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

摘要

OH Laser Induced Fluorescence (LIF) and picosecond (ps), broadband Coherent Anti-Stokes Raman Spec-troscopy (CARS) are used for time-resolved temperature and time-resolved, absolute OH number density measurements in lean H_2-air, CH_4-air, C_2H_4-air, and C_3H_8-air mixtures in a nanosecond (ns) pulse discharge cell/plasma flow reactor. The premixed fuel-air flow in the reactor, initially at T_0 = 500 K and P= 100torr, is excited by a repetitive ns pulse discharge in a plane-to-plane geometry (peak voltage 28 kV, discharge gap 10 mm, estimated pulse energy 1.25 mJ/pulse), operated in burst mode at 10 kHz 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 = 550-600 K, using both OH LIF and CARS, and remain essentially unchanged for up to 10 ms after the burst. Time-resolved temperature measured by CARS during plasma-assisted ignition of H_2-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 C_2H_4-air, C_3H_8-air, and CH_4 is highest in lean mixtures. In H_2-air, OH concentration is nearly independent of the equivalence ratio. In C_2H_4-air and C_3H_8-air, unlike in CH_4-air and in H_2-air, transient OH-concentration overshoot after the discharge is detected. In C_2H_4-air and C_3H_8-air, OH decays after the discharge on the time scale of ～0.02-0.1 ms, suggesting little accumulation during the burst of pulses repeated at 10 kHz. In CH_4-air and H_2-air, OH concentration decays within ～0.1-1.0 ms and 0.5-1.0 ms, 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 H_2-air and hydrocarbon-air chemistry mechanisms. Kinetic mechanisms for H_2-air, CH_4-air, and C_2H_4-air developed by A. Konnov provide the best overall agreement with OH measurements. In C_3H_8-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 model applicable to fuels C_3 and higher.