Michael A. Chaszeyka Non-Equilibrium Thermodynamics Laboratory Absolute OH Number Density...

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


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


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


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?


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


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


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


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


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


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


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


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



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


C2H4-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



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


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)


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?


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)


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


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

Michael A. Chaszeyka Non-Equilibrium Thermodynamics Laboratory Absolute OH Number Density...

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