International journal of COMADEM Vol 22 No 02 (April 2019) p. Effects of asymmetric flow within vaneless diffuser on the performance characteristics of the compressor stage of a turbocharger Noukhez Ahmed a *, Taimoor Asim b , Rakesh Mishra c a Faculty of Science and Engineering, University of Wolverhampton, Wulfruna Street, Wolverhampton, UK WV1 1LY b School of Engineering, Robert Gordon University, Garthdee Road, Aberdeen, UK AB10 7GJ c School of Computing and Engineering, University of Huddersfield, Queensgate, Huddersfield, UK HD1 3DH  Corresponding author. Tel.: +44-190-232-2703; email: n.ahmed14@wlv.ac.uk 1. Introduction International legislation requires sustained lowering of the emissions and the fuel consumption in internal combustion engines. Reducing the fuel consumption without sacrificing the level of performance delivered by the current vehicles has become a challenge for automotive industries. Turbocharging is one of the solutions to reduce emission levels by utilising the exhaust gas energy. The turbocharger’s performance is explicitly dependent upon the compressor stage efficiency. The compressor stage efficiency is further dependent upon the flow characteristics within the different components of the compressor, especially the diffuser passage. Diffuser plays a significant role in increasing the stage efficiency. However, the flow variations within the diffuser are dependent upon the incoming flow from the impeller. Estimation of local flow variations across the diffuser is very difficult as these depend on the shape and number of blades used in the impeller. Currently the performance characteristics of compressor stage are defined by the total-to-total stage isentropic efficiency as per equation (1). It has been found in literature that the compressor stage components in general and diffuser in particular have impact on the total-to-total stage performance. η t = T o,in ×[PR C γ−1 γ −1] T o,out −T o,in (1) Japikse and Goebel [1] have carried out experimental studies to develop techniques in order to estimate the flow behaviour within the compressor stage, focusing on the diffuser passage. It has been found that when the flow enters a straight diffuser, it follows the hub wall of the passage at the impeller discharge due to the impeller passage direction. Backflow appears near the shroud wall of the diffuser passage. This flow behaviour reverses midway to the diffuser passage, whereby the flow attaches the shroud surface after the midway and backflow is obtained near the hub wall. Adachi et al. [2] have carried out experiments to improve the performance of the vaneless diffuser passage of a compressor stage. This study focused on the optimisation methods for annular vaneless diffusers for improving the overall compressor stage performance. The aim of this investigation was to improve both the surge margin and isentropic efficiency at COMADEM International Journal of Condition Monitoring and Diagnostic Engineering Management 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.