plasma assisted combustion & chemical conversion
TRANSCRIPT
Yiguang Ju
Princeton University
Plasma Assisted Combustion & Chemical Conversion
2021. 6. 21
Princeton Combustion Summer School
Sang Hee Won
Associate professor,
Univ. South Carolina
Joseph Lefkowitz
Research fellow
AFRL
Wenting Sun
Assistant professor,
Georgia Tech
Prof. Richard B Miles
Princeton/Texas A.M.
Prof. Christophe Laux
Ecole Centrale Paris
Aric Rousso
Graduate student
Timothy Ombrello,
Senior research
engineer
AFRL
Prof. Igor V. Adamovich
Ohio State University
Prof. Svetlana Starikovskaya
Ecole Polytechnique
Dr. Andrey Starikovskiy
Princeton
Acknowledgement
Prof. Bruce Koel
Princeton Univ.
Prof. Haibao Mu
XiAn Jiaotong
Visiting
Timothy Chen
Graduate student
Xingqian Mao
Princeton
Prof. Min Suk Cha
KAUST
Prof.Qi Chen
Beijing Jiaotong
Visiting
Hongtao Zhong
Princeton
Chao Yan
Princeton
Dr. Nils Hansen
Sandia National Lab
Dr. Chris Kliewer
Sandia National Lab
Annemie Bogaerts
Univ. Antwerp
Low carbon energy conversion: From Fossil Fuel Energy to Electron Energy:
Fossil fuels
High carbon
• Power• Fuels• Chemicals• Fertilizers (NH3)• Materials • E-fuels• H2
• Energy storage…
Low carbon
CO2Electricity
Electrons
Catalysis
Plasma/photons
Hot flame
Cool flame
N-heptane/He
O2/He
BiomassPhotons Low efficiency
Higher efficiency Energy storage
EVs
eCO2
• Transformation of fossil energy industry to low carbon chemical manufacturing industry.
• Development of plasma assisted advanced engines
Syngasl
Te >> Tv>>Tn
Non-Equilibrium plasma
Te ≈ Tv ≈ Tn Near Equilibrium plasma
arc
Gliding arc
Corona
Tn~n*10,000 K
Tn~n*1000 K
Tn~n*100 K
Te > Tv>Tn
What is plasma?
4
Plasma: 4th state of matter: A partially ionized, quasi-neutral charged reactive mixture…
Energy transfer and active species production in Non-Equilibrium Plasma
6
Photoionization
Slow heating (ms)
5
High energy electrons/ions (ns) Fast heating (ns)
Active species (ns)
Non-equilibrium
Equilibrium
Electronically excited species
Vibrationally excited species
Temperature rise (ms)
• Thermal effect (μs-ms)• Chemistry effect (ns-ms)
Plasma
Advanced engines Nano materials
Electrode
Electrode
CO2, CH4 → CnH2n
Fuel/NH3 synthesis
+
-Pollutants control
AgricultureBiochemical……
Plasma-Chemistry: a new pathway to Low Carbon Electron Energy
Chemistry & Dynamics
1. Introduction and plasma discharge
2. Plasma Assisted Combustion and Applications in Engines
3. Effects of plasma on ignition, flame propagation, minimum ignition energy, and cool flames
4. Effects of electric field on combustion: Joule heating and ionic wind
5. Plasma chemistry and kinetic studies
6. In situ laser diagnostics
7. Thermal-chemical instability and chemical mode analysis
8. Modeling of plasma assisted combustion and chemical conversion
9. Perspectives of future researchReview papers of plasma assisted combustion and chemical conversion1. Ju, Y. and Sun, W., 2015. Plasma assisted combustion: dynamics and chemistry. Progress in Energy and Combustion Science, 48, pp.21-83.2. Starik AM, Loukhovitski BI, Sharipov AS, Titova NS. 2015 Physics and chemistry of the influence of excited molecules on combustion
enhancement. Phil. Trans. R. Soc. A 373: 20140341. 3. Igor V Adamovich and Walter R Lempert, 2015, Challenges in understanding and predictive model development of plasma-assisted
combustion, Plasma Physics and Controlled Fusion, Volume 57, Number 1.4. Starikovskiy A, Aleksandrov N. Plasma assisted ignition and combustion. Prog. Energy Combust. Sci. 2013;39:61–110.5. Starikovskaia SM. Plasma assisted ignition and combustion. 2006; J. Phys. D: Appl. Phys. 39:R265–R299.6. CO Laux, TG Spence, CH Kruger, RN Zare, Optical diagnostics of atmospheric pressure air plasmas, Plasma Sources Science and Technology 12
(2), 1257. A Fridman, S Nester, LA Kennedy, A Saveliev, O Mutaf-Yardimci, Gliding arc gas discharge, Prog. Energy Combust. Sci. 25 (2), 211-231 8. Bogaerts, A., et al., 2020. The 2020 plasma catalysis roadmap. Journal of Physics D: Applied Physics, 53(44), p.443001.9. Ju Y, Reuter CB, Yehia OR, Farouk TI, Won SH. Dynamics of cool flames. Progress in Energy and Combustion Science. 2019 Nov 1;75:100787.
Lecture contents and review articles
7
Milestones of Plasma assisted combustion
1860 Étienne Lenoir used an electric spark plug in his gas
engine, the first internal combustion piston engine.
(Spark ignitor for engines)
1814 – W.T. Brande. Phil.Trans.Roy.Soc, 104, 51. (Electric field-flame interaction)
1948, Calcote, 3rd Symposium on Combustion and Flame, and Explosion
Phenomena (Vol. 3, No. 1, pp. 245-253) (Ionic wind)(a) XH2,PJ = 0.3, PIN = 3.0 kW, PIN total = 4.3 kW
(b) XH2,PJ = 0.5, PIN = 6.0 kW, PIN total = 8.2 kW
1981: Kimura, L, et al., Combustion and Flame, Vol. 42, No. 3,
pp. 297- 305 (Plasma jet in supersonic combustion)
2013: Leonov, S.B., Firsov, A.A., Shurupov, M.A., Michael,
J.B., Shneider, M.N., Miles, R.B. and Popov, N.A., 2012.
(laser guiding plasma discharge/fs-ps diagnostics)
1998: Starikovskaia, S.M., Starikovskii, A.Y. and Zatsepin, D.V., Journal of
Physics D: Applied Physics, 31(9), p.1118. ]Anikin N B and Marchenko N 2005
(Nanosecond discharge).
2014: Won, S.H., Jiang, B., Diévart, P.,
Sohn, C.H. and Ju, Y., Proc. Comb Inst.
(Plasma assisted cool flames and LTC)(a) Hot flame (b) Cool flame
2019: Zhong, Shneider, Ju, 2019.
(Plasma thermal-chemical instability)
2020s (Plasma catalysis?)
How does plasma enhance combustion?
Ju and Sun: Plasma assisted combustion, Progress of Energy & Combustion Science, 2015
New reaction pathways with fuel
Plasma discharge
Temperature
increase
Thermal
Radicals
Int. species
Ions/electrons
Excited
species
Kinetic
Fuelfragments
Transport
Ionic windInstability
NO, O3
O, H, OH
N2*, N2(v)
O2 (a1Δg)
H2 , CO
CH4
CH2O
Combustion Enhancement
O2+, N2
+
T0 B, E Le, Rc
O(1D)
Large Ignition VolumePlasma instabilityTurbulence
Excited states
Mild Combustion
Cool Flames
Fuel/CO2
Reforming
New enginetechnology
Scramjet/ RDE engines
LowEmissions
Plasma assisted combustion
Applications of plasma assisted combustion
10
• Materials synthesis• Plasma medicine• Plasma cleaning• Plasma water treatment• …
• Thermal plasma (Equilibrium) Tgas ~ 2000K-20000KHot plasma: Ttrans = Trot = Tvib = Te Example: (Arc, lightening)
2. Plasma Discharges
Temperature:
• Non-thermal (Non-Equilibrium plasma) Tgas ~ 300K-2000KCold plasma: Ttrans = Trot < Tvib < Te
Low pressure – DC, RF glow discharges
Atmospheric pressure – DBD, Microwave, Corona & Micro-plasmas
Frequency: DC, AC, RF (MHz), MW(GHz), Pulsed…
Discharge processes:
Discharge types:
Electric field, streamer, Corona, glow, arc
Electron-beam, Corona, Dielectric barrier discharge (DBD),
gliding arc, arc, micro discharge, surface discharge…
Plasma: A partially ionized, quasi-neutral charged mixture
in which electrons and ions are separately free.
11
------
++++++
++++++
++++++
------
------
++++++
++++++
++++++
------
------
------
Perturbation of a neutral plasmaTotal charge number: 𝑄 = 𝑒𝑉𝑛𝑒Electric field (between two slabs): E=
𝑄
𝜀𝐴=𝑒𝑉𝑛
𝑒
𝜀𝐴=
𝑒𝑛𝑒
𝜀x
Coulomb force on an electron: F=-eE=−𝑒2𝑛
𝑒
𝜀𝑥
x
Equation of electron motion: F=mea 𝑚𝑒
𝑑2𝑥
𝑑𝑡2= −
𝑒2𝑛𝑒𝜀
𝑥
The frequency of electron plasma oscillation is: 𝜔𝑝 =𝑒2𝑛
𝑒
𝑚𝑒𝜀=9000 𝑛𝑒 (Hz)
Plasma frequency
If the electron density is 109 cm-3 , thefrequency is about 300 MHz. Therefore,plasma is very fast to restore chargeneutral properties.
ωp
The electron plasma frequency is critical to the propagation of electromagnetic wave in plasma. If
the electromagnetic wave frequency (ω) is less than ωp, ω<ωp, electrons in the plasma will response
and extracts energy from the electric field and reflect the incident wave. If ω>ωp, electrons in
plasma can not response and the electric field will transmit through the plasma without reflection.
Therefore, for a given ω, there is a critical plasma electron number density (cm-3), ne,c:
ω
2
2
,e
mn
ep
ce
At microwave frequency of 2.45 GHz, if the electron density is 9×1010 cm-3 , ωp =2.7GHz > ω=2.45Gz, microwave will not penetrate to the interior of the plasma, but reflect on the plasma surface within a small length scale.
ε：permittivity
V: volume
ne: electron number density (cm-3)
e: electron charge
12
E
Perturb
Mean free path and collisional frequency
d u
A
B
Mean free path
λ
nd v :unit timeper number collision The 2
Mean free path:
the mean free path
ndvnd
u22
2
1
time unit per collisions of number
time unit per distance traveling
For electron neutral molecule collisions in weakly ionized gas, ndudn
u
e
ene 22,
4
)4/(
25210 105.2)105.3(2
1
m=0.075
T=300K, p=1 atm, Molecule diameter: d=3.5A0, the molecule number density:
325
23
5
/105.23001038.1
10013.1m
319 /10 cm=2.45
Collision frequency
u: mean velocityv: relative mean velocityd: neutral particle diameter
e
eBneene
m
Tkndu
8
4/
2
,,
,8
)(0
m
Tkduuvfu B
sec/107.610075.0/500/: 96 ufrequencyCollison c
2 :areasection Cross d
2uv
n=p/(kBT)=
13
• Plasma shielding effect: If one inserts a perturbing charge objective (Q)
into a neutral plasma, as the free charges move towards a perturbing
charge objective (Q). The perturbing electric field effect will be
neutralized in a characteristic distance of D. D is the Debye length.
Debye Shielding and plasma sheath
+
E~0
E>0
+Q+
+
+
++
+
+
+
+
+
+
+
+
+
+
D
++
+
-- - - -
Cathode (perturbing objective)
++
+ + + ++
D
Ion bombardment
Sheath
Neutral plasma
Potential distribution
How large is Debye length? 14
𝛻 ∙ 𝑬 =𝑒
𝜀(ni-ne)In the Maxwell’s equation:
For a steady state problem:
ε: the permittivity of the plasma
𝑬 = −𝛻𝜑 𝛻𝟐𝝋 = −𝑒
𝜀(ni-ne)
𝛻𝟐𝒆𝝋
𝒌𝑩𝑻𝒆= −
1
λ𝑫𝟐(ni
n0-ne
n0)
λ𝑫 =𝒌𝑩𝑻𝒆𝜀
n0𝑒2
In air, if Te = 1000K and n0 = 1013 cm-3,
we have λ D = 6.9 × 10−5 cm= 6.9 × 10−7 m.
Debye length
• Therefore, when L >> λD and there is enough electrons in the Debye sphere
to produce shielding, plasma will be almost quasi-neutral everywhere.
• The equation means that the net charge potential will decrease exponentially in a length scale of λ D
+
E~0
E>0
+Q+
+
+
++
+
+
+
+
+
+
+
+
+
+
D
++
Debye length
15
ε0 of a vacuum) :
8.85 x 10-12 (Faraday/m)
N𝒆 = n𝒆𝟒𝝅
𝟑𝝀𝑫𝟑 >> 1
• Response time of plasma shielding to recover quasi-neutrality
in plasma :𝜏𝐷~
λ𝐷
𝑣𝑒=
λ𝐷
𝑘𝐵𝑇𝑒/𝑚𝑒= 1/𝜔𝑝
16
If the plasma response time (τD) is shorter than the characteristic time of an
external electromagnetic field: τD < τp (such as laser, nanosecond pulse discharge, microwave), then this
radiation will be shielded out, otherwise, will be refractively transmitted.
2014 Paul Gibbon, CERN School on Plasma Wave Acceleration
𝜔𝑝2
𝜔2 =𝑒2𝑛𝑒𝜀𝑚𝑒
λ2
4𝜋2𝑐2
λ: laser wavelengthc: light speed
shielded outTransmission
tc
t
nne
B
ei
EjB
BE
B
E
2
1
0
)(
E: electric field, B: magnetic fieldj: currentε: permittivity μB: permeability
)n -e(n v)n - e(n )vn - ve(n eeiieieeii VVj
Maxwell equation
Diffusion with an external electric field: diffusion velocity and drifting velocity
E
E
eeeeee
iiiiii
nnDVn
nnDVn
Di Charge diffusivities,
μi electron and ion mobilities
For weakly ionized plasma: ni << n; ne << n
eBe
e
Bi
i
Tk
e
D
Tk
e
D
Einstein relation
ii v v V
Mean + relative velocity
17
ee v v V
Question: can the ions and electrons diffuse independently?
field electricambipolar : )(
)(-
eie
eie
n
nDD
E
Ambipolar diffusion (steady state neutral plasma)
In one-dimensional plasma (zero flow velocity): The ion and electrons fluxes are,
In steady state and quasi-neutral plasma:
iambi
eii
iieiiii
iiiie
nD
n
nDDnnD
nnD
)(
)( -
- i
ETherefore:
E
E
eeeeeee
iiiiiii
nnDVn
nnDVn
ei
)1( )(
)(
i
eie
e
ii
ei
ieeiambi
T
TDDD
DDD
)( ei
In non-equilibrium plasma, Te is much greater than Ti, the ambipolar diffusivity is much higher than ion diffusivity.
18
Energy transfer and active species production in Non-Equilibrium Plasma
6
Photoionization
Slow heating (ms)
19
High energy electrons/ions (ns) Fast heating (ns)
Active species (ns)
Non-equilibrium
Equilibrium
Electronically excited species
Vibrationally excited species
Temperature rise (ms)
• Thermal effect (μs-ms)• Chemistry effect (ns-ms)
Figure 1. Experimental and predicted temperature and N2
vibrational temperature during and after a ns pulse
discharge in air between two spherical electrodes 1 cm
apart at 100 Torr.
Figure 2. Experimental and predicted temperature during and
after a ns discharge pulse in an H2-air mixture (ϕ=0.14) between
two spherical electrodes 0.9 cm apart at 40 Torr, plotted together
with predicted number density of electronically excited N2
molecules and Tv(N2).
Igor V Adamovich and Walter R Lempert, 2015, Challenges in understanding and predictive model development of plasma-assisted combustion, Plasma Physics and Controlled Fusion, Volume 57, Number 1.
Energy transfer in Plasmas: fast heating and slow heating (vibrational relaxation)time-resolved and spatially-resolved measurements of N2 vibrational temperature
Two-step thermalization
20
-10 0 10 20 30 40 5010
12
1013
1014
1015
1016
1017
0
1
2
3
4
5
6
0
50
100
150
200
250
300
Time (ns)
N2(A)
N2(C)
Ab
so
lute
de
nsitie
s [
cm
-3]
N2(B)
02.0x10
17
4.0x1017
6.0x1017
8.0x1017
1.0x1018
1.2x1018
O (3P) density
1500
2000
2500 from N
2(C-B)
from N2(B-A)
Te
mp
era
ture
[K
]
Temperature
0
10
20
30
40
Cu
rre
nt [A
]
Vo
lta
ge
(V
)
V Iconduction
E/N
[T
d]
Timescales of plasma chemistry
hheating =21±5%
hdiss. = 35±5%
Ultrafast heating:
900 K in 20 ns
Ultrafast
dissociation of O2
Rusterholtz et al, J. Phys.D, 46, 464010, Dec 2013
Plasma chemistry occurs in nanoseconds! Christophe Laux, 2014
Ionization processes: Thermal ionization, electron impact ionization, photo-ionization for second electrons
Plasma and Plasma Properties
e + O2= 2e + O2+ (electron-impact)
hν + O2= O2+ + e (photon ionization)
M+O2= M+O2++ e (Thermal)
e + O2= O2-
e + O2+=O(3P) + O(1D)
Electron quenching processes: recombination and attachment
+
x d0
Plasma temperature: Electron temperature, vibrational and rotational temperatureElectron temperature: 1 eV = 11600 K = 1.6 ×10−19 Joules.E/N: Townsend, E: V/m; N: 1/m3; 1Td=10-21 V/m2
Second electron
Equilibrium and non-equilibrium plasma:Equilibrium: Distribution function: Boltzmann
Non-equilibrium: Distribution function: non-Maxwell-Boltzmann distribution
)/exp(),,1(.
TkgVEQ Bi
i
i
𝜕𝑓
𝜕𝑡+ 𝒗 ∙ 𝛻𝑓 −
𝑒
𝑚𝑬 ∙ 𝛻𝑣𝑓 = 𝐶 𝑓
Temperature: Te ≈ Tv ≈ Tn
Temperature: Te >> Tv>>Tn 22
Te >> Tv>>Tn
Non-Equilibriumplasma
Te ≈ Tv ≈ Tn Near Equilibrium plasma
arc
Gliding arc
Corona
Tn~n*10,000 K
Tn~n*1000 K
Tn~n*100 K
Te > Tv>Tn
A few examples
23
Electron impact avalanche𝑑𝑛
𝑒
𝑑𝑥= α𝑛𝑒
𝑛𝑒=eα𝑥
α: The 1st Townsend coefficient, inverse of net ionization length scale. It is determined exponentially by E/p or E/N (electric field strength)
If α > 0, electron avalanche phenomenon.
α = 𝐴𝑝𝑒𝑥𝑝 −𝐵𝑝
𝐸𝑛𝑒
A,B: constantsE: electric field p: pressurene: electron number density
+
x d0
Breakdown voltage:
The minimum voltage between two electrodes that causes an arc. At the breakdown voltage, the rates of ionization and dissociative attachment becomes equal.
Paschen's law: pdmin=1 torr cm. The nonlinear dependence of breakdown voltage is
to due to electron impact avalanche via collisioinal energy transfer
Lieberman, Michael A.; Lichtenberg, Allan J. (2005)
The breakdown field for atmospheric air~28.7 kV cm−1
Mean free path (1 atm, air): Molecules: 0.1 µm, Electron-molecule: 5.5 µm
pdmin=1 torr cm at 760 Torr, dmin=13.2 µm, twice of the mean free path.
Few
co
llisi
on
More collisions
How to produce uniform plasma at high pressure?24
Meek and Loeb criterion: Streamer is formed once the total number of electrons in the electron avalanche is so large that their space charge field becomes comparable to E0, the avalanche-to-streamer transition occurs. α𝑑~18 − 20 and ne= 1013 cm-1
Stream propagation: A space charge wave, which can penetrate into neutral gas
with a velocity much higher than the electron drift velocity, up to a fraction of the
speed of light.
anodecathode
--
++
ℎν
E
+-
Streamer
- +
α𝑑~18 − 20
Positive streamer
E0
neutral
Space chargeStreamer head
+
Power input by external field: N0𝑞𝑒𝐸0𝑉𝑠Power consumed in ionization:
𝑑𝑛0
𝑑𝑡𝑄𝑒
Plasma DischargesStreamer discharge: a non-thermal narrow filamentary discharge channels formed at the initial stage of a sparkbreakdown by a high voltage pulse (1-100 ns). A streamer has a streamer head (space charge) with a high reduced electricfield (~100 Td, 5-10 kV cm−1 for air at atmospheric pressure) followed by a streamer channel with lower electric field andhigher conductivity (charge number density). Formation of a streamer discharge occurs when the electric field in thestreamer head is at the same magnitude or greater than a critical external electric field (4.4 kV cm−1 in air at atmosphericpressure for a positive streamer, 8–12.5 kV cm−1 for negative streamer). Streamers are fundamental components in manykinds of discharges such as the dielectric-barrier discharges, corona discharge, and spark. It is widely used in industrialozone production, biomedical treatment, plasma assisted combustion, pollution control. Note: a microwave streamer is ahot plasma not a streamer.
Energy balance:
25
Fig. (a) Geometry of the simulation domain. (b) Propagation of growing and decaying positive streamers in an external field of10 kV cm−1. Both positive streamers are initiated from a Gaussian distributed plasma cloud with a peak density of 1020 m−3 and acharacteristic size σ0 of 0.05 mm. The radius of the spherical electrode Rsph is 0.5 mm. The only difference is that in the left panelthe spherical electrode has a potential Usph = 3.5 kV, whereas in the right panel Usph = 3.2 kV. (c) Propagation of negativestreamers in an external field of 20 kV cm−1. For both negative streamers, the initial plasma cloud has a peak density of 1018 m−3and a characteristic size of 0.10 mm. The electrode radius Rsph = 1.0 mm, and in the left panel Usph = 4.0 kV, whereas in the rightpanel Usph = 3.4 kV
Qin et al., J. Phys. D: Appl. Phys. 47 (2014) 435202 (9pp)
Flame front: auto-ignition and diffusive heat transfer, self-supported propagationStreamer: ionization and space charge transfer, external field supported propagation
Positive and negative streamers: Propagation of negative streamer requires a much stronger space charge field.
Streamer propagation vs. Flame propagation
26
Corona DischargeAn discharge around a highly curved conducting electrode induced by a high electric field, but the
external electric field is not high enough to cause a breakdown or arc. Widely used in ozone generation.
Positive corona: electrons are attracted to curved positive electrode and have enough energy to cause
electron avalanche. Electron energy is high, density is low.
Negative corona: ions are attracted to curved negative electrode. The photon emissions via ion-
bombardment on electrode surface cause electron avalanche. Electrons have lower energy but higher
density.
Fig. Pulsed corona discharge and positive streamer development: CCD photos of the point-wire discharge in air using 5μs optical gate. Applied voltages: (a) at
7.5 kV, (b) and (c) at 12.5 kV. For (a) and (b) the semiconductor switch is used, for (c) the spark gap. The electron temperature is about 5-10 eV. By E M van
Veldhuizen and W R Rutgers, J. Phys. D: Appl. Phys. 35 (2002) 2169–2179 PII: S0022
(a) (b) (c)
27
A discharge that occurs between electrodes with at least one electrode is covered by dielectric materials. It is a corona
discharge with a dielectric electrode. The existence of dielectric barrier limits the current and restricts transition of DBD
discharge to arcing. DBD discharge often has filamentary micro discharge structures and is physically behaving like an
incomplete streamer breakdown. DBD discharge has low electron number density and high electron energy and been widely
used in ozone generators.
Dielectric barrier discharge (DBD) and NS DBD
Rectangular quartz channel 22 mm x 10
mm in cross-section and 280 mm in
length. Rectangular copper electrodes, 15
x 60 mm. High-voltage pulses 20 kV on
the high voltage electrode, 25 ns duration
at the half-amplitude, up to 20 KHz.
Nanosecond DBD discharge in air: 20 kV, 10 kHz, pulse N10. Left:
Front view; Right: side view
Conclusion: NS discharge in DBD geometry in air is non-uniform. Initial electrical
field’s distribution and thermal ionization instability development form the non-
uniform energy distribution in the discharge. This non-uniformity can play a key
role in kinetic experiments in this type of the discharge.Andrey Starikovskiy et al., 2014, AIAA-paper
40 Torr/AR, NS BDB
28
DC Glow Discharge (high special uniformity and volumetric)
The glow discharge is stable in a low pressure, but it is possibleto stabilize such a plasma at atmospheric pressure if threerequirements are met: (i) use of a source frequency of over 1kHz, (U) insertion of a dielectric plate (or plates) between thetwo metal electrodes, (iii) use of a helium dilution gas.
A self sustained weakly ionized volumetric (non-filamentary) discharge supported by thesecondary electron emission from the cathode. Ithas three distinctive structures: Negative glow,Faraday dark space, and positive column.
http://en.wikipedia.org/wiki/File:Electric_glow_discharge_schematic.png
Princeton Plasma Physics Laboratory
29
5FIG. 2. 10 ns exposure time photograph of the gap taken when
the discharge current is maximum. The gap length is 5 mm and
the cathode is located at the bottom.
Cathode is at the bottom
FIG. 1 100 ns exposure time photographs of the gap taken during the discharge initiation, the discharge current being
periodic. The number on the current wave form a) corresponds to the number on the left side of the picture b) and indicates
the time when the picture was taken. The gap length is 5 mm. In each picture, the cathode is located at the bottom. Francoise
Massines et al., J. Appl. Phys., Vol. 83, No. 6, 15 March 1998
Positive column
Faraday dark space
Negative glow
30
Atmospheric pressure DC glow discharge
Figure 2. Images of glow discharge in atmospheric
pressure air at (a) 0.1 mm, (b) 0.5 mm, (c) 1mm
and (d) 3mm electrode spacing
Figure 3. Image of the glow discharge in atmospheric
pressure hydrogen. Positive column and negative glow
are visible. In addition standing striations are visible in the
positive column.
David Staack, Bakhtier Farouk, Alexander
Gutsol and Alexander Fridman, Plasma
Sources Sci. Technol. 14 (2005) 700–711
31
•Rotational temperature (Trot) increases with vibrational temperature (Tvib) decreases with increase in pressure.• Above 100 psi, they are measured to be within 500K of each other which is equal to the uncertainty in Tvib fitting.
David Staack, UTAM
Transition from micro glow discharge to equilibrium arc discharge
32
Spark Discharge
A small volume, high temperature, and high current equilibrium arc initiated by ahigh voltage breakdown discharge (~10 ns). It has high current (1-1000 A), lowvoltage (10-100 V), and low electron temperature (~1 eV). Spark discharge iswidely used in gasoline engines. The role of spark discharge is to create hightemperature environment for ignition. Laser ignition is also to create a spark.
Plasma torch
Meghnad Saha derived an equation for the relative number of atoms ineach ionization state in an equilibrium plasma:
Tk
EE
Be
ie
i
i
i B
ii
eh
Tkm
gn
g
n
n
12/3
2
11 22
It depends on the number density of electrons, ne. This is because as the number density of electrons increase, the electric field decreases and thus lower the ionization state.
Plasma torch is also a continuous electric arc. It is high temperature nearequilibrium plasma. It is widely used in ignition and materials processing.The temperature, power, and electron number density is very high. Itmostly places a thermal effect in dissociating reactants and acceleratingchemical reactions.
33
Fig. 1 Left: Schematic of a traditional gliding arc plasma discharge with
the numbers corresponding to the sequence in time evolution of the arc as
it moves along the electrodes (Ombrello and Ju). Right: direct image of a
gliding arc time trajectory [Courtesy from Dr. Z.S. Li at Lund University].
Fig. 3 Short exposure grayscale photograph of the
magnetic gliding arc discharge once stabilized at the
largest gap, with the cathode spot (CS) and positive
column (PC) shown.
Fig. 2 Pictures of the gliding arc plasma system with the (a) side view of central
electrode, (b) top view of system, and (c) time integrated top view photograph of
the magnetic gliding arc creating a plasma disk to quasi-uniformly activate the
flow. The numbers in (a) and (b) indicate the path of the gliding arc from
initiation, point 1, to arc rotation/elongation, points 2 and 3, and final arc
stabilization, point 4.
a gliding non-equilibrium electric discharges invented by Lesueur et al. [1]. The main distinctive aspect of the gliding
arc is a high level of non-equilibrium with both high electron temperature (1-2 eV) and high electron density as well as
high gas temperature (~2000 K). It can be inexpensively generated under near-to-atmospheric pressures.
[1]. H. Lesueur, A. Czemichowski and J. Chapelle, Frenchpatent 2 639 172.
[2] A Fridman, S Nester, LA Kennedy, A Saveliev, O Mutaf-Yardimci, Gliding arc gas discharge, Prog. Energy Combust. Sci. 25 (2), 211-231
Gliding arc
[3] Ombrello et al., AIAA Journal 2006. 34
)1()()(1 2ET
r
TTr
rr
2
)/exp(/)( 0
2
0
2 TkET BElectrical conductivity.
0
2
00 /)(16)(2 ETkTr
TTrW B
IWlRIV /0
),4/(,2/
),2/(),4/(
2
00
0
2
0
RVWVV
RVIWRVl
critcrit
critcrit
,
)4/(2 2
00 WlRVVWRI
WE Electric field:
Energy conservation equation
Temperature: T
Effective electric field strength: E0
Conductive arc heat loss per unit length from above equation:
From Ohm’s law:
RWlRVVI 2/)4( 2
00 We have:
Corresponding to steady and unsteady gliding arc.
042
0 WlRVCritical condition:
35
RI
V=Wl/I
V0
(a)
Fig. 2 Left: Plot of the increase in electric field in plasma after the transition point in a gliding arc discharge [8]. Right: three sequentialframes of gliding arc images recorded by a high-frame-rate camera, showing the conversion from a glow-type discharge to a muchbrighter spark-type discharge [7]. (Courtesy from Dr. Z.S. Li at Lund University)
Gliding arc voltage
36
Short-cut
Gliding arc dynamics and radial production
Fig. 3 Left: A short-cut event recorded at 20 kHz framing rate using an exposure time of 13.9 μs. The short-cut current path is indicated by the arrow in the frame of t
= 50 μs. Right: Three typical single-shot OH PLIF images of a gliding arc using an exposure time of 2 µs, at two flow rates (a) 17.5 SLM, (b) 42 SLM. The typical
thickness of the OH distribution is labelled in the images with unit of centimeters [6, 7] (Courtesy from Dr. Z.S. Li at Lund University)
Magnetic gliding arcs
37
RF and Microwave discharges
In DC and AC discharges, electrical power is delivered to plasma by moving electrons/ions to the electrodes across the cathode and anode sheaths. When the electrical frequency is very high like RF and MW, the time required for charge particles to move across the sheath becomes comparable or longer than the wave period of electrical field. Therefore, the interaction between electrical field and plasma is exclusively by charge displacement current, not by a directed current to electrodes. Therefore, it can be delivered without requirement of an electrode in contact with plasma by a sheath.
RF discharge (10k-100M Hz) Microwave discharge (1G-300G Hz)
p pe cmn )107( 110
Low pressure-1 atmField wavelength: metersLower electron energy (1-2 eV)Some sheath
Low pressure-high pressureField wave length: 12.24 cm at 2.45 GHzHigher electron energy (5-15eV)No high voltage sheath
• Inductive coupling: via oscillating magnetic field• Capacitive coupling: via oscillating electric field
RF & MW plasma coupling
Particle interaction Collective interaction
38
microwave resonator
Qiang Wang et al, APPLIED PHYSICS LETTERS 104, 074107 (2014)
Miles et al., PrincetonIkeda et al., Imagineering Inc.
Microwave discharge for ignition and flames
39
Micro-discharge 1
Microscale Discharges in Liquids
Microdischarge between ceramic
spheres (Tomohiro Nozaki, 2015)
Applications• Largescale surface ignition• Crude Oil Fuel Reforming• Medical Treatments• Plasma catalysts• Aerodynamic Control• High pressure materials processing
40
Electrode
Electrode
Preliminary tests of single (top) and four channels (bottom) micro-discharge using single a RF power supply.
The channel is 76mm Χ 26 mm with a gap distance about 0.5 mm, (Princeton, 2017)
Courtesy by David Staack
Microscale Discharge micro tips
Micro-discharge in CH4/He250 torr (Princeton, 2017)
41
Nanostructured discharge
Parallel Plate electrodes Dielectrics(Al2O3 of 0.6 mm with AAO AAO film of 50 μm, holes of 200nm p= 60Torr d= 1 inches rf power =1~100W
α to ϒ discharge mode transition is observed Under α discharge mode, dielectric materials has small
effect on the discharge Under ϒ discharge mode, the discharge on AAO area is
difficult to transit from α to ϒ mode than Al2O3
Possibility of plasma control using nanostructures
Haibo Mu and Yiguang Ju, 2017, unpublished work
Findings
eaiee nDn
dt
dn 2)(
Breakdown condition of plasma discharge
eaiee n
Dn
dt
dn
2
)(
electrons oflength diffusion sticcharacteri:
tD
ee
ai
enn)(
02
2
)/()/(
D
NENEai
Breakdown condition:
ec
e
c m
eTvvlD
3
8
33
1 2
Electron production, attachment, and diffusion:
Diffusivity of electrons (no Ambipolar diffusion):
Introducing a diffusion length scale:
Electron production, attachment, and diffusion:
νi: ionization rate, νa: attachment rate
42
43
Plasma thermal instability
Lightening arc
Streamer-leader transition
Gallimberti [1979, 2002] Bazelyan et al. [2007]
Positive feedback between Joule heating and electric field.
δT↑
δN ↓δ(E/N) ↑ δTe ↑
δne ↑ δ(jE) ↑ T: gas temperature
N: gas number density
E/N: reduced electric field
Te: electron temperature
ne: electron number density
jE: Joule heating
Red Sprites, Blue Jets and Elves
http://www.albany.edu/faculty/rgk/atm101/sprite.htm
44
Air pollution helps wildfires create their own lightning
Science, 2021 doi:10.1126/science.abj6782Nikk Ogasa
How do we explain this and how to control plasma discharge?
Pulse 50
150
200
300
450
600
Pulse 50
150
200
300
450
600
Stoichiometric 2.8% C5H12, 22.2% O2, 75% Ar Dilution
Aric Rousso et al., Plasma Sources Science and Technology, 2020
Electron energy and number density in various Equilibrium & Non-equilibrium Various Plasmas
1010 1015
0.1
1
10
Ele
ctr
on
te
mp
era
ture
, e
V
Electron number density, 1/m3
Corona DBD
Arc
Flame MHD
RF
Glow
Nanosec
0.1
1
10 Corona DBD
Gliding Arc
Arc/Spark
Flame MHD
DC, MW
Micro dis.
45Ju and Sun: Plasma assisted combustion, Progress of Energy & Combustion Science, 2015
46
Table 1: LTPs examples used in modern plasma
applications: Te, Ti, Tg, Tnp are the effective
temperatures (or mean energy) of electrons,
ions, gas, nanoparticles, respectively. Ne, Nnp are
the densities of electrons and nanoparticles,
respectively. εI is the ion energy. Z is the
ionization degree defined as the ratio of the
plasma density to the sum of atomic, molecular,
and plasma densities. Abbreviations: APPJ
(atmospheric pressure plasma jet), DBD
(dielectric barrier discharge), ExB (crossed
electric and magnetic fields)
Yevgeny Raitses, 2019
Non-equilibrium Low Temperature Plasma (LTP) Properties
47
What are our questions?
CombustionPlasma
Chemistry
PhysicsInstability Catalysis
Pri
nce
ton
Un
iver
sity
Combustion lab.
2. Plasma Assisted Combustion and Chemical Conversion
Yiguang Ju
1. PAC in scramjet engines and detonation engines
2. PAC in spark ignition engines
3. PAC in flame stabilizations for gas turbine engines
4. PAC in ignition, combustion, cool flames, and emission control
5. Plasma assisted chemical conversion: CH4/CO2 and NH3
Yiguang Ju, Princeton University
Princeton Combustion Summer School
2021.6.21
Pri
nce
ton
Un
iver
sity
Combustion lab.
MotivationDevelopment of High Speed/Low Emission Power Systems & Synfuels
•Enhanced combustion efficiency and flame stability
•Predictive new engine design with alternative fuels for low emissions
Advanced Gas Turbines
(Low NOx, after burner, renewable fuels)
Ignition and Flame Stabilization
Internal combustion engines:
Ignition timing control
Conventional discharge?
Pulse Detonation Engines
Ignition and DDT
Pri
nce
ton
Un
iver
sity
Combustion lab.
1. PAC for Ramjets and Scramjets
Flow Residence Time
Chemical Reaction Time<< 1
•Flow time scale <1 ms
Mach 10
X43: Hydrogen
Silane: ignition enhancer
X-51 (AFOSR)
• Mach 4-8
• Hydrocarbon fuels
(JP-8, JP-7)
Difficulty in ignition, flame stabilization, and
combustion completion
Pri
nce
ton
Un
iver
sity
Combustion lab.
Increase residence time or reduce combustion time
Solutions:
•Cavity
•Oblique shocks
Niioka et al.1998
•Plasma
Takita et al.
Kimura et al. 1981
Masuya et al. 1993
Pri
nce
ton
Un
iver
sity
Combustion lab.
Plasma-Assisted Combustion
Ignition & Flame speed ~(a, Dfuel, , Φ, Rc, e , …)Tf-Ea
Plasma discharge
Temperature
increase
Thermal Enhancement
Radicals
Ions/electrons
Excited
species
Kinetic enhancement
Fuelfragments
Transport enhancement
Ionic wind
O, NO
N2 *(A)
O2 (a1Δg)
H2, CH4
C2H4
O2+
Mixing, flow
Pri
nce
ton
Un
iver
sity
Combustion lab.
O Radical Production by Plasma on ignition and flame extinction
Takita and Ju (2006)
He/O2 = 0.45:0.55 He/O2 = 0.38:0.62
Effect of PAC on methane flame ignition and extinction, Sun W. et al. 2010, 2011
Pri
nce
ton
Un
iver
sity
Combustion lab.
(a) XH2,PJ = 0.3, PIN = 3.0 kW, PIN total = 4.3 kW
(b) XH2,PJ = 0.5, PIN = 6.0 kW, PIN total = 8.2 kW
Fig. 2.1 H2 ignition by plasma torch in a M=2.3 flow and the
effect of total heat addition on pre-combustion shock wave, XH2
is hydrogen mole fraction in H2/N2 plasma torch; Pin: plasma
torch electric power, Pin total: total heat addition (Pin+H2 enthalpy
flux) [192]
Electron-impact reactions (CH4, O2, N2, H2)
e + CH4 = CH4++2e
e + CH4 = CH3 + H + e
e + O2 = 2O + e
e + N2 = 2N + e
N+O2 = NO+N
NO + HO2 = NO2 + OH
:
Pyrex Glass Window
30
Plasma
Torch
x
yPlasma
Jet
320
170
Main Stream
Fuel Injector
z
30
Nozzle
M=1.8
Feedstock
Pyrex Glass Window
30
Plasma
Torch
x
yPlasma
Jet
320
170
Main Stream
Fuel Injector
z
30
Nozzle
M=1.8
Feedstock
Effect of mixing ratio of N2/O2 feedstock on wall pressure
increase due to combustion of fuel injected at Xi = 24 mm in
experiment, H2/air mixture, Takita, Abe, Masuya, and Ju,
2006,
Plasma jet ignition enhancement
Pri
nce
ton
Un
iver
sity
Combustion lab.Fig. 2.2b Direct photographs of DBD plasma in M=2.0
supersonic flow [181]
Fig. 2.2a Schematic of the test section with
DBD and plasma torch [181]
DBD/plasma jet ignition
Wall pressure distributions when H2 fuel was injected
from xi = −40 mm under simultaneous operations of
DBD device and PJ torch. (a) Pin = 1.75 kW.
(b) Pin = 2.4 kW. (c) Pin = 3.3 kW. (d) Pin = 4.05 kW.
Pri
nce
ton
Un
iver
sity
Combustion lab.
Fig. 2.4 Schematic of the cavity model [182]Fig. 2.5 OH PLIF images of a cavity flame in supersonic flows
of two different enthalpies: (a) without the plasma and (b) with
plasma at M=2.9, Jn~4 of H2 jet, and (c) without plasma and
(d) with plasma at M=2.6 Jn~3.5 of H2 jet [182]
A nanosecond pulsed plasma discharge, arc jet vitiated
ignition
0.15
0.48
1.95
2.72
0.93
1.34
2.26
3.51
N2 arc jet heated air free stream: T0 = 2,500 K, P0 = 1
bar, M=4.5
ISO 200, Exp. 3
ms
Cavity
Equivalence
ratio
1)
Partially-
Premixed
Flame
2) Cavity
Flame
Holding
3) Non-
Premixed
Flame
Arc jet vitiated ignition, Do et al. 2013
Pri
nce
ton
Un
iver
sity
Combustion lab.
Fig. 2.6 Schematic of experimental setup and electrodes arrangement
[184]
Fig. 2.7 Left: discharge without fuel injection.
Right: discharge interaction with H2 injection [184]
Gliding arc flame stabilization
large volume and forced ignition
Pri
nce
ton
Un
iver
sity
Combustion lab.Esakov et al. AIAA-paper-2005
I. Esakov et al., IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 37,
NO. 12, DECEMBER 2009
Subcritical streamer microwave discharge
The discharge in the undercritical field can be initiated,
for example, at the location of a cylindrical MW
vibrator in the EM beam. Conditions of local electrical
breakdown E ≥ Ecr in this case are realized at the
ends of the vibrator.
vibrator
Pri
nce
ton
Un
iver
sity
Combustion lab.
Microwave flame stabilization in a high speed flow (200 m/s) as a
preburner
Precombustor
Fig. 2.9 Schematic of experimental setup [186]
Van Wie et al., AIAA 2006-1212
cylindrical MW vibrator
Pri
nce
ton
Un
iver
sity
Combustion lab.
Fig. 2.22 Left: A valve less PDE setup at the Naval Postgraduate School. This type of architecture requires a
booster and its anticipated applications are missiles or rockets. Right: Comparison of ignition delays for C2H4/air
mixture using spark plug and transient plasma igniter [178]
Fig. 2.23 (a) PDE engine facility at the Air Force Research Lab at Wright-Patterson Air Force Base, (b) Schlieren imaging of nanosecond pulsed
discharge igniter in CH4/air mixture, Φ=1, (c) Schlieren imaging of nanosecond pulsed discharge igniter in CH4/air mixture, Φ=0.8 [62]
Plasma assisted ignition and DDT in PDE/RDE
0
1
2
3
4
5
6
7
1 10 100 1000
Fla
me
-De
ve
lop
me
nt
Tim
e [
ms
]
Total Energy [mJ]
MSD
NRP discharge, f = 1-5 kHz
NRP discharge, f = 10-40 kHz
Figure 3: Ignition kernel development for 5
pulses of 3.2 mJ per pulse. Left images:
pulse repetition frequency of 2 kHz. Right
images: pulse repetition frequency of 40
kHz.
0.01 ms
0.5 ms
1.0 ms
2.0 ms
4.0 ms
6.0 ms
2 kHz 40 kHz
0
200
400
600
800
1000
1200
1400
1.E-8
1.E-6
1.E-4
1.E-2
1.E+0
1.E+2
0 0.05 0.1 0.15 0.2 0.25
Te
mp
era
ture
Mo
le F
rac
tio
n
Time (ms)
O atom, 5 kHz O atom, 10 kHzO atom, 40 kHz T, 5 kHzT, 10 kHz T, 40 kHz
a.
Figure 6. a) Computed atomic oxygen concentration and temperature
as a function of time with 1% oxygen dissociation repeated at 5 kHz,
10 kHz, and 40 kHz frequencies for stoichiometric methane-air
mixtures at 850 K. b) . Computed atomic oxygen concentration and
temperature as a function of time with 0.1%, 0.5%, and 1% oxygen
dissociation repeated at 40 kHz frequency for stoichiometric
methane-air mixtures at 850 K.
• Increase of ignition kernel volume• Reactivate chemical radicals before quenching
Lefkowitz, J.K., Guo, P., Ombrello, T., Won, S.H., Stevens, C.A., Hoke, J.L., Schauer, F. and Ju, Y.,
2015. Combustion and Flame, 162(6), pp.2496-2507.
Plasma assisted DDT for PDE/RDE
15
• Control of detonation formation in pressure gain engines
Detonation failure
Kawasaki, A et al, 2019. Proceedings of the
Combustion Institute, 37(3), pp.3461-3469.
Pratt and Whitney Rocketdyne (PWR)
16
No Ozone
Ozone
Kinetic Effect of Ozone Addition on DDD of C2H2/O2 Mixtures in A Microchannel
Sepulveda, J., et al., AIAA journal, 57(2), pp.476-481. DOI: 10.2514/1.J057773
Plasma may enhance DDT ?
Pri
nce
ton
Un
iver
sity
Combustion lab.
Pressure SensorThermocouple
Air Flow Sensor
Carburetor
Throttle Rod Choke Rod
Exhaust
2. PAC for IC engines
PU-Imagineering Inc.
SP-HCCI/RCCI
Challenge: Development of 60+ % Efficient Engines
Gross thermal efficiency ~ 42.3%
Heat loss ~ 54.6%
Carnot efficiency ~ 1-300/2100=86%
Otto efficiency: 68%
18
Diesel engines
Diesel engine (Nissan)
Output
Efficiency of lean burn gasoline engine
SIP program in Japan, 2019
We need low temperature combustion (LTC)
• Lean burn• Ignition assisted LTC engines like
HCCI (homogenous charge compression ignition)
RCCI (Reactivity controlled compression ignition)
• Higher compression ratio, Const Vol. comb.• Green fuels …
Pri
nce
ton
Un
iver
sity
Combustion lab.Gundersen et al., 2003
Transient corona discharge
Disk electrode & streamers
Corona enhanced ignition
Pri
nce
ton
Un
iver
sity
Combustion lab.
Fig. 2.19 Left: streamers generated by a single 370 mJ, 56 kV, 54 ns pulse
(maximum E/N~400 Td) in air (10 s gate time); Right: flame propagation from
multiple ignition sites at the base of the streamers after a single pulse in F=1.1
C2H4/air mixture (1 ms gate time) [81]
Fig. 2.20 Images of flame development in F=1.1 C2H4/air mixture, 6 ms after ignition. A 300 ms gate
time was used with equal sensitivity for both images and 996×990 resolution. Left: spark ignition
using a standard 105 mJ, 10 ms, 15 kV spark ignition system and a spark plug with a 1 mm gap. Right:
transient plasma ignition using a 70 mJ, 12 ns, 54 kV pulse with a 6 mm gap [81]
1. Shiraishi T, Urushihara T, Gundersen MA. A trial of ignition innovation of gasoline engine by nanosecond
pulsed low temperature plasma ignition. J. Phys. D: Appl. Phys. 2009;42:135208.
Pri
nce
ton
Un
iver
sity
Combustion lab.
Microwave and nanosecond plasma assisted ignition
Nanosec pulses
Less
Heat Loss
Larger
volume
t1 t2
O, OH, NO, C2H4… production
Microwave
Spark Large
heat
Loss,
Small
volume
t1 t2
Fig.1 Current spark ignition plug: large heat loss, small volume
microwave repetitive nanosecond ignition with radical production,
Increased volume, less heat loss
Pri
nce
ton
Un
iver
sity
Combustion lab.
Spark, Microwave, Gliding arc
Microwavepulsed power
generator
Synchronizationpulse
generator
Gliding arc power generator
Nanosecond pulsed plasma
generator
Microwave antenna(Imaging Eng. Inc)
SparkMGANSDelectrodes
OH* Comparison between Spark and MW ignition.
Imagineering Inc.
Pri
nce
ton
Un
iver
sity
Combustion lab.
Ikeda et al., Imagineering Inc.
Pri
nce
ton
Un
iver
sity
Combustion lab.
Fig. 2.16(a) direct photograph of plasma assisted 34 cc Fuji engine test setup and (b) the comparison of
limits of stable engine operating conditions with and without microwave (MW) discharge at 2000 rpm [69] .
Lefkowitz, J.K., Ju, Y., Tsuruoka, R. and Ikeda, Y., 2012. A study of plasma-assisted ignition in a small
internal combustion engine. AIAA paper-2012-1133.
Pri
nce
ton
Un
iver
sity
Combustion lab.Q Wang et al., Applied Physics Letters 103, 204104 (2013); doi:
10.1063/1.4830272
Spark plug
ignition (Φ)
Microwave
ignition
1 bar >2 1.6
2 bar 1.8 1.0
4 bar 0.9 0.7
6 bar 0.9 0.7
8 bar 0.9 0.7
Table 1. The lean burn limits at different initial pressures
The pressure curve of MW ignition at 8 bar
Microwave/spark ignition
Pri
nce
ton
Un
iver
sity
Combustion lab.
Fig. 2.13 The effect of spark ignition and microwave enhanced spark ignition on COVIMEP, fuel consumption and exhaust
emission [172]
Fig. 2.14 the SI and SI+MW modes as a function of equivalence ratio at an initial pressure of 1.08 bar and
300 K (a) for FDT, (b) for FRT [70]
Fig. 2.12 Comparison of C3H8 flame images
in a compression-expansion engine using
conventional spark plug and microwave
enhanced spark plug, F=1, initial pressure 600
kPa, initial engine speed 600 rpm [172]
Microwave/spark ignition in engine
Fig. 2.11 Comparison of ignition using spark plug (left), microwave
(middle), gliding arc (right) (Photos were provided by Knite Inc. and
Imagineering Inc.) [197]
Ignition to flame transition(critical radius, Rc)
Flame Radius, R
Fla
me
Pro
pa
ga
ting
Sp
ee
d,U
10-1
100
101
102
10-2
10-1
100
101
Q=0.0
Q=0.1
Q=0.15
Q=0.6
(a), Le=1.2, h=0.0
a
b
cd
e
f
g
h
i
j
Critical ignition radius
Rc
Q?
Role of plasma:
• Mainly increase the initial ignition volume,
Rc; not increase flame speed!
• The thermal effect is not very large.
Why do we see a significant extension of lean burn in engines with microwave?
Ignition by nanosecond surface dielectric barrier discharge (SDBD)
S M Starikovskaia, J. Phys. D: Appl. Phys. 47 (2014) 353001 (34pp)
S.A. Stepanyan, M.A. Boumehdi, G. Vanhove, P. Desgroux, N.A. Popov, S.M. Starikovskaia, Comb. & Flame, 162 (2015) 1336-1349
600 650 700 750 800 850 900 950 10006
8
10
12
14
16 (CH4:O2, =1) + 76% Ar
(n-C4H
10:O
2, =1) + 76% Ar
(n-C4H
10:O
2, =1) + 76% N
2
(CH4:O2, =0.3) + 75% Ar
(CH4:O2, =0.5) + 75% Ar
No autoignition
Pre
ssu
re P
d,
atm
Temperature Tc, K
S M Starikovskaia
29
Dissociation and fast gas heating via
electronic excitation of molecular nitrogen
0 500 1000 1500 2000
500
1000
1500
2000
2500
U=10 kV, T=20 ns
1.5 mm ID, 80 mm length
Tem
pera
ture
, K
Time, ns
Capillary ns discharge in air,
P=25 Torr, T0=300K
N2 + e -> N2 (C3Pu) + e
N2 (C3Pu) + O2 -> N2 + O + O(1D)
O. Dutuit, N. Carrasco, R. Thissen et al. 2012 The Astrophysical J. Suppl. Series, 204/2
N. Popov, 2011, J. Phys. D: Appl. Phys. 44, 285201 (16pp)
0 50 100 150 200
0
10
20
30
40
Plasma assisted ignition, U=-24 kV
PTDC=14.7 atm
TC
=972 K
discharge
initiation
(blue step)
Pre
ssu
re,
atm
Time, ms
30
Autoignition vs plasma ignition in RCM
at PTDC=15 bar and TC=970 K, (CH4:O2)+76%Ar
0 50 100 150 200
0
10
20
30
40
PTDC=14.7 atm
TC
=972 K
Pre
ssu
re,
atm
Time, ms
Autoignition
E=0.1-5 mJ, 100 “kernels”
S.A. Stepanyan, M.A. Boumehdi, G. Vanhove, P. Desgroux, N.A. Popov, S.M. Starikovskaia, Comb. & Flame, 162 (2015) 1336-1349
31
Pressure trace and corresponding fast
imaging of flame propagation
200 205 210 215 2200
5
10
15
20
25
30
35
40
discharge
initiation
Pre
ss
ion
/ b
ar
Time / ms
0.8 ms 1 ms 1.2 ms 1.4 ms
1.6 ms 1.8 ms 2 ms
Pressure detector
CH4:O2, ER=1 + 70% Ar, TC=947 K, PTDC=15.4 bar
S.A. Stepanyan, M.A. Boumehdi, G. Vanhove, P. Desgroux, N.A. Popov, S.M. Starikovskaia, Comb. & Flame, 162 (2015) 1336-1349
32
Experiments in n–C7H16:O2:N2
Autoignition (black) and plasma ignition (red)
100 200 300 400
0
1
2
3
4
5P
TDC=2,3 bar
TC=646 K
V = 46 kV
Pre
ss
ure
, b
ar
Time, ms
No discharge
With discharge
Discharge initiation
100 200 300 400
0.0
0.4
0.8
1.2
1.6
Pre
ss
ure
, b
ar
Time, ms
No discharge
With dischargePTDC
=1,6 bar
TC=648 K
V = 46 kV
Discharge initiation
The discharge is able to modify gradually a cool flame
(U increase or P increase) and to initiate a 2-stage flame
33
Polarity: U>0
Energy deposition
W= 4.8 mJ
Quasiuniform ignition around HV electrode. Streamer discharge. Pressure 6 bar, Temperature 300 K.
Second regime of ignition:
Ignition along the
perimeter of HV electrode
Flame Initiation in H2/Air ER=0.5, P=6 bar
S.A. Shcherbanev, N.A. Popov, S.M. Starikovskaia, Comb. & Flame, 176 (2017) 272-284
34
Polarity: U>0
Energy deposition
W= 12 mJ
Ignition along the channels. Filamentary discharge. Pressure 6 bar, Temperature 300 K.
Third regime of ignition:
Ignition along the
discharge channels
Flame Initiation in H2/Air ER=0.5, P=6 bar
S.A. Shcherbanev, N.A. Popov, S.M. Starikovskaia, Comb. & Flame, 176 (2017) 272-284
Pri
nce
ton
Un
iver
sity
Combustion lab.
Fig. 2.21 Direct photograph of a prototype laser igniter
showing breakdown in air at multiple points [200]
Controlled plasma discharge for volumetric ignition
1
cm
(a)
(b)
(c)
Laser ignition and laser guided
discharge control, Miles et al. 2013
Fig. 3.16 Arc produced flow instability and jets [238]
Is low temperature combustion (LTC) relevant to engines?
36
TimescalesLow temperature ignition 0.1-1 msGasoline/diesel engine (30 CAD) 5 msGas turbine engine 1-5 msTurbulence eddy, l/u’ 0.1-100 msFlame time scale, δ/ub 0.1-1ms
Skeen et al. 2015
n-dodecane spray
Plasm assisted cool flame ignition
CH2O
Rainer N. Dahms et al., Proceedings
of the Combustion Institute 36 (2017)
37
Prometheus bringing fire to the earth
History of flame discovery: 3 different flames
1M B.C. 1817
FoodTools
Industrial revolutions
Space Exploration
2000 2010 2012-2013 2015 2017-2019
New enginesNew fuels
Premixed flames
Lean flame stabilization demonstrations
• MINI-PAC Bluff-body stabilized flame (propane or methane, 1 bar, 11 kW)
• TWO-STAGE SWIRLED INJECTOR (Propane air, 1 bar, 52 kW)
• AERODYNAMIC INJECTOR (MERCATO, Kerosene/air, 3 bar, 200 kW)
38
10
0m
m
• LEL: reduced by 10%
• Plasma power = 75 W
• LEL: reduced
from 0.4 to 0.11
• Plasma power = 300 W
• LEL: reduced from 0.44 to 0.21
• Plasma power = 1 kW
Pri
nce
ton
Un
iver
sity
Combustion lab.Laux et al., 2007
Nanosecond discharge on fuel lean flame stabilization
Plasma aided Ammonia Combustion
40J. Choe, W. Sun, T. Ombrello, C. Carter, “Plasma assisted ammonia combustion: Simultaneous NOx reduction and flame
enhancement.” 2021 Combustion and Flame, 228, 430-432.
(a) Schematic of the experimental setup, and direct photographs (ISO100, F/10, 2 s) of flames
(b) no plasma, ϕ=0.94 (c) with plasma, ϕ=0.94, (d) no plasma, ϕ=0.71 (c) with plasma, ϕ=0.71
Extension on LBO and Decreased NOx
41
(a) lean blowoff limits of ammonia/air flames with and without plasma,
(b) NOx emissions without and with plasma (ϕ = 0.94)
Plasma extended lean blowout (LBO) limits of ammonia flame and
simultaneously decreased NOx formation. Detailed mechanism is not clear.
O3 assisted C2H4 lifted flames -
Two-way phenomenon
42
O3 added after steady lifted flame established, with corresponding uf
Nozzle exit
uf = 3.29 m/s uf = 4.50 m/sO3 addition O3 addition
13% O2 + 87% N2 co-flow
Uco= 0.014 m/s
3500
ppm
0
ppm
0 ppm
2200
ppm
Blow-
out
If flame ascends, blow-out finally happens if O3 too much
Reaction of flame after O3 addition depends on HL,0, both ascend and descend
possible
Effect of ozonolysis reactions
B. Wu, H. Mitchell, W. Sun, T. Ombrello, and C. Carter. “Dynamics of laminar ethylene lifted flame with ozone addition.”
2020 Proceedings of the Combustion Institute, 38, 6773-6780.
CH2O PLIF
43
uf = 3.10 m/s uf = 3.57 m/s
Significant amount of CH2O production owing to ozonolysis
reactions upstream changing flame dynamics
CH2O PLIF
Flame
chemiluminescence
Pri
nce
ton
Un
iver
sity
Combustion lab.
Plasma assisted non-equilibrium chemical synthesis
Pri
nce
ton
Un
iver
sity
Combustion lab.
Plasma assisted chemical reforming: CH4/CO2
400 0C
Ozaki, 22nd ISPC, 2015
Pri
nce
ton
Un
iver
sity
Combustion lab.
Chemical looping?
Catalysis ammonia
Plasma assisted chemical looping: nitrogen fixation
Fig.1 Diagrammatic illustration of nitric acid, hydrogen, ammonia and fertilizer production from air and water through
catalytic plasma nitrogen oxidation and catalyst re-oxidation with water for hydrogen production for ammonia and
fertilizer synthesis.
Pri
nce
ton
Un
iver
sity
Combustion lab.
Annemie Bogaerts, Online Catalysts
Seminar, June 18, 2021
Schneider et al., Nature Catalysis 2018, 1, 269
Plasma assisted chemical reforming: NH3
Plasma catalysis Vibrational excitation
• Which non-equilibrium excitation mode plays a
critical role in ammonia catalysis?
• How much do we understand the mechanism?
Pri
nce
ton
Un
iver
sity
Combustion lab.
Technical questions:
1. Plasma can do a lot of “magics” in combustion
enhancement. Does it really have any “kinetic
merits” on combustion enhancement?
2. How does plasma kinetically enhance ignition, flame
speed, and minimum ignition energy?
3. What are the reaction pathways of plasma assisted
combustion?
4. How does non-equilibrium plasma excitation change
the yield and selectivity in catalysis?
Lecture 3 Plasma assisted combustion: ignition, flame propagation, burning limits, cool flames, and
the minimum ignition energy
Yiguang JuPrinceton University
• Plasma chemistry effect on ignition and ignition limits
• Flame propagation and the effects of heat loss and stretch
• Extinction, quenching distance, and flammability limits
• Ignition assisted flames and cool flames
• The Minimum ignition energy and the critical flame initiation radius
Princeton Combustion Summer School2021.6.21
SL TadFuel/O2 q
Premixed flames
Tad FuelqO2
Diffusion flames
air
Fuel
Ignition, flames and burning limits
2
Auto-ignition: An exothermic, self-accelerating, chain-branching process
Daf=𝑻𝒓𝒂𝒏𝒔𝒑𝒐𝒓𝒕 𝒕𝒊𝒎𝒆𝒔𝒄𝒂𝒍𝒆
𝑹𝒆𝒂𝒄𝒕𝒊𝒐𝒏 𝒕𝒊𝒎𝒆𝒔𝒄𝒂𝒍𝒆 𝒐𝒇 𝒇𝒍𝒂𝒎𝒆
Daf ~ O(1), but Daig → 0 (chemically frozen)
Premixed hot flame
Flam
e t
em
pe
ratu
re, K
Equivalence ratioΦ0L
1200
Lean limit Rich limit
Φ0R
𝑫𝒂𝒇,𝑯 < 𝟏
Chain-branching reaction: H+O2=O+OH
Tem
pe
ratu
re, K
Flame
AB
C D
E
FIgnition
Diffusion hot flames
Stre
tch
exti
nct
ion
Rad
iati
on
exti
nct
ion
Flow residence time
1200
extinction
Flame: An exothermic, luminescent, Diffusion-Ignition Front with chain-branching processes
3.1.1 Ignition and ignition limits
00
/
/
)0(,)0( FF
RTE
FF
RTE
Fp
YYTT
eYBdt
dY
eYBQdt
dTC
P, T0, YF0
0)/( Fp YQTCdt
d
)1(/ 000 qTCQYTT pFad
)1(/)(0
00 qQ
TCQTTCYY
p
pFF
1/
0
0
0
00
0
)(
/
/
/
/
0
RTE
pF
Be
t
RTE
TCQYq
TT
/)1(///// 000 eeeeeRTERTERTERTERTE
Auto-ignition Considering an auto-ignition problem at constant pressure, p, at initial temperature of T0, and fuel mass fraction of YF0.
ρ:densityT: temperatureY: mass fractionQ: heat release per unit mass of fuelE: activation energy
Conservation equation
Adding the mass and energy equations:
0/// 00 QTCYQTCYQTC adpFpFpNormalization:
, oflimit in the : theoryAsymptotic
)/1(1 O
1)0(
)1(1 /)1(
eqd
d
/1
0)0(
qed
d
)1ln( q
growth lexponentia ;/1:imeIgnition t qig
00 /
0
2
01/1
0 )(RTE
F
pRTE
igig eEBQY
TRCBeqt
T
T0
tig
Normalized equation:
a small change in temperature will lead to dramatic change in the reaction rate, therefore, in this limiting case, we have
Solution:
Define: q/1
Plasma effects on homogenous ignition (B, E, T): 1. Increase reaction rate B; 2. Reduce activation energy E, 3. change temperature (heat loss or addition)
00
/
3
0
2/
)0(,)0(
3/4
)(4
FF
RTE
FF
RTE
Fp
YYTT
eYBdt
dY
R
TThReYBQ
dt
dTC
Auto-ignition with heat loss or heat addition Assume: heat addition or loss is a small perturbation O(1/β):
0)0(
Hqed
d
pCR
htH
03
T
T0
tig
h<0
h>0Plasma heat addition/loss will shorten/extend the ignition delay time
Electron impact ionization/dissociation/excitation
e +O2 =O++O+2e (R1a)
e +O2 =O+O(1D) (R1b)
e +O2 =O2(1Δg)+e (R1c)
e +O2 =O2(v)+e (R1d)
Electron ion recombination, attachment, charge transfer
e+O2+ =O+O(1D) (R2a)
O2+ +O2
- =2O2 (R2b)
e+O2 +M = O2- +M (R2c)
H2O+N2+ =H2O
++N2 (R2d)
Dissociation and energy transfer by ions and excited species
N2(A,B,C)+O2 =O+O+N2 (R3a)
O(1D)+H2 = OH+H (R3b)
H+ O2(1Δg)= O+OH (R3c)
N++O2= O++NO (R3d)
CH3+HO2(v)=CH2O+OH (R3e)
N2(v=5) +N2 = N2(v=3) + N2 (R3f)
N2(v) + HO2 → N2 + HO2(v) (R3g)
3.1.2 Plasma chemistry for radical production and gas heating
Recombination/fast heatingRecombination/fast heatingAttachmentCharge Transfer
Radical productionExcitation
Slow heating
>10 eV
~10 eV~1 eV0.2-2 eV
Radical production & fast heating
Pri
nce
ton
Un
iver
sity
Combustion lab.
Kinetic ignition enhancement by radical addition
10-7
10-6
10-5
10-4
Mole fraction of radicals added into mixture
10-5
5
10-4
5
Ign
itio
n t
ime
(s)
H2:O2=2:1
T=1000 K
H
O
OH
Chain initiation
Chain-termination
H+O2+(M) → HO2 +(M)
H+OH+(M) → H2O+(M)
H2 +O2 → H+HO2
H2 +O2 → OH+OH
Chain branching and propagation
H+O2 → OH + O
O+H2 → OH + H
OH + H2 → H2O+H
H2O2+M =2OH +M
Slow
Rate limiting
(low P & high T)
Rate limiting
(high P)
Air Air
1
2
5
15
16
3
14
N2
H2 &
N2
7
10
11 12
13
8 6
9
Fuel Fuel 4
N2 N2
1. Silicon Controlled Rectifier, 2. Silicon carbide
heater, 3. R-type thermocouple, 4. Fuel injection
spacer 5. MGA plasma power supply, 5. MGA device,
6. MGA power supply, 7. Cathode, 8. Anode, 9.
Magnets, 10. Gliding arc initiation wire, 11. MGA, 12.
Insulator, 13. Nozzle with N2 co-flow, 14. K-type
thermocouple & FT-IR probe, 15. Diffusion flame, 16.
Water-cooled nozzle with N2 co-flow.
Kinetic effect by NO production on counterflow ignition
Diffusion Flame
Temperature &
Species Measurements
• FTIR, PLIF, Rayleigh
H2/N2
Air/H2/CH4
8
Plasma assisted ignition: H2 Ignition by gliding arc
825
850
875
900
925
950
975
1000
1025
175 200 225 250 275 300 325
Strain Rate, s-1
Ign
itio
n T
em
pera
ture
, K
NP + NF NP + 1% H2NP + 2% H2 P + NFP + 1% H2 P + 2% H2Comp. NP + NF Comp. P + NFComp. P + 2% H2 Comp. NP + 2% H2
NO+HO2=NO2+OH
NO2+H=NO+OH
H+O2+H2O=HO2+H2O
Plasma catalytic effects reduce H2 ignition temperature (Ombrello, T., Ju, Y. and Fridman, A., 2008. AIAA journal, 46(10), pp.2424-2433.)
9
d[H]/dt→infinity
1][
2
2
1 Mk
kNot explosive
1 explosive
H+O2
OH+O (R1)
+(M) HO2 (R2)
HO2+H=OH+OH
H+O2+(M) → HO2 +(M)
H+O2 → OH + O
• Radical and heat production by plasma can extend the explosion limit.
Plasma can break the conventional explosion limit
Pri
nce
ton
Un
iver
sity
Combustion lab.
Ignition Chemistry: Elementary chain reactions of CH4-O2 system
Chain initiation:
CH4 +O2 → CH3 +HO2
CH4+(M) → CH3 +H+(M)
Chain-branching and propagation
H+O2 → OH + O
CH3 +O2 → CH3O+O
CH3 +O2 → CH2O+OH
CH3 +HO2 → CH3O+OH
CH3O + O2 →CH2O+ HO2
CH3O + M →CH2O+ H+M
CH2O +X →HCO+XH (X=H, OH, O, HO2)
HCO+M→CO+H+M
HCO+O2→HO2+CO
CO+HO2→CO2+OH
Termination reaction
H+O2+M → HO2 +M
CO+OH→CO2+H
Slow
Slow
Opportunity of plasma
12
Kinetics of the ignition: CH4:O2:Ar mixture
(T5=1530 K, n5=5x1018 cm-3)
10-1
100
101
102
103
104
10-6
1x10-5
1x10-4
10-3
10-2
10-1
OH,H,CO2,O
Tem
pera
ture
, K
CH3,H
2O,H
2,CO
CH4
O2
Mo
le f
racti
on
Time, s
1500
2000
2500
Autoignition
10-1
100
101
102
103
104
10-6
1x10-5
1x10-4
10-3
10-2
10-1
Plasma Assisted Ignition
CH2O
H2O
CO2
CO2
H2
CO
H2O
CH3
H
OH
O
CH4
O2
Mo
le f
racti
on
Time, s
1500
2000
2500
Tem
pera
ture
, K
Plasma assisted ignition is characterized by:
– slow increase of gas temperature – developed kinetics of intermediates– partial fuel conversion during induction time
I N Kosarev, N L Aleksandrov, S V Kindysheva, S M Starikovskaia, A Yu Starikovskii, Combustion and Flame, 154 (2008) 569-586
13
Plasma assisted ignition: experiments and
numerical modeling: (CH4-C5H12):O2 + 90% Ar
0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85
100
101
102
103
104
105
CH4-C
5H
12,
PAI, 0.2-0.7 atm
C2H
6-C
5H
12,
auto, 0.2-0.7 atm
CH4, auto, 0.4-0.7 atm
CH4, auto, 2 atm
Ign
itio
n d
ela
y t
ime, s
1000/T, K-1
Auto Exp, C2H6
Auto Calc, C2H6
PAI Exp, C2H6
PAI Calc, C2H6
, , , C3H8
, , , C4H10
, , , C5H12
I N Kosarev, N L Aleksandrov, S V Kindysheva, S M Starikovskaia, A Yu Starikovskii, Combustion and Flame, 156 (2009) 221-233
Shock tube/nanosecond dsicharge experiments
Plasma kinetic effect on CH4 ignition (gliding arc)
Heated Air (model)
MGA (model)
1000
1050
1100
1150
1200
1250
1300
1350
1400
1450
1500
150 200 250 300 350 400
Strain Rate, s-1
Ign
itio
n T
em
pera
ture
, K
Heated Air (Fotache, Kreutz and Law, 1997)
Heated Air (experiment)
MGA (experiment)
AIAA paper-2007-1025
14Plasma catalytic effects reduce CH4 ignition temperature (Ombrello, T., Ju, Y. and Fridman, A., 2008. AIAA journal, 46(10), pp.2424-2433.)
Residence time
Te
mp
era
ture
Scramjet, afterburner
Plasma generated
species:
O, H, O2(a∆g) …
New “S-curve” by Plasma assisted combustion forsmall molecule fuel such as H2, CH4
the classical S-curve
3.1.3 Plasma Assisted Combustion: The impact of plasma on the ignition and extinction S-curve
The effect of kinetic enhancement (μs ~ ms, 800-1200 K)
Ignition
Extinction
Sun et al. Proc. Comb. Inst. 34, 2010, Combust. Flame 2011, 2012Ombrello et al. 2008
•Strong kinetic enhancement at intermediate temperature•Less effect at high temperature
Plasma
CH4
0.05 0.10 0.15 0.20 0.25 0.30 0.35
1x1015
2x1015
3x1015
4x1015
5x1015
6x1015
7x1015
Smooth
Transition
Extinction
Ignition
O2
=34%
O2
=62%
Fuel mole fraction
OH
nu
mb
er d
en
sity
(cm
-3)
plasma S-curve
Non-thermal plasma dramatically enhances ignition chemistry, but less impact on flame speed/extinction limit!
Nanosecond plasma assisted low temperature ignition of
dimethyl ether ignition in a diffusion counterflow flame
16
0.00 0.02 0.04 0.06 0.08 0.10 0.12
1x105
2x105
3x105
4x105
5x105
6x105
Extinction
increase
decrease
CH
2O
PL
IF (
a.u
.)
Fuel mole fraction
Hot Ignition
P = 72 Torr, a= 250 1/s, f = 24 kHz
XO2=40%, varying Xf
LTC
HTC
S-Curve0.00 0.02 0.04 0.06 0.08 0.10 0.12
1x105
2x105
3x105
4x105
5x105
6x105
increase
decrease
CH
2O
PL
IF (
a.u
.)
Fuel mole fraction
P = 72 Torr, a= 250 1/s, f = 34 kHz,
XO2=60%, varying Xf
New ignition/extinction curve without
extinction limit
LTCHTC
Radical production by plasma can activate LTC at much shorter timescale.
Sun, W., Won, S.H. and Ju, Y., 2014. Combustion and Flame, 161(8), pp.2054-2063.
Plasma activated low temperature combustion pathway
LTC
H+O2=OH+O
Radical production by plasma
Plasma activated high temperature combustion pathway
Plasma activated low temperature
combustion pathway
O+RH → R+OH
R → R’’+2OH
O+RH → R’’+ 3OH
A schematic of the key reaction pathways for high pressure fuel oxidation of at different temperatures
(blue arrow: Below 700 K; yellow arrow: 700-1050 K; red arrow: above 1050 K).
Fuel (RH)
+(OH, HO2)
R
nOH
aldehyde C2H3/CH2O
H/HCO+O2+(M)
+M
+O2
CO2
CO
OQ’O
PlasmaO(1D), O, R, O3
O2(1Δ), N2(v), …
alkene +
+O2
RO2
QOOH
O2QOOH
+O2
KOOH +O2
O2P(OOH)2
H2O2
+O2
HO2
+HO2
Plasma Assisted Combustion Chemistry
• Three chain-branching pathways at low, intermediate, and high temperature• Plasma accelerate low and intermediate chemistry
e +O2=O+O(1D) +e
H+O2(1Δg) =O+OH
O(1D)+RH =OH+R
N2(A,B,C)+O2=O+O+N2
N2(v)+HO2 =OH+O+N2
R(v,*)+O2=RO+OH
=???
O3+M =O+O2+M
Plasma assisted three different flame regimes: cool flame, warm flame, and hot flame
1 2
Hot ignitionLow temperature ignition
0.0 0.1 0.2300
600
900
1200
1500
Tem
pera
ture
(K
)
Time (sec)
R+O2=RO2
HCO+O2=CO+HO2
2HO2=H2O2+O2
H2O2=2OH
H+O2=O+OHO+H2=H+OH
RO2→QOOH →R’+OHO2QOOH →R’’+2OH
Thermal effectKinetic effect
Plasma Activated Low Temperature Combustion for large hydrocarbon fuels
500-800 K
800-1100 K
>1100 K
High
Intermediate
Low
More kinetics effect of PAC at low temperature combustion?
Two-stage ignition: n-heptane
Large molecules Fuel fragments Small molecules
CH2O+X=HCO+XH
τig,Hτig,L
0.01
0.1
1
10
100
0.8 1 1.2 1.4 1.6
Ign
itio
n d
ela
y t
ime
s [
ms
]
1000/T [1/K]
High T ignitionLow T ignitionHigh T ignition (100ppm OH)Low T ignition (100ppm OH)
Stoichiometric n-heptane/air premixture at 20 atm
Constant pressure calculation
1000 K 800 K1250 K
ab
n-heptane HTI
LTI
τig,C~ τf, Daig,C > 0Fast low temperature ignition:
w. 100 ppm OH
Chain-branching reactions
Nu
mb
er
of
oxy
gen
1
2R→RO2→QOOH→O2QOOH→OQ’O +2OH
O2
O2
H2O2→2OH
R’+ HO2→ RO+OH,
O2
H + O2→H+OH
O2
600 KLow Temp.
1200 KHigh Temp.
800 KInt. Temp.
Plasma Activated Low Temperature Combustion for large hydrocarbon fuels
Plasma activated self-sustaining Cool Flames: Radical production
Heated N2 @ 550 K
N2 @ 300 K
Stagnation
plane
Oxidizer @ 300 K
with plasma discharge/
O3
Fuel/N2 @ 550 K
Fig. 1 Schematic of experimental setup
Fig. 2 Plasma and ozone assisted n-heptane cool flames
Cool flame
Hot flame
N-heptane/He
O2/He
Low pressure NSD (60 torr)
(a) Hot diffusion flame
(b) Cool diffusion flame
Tf~1900 K
Tf~650 K
1 atm O3 addition
O3+M=O+O2+M
Sun, W., Won, S.H., Ju, Y. (2014),, Combustion and Flame, 161 (2014) 2054–2063.
Won, S.H., Jiang, B., Diévart, P., Sohn, C.H., Ju, Y., (2015), Proceedings of Combustion Institute, 35, 881-888.
Premixed flames
6/20/2021
Prometheus bringing fire to the earth
History of flame discovery: 3 different stable flames with plasma
1M B.C. 1817
FoodTools
Industrial revolutions
Space Exploration
2000 2010 2012-2013 2015 2017-2018
New enginesNew fuels
Control of low temperature combustion: O3 effects
ELTC
O3 enhanced Extreme LTC of Methyl Hexanoate
Rousso, A.C., Jasper, A.W., Ju, Y. and Hansen, N., 2020. JPC A, 124(48), pp.9897-9914.
23
FuelFig. 1: Ethylene ozonolysis reaction pathway
(primary ozonide, POZ; secondary ozonide: SOZ).
Fuel/O3
10x10-3
8
6
4
2
0
Mo
le F
ract
ion
n-pentane + 500 ppm NO
neat n-pentane
600
500
400
300
200
100
0
Mo
le F
ract
ion
(p
pm
)
NO
NO2
N balance
400
300
200
100
0
Mo
le F
ract
ion
(p
pm
)
775725675625575525
Temperature (K)
HONO
6643.17 cm-1
6642.51 cm-1
6638.26 cm-1
Modeling
• L. Marrodán, Oliver Herbinetet et al., 2019. Chem. Phys. Lett., 719, 22-26.
• Zhao, H. et al., 2018.. C&F 197, 78-87.
RO2 + NO = RO + NO2
NO+HO2<=>OH+NO2
NO2+HO2=HONO+OH
Control of cool flames and warm flames: NOx effect by Plasma
0.1 1500
750
1000
1250
1500
Warm
Hot
Cool
Warm
Ma
xim
um
te
mp
era
ture
Tm
ax [
K]
Strain rate a [s-1
]
0 ppm NO
300 ppm NOTf = 550 K, To = 300 K
Xf = 0.1, Xo = 0.1
P = 8 atm
nC12H26/N2/NO - O2/N2
Cool
100
Cool Flame
Warm Flame
10-1
100
10-1
M. Zhou et al., PCI, vol. 38, 2021
• NO inhibit cool flames• Promote warm flame• Accelerate warm flame reignition
w. NO
N-pentaneSuppressing effect
Enhancing effect
24
0 NOw. NO
50
40
30
20
10
0
ring d
ow
n tim
e (
ms)
6642.86641.86640.86639.86638.86637.8
wave number (cm-1
)
* * **
n-pentane+ NO
HONO(+ NO2 + …)
HONO lines
Flow reactor studies of non-equilibrium plasma-assisted oxidation of n-alkanes
Tsolas, N., Lee, J.G. and Yetter, R.A., 2015. Phil. Trans.
R. Soc. A, 373(2048), p.20140344.
Pri
nce
ton
Un
iver
sity
Combustion lab.
Gundersen et al.
Ignition enhancement by transient corona discharge
Disk electrode & streamers
2-10KV, 20-200ns
•Increased volume
•Transient discharge
Pri
nce
ton
Un
iver
sity
Combustion lab.
Ignition delay time: corona discharge vs. spark
CnHm+e=CnHm-1+H*+e
O2+e=O(1D)+O(3P)+e
Radical production
Large ignition volume?
Liu J, Wang F, Lee L, Ronney P, Gundersen M. In42nd AIAA Aerospace Sciences Meeting and Exhibit 2004 (p. 837).
3.2.1 Adiabatic flame propagation A flame is a self-propagating auto-ignition and thermal diffusion front. The propagation speed of a one-dimensional flame front relative to the far field unburned mixture is the flame speed.
SL
Fuel/airT
Heat conduction
ignition
Reaction zone
YF
0)(,)(
)(,)(
)2(
)1(,
00
2
2
2
2
Fad
FF
FF
p
YTT
YYTT
Wdx
YdD
dx
dYu
qdx
Td
dx
dTuC
Governing equations
RTE
F
F
eYB
QPY
/
2
Enthalpy conservation outside diffusion zone:
q
TC
W
Y
q
TC adpFp
0,0
WC
qYTT
p
F
ad
0,
0
dxWEqqEq
]/)2.(/)1.([
xf
0,0
1
Fad
ad
Y
Y
LeTT
TT
termsconvectionneglect and ]/)2.(/)1.([ dxWEqqEq
fx
x
LeY
Y
RT
E
T
TT
T
TT
F
F
adad
ad
ad
ad
0,
00 ;;
)/1( adTTDefine:
In reaction-diffusion zone:
1st order reaction
D
W
dx
Yd F 2
2
FF dY
D
W
dx
dYd
2][ 2
1)(2)(2
222][
/20,
0
/20,
/0,0,2
adad
ff
F
Ff
RTEFfRTEFf
F
RTE
F
FF
F
Y
Y
F
x
x
eLeY
D
WBdee
LeY
D
WB
deYBLeY
D
Wd
LeY
D
WdY
D
W
dx
dYd
ad
f
RTEFf
xF e
LeY
D
WB
dx
dY /20,2)(2 ][
dxWdx
YdD
dx
dYu F
x
F
x ff
][2
2
fxF
Fdx
dYDuY ][0,
adRTE
p
eC
WBLe
/
2
22 2u)(m
In reaction-diffusion zone (neglect convection):
Rewriting:
Integrating from flame front to a location x in the reaction zone:
Integrating from unburned region to flame front to find the fuel concentration gradient at flame front:
(1)
(2)
Flame speed is affected by Le, B, E, and T. How does plasma affect flame speed?
orderreaction :n,12/ n
adU Mass burning rate:
3.2.2 Flame propagation speed with heat loss or heat addition
0)()(
)(,)(
)2(
)1()(4
00
2
2
4
0
4
2
2
dx
dY
dx
dT
YYTT
Wdx
YdD
dx
dYu
TTKpqdx
Td
dx
dTuC
F
FF
FF
p
tcoefficien absorptionmean :
/
/
/
/
)/()(
0
0
00
PlanckKp
U
xt
U
TCx
Uum
xxX
YYy
TTTT
ad
ref
ref
ad
p
ref
ad
ref
FF
ad
adpadp
ad
UCUC
TKpH
yLe
W
WdX
yd
LedX
dym
HW
dX
d
dX
dm
4
2
2
2
2
2
4
)1(1
)1(exp
2
1
Outer solution (convection diffusion zone):
0X 0
0X )exp(10
mLeXy
0X 1
0X )exp(0
mX
Fuel/air
T
Reaction-diffusion
YF
Convection-diffusion
X=0
dXH
dX
d
dX
dy
LedX
dm
dX
dym ))
1()(
0
00
dXH
dX
d
dX
yd
LedX
dX
dm
dX
dym )
1()(
2
2
2
200
1010 , yyy
m
H
dX
d
dX
dy
Lem
0
11
1)]()0([
2
1100
0
0
ln)0(or 2
)0(exp,0
1m
dX
d
dX
d
dX
dy
Le
m
H
dX
d
m
H
dX
d
m
H
dX
d
dX
dy
Lemm
0
1
00
2 1ln
m
H
dX
d
0
1
Hmm 2ln 22
Adding the mass and energy equation and integrate from upstream boundary to flame front:
Perturbation: assume heat loss or addition only perturb the temperature and mole fraction in O(1/β)
Rewrite the equation above:
Using the jump condition across the reaction zone:
Fuel/air
T
Reaction-diffusion
YF
Convection-diffusion
0- 0+
Find the perturbation in the burned gas zone:
Flame speed with heat loss/addition: How does heat loss/addition affect flame?
Extinction limit and flammability limit:
Hmm 2ln 22
Fig. The dependence of the normalized burning
velocity on the normalized radiative heat loss
of a one-dimensional planar flame.
0.0 0.1 0.2 0.3 0.4 0.50.0
0.2
0.4
0.6
0.8
1.0
Extinction
Limit
e-1
e-1/2
No
rma
lize
d b
urn
ing
ve
loci
ty,
m
Normalized heat loss, HNormalized heat loss 2H
adpadp
ad
UCUC
TKpH
44
adUum /
• For a given mixture with a constant adiabatic flame speed, the increase of heat loss will reduce the flame speed and lead to flame extinction at 2H=1/e and the normalized flame speed at extinction limit is e-1/2
• For a given heat loss intensity (e.g. Kp), as the mixture fuel concentration decreases, the normalized heat loss H will increases. Therefore, at a critical fuel concentration, 2H becomes 1/e, and no flame is available below this fuel concentration. This defines the lean flammability limit.
• How does plasma can change the flammability limit?
Quenching diameter:
Nud
Hf
2
24
releaseheat chemical Total
wall the tolossHeat 2
Nud
mmf
2
2
224
ln
fNued 20
Fuel
Air
wall
wall
Convection Heat losses
Uf
Heat recirculation
For a flame propagating into a tube, the heat loss from the flame to the wall is governed by the convective heat transfer to the wall
0.1 0.2 0.3 0.4 0.5
0.6
0
.7
0.8
0
.9
1
.0
df /20.1 0.2 0.3 0.4 0.5
0.6
0
.7
0.8
0
.9
1
.0
df /2
Fig. Burning rate (solid line) and normalized flamepropagation speed U (U=m in this figure) plotted against
the ratio of flame thickness to channel width (d) for
selected values of reduced heat transfer coefficient (k)
with in a quiescent, two dimensional channel flow.
(Matalon et al. 2003)
Quenching diameter:
Flame speed:
eNu
dH
f 142
2
2
d0: the minimum diameter in which a laminar flame canPropagate.
How does plasma discharge affect the quenching diameter?
3.2.3 Flame speed, extinction and flammability limits with by flame stretch (Le)
0,0
2/
0,2
2
0
2/
0,2
2
)(,)(
0)0(0)0(
)(
)()(
FF
F
f
RTE
FFF
rf
RTE
Fp
YYTT
dx
dY
dx
dT
xxBeYdx
YdD
dx
dYax
TTQxxBeQYdx
Td
dx
dTaxC
ad
ad
u=-ax
Potential flow (outside)ad
ad
ad
RTE
ad
pFad
adpr
T
TT
RT
E
BeU
CQYTT
dxdua
TTKQ
ad
0
2/
0,0
3
0
3
function delta Dirac:
/
/
)(4
adrefref
ad
p
ref
RTE
ad
pFad
ref
FF
ad
Uxt
U
Cx
BeU
CQYTT
xxX
YYy
TT
TT
ad
/
/
/
,/
/
,
2/
0,0
0,
0
0
,2
4 3
Xat
DCLe
UCUC
TKH
ref
p
adpadp
adp
In the limit of large β
0)(21
2
0)(22
2
2/)1(
2
2
2/)1(
2
2
fFF
f
f
f
ead
yd
Led
dy
ea
H
ad
d
d
d
Here the stretch rate a is non-dimensional
dt
dA
Aa
f
f
1Flame stretch:
dxdua /axu
Perturbation
Outer solution in convection-diffusion zone
,.../1...,/...,/111010
FFFF YTplLeYYYTTT
),(at /1,/2222 00
f
tttt dtedteydtedte
ff
),0(at 0,1 00
fy
Jump conditions
0)(2 2/)1(
2
2
ffe
ad
d
(1) Integrating from flame front (-) to end of reaction zone (+)
02 2/1
fead
d
(2) Integrating the summation of mass and energy equation in reaction-diffusion zone,
0
0
d
dyl
d
dp F
02
20
2
02
2
2
H
ad
ydl
d
pd
d
dp F
Governing equation for perturbed variables: and ,0,0 p
Solution: ,2
2/
1
fp
f
eg
a
)/(2
)1
2
1( 11321
2
1
2gIg
a
Hgg
a
H
glp fff
ndeg fn
1 )1(
1
22
1
0
)1(
2
22
dneg fn
dndkk
eeI
f
f
nkn
1
2
)1(1 )1(
11
1222
22
dnn
eg
fn
1
2
)1(
31
122
If ,12, 1
2 gff Hmm 2ln 22
Enthalpy change
am f 2
Flame propagation speed with stretch and heat loss: sublimit combustion
0.1 1 10
0
5
10
15
20
0.1 1 100
5
10
15
20
25
0.1 1 100
5
10
15
20
0.1 1 100.0
0.5
1.0
1.5
2.0
YF=0.0295
X
f
a
YF
=0.0296
X
f
a
YF=0.03
Xf
a
YF=0.029
Xf
a
Dependence of flame location on stretch at Le=0.9
• At low fuel concentration (YF=0.029) below the flammability limit, flame can exist in a narrow range of stretch rate bounded by a radiation extinction limit and a stretched extinction limit.
• As the fuel concentration increase close to the flammability limit, there exist two flame islands, respectively, close and away from the stagnation plane.
• As the fuel concentration further increases to slightly above the flammability limit, there exist both planar flame at zero stretch rate and a near stagnation flame island at Lewis number below unity.
• As the equivalence ratio becomes above flammability limit, the two flame islands merge together and a stretched flame can becomes a planar flame as the stretch rate decreases.
Le=0.9
Ju, Y. and Minaev, S., 2002. Proceedings of the Combustion Institute, 29(1), pp.949-956.
Microgravity experiments
Maruta, K., Yoshida, M., Ju, Y. and Niioka, T., 1996, Symposium
(International) on Combustion (Vol. 26, No. 1, pp. 1283-1289). Elsevier.
0 10 20 30 40
1200
1250
1300
1350
1400
1450
m
l
T0=1358.
0.46
k.
j
i
h..
..
...
.
. .
0.450.46
0.469
0.469 =0.48
g
fed
cb
a
Le=1.4
Fla
me
tem
per
ature
(K
)
Stretch rate (s-1)
x
T
NFDF NSF WF
Numerical simulation: detailed chemistryCH4-O2-N2-He
Stan
dar
dlim
it
Temperature curve of 1D planar propagating flame
B
CE(FSWSF limit)
DF
Stretched flame
AG
WF
Stretch rate, a
0
0'
G-curve(Le < Lecr)
Limit of NSF
The G-Curve
0.4 0.6 0.8 1
Equivalence ratio
10-1
100
101
102
103
Str
etc
h r
ate
at
exti
ncti
on
(1/s
)
Le=0.967
F
B
C
D
E
Stretch limit of normal flame
CH4/AIR A
Radiation limit of weak flame
Jump limit of weak flame
Radiation limit
of NSF
0.488
,
F
G
experiment
The G-curve (Le < Lecr )
Ju, Y., Guo, H., Maruta, K. and Liu, F., 1997..JFM, 342, pp.315-334.Guo, H., Ju, Y. and Niioka, T., 2000. CTM, 4(4), pp.459-475.
Φ0
Φ0
When a mixture has a low Lewis number, the flammability (Φ0) region can be extended significantly by stretch!
0.4 0.6 0.8 1
Equivalence ratio
10-1
100
101
102
103
Str
etc
h r
ate
at
exti
nct
ion
lim
it (
1/s
)
C3H8/Air
Standard limit
A
B
E
C
D
F
G
, Experiment
The K-curve (Le > Lecr )Now we can understand the experimental data on the figure below
How does plasma change the flammable region of stretched flames?
3.3.1 Plasma enhancement and flame speed Gliding arc on flame extinction experiment
Diffusion Flame
H2/N2
Air/H2/CH4
0
50
100
150
200
250
300
350
19 20 21 22 23 24 25 26
Percent Methane Diluted in Nitrogen
Str
ain
Rat
e, 1
/s
Bundy et al.
Puri & Seshadri
No Plasma
33 Watts44 Watts
60 Watts
78 Watts
78W
60W
44W
33W
0W
0.00E+00
6.00E+15
1.20E+16
1.80E+16
-0.4 -0.2 0 0.2 0.4
Distance Between Nozzles, cm
Nu
mb
er
De
nis
ty o
f O
H
0 Watts, a=83.3 1/s
48 Watts, a=183 1/s
78 Watts, a=127.7 1/s
Computation
0 W 48 W 78 W
Role of plasma: mainly thermal effect
Ombrello, T., Qin, X., Ju, Y., Gutsol, A., Fridman, A. and Carter, C., 2006. AIAA journal, 44(1), pp.142-150.
Pressure
Sp
ecie
s L
ifet
ime
1 atm
ramjets &
scramjetsgas turbinesICE’s PDE’s
Plasma Generated Active Species
Lifetime vs. Pressure
long lifetime
10-1000 times
more reactive
than O2
What are the effects of O3, O2(a1Δg), O, … on flame propagation?
O2(a1Δg)
Enhancement of flame Speed by plasma generated O3
u
bliftedL SS
10 mm
0.2
0.3
0.4
0.5
0 0.005 0.01 0.015
Mixture fraction gradient dY F /dR
Sli
fted
[m
/s]
0
2
4
6
8
10
12
14
En
ha
nce
men
t [%
]
0 ppm O3 592 ppm O3
1110 ppm O3 1299 ppm O3
1299 ppm O3
1110 ppm O3
592 ppm O3
(~ 1/axial distance)
Flame speed extraction
Nozzle Tip
Lifted C3H8/O2/N2 flames
Ombrello, T., Won, S.H., Ju, Y. and Williams, S., 2010. Part I:
Effects of O 3. Combustion and flame, 157(10), pp.1906-1915.
Kinetic Thermal Enhancement Mechanism by O3
0
2
4
6
8
10
12
14
0 1000 2000
Fla
me S
peed
En
ha
ncem
en
t [%
]
Concentration of O3 [ppm]
SL (O3 decomposing in pre-heat zone)
SL (O3 to O2 far upstream of pre-heat zone)
Extrapolated Enhancement (experiment)
O3
O2
O
C3H8
C3H7+OH
O3
Fla
me
Pre
-He
at Z
on
e
Reactants
Products+H2O
+HEAT
or other stable
species
O3+O3 O2+O2+O2
Kinetic, Curvature and Stretch Effects
Radical production by plasma mainly accelerates heat release (not change branching).
Kinetic-thermal effect!
Kinetic Effect by O2(a1Δg) on flame propagation
O2 (a1Δg) at 0.98 eV
O2 (b1Σg+) at 1.6 eV
O2 (a1Δg) + H = OH+O fast
O2 + H = OH +O slow
Lifted flame experimental system
microwave
power supply
T3
T2 T1 fuel
oxidizer
φ=1camera
O2
Ar
ignition
system
P1
P2
Lifted Flame
NO
ICOS Cavity
FTIR
C3H8 or C2H4
O3
Absorption
Cell
vacuum
pump
3-way
valve
vacuum
pumpIn
ten
sity
254 nmWavelength
w/o O3
w/ O3
Hg light
emissionlower light
intensity
254nm
Detector
Notch
Filter
Flow
Hg Light
O3
O3
O3
O3
O3
O3
O3
O3 O
3
Off-Axis ICOS Cavity
Diode LaserComputer
PD
LensMirror
LensMirror
Flow Flow
NOx
O3
O2(a1Δg)
6636.16 6636.20 6636.24
Frequency [cm-1]
Cro
ss-Sectio
n
(x1
0-2
3) [cm
2]
Ca
vit
y E
nh
an
ced
Ab
sorp
tio
n (
GA
)
Q(12) Experimental measurement
Q(12) Curve fit
O2(a1Δg) Enhancement of C2H4 Flame Speed
[O2(a1Δg)], ppm ΔHL, mm
3137 4.76
4470 6.82
4627 6.83
5098 7.31
0
1000
2000
3000
4000
5000
6000
4 5 6 7 8
Change of Flame Liftoff Height, ΔHL [cm]
Con
cen
trati
on
[p
pm
]
SDO (w/ NO)
SDO (w/o NO)
O3 (w/o NO)
Energy Coupling Into Flow
≈ 1 eV to produce O2(a1Δg)
≈ 5000 ppm O2(a1Δg) 2-3 % Lifted Flame Speed Enhancement
Microwave Power = 80 Watts
Nozzle Tip
O2 (a1Δg) + H = OH+O fast
O2 + H = OH +O slow
Hydrocarbon quenching?
Far less than
Ombrello, T., Won, S.H., Ju, Y. and Williams, S., 2010.
Part II: Effects of O 2 (a 1 Δ g). Combustion and
Flame, 157(10), pp.1916-1928.
The kinetic effect of O3 and O2(a1Δg)
on flame speeds is small.
Effect of O production in nanosec plasma on
flame extinction
50-20 -15 -10 -5 0 5 10 15 20
-2000
0
2000
4000
6000
8000
Vo
lta
ge
(V)
Time (ns)
FWHM= 6 ns
f = 5~50 kHz
20 & 28 mm ID
15.24 mm × 22 mm
10 mm
E/N~10-15 Vcm2
10 mm away from exit
Power~0.7 mJ
f=40 kHz
Atomic O measurement (TALIF) and effect of extinction limit
51
Ar diluted CH4/O2 diffusion flame:
XO2=0.28, Peak voltage= 7 kV, P= 60 Torr
0.30 0.31 0.32 0.33 0.34 0.35 0.36
150
225
300
375
450
525
computation (T=348 K)
computation (T=398 K)
computation (T=528 K)
computation (T=528 K)
2000 ppm O addition
Fuel mole fraction XfE
xti
nct
ion
str
ain
ra
te (
1/s
)
no plasma
with plasma (f=5 kHz)
with plasma (f=20 kHz)
heated flow (T=398 K)
heated flow (T=528 K)
0 5 10 15 20 25 30 35 40 45
0
2
4
6
8
10
12
14
Ato
mic
ox
yg
en c
on
cen
tra
tio
n (
10
15
cm
-3)
Pulse repetition frequency (kHz)
Sun, W., Uddi, M., Ombrello, T., Won, S.H., Carter, C. and Ju, Y.,
2011. Proceedings of the Combustion Institute, 33(2), pp.3211-3218.
“O production has minor kinetic effect on flame extinction!”
Flame speeds vs. O3 addition: Steady C2H4 lifted flames
with - Two-way phenomenon
52
O3 added after steady lifted flame established, with corresponding uf
Nozzle exit
uf = 3.29 m/s uf = 4.50 m/sO3 addition O3 addition
13% O2 + 87% N2 co-flow
Uco= 0.014 m/s
3500
ppm
0
ppm
0 ppm
2200
ppm
Blow-
out
If flame ascends, blow-out finally happens if O3 too much
Reaction of flame after O3 addition depends on HL,0, both ascend and descend
possible
Effect of ozonolysis reactions B. Wu, H. Mitchell, W. Sun, T. Ombrello, and C. Carter. 2020 Proceedings of the
Combustion Institute, 38, 6773-6780.
CH2O PLIF
53
uf = 3.10 m/s uf = 3.57 m/s
Significant amount of CH2O production owing to ozonolysis
reactions upstream changing flame dynamics
CH2O PLIF
Flame
chemiluminescence
54
SL=SL(Daig) ?
x
T
T0
Tp
Ti
Tf
𝑺𝑳~𝑺𝑳,𝑫𝒂,𝒊𝒈=𝟎 ∗𝑻𝒇 − 𝑻𝟎
𝑻𝒊 − 𝑻𝒑
𝑻𝒑 =𝑻𝒂
𝐥𝐧[ 𝟏 − 𝑫𝒂, 𝒊𝒈
𝒆𝑻𝒂𝑻𝟎 +𝑫𝒂
, 𝒊𝒈𝒆𝑻𝒂𝑻𝒇]
𝑫𝒂𝒊𝒈 =𝝉𝒇
𝝉𝒊𝒈 𝑻𝟎, 𝑷, 𝝓
𝝎 = 𝑨𝑻𝒃 𝐞𝐱𝐩 −𝑻𝒂𝑻
Ignition Flame
𝑇𝑎 → ∞, 𝑻𝒊→ 𝑻𝒇
b=2
Zhang, T. and Ju, Y., 2020. C&F, 211, pp.8-17.
Cool flame speed, SL, exponentially increases with Daig
Flames in cavityRCCI
Convection-diffusion-reaction front
3.3.2 Ignition assisted flame propagation: the role of plasma in enhancing flame speed
3.4 Plasma effect on the minimum ignition energy and the critical flame initiation radius
Lefkowitz et al. 2012, Ikeda et al. 2009
Internal combustion engine, microwave
Flammability limit?
Spark Microwave gliding arc Multi sparks
Why does a flammable mixture can not be ignited by a spark for a small engine or at lower pressure?
Puzzle of high altitude relight: an unresolved ignition problem or a flame problem?
Altitude
[1/p]
Flight speed
Flow speed
Is the flame speed really a problem for relight?
Flame speed ~ pn/2-1
Engineinstability Flow
speed
Q ?
• What governs the ignition & Eig?
• What are the chemistry and
transport effects?
•Eig,min: Defined by stable “flame ball” size?Zeldovich et al. (1985), Champion et al. (1986)
LeTTCRE adpZig ~)(3
4 3
Larger fuel molecules larger Eig
•Eig,min: Defined by flame thickness, δ (make a guess)?
B. Lewis and Von Elbe (1961), Ronney, 2004, Glassman (2008)
11
)(3
42/33
3
LeSTTCE
u
adpig
Larger fuel molecules smaller Eig
volume heat capacity
Ignition sparkto a flame
δ
fuelJet
oxygenLe
ydiffusivit Mass
ydiffusivit Thermal
Temperature
Fuel concentration
T ~ 1/r
Reaction zone
Interior filled with combustion
products
Fuel & oxygen diffuse inward
Heat & products
diffuse outward
C ~ 1-1/r
T*
T•Q
Assumptions and simplification: • 1D quasi-steady state, Constant properties• One-step chemistry• Center energy deposition
H
r
Tr
rrr
TU
t
T)(
1 2
2
)( 2
2
1
r
Yr
rr
Le
r
YU
t
Y
)~~
(~~~
~~
0
0
TTSC
HH
aduP
f
)()1(
1
2exp Rr
T
TZ
f
f
0,/,0 2 YQrTrr
0,, YTTRr f
1,0, YTr
00 ~
~
,~
~
f
f
f
rR
rr
f
f
R
ULeULeR
QfT
TZdeeR
LeQT
)1(
1
2exp/
1 22
Flame speed: effect of flame radius, heat addition and Lewis number
Chen, Z. and Ju, Y., 2007. Combustion Theory and Modelling,11(3), pp.427-453.
Le: Lewis numberZ: activation energyσ: density ratioU: flame speedQ: ignition energyΩ: analytic functions
Flame Radius, R
Fla
me
Pro
pa
ga
tin
gS
pe
ed
,U
10-2
10-1
100
101
102
10-3
10-2
10-1
100
101
Q=0.00
Q=0.05
Q=0.092
Q=0.10
Q=0.20
a
bc
d
e
f
g
h
i (a), Le=1.0, h=0.0
Flame Radius, R
Fla
me
Pro
pa
ga
tin
gS
pe
ed
,U
10-1
100
101
102
10-2
10-1
100
101
Q=0.0
Q=0.1
Q=0.15
Q=0.6
(a), Le=1.2, h=0.0
a
b
cd
e
f
g
h
i
j
Flame radius, R
Fla
me
pro
pa
ga
tin
gsp
ee
d,U
10-1
100
101
102
0.0
0.4
0.8
1.2
1.6
Le=0.5
0.8
1.01.2
2.0
OO O O O
adiabatic(h=0.0)
O
1.4U=0:
Flame
ball
Extinction
limit
1. The critical ignition size and energy is governed by two different length scales:
•Flame ball size (small Le)•Extinction diameter (large Le)
Chen & Ju, Comb. Theo. Modeling, 2007
2. With ignition energy, there is a critical flameinitiation radius, below which, ignition will fail eventhe mixture is above the flammability limit.
Q ?
The Critical Ignition Radius
60
Ignition by heat and radical deposition
LeF = 2.2
R
U
10-1
100
101
102
10-3
10-2
10-1
100 Le
Z= 1.0
LeF
= 1.0
qt
= 0.0
12
3
4
5
1: qc
= 0.0
2: qc
= 0.4
3: qc
= 0.8
4: qc
= 1.0
5: qc
= 1.2
R
U
0 0.05 0.1 0.15 0.20
0.05
0.1
4
5
3
Radical Only
R
U
10-1
100
101
102
10-3
10-2
10-1
100
LeF
= 2.2
LeZ
= 1.0
qt
= 0.05
1: qc
= 0.0
1
(b)
R
U
10-1
100
101
102
10-3
10-2
10-1
100
LeF
= 2.2
LeZ
= 1.0
qt
= 0.05
1: qc
= 0.0
2: qc
= 0.5
2
21
(b)
R
U
10-1
100
101
102
10-3
10-2
10-1
100
LeF
= 2.2
LeZ
= 1.0
qt
= 0.05
1: qc
= 0.0
2: qc
= 0.5
3: qc
= 0.675
2
2
3
3
3
1
(b)
R
U
10-1
100
101
102
10-3
10-2
10-1
100
LeF
= 2.2
LeZ
= 1.0
qt
= 0.05
1: qc
= 0.0
2: qc
= 0.5
3: qc
= 0.675
4: qc
= 0.7
2
2
3
4
3
3
44
1
(b)
1st
flame
bifurcation
R
U
10-1
100
101
102
10-3
10-2
10-1
100
LeF
= 2.2
LeZ
= 1.0
qt
= 0.05
1: qc
= 0.0
2: qc
= 0.5
3: qc
= 0.675
4: qc
= 0.7
5: qc
= 0.73
2
2
3
4
3
3
44
5
5
1
(b)
2nd
flame
bifurcation
R
U
10-1
100
101
102
10-3
10-2
10-1
100
LeF
= 2.2
LeZ
= 1.0
qt
= 0.05
1: qc
= 0.0
2: qc
= 0.5
3: qc
= 0.675
4: qc
= 0.7
5: qc
= 0.73
6: qc
= 1.0
2
2
3
4
3
3
6
44
5
5
6
1
(b)
Chen et al. 2011
Critical flame initiation radius
• Outwardly propagating flames• n-Decane/Air at ϕ 0.7, 1 atm, 400 K• Schlieren imaging 15000 fps
60
1 cm
0 2 4 6 8 10 12 14 16 18
0.0
0.5
1.0
1.5
2.0
2.5
Time [ms]
Fla
me
ra
diu
s R
f [c
m]
60
80
100
120
140
160
180
Fla
me
sp
ee
ds
dR
f / d
t [c
m/s
]
Flame radius
Flame speed
Won, Santer, Dryer, Ju, 2012
Cube of critical flame radius, RC
3
Min
imu
nig
nitio
np
ow
er,
Qm
in
0 500 1000 1500 2000 25000
0.5
1
1.5
2
Z = 10
Cube of critical flame radius, RC
3
Min
imu
nig
nitio
np
ow
er,
Qm
in
0 500 1000 1500 2000 25000
0.5
1
1.5
2
Z = 13
2.0
1.4
1.6
1.7
1.8 = Le
1.9
1.51.4
2.5
2.0
1.9
1.8
1.7
1.6
1.5
Le = 2.1
2.3
2.4
2.2
Minimum Ignition Energy vs. Critical ignition radius:
impacts of flame chemistry and transport
Chen, Burke, Ju, Proc. Comb. Inst. Vol.33, 2010
Activation energy
0
0.5
1
1.5
2
2.5
3
0.6 0.7 0.8 0.9 1
Cri
tic
al r
ad
ius
[c
m]
Equivalence ratio
JP8 POSF 6169
SHELL SPK POSF 5729
@ 1 atm
Unburned Temperature = 450 KFuel/Air (21% O2) mixture
Fuel Mean molecular
weight
Radical Index
JP8 POSF 6169 153.9 0.80
SHELL SPK POSF 5729
136.7 0.85
Won, Santer, Dryer, Ju, 2012
What controls ignition in engine? Critical Ignition Radius
Theory: Ignition to flame transition(critical ignition radius, Rc)
Flame Radius, R
Fla
me
Pro
pa
ga
tin
gS
pe
ed
,U
10-1
100
101
102
10-2
10-1
100
101
Q=0.0
Q=0.1
Q=0.15
Q=0.6
(a), Le=1.2, h=0.0
a
b
cd
e
f
g
h
i
j
Critical ignition radius
Rc
Q
?
0
0.5
1
1.5
2
2.5
3
0.6 0.7 0.8 0.9 1
Cri
tic
al r
ad
ius
[c
m]
Equivalence ratio
JP8 POSF 6169
SHELL SPK POSF 5729
@ 1 atm
Unburned Temperature = 450 KFuel/Air (21% O2) mixture
• Spark size > Rc, but…• Rc increases with leaner mixtures• Rc increases with EGR• Rc increases with smaller engines
How to successfully ignited lean mixtures?
Solution? Turbulent jet or volumetric ignition?
How big is the critical ignition radius?
1.5-2.5 cm at 1atm
t=0.5ms t=28.5msSingle spark
3 cascading sparks at the same total energyCH4/air
Z. Chen and Y. Ju, CST, 2007
H Zhao et al., CNF, 2019
Advanced engine
63
Subsonic Ignition Tunnel Utilized
to Elucidate Fundamental Interactions
• Subsonic Wind Tunnel ◦ Premixed methane/air at room temperature and pressure
◦ U = 1 - 10 m/s
◦ Re = 6,000 - 24,000
◦ Optical access through windows on three sides
•Transient Plasma Systems Pulsed Power Supply ◦ 10 ns FWHM
◦ Pulse repetition frequency (PRF) up to 330 kHz
◦ Peak voltage of 10 kV into 50 Ω resistor
◦ Maximum Energy Per Pulse ≈3 mJ
•Electrodes◦ Lanthanated tungsten
◦ Pin-to-pin configuration
◦ Micrometer controlled inter-electrode gap distance
◦ Tip angle of 20° Courtesy of Timothy Ombrello
64
300 kHz 100 kHz 20 kHz 10 kHz 5 kHz 2 kHz 1 kHz3.3 kHz 2.5 kHz
Fully-Coupled Partially-Coupled Decoupled
Effect of Time Scale of Energy DepositionFixed Total Energy and Varying Pulse Repetition Frequency (PRF)
3.3 µs 10 µs 50 µs 100 µs 200 µs 500 µs 1000 µs300 µs 400 µs
CH4-Air, φ = 0.6, U = 10 m/s, D = 2 mm, and N = 20
Three Distinct Regimes Identified
J.K. Lefkowitz, T. Ombrello / Combustion and Flame 180 (2017) 136–147
Larger ignition size, leaner mixture ignition
Effect of ignition kernel size on ignition probability
66
• Increasing power deposition rate (high PRF) is a superior method to ensure ignition
• In partially-coupled regime, more pulses increases ignition probability, but not to 100%
• In decoupled regime, ignition probability is a linear function of number of pulses
MIP = Minimum Ignition Power (determined for 50% ignition probability)
Fully-Coupled Partially-Coupled
De-
coupled
Effect of Inter-Pulse Time
and Number of Pulses
0
10
20
30
40
0 5 10 15 20
MIP
(W
)
Number of Pulses
CH4-Air, φ = 0.6, U = 10 m/s, D = 2 mm, and N = 20
Ignition probability is dependent on PRF (inter-pulse time), not total energy deposition!
Overlaid Schlieren and High Speed OH-PLIF Diagnostics
February 27th, 2021 Combustion Webinar 67
Flow𝑼 = 10 m/s• Schlieren images (grey-scale) show warm gas
• OH-PLIF images show burned gas
• Higher PRF Schlieren and OH-PLIF largely overlap
Larger OH region
Greater intensity of OH-PLIF signal
• Lower PRF Schlieren and OH-PLIF differ drastically
More likely to be quenched
PRF (kHz) = 2 5 10 25 50 100 200
Scale × 2
J. Lefkowitz, Combustion Webinar, Lecture 28, 2021. https://youtu.be/ckIOU4PnyTA
68
How Does This Translate to a More Realistic FlowImplications in a Recirculating Turbulent
Reactive Flow: Mach 2 Cavity
NP
HF
D (
300 k
Hz)
Cap
acit
ive D
isch
arg
e
M=21.65 cm
22.5°
6.6 cm
2.54 cm
1.9 cm
Steady-State
Chemiluminescence
Time to Ignition
for NPHFD
Time to Ignition for
Capacitive Discharge
69
Time to Ignition for Lean Cavity (~Φ=0.8)Energy Deposition of 50-800 mJ
Drastic Change in Ignition
Time Below ~ 100 mJ
Approximately 1
Cavity Cycle Time
Factor of 7 Difference in Energy
Deposition, But Same Ignition Time
Directly Ties to The Subsonic
Benchtop Experiments to Highlight
Synergy Between Pulses and the
Effect on Flame Growth Rates
70
Bluff-body stabilised premixed flames (Dawson et al, Proc Comb Inst 33:1559-1566, 2011)
same, U increases
OPEN
ENCLOSED<OH*>
Prior to BO
E. Mastorakos
71
Build-up of CH2O in the RZ close to LBO
Kariuki et al, PROCI 35
Simultaneous CH2O – OH PLIF
Inst
Mean
Close to LBO
Close to LBOFar from LBO
E. Mastorakos
OH+CH2O=HCO+H2O
O(L/U) ~ O(α/SL2)
Plasma assisted flame stabilization
72
(c)
Ignition assisted turbulent flame propagation SL=SL(Daig)
• N-heptane/air
• OH PLIF 10 kHz, 40 ms duration
Reactor-assisted turbulent slot (RATS) burner
B. Windom et al., C&F 169, 2016, pp.19-29
Summary: The impact of plasma on fundamental combustion properties:
How does plasma assist combustion? Ignition, Flame speed/limit, Emin
Ignition/extinction S-curveFlame speed and propagation
(Flammability limit)
High temperature flame
Flam
e t
em
pe
ratu
re, K
Equivalence ratioΦ0
1200
Φ0,r
Flow residence time
Te
mp
era
ture
, K
Ignition
Extinction
1500
Ignition to flame transition(critical radius, Rc)
Flame Radius, R
Fla
me
Pro
pa
ga
ting
Sp
ee
d,U
10-1
100
101
102
10-2
10-1
100
101
Q=0.0
Q=0.1
Q=0.15
Q=0.6
(a), Le=1.2, h=0.0
a
b
cd
e
f
g
h
i
j
Critical ignition radius
Rc
Q?
• Shorten ignition time• Extend extinction limit
Plasma
Plasma
• Increase flame speed• Extend flammability limit
• Make ignition kernel > Rc• Accelerate ignition to flame transition
0/
0
2
0 RTE
F
p
ig eEBQY
TRCt adRTE
p
L eC
WBLeS
/
22
Ignition delay Mass burning rate
3
0min )( cadp RTTCE
Minimum ignition energy
Summary
1. Plasma has both kinetic and thermal effects on ignition enhancement.
2. Plasma has only minor kinetic effect on flame propagation speed at high temperature. The main effect for the extension of extinction limit is thermal. However, plasma can enhance ignition assisted flame speed significantly.
3. Plasma may have strong kinetic effect on cool flame and warm flame propagation speed and limits.
4. Plasma can cause fuel fragmentation and reduce the fuel Lewis number, thus enhance flame speed via the Lewis number effect (Transport).
5. The minimum ignition energy is governed by a Critical Radius. Plasma can create a large volumetric discharge greater than the critical radius to reduce the minimum ignition energy, especially at low pressure and fuel lean conditions.