Ignition Delay and Time-Resolved Temperature Measurements in Nanosecond Pulse Hydrogen-Air and Ethylene-Air Plasmas at Elevated Initial Temperatures

摘要

Ignition time is measured in premixed, preheated hydrogen-air and ethylene-air flows excited by a repetitive nanosecond pulse discharge in a plane-to-plane geometry. ICCD images of the plasma and the flame demonstrate that mild preheating of the fuel-air flow greatly improves plasma stability and precludes filament formation. At the initial temperatures of T_0= 100-200~0 C, hydrogen-air plasmas remain stable and uniform up to at least P=150 torr, and ignition occurs in a large volume. Preheated ethylene-air plasmas appear less uniform. Even at T=200~0 C, ethylene-air ignition begins near the electrode edges, with flame propagating toward the center of the plasma. Ignition time in hydrogen-air and ethylene-air mixtures is measured at initial temperatures of T_0= 100-200~0 C, pressures of P=40-150 torr, equivalence ratios of ￠=0.5-l .2, and pulse repetition rates of v=10-40 kHz. In hydrogen-air, ignition time is reduced as the initial temperature or the pressure of the mixture is increased. At high discharge pulse repetition rates, ignition time is nearly independent of the equivalence ratio. At low pulse repetition rates, near ignition threshold, ignition time in fuel-rich mixtures increases considerably. Ignition time exhibits a minimum at a certain optimum pulse repetition rate, which shifts toward lower values at higher initial temperatures and pressures. In ethylene-air mixture, similar trends are observed, except that ignition time exhibits a weaker dependence on equivalence ratio. Time-resolved N_2(C3~II→B~3FI, v'=0→v"=0) emission spectroscopy is used to measure the temperature in hydrogen-air plasmas. The results show that ignition begins at the plasma temperature of approximately T=700 K, and results in a rapid temperature rise (over ~1 msec). Peak temperature achieved during combustion is T=1600 K. Temperature measured near the electrode edges is essentially the same as in the center of the discharge, within the experimental uncertainty, indicating that the plasma remains uniform through the discharge region. The results of ignition time measurements in hydrogen-air mixtures are compared with predictions of the hydrogen-air plasma chemistry model, showing satisfactory agreement. Plasma temperature measurements at the end of the discharge pulse burst and ignition delay time measurements after the burst demonstrate that ignition at the present conditions is not thermal and is affected by reactions of radical species generated in the plasma, which reduce ignition temperature and shorten ignition delay time.