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

TL;DR

This study models and analyzes transient low-power metal vapour discharges in near-limit ignitions, revealing significant deviations from local thermodynamic equilibrium and providing insights into their physics and potential safety implications.

Contribution

It introduces an empirical and physics-based combined model for transient metal vapour discharges, highlighting their non-LTE behavior and advancing understanding of ignition processes.

Findings

Predicted electrical waveforms match experimental data well.

Simulation indicates the discharge likely deviates from LTE assumptions.

Discrepancy between resistance calculations suggests non-LTE conditions.

Abstract

This article presents an investigation of a transient (30 {\mu}s - 5 ms) electrical discharge in metal vapour with low voltage (< 50 V) and current (< 1 A), 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 including 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 simulated temperature fields, 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 comparison shows a significant discrepancy, yielding the important insight that the studied discharge very likely deviates strongly from LTE.