Characterising the Influence of Impeller Exit Recirculation on Centrifugal Compressor Work Input

Impeller recirculation is a loss which has long been considered in 1-D modelling, however the full extent of its impact on stage performance has not been analysed. Recirculation has traditionally been considered purely as a parasitic (or external) loss, i.e. one which absorbs work but does not contribute to total pressure rise across the stage. Having extensively analysed the impact of recirculation on the impeller exit flow field, it was possible to show that it has far reaching consequences beyond that of increasing total temperature. The overall aim of this package of work was to apply a much more physical treatment to the impact of impeller exit recirculation (and the aerodynamic blockage associated with it), and hence realise an improvement in the 1-D stage performance prediction of a number of turbocharger centrifugal compressors.
The factors influencing the presence and extent of this recirculation are numerous, requiring detailed investigations to successfully understand its sources and to characterise its extent. A combination of validated 3-D Computational Fluid Dynamics (CFD) data and gas stand test data for six automotive turbocharger compressor stages were employed to achieve this aim. In order to capture the variation of the blockage presented to the flow with both geometry and operating condition, an approach involving the impeller outlet to inlet area ratio and a novel flow coefficient term were employed. The resulting data permitted the generation of a single equation to represent the trailing edge blockage levels for the entire operating map of all six compressor stages under investigation.
With an understanding of the extent of the region of recirculating flow realised, and the key drivers leading to its creation identified, it was necessary to comprehend how the resulting blockage influenced compressor performance. Consideration was given to the impact on impeller work input through modification of the impeller exit velocity triangle, incorporating the introduction of the concept of an “aerodynamic meanline” to account for the reduction in the size of the active flow region due to the presence of blockage. The sensitivity of the stage to this change was then related back to the level of backsweep applied to impeller. As a result of this analysis, the improvement in the 1-D performance prediction of the aforementioned automotive turbocharger centrifugal compressor stages was presented and the results disseminated. In addition, a number of design recommendations were presented to ensure that the detrimental effects associated with the presence of impeller trailing edge recirculation could be minimised.