Download - Non-Equilibrium Kinetic Studies of Plasma-Assisted Combustion using Laser-Based Diagnostics

Z. Phys. Chem. 225 (2011) 1193–1205 / DOI 10.1524/zpch.2011.0159© by Oldenbourg Wissenschaftsverlag, München

Non-Equilibrium Kinetic Studies of Plasma-AssistedCombustion using Laser-Based Diagnostics

By F. Grisch1,∗, G. A. Grandin2, D. Messina2, and B. Attal-Tretout2

1 INSA-Rouen, UMR-CNRS 6614, CORIA, BP 8, 76801 Saint Etienne du Rouvray, Cedex France2 Office National d’Etudes et de Recherches Aerospatiales, Fort de Palaiseau, 91761 Palaiseau, Cedex

France

Dedicated to Katharina Kohse-Höinghaus on the occasion of her 60th birthday

(Received July 28, 2011; accepted in revised form November 14, 2011)

Plasma / Combustion / Nanosecond Pulsed Discharge / Kinetic Mechanism /Temperature / Concentration / Methane / OH / CH / CH2O /Coherent Anti-Stokes Raman Scattering / Planar Laser-Induced Fluorescence /Thomson Laser Scattering / Electron Temperature / Electron Density

Experimental investigation of nanosecond pulsed discharge in premixed CH4/air mixtures atatmospheric pressure has been carried out using laser diagnostics. Electron temperature andnumber density are measured using laser Thomson scattering. Temperature of neutral moleculesis measured by CARS. Finally, OH, CH and CH2O are probed using PLIF to identify their rolein the reduction of ignition delay and in the improvement of lean burn capability relative toconventional spark ignition. Measurements are compared with numerical simulations performedusing CHEMKIN-based code.

1. IntroductionIgnition and flame stabilization of combustible mixtures are still challenging problemsin combustion research. Reduction of ignition delay time, flame-holding and flame sta-bility improvement, flame blow-off prevention, possibility of high-altitude relightning,reduction of nitric oxide and hydrocarbon emissions from non-stoichiometric flamesand extension of fuel flammability limits are some of the key technical issues in thisfield. In particular, control of ignition and combustion in aircraft jet engines results incontrol of a variety of parameters, such as flight speed, altitude, and thrust [1]. Also,engine relight in case of a flameout at high altitudes and low combustor pressures isextremely challenging. Another critical issue is combustion stabilization at fuel leanconditions, which would help reduce NOx emissions. An ignition method occurringwithin short times and applicable at low combustor pressures, high flow velocities, andlow equivalence ratios may help to resolve these issues.

* Corresponding author. E-mail: [email protected]

mailto:[email protected]

1194 F. Grisch et al.

Conventional ignition of combustion includes the use of heated surfaces or fila-ments, pilot flames, electric discharges (spark or arc discharges and plasma torches), andhigh-power lasers. Among these, ignition of combustible flows using low-temperaturenon-equilibrium plasma has attracted considerable attention [2]. Various types of non-equilibrium plasmas have been used such as pulsed corona discharge, microwavedischarge, RF discharge, “gliding arc” discharge, nanosecond pulse duration Fast-Ionization Wave (FIW) discharge, and uniform, volume-filling, periodically pulsednanosecond discharge. Fuel/air mixtures exposed to short duration electrical pulsesusually form non-equilibrium plasmas, in which the electric fields accelerates the freeelectrons to high energy without significantly raising the temperature. Electrons/neutralgas molecule collisions, excite the molecular vibrational modes. If the collisional en-ergy is large enough, many molecular bonds can be broken, thus forming radical species.These radicals readily react with the surrounding fuel and oxidizer molecules releasingheat that in turn generates additional radicals, thus initiating a sequence that could resultin ignition. Chemical reactions produce more thermal energy than is consumed to formthe radicals so that, if the dissociation is efficient, less energy is required to ignite withradicals than to thermally ignite using a spark plug. The presence of radicals and ions canalso reduce considerably the ignition delay time due to a faster reactivity. For instance,a typical delay time for thermal ignition is of the order of 10−2 s while radical-inducedignition can be as short as 500–800 ns. The control of the dissociation of fresh moleculesinto radicals is driven by the voltage between electrodes.

The present study is focused on the combustion kinetics of nonequilibrium plasmaignition assisted by a nanosecond discharge at atmospheric pressure. To this end, ex-perimental and numerical studies are performed to clarify the chemical kinetic pathwaysthat lead to ignition. This work is an extension of an earlier study [3] in which tem-perature measurements inside the nanosecond pulse discharge were performed usingCoherent-Anti-Stokes-Raman-Scattering (CARS). This experimental study is extended:1) to investigate the impact of the nanosecond pulse discharge on ignition and 2) to per-form simulation with CHEMKIN-based code [4] and the GRI-MECH 3.0 methane/airdetailed chemical kinetic mechanism [5]. Laser diagnostics measurements of electrontemperature and electron number density, gas temperature (rotational and vibrationaltemperatures), concentrations of CH4, H2 and C2H2, and of radical species such as CH,OH and CH2O are obtained. Radicals are the key species involved in the oxidation andchain-reaction processes induced by the plasma/gas interaction. The temporal and spa-tial evolutions of the plasma/gas mixture are investigated by delaying the laser relativeto the pulsed discharge. Laser Thomson scattering allows for electron temperature anddensity measurements. CARS provides gas temperature and major species concentra-tion. Finally, Planar Laser-Induced Fluorescence (PLIF) is well suited to probe radicals.

2. Experimental procedure

The non-equilibrium plasma is produced by a periodically pulsed generator producingnegative electric pulses ranging from −10 to −40 kV in amplitude, 70 ns in durationwith a repetition rate up to 200 Hz [3]. The high-voltage generator is built using a hy-drogen dual-grid thyratron for improving the triggering speed, associated with a hollow

Non-Equilibrium Kinetic Studies of Plasma-Assisted Combustion using Laser-Based Diagnostics 1195

Fig. 1. Arrangement of the electrodes downstream from the burner nozzle exit.

cathode for protection against voltage reflections. Triggering and laser/discharge syn-chronization is achieved thanks to a delay generator with temporal jitter below 10 ns.Two stainless-steel needles, with a tip curvature radius of 0.1 mm are placed above theburner nozzle exit to produce the pulsed discharge. Position of the needles with respectto the nozzle can be varied horizontally and vertically. Both electrodes are tilted verti-cally by 9◦ with respect to the laser beams axis (Fig. 1). The beams are focussed at thecentre of the discharge.

The atmospheric burner consists of two concentric nozzles made of ceramic whichis an electrically non-conducting. The inner injection nozzle is 10 mm in diameter andit is surrounded by an annular nozzle which is 20 mm in diameter. Depending on the ex-periments, the inner nozzle operates with a room temperature methane/air mixture withvariable species composition, while inert gas is injected through the surrounding nozzleto prevent chemical and hydrodynamic disturbances of the inner flow. Gas flow rates areregulated by mass flow meters.

Thomson scattering measurements are performed with the experimental set-up pre-sented in Fig. 2. A seeded frequency-doubled Nd : YAG laser (Quantel Laser) delivering200 mJ of average power at 532 nm with a pulse energy stability of 0.5% rms is fo-cused at the centre of the discharge with a 500 mm focal length lens. This systemoperates at a pulse repetition rate of 10 Hz with a 13 ns pulse width and a spectralline width less than 0.003 cm−1. The Thomson scattering is produced through the elas-tic scattering of the laser light by electrons. Signal is collected at an angle of 90◦

to the laser beam axis, with an 80 mm focal length lens and is imaged onto a triplegrating spectrograph entrance slit through a 200 mm focal length lens. The probedvolume is 1 mm long and 100 μm in diameter. The Raman spectrograph (Jobin-YvonT 64000) has a 640 mm focal length and an f-number of 7.5. It is equipped with three1800 grooves × mm−1 diffraction gratings allowing good rejection of signals emit-ted at the excitation wavelength (Rayleigh scattering, Mie scattering and stray light).The Raman spectrum is detected on a back-thinned liquid nitrogen cooled CCD cam-era equipped with a 800 × 2000 chip having a quantum efficiency of 85% at 500 nm.


Fig. 2. Schematic diagram for laser Thomson scattering measurements.

The spectral resolution of the optical system is equal to the spectrograph resolution(∼ 0.003 nm/pixel). The Thomson scattering spectrum is recorded on a spectral rangeof 200 cm−1 shifted from the laser excitation wavelength by 30 cm−1. No interferencesbetween the Thomson signal and the Rayleigh and laser light noises are observed,demonstrating the good noise rejection of the detection system.

Data acquisition is performed by accumulating single-shot measurements in succes-sive pulsed discharges at 10 Hz rep. rate. The laser energy is 10 mJ per pulse to preventlaser breakdown inside the plasma. The Thomson scattering spectrum is integrated over60 s in order to have a good signal-to-noise ratio. The peak intensity of the Thomsonsignal is typically equal to 500 photoelectrons. To remove the plasma emission and thestray light from the Thomson signal, a specific experimental sequence is adopted forthe recording. We initially record the plasma emission without any laser beam. The rawThomson signal (on top of the plasma emission) is then recorded with the laser beampropagating within the plasma. Finally, the Thomson spectrum is obtained by subtract-ing the plasma emission from the raw Thomson signal. The calibration of the resultingThomson signal is performed before each measurement, by comparison with a rota-tional Raman spectrum of air. The electron temperature and the electron density arededuced by fitting the Thomson spectrum with theoretical simulations calculated witha Gaussian electron velocity distribution function and a least-squared method [6].

Gas temperature was inferred from the rotational and vibrational temperaturesof neutral gases measured with the N2 broadband coherent anti-Stokes Raman spec-troscopy (CARS) [3], which is appropriate in pulsed discharge since the signal effi-ciency is high and the corresponding Raman shift of N2 is well isolated from thoseof other chemical species. The temperature is derived from the observed Q-branch theCARS spectrum of N2, which is fitted to a library of theoretical shapes simulated as-suming two distinct temperatures, one for vibration and the other for the rotation ofN2 [7]. Based on histograms of single shot data, a statistical accuracy of 100 to 150 Kfor both rotational and vibrational single-shot temperatures is inferred a value whichincludes the contribution of fluctuations in the discharge itself. A frequency doubledinjection-seeded Q-switched Nd : YAG laser is used as a pump laser; it emits pulses at532 nm wavelength, 13 ns in duration, 0.003 cm−1 in line width, and 50 mJ in energy.A dye laser having 100 cm−1 bandwidth, 607 nm central wavelength, and 5 mJ energy


Fig. 3. Geometric arrangement of the CARS laser beams.

per pulse provides the Stokes beam. Reference and sample CARS spectra are dis-persed in separate spectrographs and detected using two optical multi-channel analyzers(Roper Instruments). Experimental details for the CARS measurement are described inGrisch et al. [3]. One key point of this single-point measurement is related to the geom-etry used to probe the discharge by CARS. A planar BOXCARS optical geometry isthen used to reduce the CARS probe volume to the dimensions of the pulsed discharge.To this end, the laser beams are focused in the centre of the discharge by means ofa 200 mm-focal-length achromatic lens. With a beam separation of 20 mm, the CARSprobe volume is considered as a 0.6 mm-long and 50 μm diameter small cylinder. Thisprobe volume is aligned along the longest dimension of the discharge in order to avoidinterferences of the CARS signal from the discharge with N2 CARS signal produced ingases surrounding the plasma (Fig. 3).

Concentrations of OH, CH and CH2O radicals have been measured using planarlaser-induced fluorescence (PLIF) which has the advantage of trace-level detection [8].Each species is probed with a separate laser system. Excitation of OH is done at282.75 nm corresponding to the Q1(5) transition in the A2Σ+–X2Π(1, 0) system andCH2O via the A–X (21

0410) at 339.47 nm. For CH, an excitation of the P1(10) rota-

tional transition of the C2Σ+–X2Π(1, 0) electronic transition at 317.11 nm is chosen.The laser beams are produced by frequency-doubled dye lasers. The output power is10–15 mJ/pulse.

The OH-LIF excitation scheme is chosen to minimize the variation of the ground-state Boltzmann fraction population over the expected temperature range, based on therotational and vibrational energy level expressions of Dieke and Crosswhite [9]. For theCH2O molecule, both the rotational and the vibrational terms have a significant influ-ence on the ground-state population distribution. The selected CH2O vibrational exci-tation transition has been shown to be the most advantageous in terms of fluorescencestrength [10]. This is due in part to an increase in the absorption coefficient which is mustbalanced against a predissociation lifetime of 28 ns for the 21

0410 excited state compared to

the 410 excited state. The rotational excitation has been selected to find a compromise be-

tween maximizing the ground-state rotational distribution and minimizing the variationwith temperature. Additional consideration for the selection of the PP3(18) and not a PQ(or RQ) rotational line is the high energy/pulse produced by the laser at this wavelengthand the reduced dependence of the fluorescence signal on temperature.

The experimental layout is shown in Fig. 4. The laser beam is shaped into a 150 μmvertical sheet with 3 mm height, which corresponds to the distance between the elec-trodes. Thus, a spatial resolution of PLIF of 20 μm × 20 μm × 150 μm is obtained in


Fig. 4. Geometric arrangement of the laser sheet for PLIF measurements.

accordance with the dimensions of the discharge. The signal-to-noise ratio is opti-mized with an image intensifier temporal gate of 20 ns. Energy in the laser sheet is 0.6,2 mJ and 3 mJ for OH, CH and CH2O experiments respectively. Fluorescence from theA–X(1, 0) and (0, 0) bands of OH is collected with an UV-Nikkor 105 mm/F4.5 lensand imaged onto an intensified CCD camera. Dichroic and WG285 optical glass fil-ters are used to isolate the 306–312 nm spectral range. Adopting this optical strategyallows good rejection of the plasma emission and incident light. For CH-PLIF measure-ments, the collection of fluorescence from the C–X(0, 0) band ranging between 313 and318 nm is done with a dichroic filter centred at 313 nm and having a HWHM of 5 nm.Finally, a high pass optical glass filter (GG 375) is used to collect the CH2O fluores-cence in the 350–550 nm spectral range. Data acquisition is performed at a repetitionrate of 5 Hz, and the laser beams are synchronized with the pulsed discharge using anexternal trigger from a pulse/delay generator which also trigs the discharge.

3. Numerical approach

Numerous examples of the ignition of CH4/air mixtures have been examined in the pastusing the CHEMKIN-based [4], SENKIN [11] code and the GRI-MECH 3.0 kineticmechanism [5]. Because no CHEMKIN models for a non-equilibrium plasma igniterhave been developed so far, we began by modelling the ideal situation of an isobaric,adiabatic reactor and thus the ignition problem was limited just to the chemical aspectsof the problem. There could be some concerns in using GRI-MECH 3.0 for plasmaignition, as it has not been optimized for cases that involve large radical concentra-tions. However, this mechanism is considered the most reliable one for predicting CH4

oxidation, it is expected that even in the presence of large amounts of radicals in theinitial reactant pool, it will correctly describe to the first order the underlying chem-ical pathways that result to ignition. Additionally, the kinetic mechanism described by


Table 1. Threshold energies for the decomposition of methane to neutral radicals by electron interactions.

Chemical reaction Energy threshold (eV)

CH4 + e → CH3 + H 8.8CH4 + e → CH2 + H2 9.4CH4 + e → CH + H2 + H 12.5CH4 + e → C + 2H2 14.0

GRI-MECH 3.0 is valid for chemical species in their ground states but contributions bypossible excited states are not considered in this work. Furthermore, many of the pro-cesses that will be discussed in the following occur very rapidly. Oxidation mechanismspredicted by GRI-MECH 3.0 may underestimate the times required to complete thesefast processes because they do not account for vibrational relaxation of the molecules.However, while vibrational relaxation may increase the duration of nanosecond scaleevents, it will not affect the kinetic pathways that are the subject of this paper. In par-ticular, vibrational relaxation should have little or no impact on the Ignition DelayTimes (IDT), simply because the IDT’s are many orders of magnitude larger than thevibrational relaxation time.

In our simulation, radicals are assumed to be produced instantaneously in thefuel/air gaseous mixture just after the application of the pulsed discharge. The dissoci-ation of methane is assumed to follow the prescriptions of Janev and Reiter [12], shownin Table 1, for the decomposition to neutral radicals by interactions with electrons pro-duced by the plasma. These prescriptions are used to determine the relative mixing ofCH radicals, CH2, CH3, H and H2. The proportion of fresh CH4 decomposition is as-sumed to be constant whatever the equivalence ratio studied. This rate is determinedfrom the CH concentration measured at equivalence ratio of 2.2 and using the chemicalreactions described in Table 1. The resulting value is then fixed to 4.5%. The proportionof fresh O2 decomposition is estimated by adjusting the results of the simulation of theOH concentration and temperature with measurements performed for an equivalenceratio of 0.05 (i.e. a typical air plasma). Under these conditions, the prescribed value forthe proportion of O2 decomposition by the pulsed discharge is found to be 10%. The in-let temperature specified by the CARS measurement depends on the equivalence ratio.Heat losses are also introduced in the simulation by adjusting the experimental drop intemperature observed for large delay time with the theoretical one (see the results anddiscussion section). Note that these heat losses are specified in order to connect our ex-perimental configuration using a non-adiabatic open reactor with variable dimensionsto our simulation configuration of an isobaric, homogeneous and adiabatic reactor.

4. Results and discussion

4.1 Experimental results

The equivalence ratios (Φ) of the CH4/air mixture in which measurements are per-formed are chosen in order to include all experimental ignition conditions for a CH4/airmixture by the pulse discharge (0.7 < Φ < 1.3) [3]. In this way, the kinetic mechanism


Fig. 5. Time evolution of (�) rotational temperature and (•) vibrational temperature for two equivalenceratio.

associated with the plasma and that of combustion can be well separated and probedusing laser diagnostics.

4.1.1 Plasma properties

Thomson measurements performed at the current peak yields an electron temperatureof about 2.6 eV and an electron density of 1015 cm−3. These values, at normal con-ditions (P = 1 bar and T = 300 K), allow one to determine the physical parametersrequired to classify this type of plasma. A Debye length of 3.7×10−7 m, a mean gapbetween electron of 10−7 m, a Landau radius of 5.7×10−10 m and an electron meanfree path of 9.6×10−4 m are deduced and they lead to a discharge classified as a “ki-netic plasmas”. Furthermore, an ionization factor of about 4×10−5 allows to assumea plasma weakly ionized. Complementary measurements performed in delays between30 and 70 ns demonstrate a fast reduction of the electron temperature (2.6 to 1 eV) com-bined with a decrease of electron density by a factor of 10. The agreement betweenthe Thomson-scattering data and prediction clearly suggests that free electrons followa Maxwellian velocity distribution. In other words, the collisions between electronsare so frequent that the fast heating of electrons within the pulsed discharge, the colli-sions between electrons and heavy molecules and the electron impact excitation processdo not significantly change the electron energy distribution. The plasma properties ofa weakly ionization associated with the production of a small reduced electric field(E/n) of 250 Td leads to a loss of electron’s energy by collisions and an absence of gainenergy from the electric field.

4.1.2 Temperature

The conversion of electron energy into the quantum states of N2 in premixed CH4/airmixtures are studied using CARS. Typical results of rotational and vibrational tempera-ture distributions as a function of time are displayed in Fig. 5.

In the lean-regime condition (equivalence ratio of 0.6), a considerable increasein both temperatures is observed when applying the pulsed discharge in the CH4/airmixture. Contrary to air plasmas in which a significant vibrational non-equilibrium isobserved [3], a small amount of the vibrational non-equilibrium is noted, demonstrat-


Fig. 6. Examples of spatial distribution of (a) OH (time delay = 1 μs), (b) CH (time delay = 20 ns) and(c) CH2O (time delay = 1 μs) recorded between the electrodes.

ing the presence of a partial thermal equilibrium probably induced by collisional energytransfer with CH4 molecules or some of their decomposition products. The rotationaland vibrational temperature distributions exhibit a quasi-flat profile over a period largerthan 100 μs with a temperature peak of 2500 K for Φ = 0.6, which leads to efficientheating of the neutral molecules. Similar results are observed for rich CH4-air mixtures(Φ = 2.2). After the fast excitation of the vibrational and rotational modes of N2 leadingto vibrational and rotational temperatures of 3500 and 2750 K respectively, the relax-ation of the vibrational and rotational states towards the ground state is observed att = 30 μs. Finally, for 0.7 < Φ < 1.3, temperatures up to 2400 K are measured and theyresult in an ignition of the CH4/air mixture. While ignition seems to occur from the fastincrease of the rotational temperature after the high-voltage pulse, the rapid productionof transient species with high concentration could also support this phenomenon. Forthat, the probing of OH, CH and CH2O transient species in their ground state has beeninvestigated within the discharge using the PLIF technique.

4.1.3 Radical species

Production of OH radicals is confirmed throughout the discharge volume between theelectrodes. Figure 6a shows a typical instantaneous OH distribution recorded in themiddle of the streamer at a delay of 1 μs after the high-voltage pulse. It is noted that theOH production is quite homogeneous within the discharge. CH detection using PLIFwas also performed under the same conditions. CH production is also observed withinthe discharge, indicating an efficient decomposition of the fuel by the plasma. For in-stance, Fig. 6b shows a typical instantaneous CH distribution recorded within a crosssection at the middle of the discharge. This measurement was recorded at a delay of20 ns after the application of the high-voltage pulse. In opposition to OH, the spatialdistribution of CH fluorescence between the electrodes is not homogeneous. CH is ob-served in the core of the pulsed discharge with a preferential maximum close to theanode. Finally, the production of CH2O is located on the periphery of the pulsed dis-charge core at locations where a decrease in temperature is noted (Fig. 6c).

4.2 Kinetics of radical-induced ignition

The temperature history shown in Fig. 7 is representative of most-radical-induced ig-nition processes at an equivalence ratio of 0.6. It occurs in different thermal steps thatreflect the different chemical kinetic stages. The first thermal step corresponds to the


Fig. 7. Experimental and theoretical time evolutions of temperature and minor species concentrations forΦ = 0.6.

decomposition of the fuel and air molecules by electron impact which raises the tem-perature to a maximum value of 1900 K. Note that the time scale of this heating is veryfast, typically 5 ns which corresponds to the duration of the pulse discharge. After thisstage, the development of kinetic processes is able to maintain a high reactivity insidethe gaseous mixture resulting in high temperatures depending of course on the radi-cal pool produced from the early stage. This phenomenon, occurring on a time scaleof 10–20 μs has the potential to produce an ignition in the gas mixture. The evolutionsof concentrations for the most significant radical species are also displayed to highlightthe time periods of production and/or consumption of these chemical species. For clar-ity, each figure is scaled with the maximum concentration of the radical obtained fromthe experiments and each has a different vertical scale. As seen in Fig. 7, the kineticmechanisms in the first period are firstly dominated by chemical reactions involving thedecomposition products of CH4. Radicals such as CH, CH2 and CH3 in reaction with O2

and a lesser degree O play a key role in the production of HCO and CH2O. Note fromthe radical history that all of the initial CHx products have been consumed in 100 nsinto the reaction scheme. Consumption of these species leads to an increase in HCO andCH2O, with a [CH2O] peak concentration obtained at a delay time of 400–500 ns. Afterthis time delay, the CH2O concentration quickly drops, enhancing the production ofOH, which begins after the energy deposition by the pulsed discharge. Note that the pro-duction of OH is also largely correlated with the temperature level. The maximum OHconcentration is flat until a time of 10–20 μs while the temperature stays at a maximumlevel. Finally, a uniform decrease in temperature and OH concentration is observed andthe gaseous mixture returns to room temperature after 200–300 μs. This cooling of thegaseous mixture is then driven by recombination kinetic mechanisms which produceneutral and chemically stable molecules (see Fig. 9).

When the equivalence ratio is equal to 2.2, the temperature level produced fromthe decomposition of fresh gases by the pulsed nanosecond discharge reaches 2500 K(Fig. 8). This temperature is maintained up to 1–2 μs. A fast decrease in temperaturefollows and temperature returns to room temperature at a maximum time delay of 50 μs.


Fig. 8. Experimental and theoretical time evolutions of temperature and minor species concentrations forΦ = 2.2.

Fig. 9. Experimental and theoretical time evolutions of temperature and main species concentrations forΦ = 2.2.

As observed for the 0.6 equivalence ratio, the production of intermediate species likeCH, CH2O and OH is also observed but in larger concentrations. CH is produced fromthe fast decomposition of CH4. This radical is consumed in favour of the productionof intermediate species like CHO and CH2O. The maximum concentration of CH2O isfound for a time delay of 1 μs. Contrary to Φ = 0.6, OH is produced quickly with a peakconcentration present at about 100 ns. After this time delay, the OH production is main-tained at a high level until a time delay of 1 μs which corresponds to the temporal regionof the maximum temperature in the mixture. The temperature and OH production thendrop together after this time delay. This fast decrease originates from the recombinationof the transient species in stable neutral molecular species. This result is supported bythe experimental results presented in Fig. 9 showing the temporal evolution of CH4, H2

and C2H2 concentration measured by CARS. The complete removal of CH4 is observed


during the period where a high reactivity is noted. H2 production is also observed inthe time region of 10 ns–1 μs. Note that the quick removal of H2 after this time delayis the consequence of the fast diffusion of H2 outside the discharge zone. Finally, cool-ing contributes to the kinetic pathway producing C2H2which undoubtedly arises fromthe recombination of hydrocarbon radicals.

The quantitative theoretical species concentration and temperature profiles can alsobe compared to the experimental ones. The correlation between quantitative data high-lights the identification of active chemical species governing the kinetic mechanism.Whatever the equivalence ratio, the temperature and the species concentration historiesindicate the same tendencies as those that are observed experimentally. Using the sameinitial conditions (Sect. 3) for both equivalence ratio, the time evolution of temperatureand species concentration are in reasonable agreement with the experimental ones. Itis remarkable to note that the time delays observed experimentally for the productionand consumption of transient species are predicted well when using the GRI-MECH3.0 chemical mechanism. Analysing the Φ = 0.6 condition reveals that the minimumignition delay time is ∼ 600 ns, typically 4–5 orders of magnitude faster than for anconventional thermal ignition. The fast decomposition of the fuel into large amounts ofradicals such as CH adds enthalpy to the mixture, which can be recovered as thermal en-ergy for the heating of the mixture. After this very fast step, an intense kinetic sequencefed with the initial radical pool takes place over a period of several μs which acceleratesthe development of the ignition of the gaseous mixture.

5. Conclusion

Experimental investigation of a non-equilibrium nanosecond pulsed discharge in pre-mixed CH4/air mixtures has been carried out. The results demonstrated improvementof combustion properties in the flame. The plasma has consistently shown significantreductions in ignition delay and increased lean burn capability relative to conventionalspark ignition. Identification of the physical processes underlying these improvementswas obtained through laser diagnostics. Thomson scattering measurements of the elec-tric properties of the plasma, i.e. electron temperature and electron density which arethe key parameters for a reliable classification of this plasma were obtained. Then,temperature measurements intended to quantify the energy transfer in the gas mixturewere performed with N2 CARS. Spatial and temporal evolutions of temperature wererecorded by delaying the laser pulse relative to the discharge in the 10 ns to 1 ms tempo-ral range. Energy transfer induced by collisions of N2 with CH4 drives the fast thermalheating of neutral molecules up to high temperatures. The discharge effect leads toignition of the CH4/air mixture at equivalence ratio of 0.7 and 1.3. OH-, CH- andCH2O-PLIF experiments support this conclusion. OH initially produced by the plasmadecays to negligible values at time delay larger than 200 μs while CH is mainly pro-duced on a time domain of 0–1 μs after the starting of the discharge. Given enoughradicals, it is then possible to achieve ignition in less than 600 ns – 5 orders of magni-tude lower than the ignition delay of thermal ignition. Future work will be devoted tothe probing of O and H atoms which also play a key role in plasma assisted oxidationand ignition of CH4/air flows.


Acknowledgement

This work was supported by Direction Generale de l’Armement which is gratefullyacknowledged.

References

1. S. B. Leonov, I. V. Kochetov, A. P. Napartovich, V. A. Sabel’nokov, and D. A. Yarantsev,IEEE T. Plasma Sci. 39 (2011) 781.

2. S. M. Starikovskaya, J. Phys. D Appl. Phys. 39 (2006) 265.3. D. Messina, B. Attal-Tretout, and F. Grisch, Proc. Combust Inst. 31 (2007) 825.4. R. J. Kee, F. M. Rupley, and J. A. Miller, Chemkin-II: A FORTRAN Chemical Kinetics Pack-

age for the Analysis of Gas-Phase Chemical Kinetics, Sandia Report SAND89-8009, UC-706,Sandia National Laboratories (1989).

5. G. P. Smith, D. M. Golden, M. Frenklach, N. W. Moriarty, B. Eiteneer, M. Goldenberg,C. T. Bowman, R. K. Hanson, S. Song, W. C. Gardiner, V. Lissianski, and Z. Qin, GRI-Mech3.0 (http://www.me.berkeley.edu/gri-mech/) (2000).

6. H. Kempkens and J. Uhlenbusch, Plasma Sources Sci. T. 9 (2000) 492.7. F. Grisch, P. Bouchardy, V. Joly, C. Marmignon, U. Koch, and A. Gülhan, AIAA J. 38, (2000)

1669.8. A. C. Eckbreth, Laser Diagnostics for Combustion Temperature and Species. 2nd edn. Gordon

and Breach Publishers, Amsterdam (1996).9. G. H. Dieke and H. M. Crosswhite, J. Quant. Spectrosc. Ra. 2 (1962) 97.

10. P. H. Paul and H. N. Najm, Proc. Combust. Inst. 27 (1998) 43.11. A. E. Lutz, R. J. Kee, and J. A. Miller, SENKIN: A Fortran Program for Predicting Homoge-

neous Gas Phase Chemical Kinetics with Sensitivity Analysis, Sandia Report SAND87-8248,UC-401, Sandia National Laboratories (1988).

12. R. K. Janev and D. Reiter, Phys. Plasmas 9 (2002) 4071.

http://www.me.berkeley.edu/gri-mech/

Download - Non-Equilibrium Kinetic Studies of Plasma-Assisted Combustion using Laser-Based Diagnostics

Top Related