Low-temperature filamentary plasma for ignition-stabilized combustion

Abstract

In this work, an experimental study investigating dielectric barrier discharge (DBD) plasma-assisted ignition in methane-air flows at near-atmospheric pressure is presented. Discharges are produced using 10 ns duration high voltage pulses at maximum 3 kHz pulse repetition rate. The goal is to check the feasibility of plasma as an ignition source for ignition-stabilized combustion in a wide range of equivalence ratio, pressure, and flow speed conditions. To that end, four important characteristics are investigated: 1) Ignition dynamics, 2) Minimum number of pulses required for ignition, 3) Plasma energy per pulse, gas temperature, and the effective reduced electric field, and 4) Plasma NOx production. In continuous plasma mode, we observe an elongated flame outside of the discharge region when a methane-air mixture flows through the discharge. The elongated flame is due to repetitively ignited kernels moving downstream with the flow. High-speed intensified imaging is used to explore plasma morphology and ignition dynamics. Plasma has a filamentary nature which is perceived as moving with the flow due to pulse-to-pulse memory effects. For fuel-air mixtures, ignition is followed by kernel expansion and splitting, flame propagation, and consecutive ignition. The time delay between consecutive ignition events is found to be a function of flow speed, because ignition is achieved only when a previously ignited kernel moves out of the plasma region, which happens faster at higher flow speeds. We performed optical emission spectroscopy in burst mode to further characterise plasma and ignition parameters. In air, plasma gas temperature and hence the reduced electric field increases because of pulse-to-pulse effects. By comparing the air plasma temperature with the methane auto-ignition temperature, we conclude that the observed ignition is low-temperature ignition. Parametric studies of NOx measurements suggest that plasma NOx emissions are higher for low flow speeds, high pulse repetition rates, and low-pressure conditions. Overall, our DBD filamentary plasma is found to be a fast, repetitive, and low-temperature ignition source that can be used for ignition-stabilized combustion at near atmospheric conditions.