Numerical Study On The Weakly Compressible Flow Characteristics Of Centrifugal Impeller

As a high-speed rotating impeller machine,the fluid in the centrifugal pump exhibits weakly compressible characteristics.The traditional turbulence model uses the incompressible assumption to simplify the N-S equation due to the change in density is negligible for the pressure change is too small.In fact,the fluid has a certain elasticity,and it shows weakly compressible properties in actual flow.Studies have shown that the use of weakly compressible models can reflect the real flow state at low Mach numbers,which is beneficial to the analysis of the internal flow characteristics of the impeller machinery.In this paper,the weakly compressible flow equation is combined with the standard k-? model and DES(Detached eddy simulation)model to reveal the flow characteristics in the centrifugal pump clear.The main work and research results are as follows:(1)The weakly compressible model is used to simulate the internal flow of the multi-elbow pipeline.The results show that the weakly compressible model can be used to calculate the negative pressure wave transmission phenomenon inside the pipeline,and its transmission is consistent with the experiment.Through the analysis of the front surface of the negative pressure wave transmission,the interaction between vortex and wave in the elbow is found.Combined with the analysis of turbulent viscosity and turbulent frequency,it is revealed that the high viscous force and pulsation frequency in the elbow accelerate the volume compression and expansion of the flow element,causing the transmission speed of the negative pressure wave in the elbow to be greater than that of the straight pipe.We analyzes the influence of each elbow parameter on the transmission characteristics of the negative pressure wave by control group,and proposes a formula for calculating the equivalent length of the elbow in the negative pressure wave leakage positioning method,which further verifies the applicability of the weakly compressible model.(2)The weakly compressible model was applied to the steady numerical simulation of the centrifugal impeller,and the head results of the weakly compressible model,incompressible model and experiment were compared.The results show that the head prediction accuracy of the weakly compressible model is higher than that of the incompressible model,and the accuracy improvement is greatest at 0.8Qd,which is 0.73%.Comparing the information of internal flow field,it is found that the area of the minimum pressure and the dimensionless quantity of the weakly compressible model at the inlet of the flow passage and the front end of the blade pressure surface is large under a small flow condition(0.3Qd,0.6Qd,0.8Qd).The area of maximum pressure gradient along the flow direction in the flow channel is small and closely distributed to the wall of blade,which has a great influence on the vortex growth and energy transportation of the blade surface and exit of the flow passage,resulting in a complicated vortex structure and large energy loss in the impeller.Through calculation and analysis of the effect of different speeds on the weakly compressible characteristics,it was found that the error of computational head was less than 4%when the speed was 1000r/min and 1200r/min,and the density and pressure distribution of the impeller center area were significantly different,and the internal flow was relatively stable.(3)Combining the DES turbulence model and the weakly compressible model,unsteady numercial simulation is used in the centrifugal impeller in low flow rate,and the change of the internal flow field information of the impeller and the pressure fluctuation at the exit and its extension of the flow passage are analyzed within a rotation period.The results show that the weakly compressible model has more flow vortex structures than the incompressible model,and the Q value,pressure gradient and enstrophy distribution on the blade wall surface,flow channel outlet change greatly with time.The increase of the enstrophy in the weakly compressible model leads to a greater loss of mechanical energy,which will affect the reduction of the head.Moreover,the increase of the vortex makes more unstable flow structure,and intensifies the pressure fluctuation at the exit of the flow passage,the maximum static pressure difference reaches 700Pa,and the amplitude of the pressure pulsation at the axial frequency is 80;The pressure fluctuation amplitude at the axial frequency of exit extension of the flow passage is larger,and the flow separation phenomenon predicted by the static pressure coefficient is more obvious.