Numerical Simulation And Experimental Study On Transient Cavitation Characteristics Of Mixed-Flow Pump During Startup

In recent years,in the field of military equipment,mixed-flow pumps have been used in the underwater launch weapon systems of our naval submarines.During the launch of the weapon,the parameters such as speed,head and flowrate of the mixed-flow pump drastically changed in a short time,resulting in very strong transient flow phenomena such as local negative pressure and cavitation.Once a serious cavitation happens during the startup of the mixed-flow pump,which can cause vibration and noise in the transmitter group and threaten the submarine's survival.This paper relies on the National Natural Science Foundation of China“Study on the Transient Cavitation Patterns and Induced Hydraulic Oscillation in a Mixed-Flow Pumps at Accelerated Flow Conditions?51579118?”.It is aimed at the transient cavitation characteristics during the start-up of mixed-flow pumps,with specific speed n_s=829 prototype pump was proportionally scaled to obtain a high specific speed mixed flow pump with a 1.5-fold ratio.The in-depth study of the transient cavitation characteristics during the startup of the mixed-flow pump was conducted.The related research results can provide the theoretical basis for the active control method of cavitation in the transient working process of the mixed-flow pump and related design optimization.And the results and conclusions achieved are as follows:?1?The steady-state numerical simulation of the mixed-flow pump under different flow conditions was performed.The head and efficiency curves of the mixed-flow pump were obtained.It was found that the efficiency of the mixed-flow pump reached the highest around the 0.96Q_optpt operating point,which was about 77%.The steady-state cavitation characteristics of the mixed-flow pump under rated conditions are numerically simulated,and the cavitation characteristics curve of the mixed-flow pump under rated conditions was obtained.It was concluded that the NPSH_r of the mixed-flow pump under rated flow was equal to 4.41 m.?2?The start-up of the mixed-flow pump under different start-up time?T_s=10s,15s and 20s?and different flow conditions(Q=0.8Q_opt,1.0Q_opt and 1.2Q_opt)without cavitation was numerically simulated.And the variation of the external characteristics of the mixed-flow pump and the distribution of the internal pressure,velocity field and turbulent kinetic energy in the start-up process under different starting time and flow conditions were obtained.It was found that the area with larger turbulent kinetic energy was concentrated at the tip region of the impeller,indicating that the mixed-flow pump's tip region was prone to unstable flow and should be noted during design.?3?For the transient cavitation characteristics during the start-up of the mixed-flow pump,numerical simulations of the transient cavitation characteristics of the start-up process under different start-up time?T_s=10s,15s and 20s?,different inlet pressures?30kPa¹00kPa?and different flow conditions(Q=0.8Q_opt,1.0Q_opt and 1.2Q_opt)were carried out respectively,in which the inlet pressure was 10kPa decreased each time and the amplitude is the same as that of the high-speed photography test.During the initial stage of the startup,the primary cavitation of the mixed-flow pump is mainly composed of vortex cavitation caused by the tip leakage vortex and clearance adhering cavitation caused by the tip leakage flow.With the increase of the rotation speed,the cavities continue to develop toward the entrainment zone and eventually join together to form a cloud of cavities resembling geometric triangles.According to the distribution of the volume fraction of cavitation bubbles among the blades,the triangular vesiculated cloud consists of three parts:tip vortex cavitation,cavitation in the entrainment zone,and gap cavitation.And the leakage cavitation and entrainment district cavitation is the main component of a cavitation cloud.During the start-up process,the cavitation is most likely to occur near the center of the blade at the back of the blade.And as the speed of the mixed-flow pump increases,it can be observed that the cavitation area on the back of the blade is gradually developed from the middle of the blade near the rim to the direction of the blade trailing edge and the hub.By analyzing the distribution of the bubble volume distribution in the impeller of the mixed-flow pump under different starting conditions,it is found that when the starting time is long,the inlet pressure is low,and the large flow conditions,the critical speed of the cavitation of the mixed-flow pump is low and the development of the cavitation is more serious.?4?The following four experiments were carried out:steady state external characteristic test,cavitation test under rated conditions,start-up external characteristic test,and high speed photography experiment to capture internal cavitation flow fields.Through the steady state experiments of the mixed-flow pump,the steady state external characteristic curve and the cavitation characteristics curve under the rated flowrate were obtained by start-up external characteristic test,and high speed photography experiment.The characteristic curve and the evolution of the internal cavitation flow field of the mixed-flow pump during the starup process with different starting conditions were obtained.And the numerical simulation results were verified by the experiments above.During the start-up test,there was a significant hysteresis effect in the head change curve due to the low speed of the pump during the initial stage of start-up and the sensitivity of the measuring instrument itself.By comparing the results of high-speed photography tests during the start-up process under different start-up conditions,it was found that the cavitation first occurred at the tip clearance of the pump impeller.It is also found that the critical speed of the cavitation of the mixed-flow pump is lower and the development of the cavitation is more serious with the longer starting time,lower inlet pressure and the larger flowrate conditions,which is consistent with the conclusion obtained by the results of numerical simulation.