Numerical Investigation On The Tip Clearance Flow Of Unshroud Centrifugal Impeller

Centrifugal compressor is used in turbochargers and turbine engines because oftheir high efficiency, high velocity and wide operating range. The aerodynamicperformance of centrifugal impeller decides the efficiency of centrifugal compressoras one of the main working parts. In the recent years, the unshroud impeller is widelyused in petrochemical and metallurgy engineering due to its high pressure ratio andcircumference velocity. However, the tip clearance flow of unshroud impellers willgenerate amount of energy loss. The interaction of tip leakage flow with secondaryflow and boundary layer flow take on a significant effect on the aerodynamicperformance and the exit flow of impeller. In order to understand the tip clearanceeffects on the aerodynamic performance, the simulation method is applied to analysisthe change of impeller performance parameters as efficiency, pressure ratio, bladeloading distribution，tip leakage, static entropy and entropy production of impellerbetween0and10%relative tip clearances.By the numerical simulation and analysis of the typical centrifugal impeller, it isfound that the impeller efficiency and the pressure ratio are linearly decreased withthe tip clearance increasing. A close correlation between the blade loading and the tipleakage flow distribution along the streamwise is obtained. This result shows the tipleakage flow rate is controlled by blade loading. In design flow condition, the tipclearance flow has a major effect on the static pressure distribution of latter half flowchannel. At off-design flow rate, the flow closed the leading edge is also changed bytip clearance. With the tip clearance increasing, there exists a significant separationflow area between50%and90%meridional chord, where the static pressure appearsrapidly drop down at the gap inlet closed the pressure side due to the strongacceleration of the tip gap flow from down blade. The result indicates that the energyloss generated by clearance flow’s self is not significant, and the interaction flow oftip clearance vortex with downstream main flow is the major factor leading toreduction the efficiency of the impeller.The results will provide a useful help to design and improve the performance ofcentrifugal impellers.