CFD-Simulation of Ignition and Combustion in Lean Burn Gas Engines

In this study, the ignition and combustion process in a lean burn, spark-ignited stationary gas engine was investigated using a level-set (G-equation) combustion model in the context of Reynolds-averaged Navier-Stokes (RANS) simulations. An ignition model based on Herweg and Maly [1] and its coupling with the G-equation combustion model was implemented in the framework of the STARCD/es-ICE solver. A first validation was performed by means of spherically expanding methane/air flame measurements in an optically accessible spherical combustion bomb at elevated pressure by Lawes et al. [2]: predictions of the kernel size and the flame expansion are in good agreement with the experimental data at both stoichiometric and lean conditions. The model was subsequently applied to study combustion in a premixed lean burn stationary gas engine with a displacement volume of roughly two liters, ignited by means of a centrally located “G-type” spark plug. A grid sensitivity analysis was performed which indicates that the employed average cell size of 1.5 mm is sufficient. A parametric study was carried out for the reference operating condition to determine the appropriate value of the flame speed parameter in the adopted Damköhler turbulent flame speed closure. The calibrated model was finally applied to study various engine operating conditions which include changes to engine speed, load, fuel/air ratio and spark advance. In general, good predictions in terms of cylinder pressure and chemical heat release rate are reported and the developed platform shows considerable promise towards the development of improved engine concepts.