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Effects of asymmetric flow within vaneless diffuser on the performance characteristics of the compressor stage of a turbocharger

Noukhez Ahmed, Taimoor Asim, Rakesh Mishra

Abstract

Modern engines use turbocharger that provides the extra boost to the engines and hence helps in downsizing. Turbochargers comprise of the turbine stage, bearing housing and the compressor stage. Compressor Stage helps in providing compressed air to the engine resulting in possibility of increasing the fuel-to-air ratio, which may provide extra power to the engine. Diffuser is one of the major components within the compressor stage, which helps in increasing the pressure and hence the density of incoming air. The shape of the diffuser has a significant effect on the performance characteristics of the compressor stage. According to the studies found in the literatures, it has been found that the variations in velocity profiles within the diffusers have impact on total-to-total compressor stage performance. Therefore, it is essential to critically evaluate the effect of diffuser shape on the velocity profiles across the diffuser passage. Published literature is severely limited in establishing the effects of the velocity profile asymmetry across the diffuser on the performance characteristics of the compressor stage. Hence, the present study focuses on using a well-validated Computational Fluid Dynamics tool to numerically simulate the flow within the diffuser of various shapes quantified in form of an asymmetric effect on the performance of the compressor stage. Both straight wall diffuser and diverged wall straight diffuser have been investigated in the present study. A full factorial based DoE have been incorporated whereby two factors (L/Lmax and b2/b1) have been selected respectively. Variations in flow related parameters within the diffuser have been discussed in detail for a wide range of geometrical parameters associated with the diffuser shape. It has been found in the analysis of this paper that flow across diffuser is highly asymmetric. Therefore, asymmetry of velocity profiles values has been used to predict the performance of the compressor stage as a function of radial and circumferential velocities across the diffuser. Furthermore, a novel semi-empirical prediction model has been developed to predict diffuser performance as a function of geometric and flow variables of the diffuser. The resulting diffuser map can be used for inverse design of diffuser for compressor stage as well.

Keywords

Computational Fluid Dynamics (CFD); Turbomachinery; Compressor stage; Vaneless diffuser; Asymmetric ratio

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