Nanosecond Pulse Burst Ignition of Ethylene and Acetylene by Uniform Low-Temperature Plasmas

Abstract

Nanosecond pulse burst plasma ignition measurements and kinetic modeling calculations have been used to analyze kinetics of low-temperature plasma assisted ignition of hydrocarbon fuels. Uniform low-temperature plasmas have been generated by high voltage, nanosecond duration pulses at high pulse repetition rates. Pulse bursts of up to 1000 pulses have been used to ignite premixed ethylene-air and acetylene-air flows. Ignition delay time has been determined by measuring time-resolved OH, CH, and C2 Swan band emission from the flow, which produces a well pronounced overshoot during ignition. Ignition delay time has been measured in a wide range of pulse repetition rates and equivalence ratios. Kinetic modeling of a high-voltage, nanosecond duration pulse discharge demonstrate that charge accumulation on dielectric plates covering the electrodes results in strong shielding of the plasma, which significantly lowers gap voltage and limits the pulse energy coupled to the plasma. The effective reduced electric field value inferred from plasma emission spectra, E/N=330 ± 30 Td, is approximately a factor of 3 lower than the value estimated from the pulse peak voltage. Discharge pulse energy predicted by the model is in good agreement with our previous work, where the pulse energy was inferred from the O atom density measurements. Experimental ignition delay times have been compared with results of kinetic modeling of repetitively pulsed hydrocarbon-air plasma. Modeling calculations predict considerable chemical energy release from the fuel species due to exothermic fuel oxidation in reactions with radicals generated by electron impact. This effect results in significant additional heating of fuel-air mixtures in the lowtemperature plasma. Comparing calculated plasma assisted ignition delay time with ignition by equilibrium heating demonstrated that radicals generated by the plasma reduce ignition temperature by up to 300 0 C and reduce ignition delay time by up to 2 orders of magnitude. This demonstrates conclusively the non-thermal nature of low-temperature plasma assisted ignition. Comparison of experimental ignition delay time in ethylene-air with kinetic modeling calculations shows very good agreement. In methane-air, the model predicts no ignition at the present conditions, consistent with the experiments, in which no methane-air ignition has been detected. In acetylene-air, the model overpredicts ignition delay time by 50100% compared with the experimental data.