Schlieren Imaging and Pulsed Detonation Engine Testing of Ignition by a Nanosecond Repetitively Pulsed Discharge
Aerospace Systems Directorate Wright-Patterson AFB United States
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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 methaneair 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 05 kHz, ignition kernels formed by the discharge are each transported away from the discharge gap before the following pulse arrives. At higher pulse-repetition frequencies P10 kHz,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 frequencies10 kHz and peak pulse amplitude 9 kV, 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 gasolineair 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.