Ignition of flammable gas mixtures by X-band microwave discharges

Mixtures of hydrogen, ethylene, propane and methane with air at NTP were ignited by sparks striking between electrodes in a cavity tuned to resonance at 9.4 GHz. The cavity was formed from a section of waveguide sealed by dielectric windows, and fed by a pulsed radar transmitter. The envelope of the radar pulse waveform reflected from the cavity was monitored using a detector and a transient data recorder; the light radiated from the sparks was observed by a photomultiplier. From the resulting signals, the behaviour of the ignition spark and the energy dissipated therein could be deduced. Values of the minimum input energy and the minimum energy absorbed in the cavity that were required for ignition were derived for each of the gas mixtures in their most easily ignited concentration, including an estimate of the error tolerance on the measurement. The measured results tended to be of the same order as, but higher than, well known earlier values for minimum ignition energy found using capacitive-discharge sparks. In the case of hydrogen, the results confirm the lower of two `accepted¿¿ values for minimum ignition energy.