The Mechanism Study Of Flow Loss In Transonic Centrifugal Impeller

Centrifugal compressor is a kind of general centrifugal turbomachinery,which can convert mechanical energy into gas energy by the high-speed rotating impeller,achieving the gas compression process.The centrifugal impeller as the core power component in centrifugal compressor,playing an important role in the aerospace,energy & power and metallurgy industries,is the key factor of aerodynamic performance of compressor,thus the characteristics of airflow in the impeller flow passage become one of the most attractive topics in this field.In transonic impeller,there are complex flow structures in flow passage,like shock wave,tip clearance leakage flow,boundary layer separation and so on,meanwhile,the interaction between these structures is influencing the whole flow.In this paper,the numerical simulation method was applied to calculate a series of unshroud impellers with different clearance values under different working conditions,which were formed by cutting the blade tip with equal ratio,to ensure the flow loss mechanism in centrifugal impeller under the transonic condition,and the effect of different tip clearance on the aerodynamic performance of impeller and flow field.First of all,the numerical simulation method was applied to calculate shroud impeller and unshroud impellers,formed by cutting the blade tip with equal ratio,operated in transonic condition,and the influence of key factors,such as blade loading,tip clearance leakage flow,shock wave,on flow performance were analyzed,at the same time the comparison with subsonic flow was made accordingly.In transonic centrifugal impeller,the tip clearance leakage distribution is not only related to the blade loading,but controlled by the blade loading and the scrapping pressure head.Through the verification and calculation of the shock wave,it is obtained that despite the shock wave exists in transonic impeller and the shock wave position also changes,but under the condition of Mach number less than 1.3,the shock wave almost does not cause energy loss.The tip clearance effect on unshroud centrifugal impeller with different tip values was mainly analyzed.Increasing tip clearance from 0% to 7.5%,the impeller aerodynamic performances were compared and the mechanism of the effect of tip clearances was analyzed in transonic operating condition.It is found that in transonic condition,the operating range of the shroud impeller will significantly narrow down and the impeller performance also decreases due to the boundary layer separation at the second half of flow passage.The tip leakage rate of the unshroud impellers with different tip clearance is strictly controlled by the blade loading and the scrapping pressure head.In addition,according to the distortion effect of the tip cross-flow acting on the main flow,the aerodynamic performance of the unshrouded impellers can be divided into two parts: higher or lower than that of the shrouded impeller,the cut-off point is of the 3% ratio value of tip clearance.