Effect of inlet guide vanes prewhirl on diffuser inlet unsteady flow in a centrifugal compressor stage

The effects of inlet guide vanes (IGV) prewhirl angles on the diffuser inlet flow filed in a centrifugal compressor were experimentally investigated under design condition by using the X-type hot-film anemometry. The results show that the ensemble-average velocity varies significantly at different IGV prewhirl angles, and the variation patterns are different in spanwise direction. At 10% spanwise position, the ensemble-average attack angle is negative under all three IGV prewhirl angles; at 50% spanwise position, this angle is negative under -20 degree and 0 degree prewhirl angles, while it is positive close to the main blade wake region under 20 degree prewhirl angle; at 90% spanwise position, the ensemble-average absolute flow angle is almost negative. Under three IGV prewhirl angles, the variations of time-average absolute flow angle are respectively 1.4, 2.2 and 1.3 degree at 10%, 50% and 90% spanwise positions, which indicates that the IGV prewhirl angle has great impact on the diffuser inlet flow angle, and this impact is more remarkable at mid-span than that close to hub and shroud; however, the variations of time-averaged random unsteady fluctuation flow angle at three spanwise positions are all less than 0.6 degree, which are relatively small. Due to the large variation of the absolute outflow angle, the twisting of the diffuser vane leading edge along the axial direction was recommended.