Geometrical study of spark ignition in two dimensions

Abstract

Spark ignition, as the first step during the combustion in Otto engines, has a profound impact on the further development of the flame kernel. Over the last ten years growing concern for environment protection, including low emission of pollutants has increased the interest in the numerical simulation of ignition phenomena to guarantee successful flame kernel development even for lean mixtures.However, the process of spark ignition in a combustible mixture is not yet fully understood. The use of detailed reaction mechanisms, combined with electrodynamical modelling of the spark, is necessary to optimize ignition of lean mixtures.This work presents simulations of the coupling of flow, chemical reactions and transport with discharge processes in order to investigate the development of a stable flame kernel initiated by an electrical spark. A two-dimensional code to simulate the early stages of flame kernel formation, shortly after the breakdown discharge, has been developed. The model includes Joule heating. The spark plasma channel formed as a consequence of the breakdown is incorporated into the initial conditions. The computations include the initial phase (1–5 µs), which is governed by pressure wave formation, but also the transition to flame propagation. A thorough study of the influence of the electrodes' geometry, i.e. shape and size, and gap width, has been performed for air and a lean H2–air mixture. Also a detailed methane-air mechanism was chosen as another example including combustion.Due to the fast expansion of the plasma channel, together with the geometrical complexity of the electrodes, a complicated flow field develops after the emission of a pressure wave by the expanding channel. Special numerical methods, including artificial viscosity, are required to resolve the complicated flow field during these first 1–5 µs. The heat release through chemical reactions and transport processes is almost negligible during this short phase. The second phase, i.e. the development of a propagating flame and the flame kernel expansion, can last up to several milliseconds and is dominated by diffusive processes and chemical reactions. It has been found that the geometry greatly influences the developing flame kernel and the flow field. As soon as chemical reactions begin to contribute significantly to the heat release, the effect becomes smaller.