Absolute OH Number Density Measurements in Lean Fuel-Air Mixtures Excited by a Repetitively Pulsed Nanosecond Discharge

OH Laser Induced Fluorescence (LIF) is used for temperature and absolute OH number density measurements in an atmospheric pressure, near stoichiometric CH4-air flame generated by a Hencken burner. OH rotational temperature is inferred with excitation scans of both the OH A-X (0,0) and (1,0) bands. OH LIF signal is corrected by considering transition-dependent total radiative decay rate, laser attenuation, and fluorescence trapping. The relative OH concentrations are put on an absolute scale by calibrating the optical collection constant using Rayleigh scattering. The measured absolute OH number density in the flame is compared with laser absorption measurements done at the same locations, showing good agreement and thus demonstrating the efficacy of our calibration approach employing Rayleigh scattering—for a low temperature and pressure, lean, fuel-air mixtures excited by a repetitively pulsed nanosecond (nsec) discharge. Here, a premixed fuel-air flow, initially at T0=500 K and P=100 torr, is excited by the discharge in a plane-to-plane geometry, operated in burst mode at 10 kHz pulse repetition rate. 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-nsec-gated images, employing an intensified charge-coupled device camera. 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 the leanest mixtures, while 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 ~0.02-0.1 msec, suggesting little accumulation during the burst of pulses repeated at 10 kHz. In CH4-air and H2-air, OH concentration decays within ~0.1-1.0 msec and 0.5-1.0 msec, 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 model applicable to fuels C3 and higher.