Impact of the inducer meridional length on the tandem impeller performance within a transonic centrifugal compressor

Tandem impeller configuration has the potential to improve the performance of highly loaded transonic centrifugal compressors, while the related research is still in the early stages and general design guidelines, such as the selection of inducer meridional length, are limited so far. In this paper, a highly loaded transonic centrifugal compressor with various tandem impeller configurations is numerically investigated. The objective is to evaluate the impact of inducer meridional length xi on the tandem impeller flow structure and performance, and further to look into the underlying flow mechanisms and design principles of the tandem impeller. The results indicate that the inducer meridional length xi has significant effect on the tandem impeller and compressor stage performance. There exits an optimized tandem impeller xi value of Appr. 38% that achieves a compressor stage pressure ratio and efficiency gain (at the P.E. points) of 2.6% and 1.8%, respectively, and deviating from the optimized xi value will lead to deteriorated compressor performance. In terms of the impeller performance, the inducer trailing edge determined by the inducer meridional length xi should be located downstream of the passage shock to reduce the shock and inducer wake losses. However, the inducer blade should also be placed in the impeller axial part to avoid the exudcer incidence angle deviation. In terms of the diffuser performance which is partly determined by the impeller discharge flow uniformity, the inducer meridional length xi directly affects the tandem impeller discharge flow uniformity by rearranging and controlling the secondary flow patterns, and the flow uniformity is prone to be insensitive to the xi variation when the xi value is large enough (i.e. >= 30%). Additionally, the xi value should be large enough (i.e. >= 20%) to generate intense inducer shedding vortexes, which can suppress the development of Coriolis vortex and finally improve the impeller flow uniformity.