Flow Mechanism In Unshrouded Centrifugal Impeller With Casing Treatment And Changing Back Pressure

Centrifugal Impeller is a kind of turbomachinery and has been used widely in many economic sectors.Under small flow rate working condition,unsteady flow phenomenon might appear such as stall and surge.Sometimes it can even cause serious accidents.For many years,numerous experiments have been conducted on centrifugal impeller to study its complicated internal flow.However,limited by accuracy of instrument and time duration of experiment,this method still has its shortcoming and cannot fully reveal the flow mechanism.According to this,the author suggest that numerical simulation with multi-passage model might be used.The content of the thesis is listed below.1)Single passage model was used to verify the numerical simulation and analysis method.2)Numerical simulation was performed with multi-passage model.Working condition with changing back pressure was studied.Periodic flow structure was found in the middle of the impeller.Unsteady flow mechanism was demonstrated.3)The working condition of recirculation holes was analyzed.Based on these work,the following conclusion was achieved.1)Both single passage model and four passage model were used.Steady and Unsteady simulation were performed.By comparison, multi-passage model simulation can capture more flow detail especially in circumferential direction.As far as characteristic curve was concerned,these two models yielded similar results.Single passage model can save computational time.2)Relative vorticity strength was suggested to study the flow.Periodic structure was found in the middle of the impeller.The frequency can be got by FFT of the pressure signal near shround.3)By comparing pressure fluctuation on pressure side and suction side at different blade height,the pressure distribution showed great difference.On the pressure side,the strongest fluctuation was near the middle of the blade and can exceed 3% of the average value.On the suction side,however,the most unsteady region was near the impeller exit and the hub.The amplitude can be as large as 6%.The discrepancy between the pressure fluctuation and space average value is larger than that to the time average value.And this suggests that researchers should pay more attention to the flow difference in circumferential direction.Single passage model is far from enough to reveal the flow mechanism in centrifugal compressor.4)Under small flow rate working condition,the mixing of tip leakage flow and main flow was strengthened.The effective flow region decreased.The tip leakage flow might flow across the adjacent passage and circulated in the tip region in circumferential direction. This direction is the same with the propagation direction of rotating stall.5)Numerical simulation on impeller without casing treatment was performed with changing back pressure.The slope of the pressure curve changed obviously.Joint time frequency analysis was used. When the slope changed,the band width of the low frequency components increased.When the compressor reached the maximum pressure ratio,new frequency component was found.6)Numerical simulation on impeller with casing treatment was performed with changing back pressure.When the back pressure decrease linearly,the working condition of the recirculation holes changed.Under 95% design flow rate working condition,the injection flow rate is 0.19% of the main flow.Under 85% design flow rate working condition,the recirculation flow rate is 1%.Circumferential difference was also observed.In the whole flow rate decreasing process,the flow condition in the casing changed dramatically while the flow condition in the impeller did not show great difference.