, which diffuse out into thesurrounding flow, serving as a flame holder.

Two papers should be mentioned as a continua-tion/extension of work with a nanosecond discharge at at-mospheric pressure: the study of the stability limit extensionof premixed and jet diffusion flames of methane, ethane, andpropane using nanosecond repetitive discharge [134] and theapplication of repetitive nanosecond discharges to ignite and tohold jet flames in supersonic crossflows over a flat wall [136].In [134], a local equivalence ratio analysis by means of laser-induced breakdown spectroscopy (LIBS) was used to optimizethe flame holding conditions; and in [136], a configurationcombining an upstream subsonic oblique jet and a downstreamsonic transverse jet was proposed to provide adequate flow con-ditions for jet flame ignition assisted by the plasma discharge.Combined Schlieren photography and PLIF imaging were usedto prove the validity of the suggested idea.

4.2.2. Transient plasma ignition. Understanding the ignitionof combustible mixtures under the conditions of internal

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Figure 14. Images of flame development in ER = 1.1 C2H4 : air, 6 ms after ignition (after initial flame kernel development). The ICCD gateis 0.3 ms; the sensitivities are equal for the two images. Left: spark ignition using a standard 105 mJ, 10 µs, 15 kV spark ignition system anda spark plug with a 1 mm gap. Right: TPI using a 70 mJ, 12 ns, 54 kV pulse with a 6 mm gap [140].

combustion engines demands specific knowledge of dischargebehaviour at elevated pressures. Nanosecond or transientplasma ignition (TPI) has been studied for spatially distributedplasma at atmospheric pressure. Decrease of the ignition delaytime by a factor of 3–10, comparing with a conventional sparkplug with approximately the same energy, for a nanoseconddischarge 50 ns in duration has been demonstrated for aC2H2 : air mixture, with ER = 1.44 at initial temperatures ofT = 296–396 K [137] in the framework of a study connected todetonation engines. Although this subject is beyond the scopeof the present review, it should be noted that the efficiencyof nanosecond discharge for initiation of detonation has alsobeen proved by experiments [138, 139], where the Chapman–Jouguet velocity was reached in a 13 cm diameter tube ata distance approximately one diameter from the electrodesystem [138] and two possible mechanisms for a deflagration-to-detonation transition, including a gradient mechanism, wereobserved experimentally [139].

Time-resolved ICCD images of the nanosecond dischargeand following TPI in a quiescent C2H2 : air mixture atP = 1 atm and T = 300 K are presented in [140]. Twodifferent nanosecond pulses, 10 and 50 ns in duration, arecompared to the conventional spark plug case as regardsthe efficiency of the ignition. The nanosecond dischargeconsists of a number of streamers propagating radially fromthe high voltage wire electrode located axially in the groundedcylinder. The following ignition happens near the high voltageelectrode and propagates radially. At high pressures, theenergy distribution in the discharge and the volumetric energydensity become the main factors determining the initiation ofthe ignition. Two different discharge/ignition systems are usedin [140]: a wire cylinder with stainless steel tubes of varyingdiameters used as the cathode (10–36 mm internal diameter)and a system with field enhancement at four protrusionsfrom the cylindrical cathode. Combustion evidently startsin the regions with high electric field. Figure 14 presents acomparison of ignition images from a conventional ignitionsystem and a TPI nanosecond discharge plug with a 12 mm

cathode diameter. In the case of the TPI igniter, the flamepassed through the holes in the cylindrical cathode forminga flower-shaped flame front, in contrast to the hemisphericalsmooth flame front in the spark-ignited case. Temperaturemeasurements obtained by recording a rotational spectrum ofthe second positive system of nitrogen showed a significantdifference with respect to pin-to-pin nanosecond discharge[129], where energy is concentrated in a single channel: notemperature increase was observed, and the temperature in thestreamer channels was reported to be close to room temperatureduring the discharge.

The importance of electrode geometry in nanosecond TPIsystems was confirmed by the paper [141], where combustionICCD images for the same cylindrical geometry of a TPIigniter were obtained with OH and CH emission filters (307 nmand 430 nm respectively), and 2D maps of the OH densityevolution have been recorded using calibrated PLIF withinthe time interval immediately from the discharge (50 ns) toinitiation and development of the combustion (20 ms). TheOH transformation in space is quite complex. The radical wasproduced in the branched structure of streamers with a maximaldensity near the wire anode (50 ns), then a structure with adiffusive distribution of OH around the anode and maximumdensity in the former streamer branches was formed (around1 µs), and finally (after hundreds of µs) a main flame startedfrom the centre of the diffuse OH kernel. The picture isvery much consistent with the ideas concerning a partial fuelconversion by plasma of a nanosecond discharge [59, 131].

To avoid the difficulties connected with 3D spatialdistribution of the discharge energy, a nanosecond dischargein a simplified point-to-plane geometry was used to identifythe main parameters of the transient plasma in combustiblemixtures [142, 143]. A stainless steel needle with tip radius75 µm was used as the high voltage anode and the sinteredbronze surface of a McKenna burner as the grounded cathode.The interelectrode distance was equal to 8 mm. Voltage andcurrent profiles were measured with nanosecond resolution.The region of interest tested with the CARS [142] and TALIF

18


Figure 15. Images of nanosecond discharge in (a) air, ER = 0; (b) C3H8, ER = 0.4; (c) C3H8, ER = 0.8; (d) C3H8, ER = 2.1. The ICCDgate is 3 ns [142].

[143] techniques was limited to a 250 × 250 µm2 zone nearthe tip of the anode. Rotational and vibrational excitation ofnitrogen molecules in the discharge afterglow in a variety offuel/air mixtures outside the limits of combustion was studied.Ethylene : air (ER = 0.25, 0.5, 2.4), methane : air (ER = 0.3,0.6) and propane : air (ER = 0.4, 0.8, 2.1) mixtures wereused. For each fuel, the equivalence ratios of the gas mixtureswere individually chosen to be just outside of the limits offlame stabilization. The flow rate was regulated to producea gas speed of 10 cm s−1 to ensure complete recycling of thegas volume in the discharge from shot to shot at 10 Hz whilemaintaining a quasi-quiescent environment.

The authors point out [142] that, despite noticeablemacroscopic differences in different mixtures—the dischargepulses in richer mixtures were audibly louder—the measuredelectrical characteristics remained essentially identical, withno more than 5% variation in pulse width, peak current,peak voltage and energy delivered. In spite of the negligiblechanges in electrical parameters, the morphology of thedischarge changed significantly: branching increased withincreasing fuel part in the mixture (figure 15 illustrates thiseffect for a propane : air mixture), and decreased in the setCH4–C3H8–C2H4.

4.2.3. Intermediate species in atmospheric pressureexperiments. CARS measurements of the vibrationalpopulation of N2(X 1�+

g ) reveal that, during the discharge, thevibrational temperature in air in the zone near the anode isclose to the ambient value. This is in contrast with the casefor a lot of measurements of the vibrational temperature innanosecond discharges in air based on optical emission fromthe excited N2(C 3�u) state. The addition of fuels alters thechemistry, and a peak of N2(X 1�+

g ) vibrational populationin the discharge (t < 40 ns) becomes evident. The decaytime of vibrational population after the discharge dependson the gas mixture and stoichiometry. While the vibrational

excitations of air and fuel : air mixtures in the discharge andnear afterglow are different, a common feature observed forall mixtures investigated is a strong 50% to 100% increase inthe v = 1 N2(X 1�+

g ) vibrational population at 1–10 µs afterthe discharge. The authors explain this in terms of energyrelaxation in the process N2(A 3�) + N2(A 3�) → N2(C 3�)+ N2(X 1�, v). Another important observation is that fuelmolecules quench vibrational states of nitrogen at a rate thatincreases monotonically with the concentration regardless ofthe fuel type.

The rotational temperatures in the region of interestwere observed to already be significantly above the ambientones for all mixtures during the discharge: about 1100 K forair and CH4 : air mixture, and about 1500 K for C2H4 : airand C3H8 : air mixtures. For all fuel : air mixtures the heatis generated immediately after the discharge and decreasesmonotonically on the microsecond–millisecond time scale, butambient temperature is not reached before the gas under studyis recycled with fresh unheated gas.

The authors estimate the part of the discharge energydeposited in the near-anode region as 0.1% to 16% of the totaldeposited energy, depending upon the nature of the gas. Tocompare the thermal action of the nanosecond discharge withthe minimal ignition energy (MIE, or minimal energy requiredfrom a conventional spark plug to initiate the combustion forgiven pressure, temperature and mixture composition), theauthors compare the energy density in the region of interestwith the MIE divided into a typical volume of a spark discharge.Their conclusion is that for all the mixtures the depositedthermal energy density, calculated on the assumption that therotational temperature is uniform throughout the region ofinterest, is higher than the deposited thermal energy densityrequired for spark ignition. For example, for both C2H4 andC3H8, the deposited thermal energy densities measured innanosecond streamer discharge are 1700 mJ cm−3, well abovethe required thermal energy densities of 100 mJ cm−3 and300 mJ cm−3 for C2H4 and C3H8, respectively. They note

19


Figure 16. TALIF measurements of O number density in CH4/airmixtures versus delay time from the discharge initiation;nanosecond discharge in non-preheated gas flow [143].

also that it remains unclear whether the total deposited thermalenergy is large enough to overcome thermal losses and producea stable flame kernel.

Measurements of the O atom density in the same region ofinterest, 250×250 µm2 near the anode, are presented in [143].The O atom density just after the discharge corresponds toabout 20% of the dissociation fraction of O2, which is coherentwith TALIF observations in NRPD [129]. It was shown that,in a case of three-body processes of recombination, O + O +M → O2 + M (M = O2, N2), when a typical decay timeis determined by the O atom density, the diffusion of O atomsfrom the axis of the discharge to the periphery significantlyinfluences the efficient decay time. Comparison of atomicoxygen production in air and fuel : air mixtures reveals thatthe addition of fuels to the gas mixture decreases the O atomdensity and increases the rate of decay. The amplitude ofthese changes increased monotonically with the concentrationof the fuel (figure 16). Preliminary analysis of the possiblechemical pathways leads to the conclusion that the changesin O atom behaviour in fuel : air mixtures are due to chain-branching reactions which are much more rapid than O atomrecombination.

2D maps resolved in time and space of radicals producedin methane : air mixtures of different stoichiometries (ER =0.05, 0.5, 0.65 and 2.2) by negative electric pulses from−10 to −40 kV in amplitude, 70 ns in duration and witha pulse repetition frequency up to 200 Hz were studied in[144, 145]. The electron temperature and number density in thedischarge were measured by using laser Thomson scattering;the temperature of the neutral molecules was measured by theCARS technique. OH, CH and CH2O were probed using PLIF.For CARS measurements, the probe volume was a 0.6 mmlong and 50 µm diameter cylinder. PLIF measurements wereperformed within the interelectrode gap (3 mm) with a spatialresolution of 20 µm × 20 µm × 150 µm.

Figure 17 illustrates the complex spatial structure offluorescence for different radicals in the interelectrode gap.The authors report high rotational and high vibrational

Figure 17. Example of fluorescence distribution between theelectrodes: (a) OH at a delay of 20 µs after the discharge; (b) CH ata delay of 500 ns. Reproduced with permission from [144],Copyright 2009 Elsevier Masson SAS.

temperatures, varying from 2500 K at ER = 0.6 to 3000 Kat ER = 2.2. The temporal evolution of OH, CH and CH2O isdescribed on the basis of numerical modelling [145] using theinitial radical composition and the GRI-Mech 3.0 combustionmechanism. The calculations, presented for ER = 0.6 andER = 2.2, correspond to the results of PLIF measurements,demonstrating CH decay at 0.1–1 µs followed by H2COformation with a peak density at about 0.5 µs, and OH decay at10−4 s for ER = 0.6 and at 10−5 s for ER = 2.2. The results ofthe work confirm the idea of the importance of the 2D spatialdistribution of the temperature and the densities of differentspecies, even in the case of localized pin-to-pin discharge atatmospheric pressure.

The paper [146] presents experimental measurements ofthe time evolution of OH radicals in premixed hydrocarbon–air flows in the late afterglow of a nanosecond dischargeat atmospheric pressure. Measurements were performed bythe saturated LIF technique with excitation of OH at λ =282.92 nm. The initial gas flow temperatures below the self-ignition threshold (300–800 K) were considered. The fuelswere CH4, C2H6, C3H8 and C4H10 at equivalence ratios ER =0.1, 1 and 3. The plasma was generated by 20 kV positivepolarity pulses 10 ns in duration at a 10 Hz repetition rate inan 8 mm gap between two point-like electrodes. The region ofinterest was a region with a characteristic size of a few mm inthe middle of the discharge gap. A 2D map of the dischargeemission was recorded with sub-nanosecond ICCD imaging.The flow rate of the gas, 20 m s−1, was chosen to provide thechange of the gas between the pulses.

Significant changes in OH behaviour with temperatureare illustrated in figure 18. At T = 300 K, for all threestoichiometries, the OH decay takes the longest for the CH4-containing mixture, decreasing in the range from methane topropane. The typical time for OH decay at room temperaturecan be easily obtained from any standard combustion scheme(scheme [117] was used for the analysis); the reactionproducing additional OH in a CH4 : air mixture with ER = 0.1on the scale of 100 µs is, mainly [147],

HO2 + O = OH + O2,

while the reactions responsible for the decay are

OH + O = H + O2,

OH + CH4 = H2O + CH3.

20


Figure 18. OH evolution over time in fuel–air flows excited by nanosecond discharge; P = 1 atm, CH4–C4H10 : air mixtures, (a) 300 K; (b)800 K. Reproduced with permission from [146], Copyright 2011 Elsevier.

It should be noted that the results obtained for ambient initialtemperature, T0 = 300 K, are in good agreement with the LIFOH measurements of other authors [144].

With temperature increase, the situation changesdramatically. At T = 400 K, the OH decay time in themethane-containing mixture (ER = 0.1) is already increasedby a factor of 3. At T = 800 K, all of the availablecombustion schemes, including the schemes developed forrelatively low temperature regions, fail to describe OH decaytimes increased by a factor of 100–500. In addition, the orderof the hydrocarbons changes: the longest OH decay at 800 Kis observed for butane-containing and propane-containingmixtures, decreasing gradually for ethane and methane. Theauthors of [146] indicate that the attempts to describe thestretching of the OH decay time with temperature increaseusing well-developed kinetic mechanisms have failed. In [27],vibrational excitation at the end of the discharge pulse leadingto the acceleration of chemical reactions of OH production inthe late afterglow was suggested as a possible mechanism forexplaining the observed elongation of the OH decay time withtemperature.

To summarize the results obtained at atmosphericpressure, we will refer to a few papers presenting useful resultsfor the analysis of PAC, although these papers are not in themain area of the current discussion.

There is a paper [148] in which the absolute OH densityand temperature are measured in 10 ns pulsed sparks betweentwo open electrodes in rich (50% of H2) and stoichiometric(30% of H2) hydrogen : air mixtures initially at ambientpressure and temperature. A tunable KrF excimer laser,instead of the traditional NdYAG : dye system, was used in theexperiments. The measurements were made at t > 3 µs; it wasfound that the absolute densities of the OH radicals are between1016 and 1017 cm−3. The gas temperature after the dischargewas found to be 1000–1200 K. The authors concluded that theflame velocity, after some initial stage, is not influenced by theignition energy.

The paper [149] presents measurements of the elec-tron density and electron–neutral collision frequency in

combustion-produced plasmas in methane–oxygen stoichio-metric mixtures at low pressures, 100–200 Torr, and providesa detailed analysis of the relative electron densities and colli-sion frequencies in a cool flame zone and in the main flame,as well as an analysis of the chemi-ionization. The reactionsresponsible for the blue fluorescence in the pre-flame zone(emission of electronically excited CO2, CH2O and HCO) arediscussed.

Finally, a significant dependence of the results upon thespatial structure of the discharge was directly confirmed bycomparative study of the ignition of C3H8 : air and C7H16 : airmixtures at atmospheric pressure and temperature [150]. Apositive high voltage (40–50 kV) was applied between aparabolic pin electrode with a radius of 50 µm and a groundedplane over a single short nanosecond range pulse with a risetime of 2 ns. The gap distance between the two electrodeswas adjusted between 1 and 2 cm and the pulse length variedbetween 10 to 60 ns, so varying the volumetric depositedenergy. The ICCD images of different discharge structurespresented, demonstrating higher branching in the air–mixturewith methane–mixture with heptane range, are consistent withthe results of [142]. A few patterns for initial flame kernelsin the discharge gap were obtained, namely, one-spot ignition,two-spot ignition near the electrodes and cylindrical ignitionalong the axis between the electrodes.

4.3. High pressure experiments

The spatial distribution of the energy release in the dischargebecomes an even more critical issue at elevated pressures.Papers on the initiation of combustion at high pressures bypulsed nanosecond discharges are not numerous.

4.3.1. Constant volume chamber experiments. Propane : airignition in a constant volume chamber for a pressurerange of 0.35–2.0 bar has been studied in [151]. Therepetition nanosecond discharge was initiated in a point-to-plane geometry (the radius of the tip of the high voltageelectrode was equal to 0.3 mm) with 1.5 mm interelectrode

21


distance. Pulses 5 kV in amplitude, 10 ns in duration andwith up to 30 kHz repetition rate were used. The mixturesunder study were synthetic air, C3H8 : air mixtures (ER =1.0 and ER = 0.7), and 10% and 30% mass N2-dilutedstoichiometric C3H8 : air mixtures. The discharge parameterswere studied in air in a pulse-to-pulse mode, and the ignitionwas investigated in combustible mixtures with the help offast imaging, spectroscopy and pressure measurements. Itshould be noted that, although no electrode erosion wasobserved visually, the spectroscopic measurements showedintense emission from the material of the electrode (Ni+, W+

and Ni).A significant decrease of the ignition delay time was

obtained using a burst of high voltage pulses. The MIE neededto ignite the gas mixtures was determined experimentally; itdecreased with pressure from 20 mJ at 0.4 bar to 1.3 mJ at 2 barfor a propane : air stoichiometric mixture. It was shown that: (i)a few initial pulses, when the discharge gap is not bridged, bringtoo low an energy to the system to ignite the mixture and socannot ignite the mixture; (2) starting from the first breakdownclosing the gap, the accumulation of pulses in the mixturedoes not, in the opinion of the authors, significantly changethe conductivity of the mixture, but increases the densitiesof dissociated species and radicals, so improving the ignitioncharacteristics.

The nanosecond repetitive pulsed plasma dischargebetween two open electrodes is different, in physicalparameters, from the conventional spark discharge, but isa discharge localized in space. NRPP causes significantdissociation and temperature increase in a thin—hundreds ofmicrometres—channel, following expansion and significanthydrodynamic perturbations [129]. This discharge can be ofgreat interest for internal combustion engine applications.

The improvement of existing engines and the developmentof new engines need new technical solutions, aimingat both increase of engine efficiency and reduction ofpollutant emission level. Multi-spot ignition, as a toolfor improving combustion speed and heat release rate,is one of the prospective directions. In [152], directexperimental comparison of conventional spark ignition andtwo-site dual-fuel ignition was performed on the basis ofpressure measurements synchronized with high speed in-cylinder images of the combustion process. The authorsobserved an increase in combustion speed for a premixed,homogeneous charge of methane with a small quantity of pilotdiesel surrogate fuel. They underline that the increase of thecombustion speed is due to: (1) multi-site ignition created bypilot fuel injection and the subsequent transition leading tothe development of several flame fronts; (2) the fact that theignition is distributed over a volume within the combustionchamber; (3) the spray-induced turbulence which is generatedby pilot fuel injection and likely increases the laminar flamespeed for dual-fuel strategies; (4) the possible increase of the‘equivalent ignition energy’ provided by using a highly reactivepilot fuel, as compared with the ignition energy supplied bythe conventional spark plug.

There is some experimental evidence that pulsednanosecond discharge, developing in a multi-streamer

configuration, can be efficient for the initiation of combustionat very high pressures. In [153], the ignition of a methane–air mixture in a static high pressure chamber at an initialpressure of 30 atm and ambient initial temperature has beendemonstrated. The authors used the same principle as waspresented earlier in [137, 140] where a nanosecond discharge,12 or 85 ns in duration, produced a TPI across a 4 mm gap usinga coaxial, 32 mm long electrode. The authors demonstratedthat, ignited with approximately the same delay time, 50 ms,a gas mixture ignited by nanosecond discharge demonstratesapproximately double the rate of pressure increase, comparingto a mixture ignited by a conventional spark, in a chamber570 cm3 in volume.

The efficiency of nanosecond discharges in the ignitionof gas mixtures at elevated pressures can be estimated fromthe results of 0D numerical modelling. The calculationswere performed using the Combustion Chemistry Centermechanism [54] with artificial dissociation of molecularoxygen at the time instant t = 0. Figure 19 a compares, fordifferent specific deposited energies, the ignition delay timesfor: (1) thermal ignition, where all of the energy is expendedfor gas heating; (2) 50% of the energy expended for heatingand 50% of the energy expended for O atom production; (3)100% of the energy expended for O atom production; and (4)autoignition. It is clearly seen that, for the whole of the rangeof parameters considered, for a methane : air stoichiometricmixture diluted (76%) with Ar at P = 15 bar and T = 960 K,the nonequilibrium energy input is more efficient than an equalamount of heat. For specific deposited energies of 0.02–1 eV mol−1, typical for a ‘root’ of a nanosecond streamerstarting from the electrode [155], thermal ignition reduces theignition delay time by a factor of 3–10, while the O atominjection decreases the delay time by a factor of 102–104. Itshould be noted that part of the energy goes to gas heating [20]and, hence, the efficiency will be in between those of cases (1)and (3), but a detailed analysis of reaction channels is necessaryto predict the portion of energy expended on the fast gas heatingin hydrocarbon-containing nitrogen-free mixtures at high gaspressures.

Parametric calculations for different mixtures show that aquite moderate density of atomic oxygen, less than 1% underthe conditions considered, artificially added to a combustiblegas mixture at t = 0 is enough to significantly reduce theignition delay time. Figure 19(b) illustrates this idea foran n-butane stoichiometric mixture diluted with 76% of Arat the initial pressure and temperature of 8.6 bar and 800 Krespectively. The oxygen densities reported for pin-to-pinnanosecond discharges with open electrodes [129, 143] alreadyreach tens of percent in the discharge during the first 10–30 ns. While low oxygen densities are sufficient to ignite themixture, it seems reasonable to distribute the discharge energybetween multiple streamers and to disable, via the geometryof the electrode system, a streamer-to-spark transition. Thecoaxial geometry of nanosecond surface dielectric barrierdischarge (SDBD) has been suggested [156, 157] for the studyof combustion by a single-shot pulsed nanosecond dischargein the combustion chamber of a RCM.

Nanosecond SDBD in coaxial geometry at elevatedpressures, P = 1–6 atm, was studied in [155, 158]. It was

22


0.0 0.2 0.4 0.6 0.8 1.01

10

T=800 KP=8.6 bar

(n-C4H

10:O

2, ER=1)+ 76 % Ar

Ign

itio

nd

elay

tim

e,m

s

Percent of initially dissociated O2

(a) (b)

0.00 0.02 0.04 0.06 0.08 0.10 0.120.01

0.1

1

10

1004

3

1

P=15 barT=960 K

(CH4:O2, ER=1)+76% ArIgn

itio

nd

elay

tim

e,m

s

Specific deposited energy, eV/molecule

2

Figure 19. Numerical calculations of ignition delay time (a) versus deposited energy in a CH4 : O2 stoichiometric mixture diluted with 76%of Ar: (1) thermal ignition, 100% of the energy is expended for heating; (2) 50% of the energy is expended for heating, 50% for atomicoxygen production; (3) 100% of the energy is expended for atomic oxygen production; (4) autoignition; (b) versus percentage of dissociatedoxygen in an n-C4H10 : O2 stoichiometric mixture diluted with 76% of Ar [154].

shown that, for both polarities of the high voltage pulse, thedischarge starts as a set of streamers propagating in the radialdirection from the high voltage electrode along the surface ofthe dielectric. Two maxima of emission were observed: at therise front and at the trailing edge of the pulse. The typicalvelocity of the discharge propagation for positive polaritywas reported to be a few mm ns−1—a few times that for thedischarge of negative polarity. The paper [155] reported,together with discharge ICCD imaging in air, the ICCDimaging of combustion initiation in a C2H6 : O2 = 2 : 7 mixtureat ambient initial temperature and P = 1 atm (figure 20). Theimages were taken using a LaVision Ultra Speed Star ICCDwith a gate of 500 ns and represent a side view and half of atop view for the time instants indicated in the lower corner ofeach frame. The discharge emission is not longer than 1 µs.The first image corresponds to the discharge; all other framescorrespond to combustion. At 50 µs, the formation of multi-spot combustion kernels can already be distinguished aroundthe electrode; at 120 µs and later, a well-developed flower-like flame structure initiating at the ‘roots’ of the streamersand propagating in the radial direction is clearly seen. Thepaper [155] suggests, on the basis of numerical modelling, anexplanation of the values obtained for the ignition delay time.It is important to note that to obtain a coincidence betweencalculated and measured values of the ignition delay, it isnecessary to assume that the main energy release happens inthe vicinity of the edge of the high voltage electrode, in a zonea few millimetres in length.

A modification of the nanosecond SDBD electrode systemwas used in [159], where regular structure of the streamersstarting from the high voltage electrode was ensured by the useof a gear-like high voltage electrode. Figure 21 demonstrates apossibility for controlling the energy per ignition spot by usinga gear-like high voltage electrode: the streamers originate inthe regions of the highest electric field, that is in the vicinityof the gear teeth. The energy per streamer is somewhat higherthat in a smooth edge electrode, and so it is possible to achieveignition at lower voltage amplitudes.

It should be noted that a number of ionized channelscan also be changed by the physics of the discharge itself:in [158], a streamer-to-filament transition in the systemwith a smooth edge electrode is observed with increase ofthe voltage amplitude and/or gas pressure for a negativepolarity nanosecond high voltage pulse. Theoretical estimatesand numerical modelling show that the ionization-heatinginstability at the boundary of the cathode layer can be suggestedas a mechanism for filamentation.

4.3.2. RCM experiments. RCMs are used to simulate asingle compression stroke of an internal combustion enginewithout complicated swirl bowl geometry, cycle-to-cyclevariation, residual gas, and other complications associatedwith engine operating conditions. Recent reviews [42, 160]contain detailed descriptions of RCM and ShT facilities andtheir applications for studies related to chemical kinetics. Theadvantage of RCM facilities for PAI experiments is that theautoignition at the same pressure and temperature can be takenas a reference.

Typically, an RCM consists of a combustion chamberequipped by a piston, a hydraulic motion control chamber anda driving mechanism (pneumatic or pneumatic/mechanical)transmitting the motion to the piston. The combustion chamberis equipped with pressure detectors and—optionally—withoptical windows or a system of selection of probes for gasanalysis using a chromatography technique. This allowsmeasurements of the pressure history, ignition uniformity andkinetic curves of intermediate components. The ignition delaytimes and kinetics of autoignition for different fuels are studiedin RCMs in a well-controlled homogeneous environment[161]. Typical measured ignition delay times range from afew to hundreds of milliseconds, depending on the durationof the compression stroke and the parameters of the reactingmixture after the compression.

The first known RCM nanosecond plasma experimentswere published in [162]. Three electrode geometries were

23


Figure 20. ICCD images of ignition of a C2H6 : O2 = 2 : 7 mixture by nanosecond SDBD [155]. Ambient initial temperature, P = 1 atm.

Figure 21. Images of nanosecond SDBD discharge in air: (a) coaxial system of electrodes for SDBD ignition suggested in (i) [155] and(ii) [159]; (b) typical ICCD image of the discharge with a smooth edge electrode taken with a 2 ns camera gate for P = 1 bar andU = +24 kV [158]; (c) snapshot of the discharge with gear-like electrode for P = 1 bar and U = +24 kV. Reproduced from [159], courtesyof N Aleksandrov.

used for the pulsed discharge: a localized nanosecond spark, aSDBD with a pin-like electrode, and a streamer corona. Theignition of combustible mixtures by the discharge was studied,the discharge occurring during and after compression. Theignition delay time was measured with the help of a pressuretransducer installed in the chamber wall. The authors report asignificant decrease of the ignition delay time, from hundredsto tens of milliseconds, for C3H8 : air stoichiometric and lean(ER = 0.4) mixtures for the pressure range 17–40 bar andtemperatures of 700–1000 K. The observed ignition delay timedepends upon the type of the discharge, and upon the instantof the discharge initiation.

To compare autoignition and ignition by pulsed plasmaunder the conditions of multi-spot ignition, a nanosecondSDBD in the geometry suggested and tested in [155, 158] wasinstalled into the combustion chamber of a RCM describedin [161]. The experiments presented in [163–165] can besummarized as follows: a single-shot nanosecond SDBDprovided a quasi-uniform two-dimensional plasma in thevicinity of the end plate of the combustion chamber. Theignition delay time and energy deposited in the gas by thedischarge were measured for different amplitudes of a highvoltage pulse of positive or negative polarity. The flamepropagation was recorded with the help of fast imaging.

24


25 30 35 40 450

2

4

6

8

10

12

14

16 ER=0.3ER=0.5ER=1

Dep

osi

ted

ener

gy

/mJ

Voltage on HV electrode / kV

(a) (b)

0 100 200 300 400 500 600

10

20

30

40

50

60

5

2

3

4

%o

fE

ner

gy

Reduced electric field / Td

O2, 8.4 eV

Ar, 15.8 eV ionizO

2, 12 eV ioniz

O2, 4.5 eV

O2, 5.6 eV

? =0.5? =0.3

1

Figure 22. (a) Experimentally measured deposited energy as a function of voltage on the electrode for different stoichiometries; negativepolarity pulses, CH4 : O2 : Ar mixtures. For ER = 0.5 the pressure PTDC = 15.1 bar and temperature TC = 943 K; for ER = 0.3,PTDC = 15.5 bar and TC = 962 K; for ER = 1, PTDC = 14.9 bar and TC = 972 K. (b) Energy branching for different reduced electric fieldsin ER = 0.5 (solid lines) and ER = 0.3 (symbols) CH4 : O2 : Ar mixtures: 1: electronic excitation of O2, threshold 8.4 eV; 2: ionization ofAr; 3: ionization of O2; 4: electronic excitation of O2, threshold 4.5 eV; 5: electronic excitation of O2, threshold 5.6 eV. This is accordingto [165].

For each set of initial parameters, ignition by the dischargewas compared with autoignition under the same conditions.The experiments were carried out for methane : oxygen andn-butane : oxygen mixtures with a stoichiometry between 0.3and 1 diluted with 70% to 76% of Ar or nitrogen for initialtemperatures between 600 and 1000 K and pressures between6 and 16 bar. The choice of the mixtures was dictated by thefacts that: (i) cross-sections of collisions with electrons are wellknown for methane; (ii) a region of NTC can be achieved foran n-C4H10-containing mixture under given conditions.

It was found experimentally that the energy deposited intothe gas by the discharge is a strong function of the mixturestoichiometry. This fact is illustrated in figure 22(a): forCH4 : O2 : Ar mixtures in the combustion chamber of the RCM,at P = 15.1 bar and T = 943 K, the deposited energy isat the level of the experimental error and does not exceed1 mJ for a stoichiometric mixture (72% of Ar dilution); theenergy is between 4 and 5 mJ for ER = 0.5 (75% of Ardilution), and, finally, it is 6–16 mJ for ER = 0.3 (76% ofAr dilution). The estimations were made with the help ofnumerical modelling using standard BOLSIG+ software [63]with cross-sections for electron collisions with CH4, O2 andAr taken from [75], [65] and [67] respectively. It is seenfrom the figure 22(b) that no visible changes are observed forthe mixtures of different stoichiometries. So, there is a highprobability that the discharge morphology of strongly dilutedfuel : air mixtures changes significantly with minor changes inthe fuel density. This conclusion is in a good correlation withobservations [143, 150], and it means that additional studies ofthe influence of small fuel additions on the discharge structureat pressures equal to or higher than atmospheric pressure arenecessary.

Stable mixture ignition and subsequent flame propagationin the combustion chamber have been observed for all theparameters investigated for 20 ns discharge duration in the

single-shot regime. An example of the pressure trace ina combustion chamber for autoignition of a stoichiometricCH4 : O2 : Ar mixture diluted with 76% of argon with thepressure at the top dead centre (at the moment of themaximum compression) equal to PTDC = 14.7 bar and thecore temperature (the temperature in the uniform zone nearthe end plate of the RCM combustion chamber) equal toTC = 972 K is given in figure 23(a). The pressure tracefor the same conditions as at initiation of the combustion bynanosecond SDBD with a U = −24 kV amplitude is presentedin figure 23(b). The pressure was measured using a Kistler601A piezoelectric pressure transducer mounted in the sidewall of the chamber between the piston and the end plate. Theflame propagation in the same mixture under PAI is illustratedin figures 23(c)–(f ). It is clearly seen that: (i) the ignitiondelay time under the action of plasma is less than 1 ms at theautoignition delay time of about 80 ms; (ii) pressure increasein the chamber is limited by the flame propagation. Althoughplasma is created as a regular set of streamers around the highvoltage electrode, the resulting combustion wave, recorded byfast imaging, represents a uniform structure on the millisecondtime scale. Taking the number of streamers starting from thehigh voltage electrode equal to Z = 100 and the depositedenergy W < 1 mJ, it is possible to estimate the energy perkernel under conditions of high pressure multi-spot ignition:ω < W/Z = 10 µJ.

Numerical analysis of the modification of cool flames inn-C4H10 : O2 : Ar (ER = 1, diluted with 76% of Ar) mixturesby plasma was carried out in [164]. The action of plasmawas simulated by the addition of atomic oxygen at the timeinstant t = 0. Numerical simulations demonstrated a well-pronounced cool flame and the main ignition starting at 37 msand 43 ms respectively at P = 8 bar and T = 700 K. Upon theaddition of 0.05% of atomic oxygen, a modified cool flame isobserved: the initial density of O radicals decreases sharply

25


0

10

20

30

40

Pre

ssu

re /

bar

Time / ms

(a)

0 50 100 150 200 0 50 100 150 2000

10

20

30

40

(b)

dischargeinitiation

Pre

ssu

re /

bar

Time / ms

(c) (d) (e) (f)

Figure 23. (a), (b) Pressure profiles for stoichiometric CH4 : O2 diluted with 76% of Ar mixture, TC = 972 K, PTDC = 14.7 bar; (a)autoignition, (b) PAI, U = −24 kV; (c)–(f ) successive images of the flame propagation in the same mixture, TC = 947 K, PTDC = 15.4 bar.The sample rate of the camera is 5000 pps; the exposure time is 196.75 µs. The electrode system is on the left, and the piston is on the rightin each frame. The window diameter is equal to 14 mm. This is according to [163–165].

at the beginning of the calculations, then increases slightly,synchronously with the pressure increase, at 2 ms, indicatingthe beginning of the cool flame, and then increases duringthe initiation of the ignition, at 6.4 ms. Addition of 1% ofatomic oxygen leads to complete modification of the kinetics:a separated cool flame is no longer observed, and some pressureincrease is observed during recombination of the radicals; afterthe recombination zone there is a zone of permanent increaseof densities of the main intermediate species, and the mainflame starts at 0.4 ms. We conclude that experiments with lowspecific deposited energy are needed to see the changes specificto low temperature combustion kinetics.

The disappearance of the NTC region under the actionof pulsed plasma is shown numerically by [166] using theexample of a propane : air mixture. Chain branching in theNTC region, considered in [166], is governed by the followingreactions:

R + O2 ↔ RO2 (1)

RO2 + C3H8 → ROOH + C3H7 (2)

ROOH → RO + OH + �E. (3)

Here R corresponds to n-C3H7, i-C3H7, CH3, C2H5,CH3CO3 radicals, and �E is a released energy. The primaryradicals in the mixtures considered are i-C3H7 and n-C3H7.The sequence of reactions (1)–(3) is a degenerate chain reactionwith participation of low reactivity alkyl peroxy radicals, i-C3H7O2, n-C3H7O2 and CH3O2. With temperature increase,the equilibrium in reaction (1) is shifted to the left, the RO2

density decreases and the overall rate of the chain reactionalso decreases. The discharge action can suppress the NTCregion by increasing the rate of production of the peroxy

Figure 24. Ignition delay time versus initial temperature andspecific deposited energy. Propane : air mixture, ER = 1.66,P0 = 5.5 bar [166]. Experimental points are taken from [167].

radicals, RO2. Figure 24 presents calculations [166] andtheir comparison with experimental data [167] for autoignition.Discharge is considered as a joint action of the heating due torelaxation of the energy stored in the electronic and vibrationalexcitation and the production of radicals. It is shown thatthe NTC is suppressed by the action of plasma with specificdeposited energy 0.01–0.02 eV/molecule at reduced electricfield E/N = 100 Td. It is derived in [166] that the NTCis suppressed when the ratio of the radicals produced inthe discharge, [O]0 + [C3H7]0, and the maximal density ofthe propylperoxy radical, [C3H7O2]max, is higher than unity: = ([O]0 + [C3H7]0)/[C3H7O2]max � 1. The condition � 1 does not depend upon the stoichiometric ratio.

26


20 25 30 35 40 45 500

5

10

15

20

25

Dep

osi

ted

ener

gy,

mJ

Voltage on HV electrode, kV

(a) (b) (c)

0 50 100 150 200

1.2

1.5

1.8

2.1-25,6 kV-29,5 kV-34 kV-39,8 kV-43,3 kV-46,9 kV

Pre

ssu

re,b

ar

Time, ms

Figure 25. Modification of a cool flame in n-heptane by nanosecond SDBD. (a) Modification of pressure traces under the action ofnanosecond SDBD, n-C7H16 : O2 : N2 = 1.8 : 19.6 : 78.6 mixture, PT DC = 1.8 bar, TC = 627 K. Negative polarity pulses; the voltageamplitude is between 24 and 46.9 kV. (b) ICCD image of discharge in air, T = 296 K, P = 870 mbar, U = −46.9 kV. The camera gate is100 ns. (c) Deposited energy as a function of voltage amplitude on the high voltage electrode in an n-heptane-containing mixture [169].

To the best of the author’s knowledge, no directexperimental observations of NTC modification by pulsednanosecond plasma are reported in the literature at the moment.Recent experimental work however suggests that cool flamesand two-stage ignition can be triggered by nanosecond pulsedplasma [168]. Nanosecond SDBD at different voltageamplitudes has been initiated in the combustion chamber ofa RCM in the configuration described in [163–165].

Figure 25 illustrates the results for an n-C7H16 : O2 : N2 =1.8 : 19.6 : 78.6 mixture at PTDC = 1.8 bar and TC = 627 K. Noautoignition is observed under these conditions. The dischargewas initiated by double 20 ns pulse; first, in the vicinity ofthe end plate at the top dead centre at t = 0 ms and second,at about t = 60 ms. Starting from a certain amplitude ofthe high voltage pulse (U = −39.8 kV in the particular caseof the experiments presented), the pressure trace is modifiedand demonstrates a low—about 0.1 bar—rise of the pressurewith the discharge action (see figure 25(a)). With voltage,the pressure rise increased progressively to 0.2 bar; a weakinfluence of the second pulse is observed. Finally, a ‘classical’pressure trace typical of two-stage ignition could be observed.The cool flame, initiated by the first pulse, is followed by astrong ignition, initiated by the second pulse and causing asharp pressure rise. The second stage, or main ignition, issimilar to that observed earlier in [163–165], while the coolflame and its modification by pulsed plasma is observed forthe first time.

To analyse the discharge behaviour, additional exper-iments in constant volume discharge chambers in air atambient temperature and gas density and for voltage ampli-tudes the same as in the RCM experiments were carried out(P = 870 mbar). Time-resolved ICCD imaging proved thatthe discharge develops as a set of Z ∼ 100 streamers, prop-agating simultaneously in the radial direction from the highvoltage electrode. Starting from a certain high voltage ampli-tude, the discharge detached from the end plate of the cham-ber (figure 25(b)). The deposited energy measured in the dis-charge in an n-heptane : air mixture changes from 5 to 20 mJ(figure 25(c)), so on the assumption that the discharge structure

500 1000 1500 20000.01

0.1

1

10 1234567891011

10.0

0.03

3.0

1.0

0.3

Pre

ssu

re,a

tm

Temperature, K

Dashed lines: 0.03, 0.1, 0.3, 1, 3 and 10 Natm

0.1

Figure 26. P –T diagram of results available in the literature fromexperiments on combustion initiated or assisted by nanoseconddischarges. The details of the symbols in the legend are given intable 2. Dashed lines represent the isolines of the gas density, N ,normalized with respect to the atmospheric gas density Natm fornormal conditions.

does not change significantly between air and an n-C7H16 : airmixture, the specific deposited energy can be estimated as50–200 µJ/channel. No spark formation was observed in theexperiments. Although the results are encouraging, additionalverification is planned to distinguish clearly between flamepropagation and the ignition event.

5. General remarks and conclusions

To summarize the knowledge relating to the experimentalconditions studied by different scientific groups, the initialexperimental pressures and temperatures from the paperscited in the current review have been plotted as a ‘pressure–temperature’ diagram (figure 26). As the initial pressure andinitial temperature are the parameters typically used to unifycombustion experiments, it seems reasonable to plot the samediagram for PAI/PAC experiments. Showing explicitly the gasnumber density and temperature for each of the experiments,

27


Table 2. References and mixtures under study for the data presented in the P –T diagram in figure 26.

N for symbol Reference Mixture composition, Fi /Oi /Di

1 [95, 100, 102] H2, CH4, C2H4, C3H8/O2/N2, Ar2 [163–165] CH4, C4H10, C7H16/O2/N2, Ar3 [16, 99, 170, 171] H2, CnH(2n+2), n = 1–6, C10H22/O2, N2O/N2, Ar, He4 [94] C2H6/O2/N2, Ar5 [119, 124] CH4, C2H4, C3H8/O2/N2, Ar6 [107] CH4, DME/O2/He, Ar7 [93] C2H2/O2/Ar8 [133, 134] CH4/O2/N2

9 [146] CnH(2n+2), n = (1–5)/O2/N2

10 [162] C3H8/O2/N2

11 [140, 143, 153] C2H2, CH4/O2/N2

the diagram allows comparison of the data obtained by differentauthors relating to the discharge development and combustionkinetics respectively.

The symbols, numbered on the right-hand side of theplot, relate to the appropriate papers in table 2. The mixturesinvestigated are schematized in the same table in the formFi /Oi /Di , where Fi is a fuel, Oi is an oxidizer, and Di isa diluent. Each set of experiments is represented in thefigure 26 as a single symbol with an error bar, where the symboldesignates the centre of the domain investigated, and the errorbar covers all of the intervals of temperatures or pressures understudy. Dashed lines represent the isolines of the gas density, N ,normalized with respect to the atmospheric gas density undernormal conditions.

It is seen from the figure that experiments at ambientinitial temperature cover a wide range of pressures: fromcombustible mixture oxidation in fast ionization waves atP = 1–10 Torr [99, 170, 171] to transient plasma ignitionat 30 atm [153]. Stationary reactors with the possibility ofpreliminary gas heating are used for pressures of 25–100 Torr[95, 100]. High temperature experiments, 700–2000 K, atatmospheric pressure and lower correspond to shock tubefacilities [16, 93, 94]. High pressure (2–50 atm) and moderatetemperature (700–100 K) conditions are covered by theexperiments using rapid compression machines [162–165].Finally, low pressure or atmospheric pressure burners areused for the intermediate range of temperatures, 500–1000 K[107, 133, 134]. Isolines of the gas number density, plottedwith dashed lines in the figure, clearly demonstrate that themain parts of the PAI/PAC experiments have been carried outat gas densities lower than normal atmospheric gas density:N < Natm. This means that a discharge can be produceduniformly in space in the majority of the aforementionedexperiments, except the high pressure RCM experiments.

From the examples listed and discussed in the presentreview it follows that the detailed kinetics of fuel–air mixturesunder the action of pulsed nonequilibrium plasmas remainsfar from understood, in spite of considerable experimentaladvances achieved over the past two decades. Hightemperature conditions, close to the ignition threshold, canbe described fairly well with existing kinetic mechanismstaking into account the integral production of radicals by thedischarge. In this case the non-thermal character of the ignitioncan easily be demonstrated, the ignition delay can be predicted

at least within an order of magnitude accuracy, and care mustbe taken mainly about making correct measurements of theenergy release in the discharge and calculations of the energybranching in the fuel-containing mixture.

With decrease of the initial temperature, the combustionchemistry becomes more sensitive to the details of the plasmaaction. Modification of S-curve, suppression of the NTCregion and changes of the cool flame chemistry are convincingexamples of plasma–combustion interaction demanding adevelopment of the links between plasma and combustionkinetics. The author considers these conditions to be the mostchallenging for studying chemical mechanisms of plasma-assisted ignition/combustion.

Another important issue concerns a practical result whichhas to be derived from numerical modelling. For most practicalconditions, the recombination of charged species and therelaxation of the greater part of the electronically excited atomsand molecules take times shorter than the ignition delay time.If the aim of calculations is to estimate the plasma-triggeredignition delay time, and if the action of plasma—that is, thedeposited energy, electric field and electron density distributionin time and in space, cross-sections of electron impact with themolecules of the mixture under study and kinetics in the nearafterglow—are known, any reasonable combustion kineticmechanism can be considered as a predictive mechanism: theshortening of the ignition delay time will be obtained withan accuracy within about an order of magnitude. But if theaim of the study is to reveal the details of the kinetics, or totrace the absolute values and time-dependent behaviour of theminor components, then no predictive mechanisms exist at themoment.

To illustrate the ideas stated above, calculations of theignition delay time and molar fractions of O, OH, CH2Oand H2O2 were carried out for a CH4 : O2 = 2 : 1 mixtureat the initial pressure and temperature of 700 Torr and800 K respectively. The choice of the mixture and of theinitial parameters was dictated by three factors: (i) themajority of PAI/PAC papers use the GRI-Mech mechanism,developed for the high temperature range, to model methanecombustion, even if ignition by plasma starts at the initial roomtemperature; (ii) mechanisms including the low temperaturekinetics for higher hydrocarbons are developed and availablein the literature; (iii) a cool flame in a methane-containingmixture has been obtained experimentally [55] under the

28


aforementioned conditions. Calculations were performed atconstant volume. The autoignition delay time derived fromthe pressure traces (figure 27(a)) calculated on the basis ofthe GRI-Mech 3.0 [97] mechanism is equal to 70 s. Theautoignition delay time for the Combustion Chemistry Center(C3) [54] mechanism is shorter: 3 s. No cool flame is observedin GRI kinetics, and a well-pronounced cool flame is seen whenmodelling with the C3 scheme (see the insets in figure 27(a)).It should be mentioned here that there is a principal differencebetween the autoignition kinetics and the kinetics of plasma-assisted ignition: for autoignition, one of the most importantfirst elementary steps is a reaction of hydrogen abstraction fromhydrocarbon; for PAI, the first step is a reaction of hydrocarbonwith O atoms produced by the discharge. Addition of 1%of atomic oxygen, with simultaneous removal of 0.5% ofO2, smooths over the contradictions between the kineticschemes: for GRI and C3 the ignition delay becomes equalto 1 s and 0.4 s, respectively. Although the ignition delaysdo not differ dramatically for the O atom activated kineticsfor the two mechanisms, the difference in density of theintermediate species is evident from figures 27(b)–(c): thedifference in formaldehyde density is limited by a coefficientof 2–3, and the differences in H2O2, O and OH densitiesare of 1–2 orders of magnitude. It can be concluded that,in order to build a kinetic mechanism for plasma-assistedignition and combustion, special attention should be paid tothe organization of experiments in order to obtain reliableunambiguous data.

A special set of conditions must be fulfilled to consider anexperiment on combustion triggered by plasma as a kineticexperiment. The author suggests formulating them in thefollowing way.

(i) The discharge action and the combustion chemistry shouldbe separated in time or in space; if not, their mutualinfluence has to be studied.

(ii) The spatial homogeneity of the discharge must beanalysed.

(iii) The plasma parameters (shapes and amplitudes of thevoltage and current, deposited energy, electric field,electron density) are the primary parameters of interest.As the detailed discharge–combustion mechanism forheavy hydrocarbons is a challenge, special attention mustbe given to discharge experiments on mixtures containinghydrocarbons, starting from the energy distributionand finishing with cross-section data sets for electroninteraction with higher hydrocarbons.

(iv) The action of plasma on the gas mixture at early afterglow,namely the formation of a ‘pool’ of active species, theirdependence upon the E/N value and the gas mixturecomposition, and the increase of the gas temperature dueto relaxation of the energy stored in the electronic degreesof freedom (in a PAI/PAC field, historically, this relaxationis called fast gas heating) must be studied. It is importantto note that, to trace the chemical transformation causedby plasma, the smallest possible energy release in thedischarge is needed, to avoid the shift of the equilibrium tohigher temperatures. The importance of the analysis of thespatial distribution of the energy release in the discharge

0.1 1 10 1000.9

1.0

1.1

1.2

1.3

1.4

1.5

4 3 2

C3/0.01

C3/auto

GRI/0.01

Pre

ssu

re,b

ar

Time, s

GRI/auto

1

0.1 1 10 10010-11

10-10

10-9

10-8

10-7

, O, OH

Mo

lar

frac

tio

n

Time, s

34 2 1

10010-6

10-5

10-4

10-3

10-2

10-1

, CH2O, H2O2

Mo

lar

frac

tio

n

34 2 1

(a)

(b)

(c)

64 66 68

1.0

1.5

2.0

2.5

Pre

ssu

re,b

ar

Time, s

2.4 2.6 2.8 3.0

1.0

1.5

2.0

2.5

Pre

ssu

re,b

ar

Time, s

Figure 27. Calculated autoignition (1, 2) and ignition upon artificialinjection of 1% of atomic oxygen (3, 4) for a CH4 : O2 = 2 : 1mixture at P0 = 700 Torr and T0 = 800 K. GRI-Mech 3.0 (1, 3) [97]and Combustion Chemistry Center (2, 4) [54] mechanisms weretaken as test mechanisms. (a) Pressure traces, (b) mole fractions ofO and OH, (c) mole fractions of CH2O and H2O2.

and the following spatial distribution of the gas heating andrelated hydrodynamic phenomena, while not discussed inthe present paper, must not be underestimated.

(v) Reliable data on ignition/combustion modification inplasma/combustion facilities must be obtained; specialanalysis is needed to clearly distinguish phenomena linkedto discharge-induced hydrodynamics or flame propagationfrom plasma-triggered kinetic phenomena.

29


(vi) It is necessary to record the kinetic curves of themain components, including products/reagents/the mostimportant intermediates within the time scale of theirchanges.

(vii) Sensitivity analysis must be considered as an integral partof PAI/PAC kinetic analysis, to reveal the most importantconditions and sub-mechanisms and to suggest ways inwhich their experimental verification can be achieved.

Acknowledgments

The author is grateful to US AFOSR, personally to ProfessorChiping Li for his leading role as AFOSR Manager in thefield of Energy Conversion and Combustion Science, and to DrJulian Tishkoff for his continuous interest in plasma-assistedcombustion. In the field of scientific exchange, an exceptionalwork has been done by Dr Jean-Pierre Taran, suggestingand organizing the biennial Aerospace Thematic Workshop‘Fundamentals of Aerodynamic Flow and Combustion Controlby Plasmas’. The author appreciates free and open scientificexchange and dissemination of knowledge in the field andshe is grateful to her colleagues working in plasma physics,combustion and kinetics for their continuous concern andunderstanding. In particular, close collaboration with LilleUniversity of Science and Technology, Dr Guillaume Vanhoveand Dr Pascale Desgroux as regards high pressure experimentsis appreciated. Special gratitude is expressed to Dr AndreyStarikovskiy, Dr Nikolay Popov, and Professor NikolayAleksandrov for sharing the ideas and aspirations of theauthor for more than 15 years, and to members of theauthor’s scientific team for their interest, enthusiasm andwork. The work on this review was partially supported bythe French National Agency, ANR (PLASMAFLAME Project,2011 BS09 025 01), AOARD AFOSR, the grant FA2386-13-1-4064, LabEx Plas@Par and PUF (the Partner UniversityFoundation).

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plasma-assisted ignition and combustion: nanosecond discharges and development of kinetic mechanisms

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