Methodology to Perform Conjugate Heat Transfer Modeling for a Piston on a Sector Geometry for Direct-Injection Internal Combustion Engine Applications 2019-01-0210
Diesel engines continue to remain the prime movers for heavy-duty applications. Increasing power density of diesel engines is a key pathway for manufacturers to improve customer value. A significant portion of the fuel energy in diesel engines is lost as heat transfer through the wall boundaries, which limits the engine efficiency. Component temperatures, such as the peak piston temperature, are key constraints in engine performance and design, especially as power density is increased. Therefore, optimizing diesel engine design and performance while including aspects of heat transfer and consideration of component temperatures is essential to diesel engine research and development programs. With the increasing computational power in recent times, multidimensional computational fluid dynamics (CFD) modeling tools are being used extensively for optimizing diesel engine design. However, it is still common practice in engine CFD modeling to use constant boundary temperatures. This is due to a combination of the cost of simulations that predict component temperatures, the difficulty in experimentally measuring the component temperatures, or the lack of measurements when simulation is being used predictively. This assumption introduces uncertainty in heat flux predictions. Conjugate heat transfer (CHT) modeling is an approach used to simultaneously model the convective heat transfer at the interface of the combusting gases and the solid engine components as well as the conductive heat transfer within the solid engine components. However, using CHT modeling within a piston design optimization study, would involve large numbers of simulations, and would be impractical considering the computational expense. Accordingly, in the current publication, an approach to perform piston CHT simulations on sector geometries by running just the closed cycle portion of an engine cycle is proposed to reduce the computational time significantly with minimal impact on the prediction accuracy. The study was performed on a heavy-duty Caterpillar C-15 engine at a high load operating condition of 20 bar gross IMEP and engine speed of 1800 rev/min. Since the open cycle portion of the cycle cannot be simulated on a sector mesh, a scaling methodology was developed to account for the contribution of the open cycle to the wall heat transfer. The average and the maximum piston temperature from the sector CHT approach were predicted within 35 K and 25K of the full geometry CHT simulation respectively. Additionally, the distribution of the temperature within the solid piston obtained from the sector CHT simulation was compared to the temperature distributions from a full CHT geometry and the results showed good agreement. The sector CHT approach resulted in a ~7.5x reduction in computational time.