CFD Evaluation of the Effects of Surface Area-to-Volume Ratio on the Heat Transfer and Flow Field in Wankel Rotary Engines

Wankel rotary engines generally present an unfavourable surface area-to-volume ratio that prevents them from obtaining the high efficiency needed for the currently challenging applications in the mobility sector. In a previous study, an optimisation of Wankel engine geometry was carried out in order to minimise the surface area-to-volume ratio, with the aim of reducing the overall heat loss during the combustion phase. The study reported a counterintuitive finding that the minimum surface area-to-volume ratio configuration actually produced the worst heat loss due to the unusual flow field inside the combustion chamber affecting the Reynolds and Nusselt numbers. The present study aims to provide insights into the surprising results using a detailed flow and heat transfer analysis by undertaking detailed CFD simulations for the most representative configurations in the previous study. The CFD results confirmed the findings of the previous study, showing that the modified Woschni model correctly predicted the convective heat transfer coefficient with overall good accuracy. The simulation data provided the detailed tumble fluid flow structures during the entire cycle. It was found that, for the best surface-to-volume ratio configuration, a significantly high velocity is generated at the top dead centre compared to the reference configuration, stretching the flame front toward the leading edge of the rotor, and thus increasing the wall heat transfer. The results provide helpful guidelines in design strategies for improving the overall efficiency and emissions of Wankel rotary engines.