Study On Stability Of Head-flow Curve Of Centrifugal Pump

In order to improve the efficiency of low specific speed centrifugal pumps,the outlet angle of impeller blades is usually increased in the design stage.However,it is easy to hump the head-flow curve of the pump,leading to unstable operation of the pump in the small flow area and even causing shutdown in production.At present,scholars at home and abroad have proposed many measures to eliminate the hump of the head-flow curve of centrifugal pumps,but the mechanism of these measures to improve the stability of the head-flow curve is still unclear,and the effect of their combined action has not been reported.The main work and research results are as follows:1.Taking a single-stage single-suction centrifugal pump with a specific rotation speed of 47 produced by a domestic water pump factory(the H-Q performance curve of the pump has obvious hump under small flow conditions)as the research object,the orthogonal test method is adopted to select five factors such as impeller outlet chamfer ratio,outlet angle,impeller axis inclination angle,blade wrap angle and blade inlet edge position,each factor adopts four levels,a total of 16 impeller design schemes,aiming at each scheme,The Fluent17.0 software is used to carry out numerical simulation,and the external characteristic curve of each scheme is obtained.The range analysis method is used to study the influence of five factors on the shape of H-Q characteristic curve and the head and efficiency of the pump under the rated working condition.The primary and secondary order of factors influencing the stability of characteristic curve is the position of blade inlet edge,oblique cutting of blade outlet edge,inclination of blade axis,outlet angle and blade wrap angle.Based on the analysis results,the optimal combination scheme of impeller is determined and verified by numerical simulation.The simulation results show that compared with the original design scheme,the hump of the head-flow curve under the condition of small flow is completely eliminated,the stability is strong,and the head and efficiency of the pump are increased by 2.2% and 1.4% respectively.2.In order to explore the mechanism of eliminating hump by various factors and compare the internal flow field of the impeller before and after optimization,the loss at the inlet of the original model pump impeller near the hump condition is found to dominate through quantitative analysis,and it is found that the forward movement of the inlet edge of the impeller blade will lead to the enhancement of the pre-rotation of the internal fluid in the pump suction pipe.The oblique cutting of the impeller outlet edge and the change of the axial surface shape reduce the change of the relativevelocity in the circumferential direction at the outlet,the phenomenon of "jet-wake" weakens or even disappears,the loss decreases under the condition of small flow rate,the hump of the head-flow curve disappears,and the pump runs more stably.3.By analyzing the theoretical head-flow curve of the centrifugal pump with pre-rotation at the inlet and the losses of each part in the pump,the actual head-flow curve with pre-rotation at the inlet of the centrifugal pump is obtained.Analysis shows that when the pre-swirl angle of the impeller inlet fluid is positive,the theoretical head-flow curve becomes steeper and the stability increases.The criterion of hump or not in the head-flow curve of centrifugal pump is obtained,and the influence of pump geometry parameters on the stability of the head-flow curve is preliminarily analyzed.The accuracy of the criterion is verified by a centrifugal pump with hump in the head-flow curve with a specific speed of 90 produced by a domestic pump factory.4.In order to study the influence of impeller outlet chamfering on the stability of centrifugal pump head-flow curve,the stability of centrifugal pump head-flow curve under different chamfering ratios is compared,and the best outlet chamfering ratio is obtained.By analyzing the hydraulic loss at the outlet of the impeller,it is concluded that the mechanism of eliminating hump by this measure is that the rear cover plate is cut off at the outlet of the impeller,which leads to weakening of the working force of the blade on one side of the rear cover plate,deflection of the discharged fluid to the front cover plate,and reduction of the change of the relative velocity in the circumferential direction at the outlet.The phenomenon of "jet-wake" is weakened or even disappeared.However,if the cutting amount is too large,the phenomenon of "jet-wake" will occur again,the loss increases,and the head-flow curve will lose stability.