An investigation into the effect of diffuser vane side clearance position on the efficiency and stability of an automotive turbocharger compressor

Variable geometry radial compressors have gained considerable attention in the automotive turbocharging industry due to their potential to improve performance at specific regions of the map. One such device is the on/off vaned diffuser in which a vaned diffuser is used to improve compressor efficiency at low engine speeds. To overcome the issue of a narrow map width due to choking in the vaned diffuser, the vanes are retracted from the main flow path when there is no longer a performance benefit. In order to allow the vanes to slide in to and out of the main flow path, a side clearance is required around the diffuser vanes and the stationary endwall. It has been shown experimentally in the open literature by Tange et al. (2003) that the side clearance can reduce the compressor efficiency by up to 3.5% pts when located at the hub. The aerodynamic mechanisms responsible for the reduced efficiency however are not well understood.

The purpose of this investigation is to describe the loss mechanisms responsible for reducing compressor efficiency when the side clearance is included, and how they change with endwall position. It will be shown that the positioning of the side clearance has a marked influence on compressor stability and peak efficiency, thus requiring careful consideration during the development process.