Schlieren imaging and pulsed detonation engine testing of ignition by a nanosecond repetitively pulsed discharge

A nanosecond repetitively pulsed (NRP) discharge in the spark regime has been investigated as to its effectiveness in reducing ignition time, both in a flow tube and a pulsed detonation engine (PDE). The flame-development time for methane-air mixtures in the flow tube is found to be a function of the total ignition energy and the pulse repetition frequency. Schlieren imaging revealed that at low pulse-repetition frequency (0-5kHz), ignition kernels formed by the discharge are each transported away from the discharge gap before the following pulse arrives. At higher pulse-repetition frequencies (≥10kHz), multiple pulses are all coupled into a single ignition kernel, thus the resulting ignition kernel size and the total energy deposition into the kernel are increased, resulting in a faster transition into a self-propagating flame. Imaging of the NRP discharge in air revealed that at high pulse frequencies (>10kHz) and peak pulse amplitude (>9kV), the plasma emission is not quenched in-between pulses, resulting in a building up of heat and radicals in the center of the ignition kernel. Optical emission spectra revealed the presence of electronically excited N₂, O, and N, as well as O⁺ and N⁺, during and between the discharge pulses. Numerical modeling of the plasma indicated that reactions of excited species mainly lead to the production of O atoms and the increase of gas temperature, which shortens induction chemistry timescales, and thus reduces the flame-development time through both kinetic and thermal mechanisms. Ignition of aviation gasoline-air mixtures by NRP discharge in a PDE also demonstrated a noticeable reduction in ignition time as compared to an automotive aftermarket multiple capacitive-discharge ignition system.