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Michael A. Chaszeyka Non-Equilibrium Thermodynamics Laboratory
Absolute OH Number Density Measurements in Lean Fuel-Air Mixtures Excited by a Repetitively Pulsed Nanosecond Discharge
Zhiyao Yin, Aaron Montello, Walter R. Lempert, and Igor V. Adamovich
International Symposium on Molecular Spectroscopy 68th meeting
Michael A. Chaszeyka Nonequilibrium Thermodynamics Laboratories,Department of Mechanical and Aerospace Engineering,
The Ohio State University, Columbus, OH 43210
Michael A. Chaszeyka Non-Equilibrium Thermodynamics Laboratory
Plasma Assisted Ignition
High-voltage, nanosecond pulse discharge assisted ignition
■ Telectron~1-10 eV
■ Tgas~ initial temperature
■ Non-thermal ignition: radical species generation
Energy branching in an applied field
Rotational Vibrational Electronic
Electron energy (eV) ~0.03 0.2-2 3-10
E/N (Td) <1 4-110 120-1000
Electron impact excitation and dissociation
Spark ignition
■ Telectron~5000-10000 K
■ Tgas~1000-10000 K
■ Thermal ignition
Heating
Michael A. Chaszeyka Non-Equilibrium Thermodynamics Laboratory
Nonequilibrium Plasma Assisted Ignition
Above self-ignition thresholdShock-preheated: T>1000 K, P~0.5 bar, single discharge pulse
Kosarev et al, Combust. Flame, 156:221-233, 2009
Kinetic modeling (plasma chemistry + combustion chemistry) reproduces experiments well
Radicals produced by the discharge trigger a more rapid fuel decomposition and chain reactions
Ignition at lower initial T
Without discharge
With discharge
Drastic reduction in ignition delay
Michael A. Chaszeyka Non-Equilibrium Thermodynamics Laboratory
Nonequilibrium Plasma Assisted Oxidation
Below self-ignition threshold
Uddi et al, J. Phys. D: Appl. Phys., 42:075205, 2009
Lean CH4-air, P=60 torr, T=300 K, a single discharge pulse
Baseline model (plasma chemistry + combustion chemistry) cannot reproduce experiments
New reaction channels with N2(v) and O2 (b 1Σ) have been proposed; but rates are not well-known
NOconcentration
Well-known combustion mechanisms:
■ GRI 3.0 (for CH4): 1000-2500 K
■ Konnov (for H2): 950-2700 K
■ Konnov (for C1-C4): >910 K Applicability to low-moderate T?
Michael A. Chaszeyka Non-Equilibrium Thermodynamics Laboratory
Objectives
Kinetic studies of plasma assisted fuel oxidation at low–moderate temperatures■ Temperature and Hydroxyl radical (OH) concentration measurements
■ Assessment of different conventional combustion mechanisms by kinetic modeling
Michael A. Chaszeyka Non-Equilibrium Thermodynamics Laboratory
Plasma flow reactor (T0=500 K, P=100 torr)
Experimental Setup
Peak voltage ~30 kV, pulse duration ~10 ns, pulse repetition rate at 10 kHz
Measured coupled pulse energy ~1.5-2 mJ
Photograph
Laser beam
Michael A. Chaszeyka Non-Equilibrium Thermodynamics Laboratory
LIF
Experimental Setup
Excitation transitions in OH A 2Σ+←X 2Π (v’=1, v’’=0) and (v’=0, v’’=0) bands
Calibration with Rayleigh scattering at 308 nm for inferring absolute OH number density
Michael A. Chaszeyka Non-Equilibrium Thermodynamics Laboratory
Experimental Setup
N2 CARS
Pump/Probe: 532 nm; Stokes: centered at near 604 nm, FWHM~5-6 nm Spectral resolution: ~0.4 cm-1, partially resolve the rotational structure in
the Q-branch of N2
Michael A. Chaszeyka Non-Equilibrium Thermodynamics Laboratory
Plasma Uniformity
Exclude thermal heating effect from hot filaments 0-D kinetic model
Single-shot ICCD imaging of broad band plasma emission (T0=500 K)
Imaging
1st pulse 10th pulse 100th pulse
AirP=200 torr
H2-airϕ=0.3
P=100 torr
C2H4-airϕ=0.3
P=100 torr
Through the Brewster window 50 nsec camera gate
electrodes
Imaging
Through the right angle prism 50 nsec camera gate, ϕ=0.3
electrodes
Michael A. Chaszeyka Non-Equilibrium Thermodynamics Laboratory
Characterization of the Discharge Afterglow
Decouple conventional fuel chemistry from reactions involving plasma generated excited species
Discharge operation
10 kHz, 50 pulses Afterglow
Electronically excited species, mainly N2 (C3Π), T0=500 K
Through the right angle prism 490-ps camera gate
Broadband plasma emission during a single discharge pulse
Vibrational non-equilibrium in the afterglow, T0=500 K
In air only, Tv(N2) ~850 K after 50 pulses
In fuel-air, Tv(N2) ~600-700 K
Measured gas temperature after 50 pulses, Tg ~570 K
End of discharge burst
Michael A. Chaszeyka Non-Equilibrium Thermodynamics Laboratory
Temperature after the discharge burst
OH LIF thermometry N2 CARS Comparison (T0=500 K, P=100 torr, 10 kHz, 50 pulses)
End of discharge burst 2µs after the end of discharge burst
Michael A. Chaszeyka Non-Equilibrium Thermodynamics Laboratory
Calibration using Rayleigh scattering
lEbfn
S JluBOH
f 4
lΩβ: Optical collection constant Rayleigh scattering signal:
The absolute OH number density is:
NElhc
SL
RRayleigh
LRayleighJluB
Rf
OH hcDEbf
Sn
4
DRayleigh
Spectrally-integrated LIF signal
N2
Absolute OH number density after the discharge burst
H2-air (T0=500 K, P=100 torr, 10 kHz, 50 pulses) Symbols: Expt.; Lines: Model
Popov H2-O2 mechanismKonnov H2-O2 mechanism Modeling
Michael A. Chaszeyka Non-Equilibrium Thermodynamics Laboratory
Absolute OH number density after the discharge burst
CH4-air (T0=500 K, P=100 torr, 10 kHz, 50 pulses) Symbols: Expt.; Lines: Model
USC/Wang mechanism
Better agreement with the Konnov mechanism
Konnov mechanism
Michael A. Chaszeyka Non-Equilibrium Thermodynamics Laboratory
C2H4-air (T0=500 K, P=100 torr, 10 kHz, 50 pulses) Symbols: Expt.; Lines: Model
Absolute OH number density after the discharge burst
USC/Wang mechanism
Better agreement with the Konnov mechanism
Konnov mechanism
Michael A. Chaszeyka Non-Equilibrium Thermodynamics Laboratory
Absolute OH number density after the discharge burst
C3H8-air (T0=500 K, P=100 torr, 10 kHz, 50 pulses) Symbols: Expt.; Lines: Model
USC/Wang mechanism Konnov mechanism
Neither mechanism reproduces the experiments
Michael A. Chaszeyka Non-Equilibrium Thermodynamics Laboratory
Summary
Discharge uniformity is verified by ICCD imaging of plasma emission Time-resolved [OH] is measured in lean H2-air, CH4-air, C2H4-air, and C3H8-air
at T0=500 K and P=100 torr, after 50-pulse discharge burst
Konnov’s mechs show better overall agreement for H2-, CH4-, and C2H4-air, compared to Popov’s and USC mechs
For C3H8-air, neither of the mechs reproduces the experiments
Needed: an accurate, predictive low-T plasma chemistry / fuel chemistry kinetic model applicable to high-C fuels (C3 or higher)
Michael A. Chaszeyka Non-Equilibrium Thermodynamics Laboratory
Acknowledgement
This work is supported by
The U.S. Air Force Office of Scientific Research MURI “Fundamental Aspects of Plasma Assisted Combustion” Chiping Li – Technical Monitor
Thanks for your attention
Questions?
Michael A. Chaszeyka Non-Equilibrium Thermodynamics Laboratory
Appendix I: Plasma Uniformity
Exclude thermal heating effect from hot filaments 0-D kinetic model
Averaged PLIF imaging 50-shot on-CCD accumulation, 100-ns camera gateC2H4-air, ϕ=0.1T0=500 K, P=100 torr
Imaging
electrodes
Plasma Relative OH concentration Temperature
T0=300 K, P=60 torr Non-uniform plasma
Two-line thermometry, with P1(1.5) and Q1(4.5) transitions in OH
A 2Σ+←X 2Π (v’=1, v’’=0)
Michael A. Chaszeyka Non-Equilibrium Thermodynamics Laboratory
Appendix II: LIF Excitation Scans
Data Processing:LIF excitation scan spectrum across 14 major transitions in the R-branch in the OH A-X (0,0)
Step I
Laser was scanned across R1(4.5) and Q21(4.5) transitions in OH A-X (0,0), at y=0 mm
Correct LIF spectrum with laser absorption ~f(vL)
R1(4.5)
Step II
R1(4.5)Step III
The dashed lines are individual transitions extracted from the fit
Integrate each fitted transition individually over a large spectral range
Michael A. Chaszeyka Non-Equilibrium Thermodynamics Laboratory
Appendix III: Kinetic Modeling
Plasma Chemistry
Air
Hyd
roge
nH
ydro
carb
ons
A1 N2 + e- = N2(A3Σ, B3Π, C3Π, a'1Σ) + e-
A2 N2 + e- = N(4S) + N(4S) + e-
A3 O2 + e- = O(3P) + O(3P,1D) + e-
A4 N2(C3Π) + O2 = N2 (B3Π ) + O2
A5 N2(a'1Σ) + O2 = N2 (B3Π) + O2
A6 N2(B3Π) + O2 = N2 (A3Σ) + O2
A7 N2(A3Σ) + O2 = N2 + O + O
H1 H2 + e- = H + H + e-
H2 N2(a'1Σ) + H2 = N2 + H + H
H2 N2(B3Π) + H2 = N2(A3Σ) + H2
H4 N2(A3Σ) + H2 = N2 + H + H
H5 O(1D) + H2 = H + OH
M1 CH4 + e- = CH3 + H + e-
M2 N2(A3Σ) + CH4 = N2 + CH3 + H
M3 N2(B3Π) + CH4 = N2 + CH3 + H
M4 N2(C3Π) + CH4 = N2 + CH3 + H
M5 N2(a'1Σ) + CH4 = N2 + CH3 + H
E1 C2H4 + e- = products3 + e-
E2 N2(A3Σ) + C2H4 = N2 + C2H3 + H
E3 N2(B3Π) + C2H4 = N2 + C2H3 + H
E4 N2(C3Π) + C2H4 = N2 + C2H3 + H
E5 N2(a'1Σ) + C2H4 = N2 + C2H3 + H
P1 N2(A3Σ) + C3H8 = N2 + C3H6 + H2
P2 N2(B3Π) + C3H8 = N2 + C3H6 + H2
P3 N2(C3Π) + C3H8 = N2 + C3H6 + H2
P4 N2(a'1Σ) + C3H8 = N2 + C3H6 + H2
H2-air
■ Popov’s Mechanism (Popov, 2008)
■ Konnov’s Mechanism (Konnov, 2008)
*only dominant processes shown here
Combustion Chemistry
Hydrocarbon (HC)-air■ GRI-Mech 3.0
■ USC Mech II
■ Konnov 0.5