Ignition of hydrocarbons and the thermalization of electrical discharges

Abstract

Using propane‐air mixtures, it is shown that the minimum ignition energy corresponds precisely to the amount that produces, in the discharge channel, the electron density (10¹⁷ cm⁻³) that changes the discharge from a cold streamer to a hot incipient arc. This electron density together with the minimum quenching distance are suggested as a physically meaningful criterion for ignition. The procurement of the critical electron density is also studied using a polyester film and water as examples of solid and deformable dielectric electrodes. It is found that thermalization is controlled by the rate of supply of charge into the discharge channel and that a stored minimum energy value applies only when two metal electrodes are involved. Ignition capability can be determined by the characteristics seen in the oscilloscope recordings of the current in the discharge. Electrode conduction effects are studied by varying the conductivity of a saltwater solution. It is found that either surface‐charge accumulation or resistive effects may prevent ignitions depending on the conductivity of the solution. Also, that except for metals, ignitions are always invariably associated with nonuniform electric fields and the procurement of electrons by cold‐streamer discharges. In the better dielectrics, surface charges prevent ignition independently of the energy supply. However, these same charges may be stored for a long time (days) and produce ignitions at very low voltages with polarity reversals. Finally, local thermalization is suggested as a necessary requirement for the propagation of long sparks characterized by stepping.