Understanding and Predictive Modeling of Plasma Assisted Combustion
Igor Adamovich, Walter Lempert, and Jeffrey Sutton
Department of Mechanical and Aerospace Engineering
Ohio State University
AIAA Paper 2015-0155 AIAA 53rd Aerospace Sciences Meeting (SciTech 2015), January 5, 2015
Studies of plasma assisted combustion: Motivation and objectives
• Ignition and combustion become unstable:
• At low equivalence ratios
• At low combustor pressures (high altitude flight)
• At high flow velocities in combustor (high-speed flight)
• Major new capabilities provided by nonequilibrium, transient plasmas:
• Large energy fraction into inelastic electron impact: efficient generation of metastables and radicals (N2*, O*, Ar*, O, H, OH, CH) at high E/N
• Radicals react with fuel, even at low temperatures
• Plasma chemical chain branching / fuel oxidation reactions
• “Nudging” conventional combustion chemistry in the right direction
• Reduction of ignition delay, lean flammability limit, flame blow-off velocity increase
• Objectives:
• New experimental data at controlled, well-characterized conditions
• Development and validation of kinetic mechanisms, predictive capability
I. Quantifying energy partition in nonequilibrium fuel-air plasmas
• Measurements and predictions of time-resolved electric field in high-pressure H2 plasmas (AIAA 2015-0935, Tue 3:30)
• Measurements and predictions of time-resolved electron density and electron temperature in O2- and H2-containing plasmas (AIAA 2015-1829, Thur 2:30)
• Measurements and predictions of time-resolved and spatially resolved temperature, N2(X,v=0-12) populations, [N], [O], and [NO] in air and H2-air plasma; experimental demonstrations of two-stage heating mechanism and role of N2* reactions on NO formation (AIAA 2015-1159, Wed 9:30)
Plasma Assisted Combustion MURI: Summary of Principal Achievements
Psec CARS / 4-wave mixing: electric field in a plane-to-plane nsec pulse discharge
• H2, P=430 Torr, two plane electrodes covered with quartz plates, 3 mm gap, 0.2 ns time res
-100 0 100 200 300 400-5
0
5
10
15
20
25
30
35
40
45
Time (nanoseconds)
Elec
tric
Fiel
d (k
V/cm
)
Applied Voltage/gap ratioElectric Field
• Kinetic modeling predictions: good agreement with the data
• E/N: controls partition of coupled energy among different molecular energy modes
Thomson scattering: electron density and electron temperature in a point-to-point nsec pulse discharge
10% O2 in He P=100 torr
ne= 6·1013 cm-3 Te= 1.7 eV
10 mm
• ne, Te: control coupled energy, partition of energy among different modes
• Psec rotational, vibrational CARS: T, [N2(X,v)]
• TALIF, LIF: absolute [N], [O], [NO]
• NO production dominated by reactions of N2 electronic states, N2
* + O → NO + N
• Rapid heating in air: N2* + O2 → N2(X) + O + O
• Slow heating in air: V-T relaxation by O atoms
• H2-air: additional chemical energy release
Psec CARS, LIF, TALIF: “Full set” of energy partition and plasma chemistry data in air, P=40-100 Torr
NO PLIF image 10 µs after discharge
II. Quantifying effect of excited electronic states of N2*, O*, and Ar* on fuel-air plasma chemistry
• Measurements and predictions of time-resolved temperature, Tv(N2), and absolute [OH] in ns pulse discharges in air, H2-air, and CxHy-air (Comb. Flame 2013)
• Measurements and predictions of [O], [N], and [NO] in ns pulse discharges in H2-air and C2H4-air; role of N atoms and OH radicals on NO formation (AIAA 2015-1159, Wed 9:30)
• Measurements and predictions of time-resolved spatial distributions of absolute [OH] and [H] in point-to-point ns discharges in H2-O2-Ar (Proc. Comb. Symp. 2015)
Plasma Assisted Combustion MURI: Summary of Principal Achievements (cont.)
LIF, psec CARS: absolute [OH] and temperature dynamics in preheated fuel-air mixtures after nsec pulse discharge burst
Data used for PAC kinetic mechanism validation (will get back to this)
H2 – air, ϕ=0.3 T0=500 K, P=100 torr
C2H4 – air, ϕ=0.3 T0=500 K, P=100 torr
Pulse #10 Pulse #100
, pulse #10
H2-air Φ=0.42
P=40 Torr
LIF, TALIF: dominant effect of N atoms and OH on NO formation in H2-air and C2H4-air plasmas
• Significant [NO] rise, [N] reduction in H2-air, C2H4-air plasmas (compared to air)
• Kinetic modeling: longer NO decay due to reaction N + OH → NO + H
• In air, N atoms contribute to NO decay, N + NO → N2 + O
• In fuel-air, N atoms produced in the plasma enhance generation of NO (major regulated pollutant)
Air P=40 Torr
Hot central region: chain branching dominates OH production
H + O2 → OH + O ; O + H2 → OH + H
Low temperature peripheral region: predominant OH accumulation
H radial diffusion ; H + O2 + M → HO2 ; H + HO2 → OH + OH
Rayleigh scattering, LIF / TALIF line imaging: coupling of plasma chemistry and transport (H2 - O2 - Ar, P=40 torr)
III. Quantifying effect of plasma chemical reactions on plasma-induced ignition dynamics
• Measurements and predictions of time-resolved temperature, T2(N2), and
absolute [OH] during repetitively pulsed plasma-enhanced ignition process in H2-air (Proc. Comb. Symp. 2013, Comb. Flame 2013)
Plasma Assisted Combustion MURI: Summary of Principal Achievements (cont.)
[OH], T, Tv(N2) during plasma assisted ignition of H2-air (ϕ=0.4, 120-pulse burst, T0=500 K, P=80-90 torr)
• Model predictions in good agreement with time-resolved T (psec CARS), [OH] (LIF)
• Threshold ignition temperature Ti ≈ 700 K, lower than autoignition temperature, Ta ≈ 900 K
Data used for PAC kinetic mechanism validation
IV. Development, validation, and delivery of kinetic mechanism of nonequilibrium plasma-assisted energy transfer / chemistry / ignition of fuel air mixtures
• Use of experimental results (Parts I-III) to incorporate detailed plasma kinetics into the mechanism and for validation
• Key issue: mechanism availability / ease of use, a. k. a.
• “Thanks for the mechanism. Can I also have your code?”
a) “No way”
b) “Yes but you’ll wish you’d never asked”
• Mechanism deliverable: freeware / commercial software platform (BOLSIG / CHEMKIN-PRO) for wide use, without relying on proprietary computer codes
Plasma Assisted Combustion MURI: Summary of Principal Achievements (cont.)
BOLSIG+
• Boltzmann equation for EEDF (two-term expansion, experimental cross sections): EEDF, rates of electron impact excitation, dissociation, and ionization processes vs. average electron energy
• Post-processor: rates imported into input reaction kinetics file of CHEMKIN-PRO; plasma input power waveform, other input conditions specified by user
CHEMKIN-PRO (Plasma PSR)
• Electron energy equation: electron temperature controlling rates of electron impact, k(Te)
• Heavy species energy equation: temperature, rates of thermal chemical reactions
• Charged species equation (dominant ionization, recombination, ion-molecule reactions): electron density in plasma
• Excited neutral species equations (electron impact excitation, non-reactive and reactive quenching): contribution to radical species formation
• Master equation for N2(X,v) populations; state-specific V-T and V-V processes: energy storage in N2 vibrational mode, its subsequent release
• Neutral species reactions: based on fuel-air air chemistry mechanism by A. Konnov, enhanced by radical production in plasma
• Quasi-0-D corrections: diffusion / conduction, gasdynamic expansion, cathode layer
Kinetic Mechanism / Software Platform Overview
Mechanism validation: plane-to-plane repetitive nsec discharge air and H2-air*
• Time-resolved T, Tv(N2), absolute [OH] in good overall agreement with data
• But predicted [OH] reduction with ϕ is not detected in experiment: room for improvement
* No computer code used
Mechanism validation: pin-to-pin nsec discharge in air*
• Power coupled to cathode layer (cathode voltage fall) needs to be subtracted
• Time-resolved T, Tv(N2), N2(v) are in agreement with the data
• Discharge energy partition is reproduced correctly
* No computer code used
10 mm
Mechanism validation: plane-to-plane repetitive nsec discharge, CxHy-air*
* No computer code used
Agreement with absolute [OH] dynamics
• H2, CH4: good
• C2H4: excellent
• C3H8: poor ; not sensitive to plasma chemical reactions, almost certainly due to conventional chemistry issues
1. Sample BOLSIG output
2. BOLSIG output post-processing routine (Matlab)
3. Sample CHEMKIN-PRO input gas-phase kinetics files (“PAC mechanism”)
4. Thermochemical data files (including excited species)
5. Sample discharge coupled power files (single-pulse and multiple pulse train)
6. Recorded video tutorial
7. Sample CHEMKIN-PRO output results
8. Sample mechanism validation results
Time frame • Items 1-8 for test drive and feedback: available upon e-mail request
• Mechanism / files / tutorial posted online: within a few weeks
• Periodic updates based on user feedback and new results
Kinetic Mechanism Deliverables
[OH]
[H]
■ Saturated H2O vapor / Ar buffer flow over liquid water, P=30 torr
■ Liquid surface at y=0
■ Electrodes powered by ns discharge pulses (20-pulse burst)
■ Saturated H2O vapor / Ar buffer flow over liquid water, P=30 torr
■ Calibration: Rayleigh scattering (OH LIF), Kr TALIF (H TALIF)
On-going work: in situ distributions of [OH], [H] in liquid-vapor interface plasmas (AIAA 2015-0934, Tuesday 3:00)
On-going work: high-pressure (~1 bar) “0-D” plasmas in “Wolverine” cell with liquid metal electrodes (SciTech 2016)
Electrodes are encapsulated in quartz cells: no corona outside, no discharge pulse energy uncertainty
[Ar*] (x,y) distribution 0.2 μs after pulse: 1% H2 in argon, T=500 K, P=300 Torr
ν=10 kHz, pulse #10
Argon T=300 K
P=300 Torr
Laser beam pattern for [Ar*] measurements by TDLAS
1% H2-Ar, T=500 K, P=300 Torr
x, mm
[Ar*], 1014 cm-3
Some Unresolved Issues (more on this in the paper)
• “Rapid” heating: at what conditions does “rapid” heating become dominant, compared to low-temperature radical species chemistry?
• Reactions of vibrationally excited molecules: do reactions such as N2(X1Σ,v) + O → NO + N and N2(X1Σ,v=1) + HO2 → N2(X1Σ,v=0) + HO2(ν2+ν3) → N2 + H + O2 matter?
• Fuel molecular structure: is there a difference between plasma chemistry of low octane number (with low-temperature cool flame chemistry) vs. high octane number fuels?
• Dynamic effect of plasma on non-premixed turbulent flames: preventing local extinction by producing radicals where combustion cannot be sustained otherwise
• Plasma assisted combustion in non-premixed compressible flows: coupling between discharge dynamics, fuel-air mixing, and combustion instability development
AFOSR MURI “Fundamental Mechanisms, Predictive Modeling, and Novel Aerospace Applications of Plasma Assisted Combustion” DOE PSAAP-2 Center “Exascale Simulation of Plasma-Coupled Combustion” (under U. Illinois at Urbana-Champaign prime) Nikolay Popov, Moscow State University Rich Yetter, Penn State Fokion Egolfopoulos, USC
Acknowledgments
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