Optimization Of Cavitation Performance Of A Centrifugal Pump Based On The Impeller Passage Area Control

Cavitation is a kind of complicated and harmful physical phenomenon produced during the operation of centrifugal pump.It has the characteristics of phase transition and multi-dimensional turbulent flow.After cavitation occurs,holes produce strong interference in the flow field region,which can change the flow field structure such as pressure and velocity,thus changing the external characteristics of the centrifugal pump,and even affecting the normal operation of the system.Impeller is the core work component of centrifugal pump,its hydraulic structure plays a decisive role in the external characteristics of centrifugal pump.This paper starts with the hydraulic design of the impeller geometric parameters,takes the impeller passage area as the starting point,so that the impeller passage area presents different changes along the flow direction,studies the influence of the impeller passage area change on the cavitation performance and hydraulic performance of the centrifugal pump,discusses the feasibility of improving the cavitation performance and hydraulic performance of the impeller at the same time,and provides theoretical guidance for the hydraulic optimization of the centrifugal pump impeller.In this paper,ICEM CFD software was applied to divide the computing domain into hexahedral grids,and the near-wall region was encrypted.The height of the first layer of grids was set as 0.35 mm,so as to ensure that the y~+value of the near-wall could meet the requirements of the turbulence model.Based on RNG k-εturbulence model,ANSYS Fluent software was used to calculate the hydraulic performance and cavitation performance of the original model pump,and compared with the test data.The numerical calculation of the hydraulic performance of the pump in the original scheme model is unsteady.In the range of 0.2Q_d～1.2Q_dworking conditions,the relative errors of the calculated values of efficiency and head are kept within 5%.In the design working conditions,the calculated errors of head and efficiency are 2.88%and 0.20%respectively.Schnerr-Sauer cavitation model with higher calculation accuracy was selected for numerical simulation.The absolute error between the calculated value and the test value of the critical cavitation margin was 0.15 m.The above numerical calculation methods are adopted in the follow-up studies of this paper.When the inlet and outlet area of the centrifugal pump impeller passage remains constant,the increase rate of the section area of the control runner along the flow direction presents three changing trends:first fast and then slow,linear increase,first slow and then fast.The influence of different changing trends of the section area of the flow channel on the cavitation performance and hydraulic performance of the pump is explored.The results show that the cavitation performance and hydraulic performance of the pump are the best when the change trend of the section area is first fast and then slow,while the pump first slow and then fast are the worst.At the design condition point,compared with the area change trend of first slow and then fast,the critical cavitation margin of first fast and then slow change trend is reduced by 7.79%,and the efficiency is increased by 1.87 percentage points.The comprehensive performance improvement effect of the pump is obvious,indicating that when the section area of the impeller passage increases rapidly along the flow direction in the first half and slowly in the second half,The cavitation performance and hydraulic performance of the pump can be improved at the same time,which lays a good foundation for the impeller to maintain high hydraulic efficiency and further improve the cavitation performance.On the basis of the overall changing trend of the section area of the impeller passage,the outlet area of the impeller is kept unchanged,and the throat area of the inlet is controlled by changing the blade envelope Angle or changing the diameter of the inlet of the impeller.The influence of the change of the throat area on the cavitation performance and hydraulic performance of the centrifugal pump is discussed.The results show that the inlet throat area is positively correlated with the cavitation performance of the centrifugal pump.The increase of the throat area reduces the critical cavitation margin of the pump,improves the fracture head when the cavitation is severe,and effectively improves the cavitation performance of the centrifugal pump.The influence of the throat area on the efficiency is also obvious.The hydraulic efficiency of the pump is the highest only when the section area of the flow channel along the flow direction shows a trend of rapid increase in the first half of the flow and basically unchanged in the second half of the flow.With the increase of throat area,the inhibition effect is strongest in the severe stage of cavitation,which can limit the expansion of cavitation distribution range,reduce the volume of cavitation and optimize the flow field structure.For the initial stage of cavitation,the inhibition effect is not obvious,and at this stage,increasing the throat area of the passage inlet by increasing the diameter of the impeller inlet will even expand the cavitation coverage area of the suction surface of the blade inlet side,so that cavitation occurs in advance.Finally,in order to further explore the influence of the change of impeller passage outlet area on the cavitation performance and hydraulic performance of the centrifugal pump,the throat area of the passage inlet remains unchanged.Based on the overall change trend of the section area of the impeller passage,the area of the middle and rear of the passage is controlled by changing the blade envelope Angle or changing the width of the impeller outlet.The results show that:The increase of the rear area of the impeller passage can improve the flow field structure when cavitation occurs,increase the absolute pressure in the impeller at each stage of cavitation and inhibit the diffusion of the low pressure region,so that the critical cavitation margin of the centrifugal pump decreases,the fracture head gradually increases,and the cavitation property of the centrifugal pump is effectively improved.In addition,the larger the rear area of the impeller passage,the more obvious the effect of improving the cavitation performance.Regardless of the size of the flow channel area,as long as the changing trend of the section area along the flow direction shows that the first half of the flow rapidly increases and the second half of the flow basically remains unchanged,the hydraulic efficiency of the pump can maintain a relatively high level on the whole,and with the increase of the overall section area,the efficiency value also increases slightly.To sum up,as long as the cross-section area of the impeller passage is increased,the cavitation performance of the pump will be improved;When the cross section area of the impeller passage changes along the flow direction,it increases rapidly in the first half of the flow,and basically remains unchanged in the second half of the flow,the hydraulic efficiency of the pump will maintain a high level.In view of this,based on the hydraulic design of the original scheme,the method of co-modifying the blade envelope Angle,impeller inlet diameter and impeller outlet width is adopted to control the section area of the flow passage to meet the above area variation characteristics of the impeller with high efficiency and high cavitation resistance.The numerical simulation results show that:At the design condition point,the head of the comprehensive optimization scheme is increased by 3.35%compared with the original scheme,and the efficiency is only reduced by 0.3 percentage points compared with the original scheme,but the critical cavitation margin is reduced by 12.21%.The specific cavitation speed is increased from 1382 before optimization to 1523 after optimization,and the cavitation performance is obviously improved,but the efficiency cost is very small.If the traditional method of only increasing the inlet diameter of the impeller is adopted,the critical cavitation allowance is only reduced by 8.45%compared with the original scheme,the cavitation specific speed is only increased to 1476,and the efficiency is reduced by 1.01 percentage points compared with the original scheme.It can be seen that the hydraulic design optimization theory of centrifugal pump impeller adopted in this paper can comprehensively improve the performance of centrifugal pump,and the relevant methods for the optimization of cavitation performance of centrifugal pump based on the control of impeller passage area are reliable and feasible,and can provide a new theoretical and method reference for improving the cavitation performance of centrifugal pump.