A detailed understanding of the processes associated with spark ignition, as a first step during combustion, is of great importance for clean operation of spark ignition engines. In the past 10 years, a growing concern for environmental 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 modeling of the spark, is necessary to optimize spark ignition for lean mixtures. This work presents the simulation of the coupling of flow, chemical reactions, and transport with discharge processes including ionization in order to investigate the development of a stable flame kernel initiated by an electrical spark in methane/air mixtures. A transport model taking into account the interactions of charged particles has been incorporated in the flow model. This model is based on the Chapman-Enskog theory with an extension for polyatomic gases and considers resonant charge transfer and ambipolar diffusion for the computation of the transport coefficients.
A two-dimensional code to simulate the early stages of flame development, shortly after the breakdown discharge, has been developed. The modeling includes an equation for the electrical field. The spark plasma channel left behind by the breakdown is incorporated into the initial conditions.
Due to the fast expansion of the plasma channel, a complicated flowfield develops after the emission of a shock wave by the expanding channel. The second phase, that is, 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.