Ignition of a hydrogen-air mixture by low voltage electrical contact arcs

This article presents an experimental and computational study of ignition caused by a low voltage electrical contact arc. The contact arc is a transient electrical discharge which occurs due to movement of electrical contacts, for example, when two energised electrodes are separated. The physical properties of this discharge are significantly different from the more conventional high voltage spark. Its potential to cause ignition is an important consideration in international explosion protection standards. As these standards are based on unreliable empirical methods, a more fundamental investigation is warranted. This study uses a specially designed apparatus and electrical circuit to create the contact arc in a hydrogen-air mixture. The transient development of the resulting flame kernel is observed using Mach-Zehnder interferometry with high spatial and temporal resolution. These experimental results are compared to simulations of a 3-D reactive flow model with detailed chemical kinetics and molecular transport. A quantitative comparison is effected by the generation of synthetic optical phase plots from the simulation output. This comparison showed reasonable correspondence between the simulated and measured flame shape. Ignition delays and thresholds were, however, under-predicted by the model. The comparison was complicated by significant statistical scattering in the experimental results. Additional investigations into the sensitivity of the model showed good robustness to grid size variations, and that inclusion of a simplified consideration of heat transfer through the electrodes produces small differences in flame shape and ignition delay.