Whole-annulus unsteady flow analysis on a centrifugal impeller with the casing treatment

The whole-annulus unsteady flow analysis are carried out on three types of Krain impeller: one is without the casing treatment, and the other two are added the casing treatments with the circumferential or axial grooves respectively. The unsteady flow characteristics under the low flow rate condition are simulated by the "mis-staggered blade" method and the "downstream variable-area nozzle" method. The results show that, the rotating stall inception is not always predicted by the use of the two methods. The unsteady fluctuation frequencies of flow parameters are mainly equal to the Nth harmonic frequency of blade passing frequency. The casing treatment with the circumferential grooves reduces the fluctuation amplitude of the flow upstream of the impeller effectively, whilst that with the axial grooves reduces the flow fluctuation amplitude slightly. The fluid in the casing treatments returns to the impeller passage mostly near the blade leading edge, which is one of the reasons for enlarging the stall margin of impeller.