Numerical investigation of shock effects on performance and flow field in a transonic centrifugal impeller

In transonic centrifugal compressors, the losses and blockage are increased dominantly by the shock, shock/tip leakage vortex (TLV) interaction and shock/boundary layers interaction on the blade suction side. Therefore, a detailed investigation of shock effects is seriously important to improve the aerodynamic performance and extend the stable operating range of a centrifugal compressor. This paper firstly analyzes the shock structures in the transonic impeller at the design and off-design conditions. It is found that the shock structure is three-dimensional and swept along the span, with the shock becoming normal to the shroud near the blade tip. As the mass flow rate decreases, the swept shock moves upstream, and the shock/TLV interaction becomes stronger, inducing the vortex breakdown at the near stall condition. Then a comparative study is conducted on the flow structures in the transonic centrifugal impeller without the tip clearance (OTC impeller). Compared with the impeller with a tip clearance (WTC impeller), the splitter shock strength in the passage between the main blade and splitter is enhanced and the separation caused by the splitter shock /boundary layers interaction is induced, due to the lack of the blockage in the inducer induced by the shock/TLV of main blade interaction in the WTC impeller. Thus, at a large mass flow rate, the efficiency of the OTC impeller is lower than that of the WTC impeller because the stronger splitter shock induces larger flow separation in the passages between the main blade and splitter. However, at a small mass flow rate, the WTC impeller has a lower efficiency because the losses in the inducer induced by the interaction between the shock and the TLV of main blade are dominant.