CARS Thermometry Studies of Plasma Assisted Combustion in Ethylene-Air and Hydrogen-Air Mixtures and of a Dielectric Barrier Discharge Actuator

Pure rotational CARS thermometry is used to study low-temperature plasma assisted oxidation kinetics in ethylene-air and hydrogen-air mixtures at stoichiometric and fuel lean conditions at 40 Torr pressure. Air and fuel-air mixtures are excited by a burst of high voltage nanosecond pulses at a pulse repetition rate of 40 kHz and a burst repetition rate of 10 Hz. The number of pulses in the burst is varied from a few pulses to a few hundred pulses and the results are compared with a fuel-air plasma chemistry model developed at The Ohio State University. Air and all fuel-air mixtures are found to agree well with the model. In ethylene-air mixtures, it is found that the heating rate is much faster than in air plasmas, primarily due to energy release in exothermic reactions of fuel with O atoms generated by the plasma. It is also found that the initial heating rate in ethylene-air mixtures is independent of equivalence ratio and is mainly controlled by the rate of radical production, specifically O atoms. At long burst durations, the heating rate in the lean mixture is significantly reduced when all of the ethylene is oxidized.In hydrogen-air mixtures, it is found that the heating rate is much faster than in air plasmas, primarily due to the heat release from reactions hydrogen with H and O atoms generated by the plasma. The pure rotational CARS temperature measurements also show a maximum in temperature after approximately 17 ms in the φ =1.0 and φ =0.5 mixtures, which is indicative of ignition. Sensitivity analysis shows that radicals generated by the plasma are important for low temperature plasma chemical fuel oxidation and associated heat release. It also shows that ignition is primarily controlled by the chain branching sequence O + H2 yields OH + H and H + O2 yields OH + O.Pure rotational CARS thermometry is also used to study a dielectric barrier discharge. It is found that in the plane to plane configuration, there is no detectable temperature rise, most likely due to the filaments moving from pulse to pulse, making it impossible to get the CARS beams into the filament. This issue is solved by using a floating electrode on the ground electrode to stabilize a filament. To ensure that the CARS beams are in a filament, for every spectrum a corresponding image is taken. Temperature measurements are taken at varying times after a 50 pulse burst and at short times after the burst (100 ns to 1 microsecond) there is approximately a 35 K increase from room temperature, while at longer times (10 – 100 microseconds) a 25 K increase is seen.