Design optimisation of a diffuser for a turbocharger compressor stage

The need for energy saving within the automotive industry becomes greater each year due to the continued pollution of the environment from the fossil fuels, and the amount of NOx emissions released in the environment from ground vehicles. Turbochargers are among those components that provide solutions to the energy saving for automotive vehicles, especially for the commercial division of ground vehicles. Significant developments in this field have been witnessed in the last decade using both the numerical and experimental investigations. However, the regulations of E.U regarding the energy saving has motivated the automotive industry to investigate new methods for improving the performance of turbochargers, decreasing the fuel consumption and lowering the emissions release in the environment from ground vehicles. Hence, this numerical investigation is primarily focused on the optimisation of the diffuser of a turbocharger compressor stage.
The primary goal of this study is to numerically model and investigate the compressor stage of a turbocharger using advanced Computational Fluid Dynamics (CFD) based techniques. The model has been investigated at lower operating speeds i.e. 60,000rpm and 80,000rpm. Multiple Reference Frame (MRF) approach has been employed for simulating the steady flow of air through the compressor stage of the turbocharger. In an effort to optimise the diffuser geometry, non-parallel walls approach has been used. In this approach, the shroud side wall of the diffuser has been made non-parallel to the hub side wall, in a number of configurations. These configurations have been numerically analysed in order to evaluate the effects of pinching and diffusion, where pinching corresponds to diverting the shroud wall of the diffuser by a certain angle and at a certain distance from the inlet of the diffuser, while diffusion corresponds to the increase in the diffuser’s width at its outlet section, as compared to its inlet section. Both qualitative and quantitative flow analysis has been carried out in detail in order to find optimum diffuser geometry. Furthermore, this thesis provides an insight on the flow structure within a compressor stage of a turbocharger, and reveals possible mechanisms that affect its performance characteristics.