A Comprehensive CFD-CHT Methodology for the Characterization of a Diesel Engine: From the Heat Transfer Prediction to the Thermal Field Evaluation

High power-density Diesel engines are characterized by remarkable thermo-mechanical loads. Therefore, compared to spark ignition engines, designers are forced to increase component strength in order to avoid failures. 3D-CFD simulations represent a powerful tool for the evaluation of the engine thermal field and may be used by designers, along with FE analyses, to ensure thermo-mechanical reliability. The present work aims at providing an integrated in-cylinder/CHT methodology for the estimation of a Diesel engine thermal field. On one hand, in-cylinder simulations are fundamental to evaluate not only the integral amount of heat transfer to the combustion chamber walls, but also its point-wise distribution. To this specific aim, an improved heat transfer model based on a modified thermal wall function is adopted to estimate correctly wall heat fluxes due to combustion. On the other hand, a detailed Conjugate Heat Transfer model including both the solid components and the coolant circuit of the engine is needed, accounting for all the thermo-mechanical effects acting simultaneously during actual operations. Such comprehensive CHT methodology is here presented, with particular emphasis on a dedicated framework for the thermal simulation of the piston, to account for the mutual influence of many interplaying phenomena such as oil jet impingement, frictional losses and conduction with the surrounding components. The predictive capabilities of the methodology are demonstrated both in terms of global thermal balance and local engine temperature distribution. In fact, numerical coolant heat rejection and thermal field are compared with experimental data provided by thermal survey and point-wise temperature measurements for two different mid-to-low revving speed operating conditions.