This paper describes the influence of a bent inlet pipe installed immediately upstream of a transonic centrifugal compressor on the aerodynamic performance and the stability. In order to clarify the influence of the bent inlet pipe, the internal flow fields in the inlet pipe, the impeller, and the diffuser of the compressor have been numerically investigated by a DES (Detached Eddy Simulation). For the purpose of comparison, the simulation was also conducted for the case of uniform axial inflow using a straight pipe. In order to make clear the influence of non-uniform flow with a bent pipe as far as possible, a 90-degree bent pipe was installed immediately upstream of the compressor, that is 0.86 times the inlet inner diameter.

In the case of installing the bent pipe on the upstream of the compressor, the pressure ratio decreased on the high flow rate side in the compressor performance characteristic, whereas it increased at the low flow rate side. At the low flow rate operating point, there is a reversed flow occurring in the compressor impeller on the shroud side near the blade leading-edge. Installation of the bent pipe promotes mixing between the reversed flow and the main flow at the inlet of the compressor thanks to occurrence of a secondary flow. Since the reversed flow comes out from inside of the impeller, it has a high circumferential velocity. Therefore, the mixing of the reversed flow makes the compressor inlet flow a pre-swirl flow, and thereby the incidence decreases. As a result, leading-edge separation on the blade tip side of the impeller is suppressed, and the flow field inside the impeller is improved. In the diffuser section, when the bent pipe is installed, the circumferential velocity of the impeller exit flow increases on the hub side, whereas the radial velocity decreases. As a result, the diffuser performance is deteriorated and the diffuser stall tends to occur.

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