Measurements of temperature and hydroxyl radical generation/decay in lean fuel–air mixtures excited by a repetitively pulsed nanosecond discharge

OH Laser Induced Fluorescence (LIF) and picosecond (ps), broadband Coherent Anti-Stokes Raman Spectroscopy (CARS) are used for time-resolved temperature and time-resolved, absolute OH number density measurements in lean H2-air, CH4-air, C2H4-air, and C3H8-air mixtures in a nanosecond (ns) pulse disc...

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Bibliographic Details
Published in:Combustion and flame 2013-09, Vol.160 (9), p.1594-1608
Main Authors: Yin, Zhiyao, Montello, Aaron, Carter, Campbell D., Lempert, Walter R., Adamovich, Igor V.
Format: Article
Language:English
Subjects:
Applied sciences
Bursting
Charge coupled devices
Combustion. Flame
Discharge
Energy
Energy. Thermal use of fuels
Exact sciences and technology
Ignition
Kinetic modeling
Laser Induced Fluorescence
Low temperature plasmas
Mathematical models
Nanosecond pulse discharge
Nanostructure
Plasma assisted combustion
Raman scattering
Reactors
Theoretical studies. Data and constants. Metering
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Summary:OH Laser Induced Fluorescence (LIF) and picosecond (ps), broadband Coherent Anti-Stokes Raman Spectroscopy (CARS) are used for time-resolved temperature and time-resolved, absolute OH number density measurements in lean H2-air, CH4-air, C2H4-air, and C3H8-air mixtures in a nanosecond (ns) pulse discharge cell/plasma flow reactor. The premixed fuel–air flow in the reactor, initially at T0=500K and P=100torr, is excited by a repetitive ns pulse discharge in a plane-to-plane geometry (peak voltage 28kV, discharge gap 10mm, estimated pulse energy 1.25mJ/pulse), operated in burst mode at 10kHz pulse repetition rate. In most measurements, burst duration is limited to 50 pulses, to preclude plasma-assisted ignition. The discharge uniformity in air and fuel–air flows is verified using sub-ns-gated images (employing an intensified charge-coupled device camera). Temperatures measured at the end of the discharge burst are in the range of T=550–600K, using both OH LIF and CARS, and remain essentially unchanged for up to 10ms after the burst. Time-resolved temperature measured by CARS during plasma-assisted ignition of H2-air is in good agreement with kinetic model predictions. Based on CARS measurement, vibrational nonequilibrium is not a significant factor at the present conditions. Time-resolved, absolute OH number density, measured after the discharge burst, demonstrates that OH concentration in C2H4-air, C3H8-air, and CH4 is highest in lean mixtures. In H2-air, OH concentration is nearly independent of the equivalence ratio. In C2H4-air and C3H8-air, unlike in CH4-air and in H2-air, transient OH-concentration overshoot after the discharge is detected. In C2H4-air and C3H8-air, OH decays after the discharge on the time scale of ∼0.02–0.1ms, suggesting little accumulation during the burst of pulses repeated at 10kHz. In CH4-air and H2-air, OH concentration decays within ∼0.1–1.0ms and 0.5–1.0ms, respectively, showing that it may accumulate during the burst. The experimental results are compared with kinetic modeling calculations using plasma/fuel chemistry model employing several H2-air and hydrocarbon-air chemistry mechanisms. Kinetic mechanisms for H2-air, CH4-air, and C2H4-air developed by A. Konnov provide the best overall agreement with OH measurements. In C3H8-air, none of the hydrocarbon chemistry mechanisms agrees well with the data. The results show the need for development of an accurate, predictive low-temperature plasma chemistry/fuel chemistry kinetic m
ISSN:0010-2180
1556-2921
DOI:10.1016/j.combustflame.2013.03.015