Detailed chemistry flamelet modeling of mixed-mode combustion in spark-assisted HCCI engines

Abstract

A detailed chemistry mixed-mode flamelet model for the prediction of combustion in spark-assisted homogeneous charge compression ignition (HCCI) engines is presented in this paper. The complex phenomena of spark-channel processes (turbulent corrugation, multiple restrikes) and of early flame kernel propagation (localized flame kernel formation, non-spherical early flame shapes), both induced by spray-guided spark-ignition combustion initiation, are captured by the recently introduced SparkCIMM ignition model. In this paper, laminar burning velocities and flame thicknesses are calculated along the mean turbulent flame front, using tabulated detailed chemistry flamelet calculations to appropriately consider locally rich, highly diluted, and auto-igniting stratified mixtures. Flame extinction criterions are formulated and incorporated into an extended G-equation flame front tracking scheme. Auto-ignition processes in the unburnt mixture, controlled by the flame-induced pressure and temperature increase, are captured by a recently developed multi-zone flamelet model that accounts for detailed chemistry and effects of scalar mixing on turbulent combustion. The laminar burning velocity is shown to increase significantly as the flame propagates into the chemically reacting mixture within the first stage of auto-ignition. After the initiation of the thermal runaway, however, flame extinction occurs rapidly. These interactions of mixed-mode combustion processes are captured by the presented flamelet model. It is developed based on a time and length scale analysis, revealing a scale separation between the ignition delay of diffusion combustion and the flame time of flame propagation. The analysis of the mixture preparation process, along with the simulation of turbulent flame front propagation and its extinction, demonstrates that the prediction of combustion in spark-assisted HCCI engines requires the on hand comprehensive mixed-mode combustion model. A comparison of simulation results from this new model with data from experiments and combustion models of reduced physical complexity proofs its qualification.