Effect of Piston Geometry on In-Cylinder Fluid Mechanics, Heat Transfer, and Ignition Delay in Rapid Compression Machines

This paper presents the results from experiments and Computational Fluid Dynamics (CFD) simulations performed to understand the impact of piston geometry on ignition delay for Dimethyl Ether (DME)/air mixtures inside a Rapid Compression Machine (RCM). Three piston shapes and two dilution ratios are studied using CFD simulations validated by experiments. The three piston geometries under consideration are: a flat piston, a piston with an enlarged crevice, and a bowl piston. Key phenomena analyzed in the study include fluid flow patterns, heat transfer, temperature homogeneity of the mixture, and ignition delay. The CFD model provides reasonable predictions of ignition delay when compared with experimental data. Simulations indicate that flat and bowl pistons show similar heat transfer, ignition delay, and combustion characteristics, while the enlarged creviced piston shows lower peak temperatures and a cooler mixture core due to higher wall heat transfer. It is concluded that piston geometry affects both fluid dynamics and heat transfer in RCMs, but its impact on heat transfer dominates ignition delay differences between the studied pistons. Thus, it is important to accurately characterize heat transfer when interpreting RCM results, especially when associating observed ignition delay with estimated temperature of reactants at the end of compression.