Analysis of Diffusers With Different Number of Vanes in a Centrifugal Compressor Stage

Abstract

In this paper, a computational analysis of a high-speed centrifugal compressor stage for turbocharger applications is presented. Emphasis is focused on the effect of different number of diffuser vanes, and for this reason four different designs of the vaned diffuser are analysed. The first three of the diffusers consist of 11, 22 and 33 vanes, respectively, with their leading edge at a radius of 1.075 times the radius of the impeller tip. The fourth one consists of 22 vanes with its leading edge at 1.15 times the radius of the impeller tip. All the above vane designs are of double circular arc shape. A steady CFD analysis is carried out using the Reynolds-Averaged Navier-Stokes solver TASCflow at design and off-design operating conditions. An averaging approach is used at the interface between the impeller and the diffuser. A detailed comparison between the predicted and the available experimental data is performed in terms of pressure rise and efficiency characteristics, and very good agreement is accomplished. In addition, detailed flow distributions are compared and critically analysed. One of the most important conclusions is that while maintaining the overall throat area and the location of the leading and trailing edges of the diffuser, as the number of diffuser vanes increases, the pressure recovery coefficient at the semi-vaneless space at surge condition was found to reduce, the wake pattern becomes more pronounced and the velocity distribution at vaneless and semi-vaneless space becomes more distorted when passing the same mass flow rate and therefore the diffuser has a narrower flow range. On the other hand, it was found that the diffuser outlet to throat area ratio is not the dominant factor to influence the flow range when the number of vanes changes.