Casing treatment is a widely employed technique to increase the stall margins of turbomachineries. In the last several decades,many researches on casing treatment have been carried out. However,the mechanism of its expanding stall margins is still not very clear. Till now,most casing teatment schemes are designed for axial compressors,while the investigations on casing treatments in centrifugal compressors are rarely reported. Moreover,current investigation methods mainly focus on experiments,and perfect theoretical analysis is not yet feasible. In order to study the effectiveness and further the mechanism of casing treatments in centrifugal compressors,in this paper,a computationally based investigation of the impact of the self-recirculating casing treatment on the performance of a radial compressor is carried out. The results indicate that,by casing bleed and injection,the casing treatment with inclined blades in the cavity expands the stall margin most. At low mass flows,the reversed flow through the cavity with inclined blades develops the counter swirl flow in front of the impeller inlet,which is considered to benefit increasing the pressure rise from the injection port to the bleed port and thereby augment the recirculating flow. At 120% design speed,the stall margin is larger than that at the design speed. However,the cost of extending the stall margin is the reduction of isentropic efficiency. A mended casing treatment by shifting the bleed port upstream is also studied. It is demonstrated that,relative to the original casing treatment,this mend can improve the efficiency evidently notwithstanding a little narrowing of the flow range.
As demonstrated by former work,the holed casing treatment can be used to expand the stall margin of a centrifugal compressor with unshrouded impeller.In addition,the choked margin can also be expanded as experimental results indicated.Moreover,the compressor performance,especially the efficiency,on the whole working range is improved.As shown by experiments,the stall margin and choked margin of the compressor are extended,and the maximum efficiency improvement is 14%at the large flowrate of 1.386.Numerical simulations were carried out to analyze the flow in the impeller and in the holes in the case of large flowrate.The results indicate that in large flowrate conditions,there is a low-pressure region on the throat part of the impeller passage,leading to the bypass flows appearing in the holes,which means the flow area at the inlet of the impeller is increased.The bypass flow can also contribute to the decrease of the Mach number at the throat part near the shroud end-wall which implies that the choked margin is expanded.Besides,as the bypass flow would inhibit the development of the vertexes in the tip clearance and suppress the flow recirculation in the shroud end-wall region,both the pressure ratio and efficiency of the compressor are improved,which agrees well with the experiments.
