Study On Transient Characteristics And Cavitation Mechanism Of Mixed Flow Pumps With Different Blade Lattice Solidity During Startup

In recent years,the mixed-flow pump has been used as a torpedo propulsion device in my country’s new-generation submarine weapon launch system.During the transient start-up process,the head,flow rate,and rotational speed all change drastically.When the submarine is in the mooring state,the sudden increase in speed and the acceleration of the fluid will cause the local pressure to decrease and transient cavitation will occur.The occurrence of transient cavitation will reduce the quality of the torpedo launch,induce the crew to produce hydraulic excitation and fluid noise,and pose a great threat to the concealment of the submarine.Under the support of the National Natural Science Foundation of China,the prototype pump with n_s=829 is scaled by 1.5 times,and the obtained high specific speed mixed flow pump is taken as the research object.Explore the influence of the blade lattice solidity on the internal flow field of the mixed-flow pump,analyze the influence of different blade lattice solidity on the transient cavitation during startup,and explore the internal relationship between the blade structure and cavitation.In present research,the primary results are as follows:（1）The turbulence model SST k-ωwas revised based on the filter model of density correction,and it was found that the phenomenon that the cavities could not fall off due to the excessive vortex viscosity could be better solved.A comparative analysis of the internal flow field characteristics and cavitation characteristics of mixed flow pumps with different blade lattice solidity under steady-state conditions is found,and it is found that the long-chord blades have strong functions under small flow conditions,with higher head and better cavitation performance.Well,it will cause congestion in the flow channel under high flow conditions,the head and efficiency will drop quickly,and the cavitation performance will be weak.（2）Using the transient differential pressure theoretical model,the transient head and transient flow curve equations of the mixed-flow pump during the start-up process are derived,which are basically consistent with the test results,which verifies the accuracy of the model.The transient flow delay effect and the transient effect of the head during the startup process of the mixed flow pump are analyzed,and it is found that the flow delay rate decreases with the decrease of the startup acceleration;with the decrease of the start acceleration and valve opening,the transient head will reduce the impact value of the head.Through the analysis of the transient characteristics of the transient head equation,it is found that the total head of the pump is mainly composed of the steady-state term,the angular acceleration term and the inertial term.The angular acceleration term is positive,which increases the head value of the mixed flow pump during the startup process.The head coefficient of the inertia term is negative,which reduces the head value of the mixed flow pump during the start-up process.The transient head of the outer pipeline is mainly composed of the system resistance and the inertia term.The gap value between the transient total head curve and the system resistance curve represents the inertia term,which is mainly caused by the inertia effect in the system pipeline.（3）Through the error bar analysis of the steady-state external characteristic test results,it is found that the maximum error occurs at 0.4Q_opt,and the standard deviation is 0.0232,which verifies the reliability of the test device.Through transient external characteristics test research,it is found that the transient head is synchronized with the transient pressure change at the outlet,and the head impact formed at the completion of acceleration decreases with the increase of the start-up time.Through transient pressure spectrum analysis,it is found that the main frequency of the transient pressure at the inlet is the blade passing frequency,and it gradually increases in a linear manner,reaching the maximum value of 96.4Hz（blade frequency）at the time the start acceleration is completed.When the start time increases,the characteristic frequency does not change,but its intensity decreases.However,the outlet pressure pulsation has no law of inlet,and its characteristic frequency is mainly low frequency,which reflects the transient effect in the impeller starting process.Comparing the frequency spectrum of the mixed flow pump in the cavitation state and the non-cavitation state start-up process,it is found that the low-frequency signal at the start of the cavitation state is significantly stronger than the non-cavitation state,indicating that the occurrence of transient cavitation during the start-up process induces more low frequencies signal.（4）The evolution of transient cavitation during the start-up process of mixed-flow pump is analyzed by high-speed photography test.It is found that the initial cavitation is caused by the vortex cavitation,then the TLV cavitation and clearance cavitation appear,and they combine with each other to form a triangular cavitation cloud when the speed is accelerated.When the cavitation number decreases,the initiation time of cavitation is advanced,and vertical vortex cavitation is formed near the trailing edge of the blade,and the cavitation phenomenon in the whole starting process is intensified.When the start-up time is reduced,the flow delay effect is enhanced,and the transient effect of the start-up process is enhanced,resulting in a delay in the appearance of the triangular-shaped cavitation cloud.When the opening of the valve decreases,the pressure gradient between the PS and SS surfaces decreases,resulting in vortex cavitation,TLV cavitation,and the resulting triangular cavitation cloud.（5）The simulation results of transient head and transient cavitation during the starting process are basically consistent with the experimental results.It verified the precision of the numerical method.Analysis of the internal flow field found that the main leakage vortex PTLV,the secondary main leakage vortex PTLV 2,the secondary leakage vortex STLV and the wall separation vortex WSV appeared in the impeller during the start-up process of mixed flow pumps with different blade lattice solidity,and the change law was similar.However,the leakage vortex structure of the long-chord blade is significantly smaller than the original blade,and its separation angle is relatively small.TLV,angular vortex,WSV and suction surface attachment vortex are the most concentrated areas of vorticity distribution,which is the direct cause of cavitation in this area.The start-up process of a mixed flow pump can be divided into four stages:no cavitation stage,cavitation growth stage,cavitation reduction stage and cavitation stabilization stage.The cavitation in the tip area of mixed-flow pumps with different blade lattice solidity is basically the same in position change and intensity evolution,but it is different under different valve openings.Due to the change of the incidence angle of the blade inlet during the startup of the mixed flow pump,the sheet cavitation is mainly divided into four stages.When the inlet of incidence angle is positive,the fluid near the leading edge flows from the PS into the impeller,and sheet cavitation develops on the SS surface;when the inlet of incidence angle is negative,the fluid flows into the impeller from the SS surface,and the sheet cavitation on the SS surface gradually decreases.If the absolute value of the negative incidence angle formed is large enough,the sheet cavitation on the SS surface disappears completely,and the sheet cavitation begins to appear on the PS surface.For impellers with different blade lattice solidity,the inlet incidence angle of the long-chord blades during the entire start-up process is smaller than that of the original blades,so that the sheet cavitation on the SS surface is obviously suppressed.