Investigation of the effect of electrode geometry on spark ignition

Abstract

High-speed schlieren visualization and numerical simulations are used to study the fluid mechanics following a spark discharge and the effect on the ignition process in a hydrogen–air mixture. A two-dimensional axisymmetric model of spark discharge in air and spark ignition was developed using the non-reactive and reactive Navier–Stokes equations including mass and heat diffusion. The numerical method employs structured adaptive mesh refinement software to produce highly-resolved simulations, which is critical for accurate resolution of all the physical scales of the complex fluid mechanics and chemistry. The simulations were performed with three different electrode geometries to investigate the effect of the geometry on the fluid mechanics of the evolving spark kernel and on flame formation. The computational results were compared with high-speed schlieren visualization of spark and ignition kernels. It was shown that the spark channel emits a blast wave that is spherical near the electrode surfaces and cylindrical near the center of the spark gap, and thus is highly influenced by the electrode geometry. The ensuing competition between spherical and cylindrical expansion in the spark gap and the boundary layer on the electrode surface both generate vorticity, resulting in the toroidal shape of the hot gas kernel and enhanced mixing. The temperature and rate of cooling of the hot kernel and mixing region are significantly effected by the electrode geometry and will have a critical impact on ignition. In the flanged electrode configuration the viscous effects generate a multidimensional flow field and lead to a curved flame front, a result not seen in previous work. Also, the high level of confinement by the flanges results in higher gas temperatures, suggesting that a lower ignition energy would be required. The results of this work provide new insights on the roles of the various physical phenomena in spark kernel formation and ignition, in particular the important effects of viscosity, pressure gradients, electrode geometry, and hot gas confinement.