Numerical And Experimental Investigation On Cavitating Flow In An Mixed-flow Pump With Vaned Diffuser

When cavitation occurs,not only the main flow in the impeller will be affected,but also the pump performances of the mixed-flow pump deteriorated.When the cavitation collapses,the main flow path in the pump will be changed and the flow components will be destroyed.In some certain conditions,it may even cause resonance to the pump and the pipe system resulting in catastrophic consequences.This phenomenon becomes a seriouse threaten to the safe,efficient and stable operation of the mixed-flow pump unit,and has also become an important factor and technical bottleneck during the design of the mixed-flow pump in the practical engineering applications.In this work,a model mixed-flow pump with a specific speed of 300 was selected.The cavitation characteristics of the mixed-flow pump with vaned diffuser was studied based on the method of numerical simulation combined with the corresponding experimental verification in order to explore the influence of cavitation on the flow in the pump.The main research work are as follows:1.The geometrical structure and feature of the mixed-flow pump was analyzed,and the purpose of research on its cavitation characteristics were demonstrated.The current research background both at home and abroad on the hydraulic design,the cavitation mechanism as well as the numerical prediction of the cavitation performances of the mixed-flow pump were briefly reviewed.2.The steady numerical simulation of the inner flow in the mixed-flow pump at different flow rates was carried out based on the commercial software ANSYS CFX.Meanwhile,a series tests of the hydraulic performance and cavitation performance have been carried out to be in comparison with the corresponding numerical data in order to to verify the numerical accuracy in present study as well.The main summery was as follows:the head-flow curve predicted by numerical simulation has the same trend of the variation with that obtained from the experimental test data,and both of them keep consistent among all flow rates.The error between the numerical and test data falls in the allowable range of the project,and the head deviation of the optimal operating point is 4.35%,with2.78%as the efficiency deviation.It can be indicated from above that the numerical method provided in this work can predict the hydraulic performance of the model mixed-flow pump and has been verified with a certain accuracy.3.The steady cavitating flow in the mixed-flow pump has been investigated based on numerical simulations.Meanwhile,the cavitation performances of the mixed-flow pump as well as the cavitation distribution in the impeller were obtained and compared with the corresponding experimental results.The main results were as follows:The cavitation performance curves of the mixed-flow pump predicted by numerical simulation under different working conditions have the same trend of variation with the corresponding experimental values,and the cavitation number corresponding to 1%reduction of the pump efficiency decreases first and then increases as the flow rate increases.Especially before the efficiency of the mixed-flow pump drops by 1%,the two head-cavitation number curves are more consistent.However,as the cavitation number decreases,the cavity volume increases in the pump,and there is an obvious difference between the two curves as mentioned with errors whose values increases with the increase of the flow rate.The absolute error of the inception cavitation number between the predicted data and its experimental value under different flow conditions is in the range between 0.03 and 0.06,which indicates that the numerical simulation method of predicting cavitation flow applied in this study can to some certain predict the efficiency drop of the mixed-flow pump when cavitation occurs.4.Based on the numerical prediction of cavitating flow in the mixed-flow pump,the cavitation distribution in the blade passage of vane-type mixed-flow pump under different cavitation condition was analyzed and also compared with the test,and it can be obtained as follows:a uniform cavitation is symmetrically distributed in each blade passage,and as the cavitation inception happens,a small quantity of the cavity volume was found on the leading edge of the blade.When the pressure in the pump continue to decrease and the cavitation number also decreases,the volume of the cavitation in the impeller is gradually increased along on the suction side of the blade towards the blade outlet,and gradually to the adjacent blade surfaces.It has been found that there is a few cavitation distributed in the blade pressure side even when cavitation number becomes very small.5.Based on the numerical results of steady cavitating flow in the mixed-flow pump,the unsteady characteristics of the inner flow in the mixed-flow pump running under four different cavitation working conditions was analyzed by applying the unsteady numerical method,and the pressure fluctuations in the cavitating pump by setting the monitoring points along different radius of the impeller inlet and outlet were studied.The variation of internal cavitating flow patterns in the impeller with time were analyzed,and both the time and frequency domain analysis of the pressure fluctuation under different cavitation status were carried out.The main results are as follows:when cavitation number is very small,the vapor volume fraction,the position and shape of the cavity cloud in the impeller shows a slight variation.The process of precipitation and collapse of cavitation is relatively regular,and the variation of flow field characteristics during one rotating circle of the impeller is small.The time-domain curves of pressure pulsation at the inlet and outlet of the impeller show a regular change with time under different working conditions.At the impeller inlet,the main frequency of the monitoring points near the blade hub is the shaft frequency f_n,and the main frequency near the blade leading edge is the blade passing frequency f_BPF.Furthermore,at the impeller outlet,the main frequency of the pressure fluctuation is the blade passing frequency f_BPF or 2f_BPF.The main frequency amplitude of the pressure fluctuation at the impeller inlet increases rapidly and then decreases gradually as the cavitation number decreases but can reach to its maximum value at the initial cavitation condition.