Numerical investigation of transient, low-power metal vapour discharges occurring in near limit ignitions of flammable gas

Abstract

This article presents an investigation of a transient (30 μs–) electrical discharge in metal vapour with low voltage () and current (), drawn between two separating electrodes. Discharges of this type are rarely studied, but are important in electrical explosion safety, as they can ignite flammable gasses. An empirical model is developed based on transient recordings of discharge voltages and currents and high speed broadband image data. The model is used for predicting the electrical waveforms and spatial power distribution of the discharge. The predicted electrical waveforms show good accuracy under various scenarios. To further investigate the underlying physics, the model is then incorporated into a simplified 3-D gas dynamics simulation of the discharge occurring in a flammable atmosphere. This simulation includes chemical reactions, molecular diffusion, heat transfer and evaporation of metal from the electrode surface. The local thermodynamic equilibrium (LTE) assumption is next used to calculate electrical conductivity from the field outputs of the simulation, which in turn is integrated to produce electrical resistance over time. This resistance is then compared to that implied by the voltage and current waveforms predicted by the empirical model. The two methods produce differing estimates, demonstrating that the studied discharge very likely deviates from LTE. Finally, assuming that electrical conductivity is approximately independent of the heavy particle temperature, the empirical model is used together with the calculated electrical conductivities to estimate the electron temperatures present in the discharge.