Establishment of an improved heat transfer model based on an enhanced thermal wall function for internal combustion engines operated under different combustion modes

Abstract

The heat transfer from the in-cylinder gases to the wall dramatically affects the combustion and emission characteristics of internal combustion engines, particularly for the engines operated with advanced low-temperature combustion modes. In the present work, an improved heat transfer model was established based on an enhanced thermal wall function using the piecewise functions, in which only the laminar Prandtl number is considered in the viscous sublayer, and the competition between the laminar Prandtl number and the turbulent Prandtl number is taken into account in the buffer layer and turbulent core regions. By implementing the improved heat transfer model into a computational fluid dynamics code, the predictions of the combustion processes and the heat transfer behaviors of an engine operated with conventional diesel combustion, homogeneous charge compression ignition, and reactivity controlled compression ignition modes were validated by the experimental data. Moreover, the computational results from two previous heat transfer models, i.e., Han and Reitz model and Rakopoulos et al. model, were presented for comparison. The results indicate that the heat flux characteristics under different combustion modes can be more satisfactorily reproduced using the improved heat transfer model than Han and Reitz model and Rakopoulos et al. model by avoiding the flaws in the thermal wall function. Finally, the influence of the dimensionless distance of the computational grid adjacent to the wall on the predicted heat flux was discussed to study the dependence of the heat transfer model on the size of the near-wall computational grid.