Effect of bend-induced inlet distortion on a centrifugal compressor performance and stall inception

Bend pipes are usually installed upstream of a centrifugal compressor because of spatial constraints. With the interaction between radial pressure gradient and centrifugal force in bend pipe, a pair of counter-rotating vortices (Dean vortices) is formed after the fluids passed through the bend, and the shape of Dean vortices keeps changing in the straight pipe between the bend and compressor. The effects of the changed Dean vortices on the aerodynamic performance and stall inception of a centrifugal compressor are investigated with an experimentally validated numerical method based on four models, in which the distance of the straight pipe located between the bend and the compressor varies from 0D to 1D through 1/4D and 1/2D. The results show that as the straight pipe length increases from 0D to 1D, both the total pressure deficit at compressor inlet and the flow separations at inlet pipe symmetry plane are enlarged, and the mass flow rate distribution of full-annulus become more fluctuant. As a consequence, the compressor aerodynamic performance degrades and its maximum flow capacity decreases. More importantly, bend-induced inlet distortion has the effect of inducing/suppressing impeller stall inception, it depends on the location and intensity of swirl distortion and total pressure distortion. The low total pressure region located at impeller tip leads to reversed flow and stall inception, while the positive pre-swirl component in swirl distortion decreases the incidence and suppresses stall inception. These results provide a reference for the design of upstream inlet curved pipes of centrifugal compressors.