Accelerating Flow Characteristics And Cavitation Mechanism Of Mixed-flow Pump With Different Blade Numbers

With the rapid development of Chinese national defense equipment,mixed-flow pump has been used in new generation submarine weapon launchers.When a submarine launches a torpedo,the mixed-flow pump has been in the acceleration phase.During this process,the parameters such as speed and head have changed greatly,and the internal flow field also has strong transient phenomena.When the submarine is offshore,due to the low inlet pressure,cavitation is very likely to occur during the start-up process.The occurrence of accelerated flow cavitation of the mixed flow pump will directly reduce the launch quality and pose a great threat to the submarine.This paper supported by 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.The high specific speed mixed flow pump which was scaled 1.5 times by specific speed n_s=829 prototype pump is model pump.The paper investigates the transient cavitation characteristics of mixed flow pumps at different blade numbers during start-up.The main results and conclusions achieved are as follows:?1?In this paper,the theoretical model of transient pressure difference established by the French Dazin researchers on the accelerated flow process of rotating machinery is used.By establishing the transient hydraulic performance equation based on the model pump in this paper and the pipeline characteristic equation under transient conditions for numerical solution,the transient head curve of the model pump during the startup process is obtained.By comparing and analyzing the results of the theoretical model with the test results,it is found that the results are in good agreement,which verifies the applicability of the theoretical model and further shows the accuracy of the test bench.?2?According to the numerical simulation of the transient cavitation characteristics of the mixed-flow pump with different blade numbers during the startup process,the flow conditions(Q=1.0Q_opt),startup time?T_s=2s?and inlet pressure(P_in=50kPa)were analyzed.The evolution of the cavitation morphology in the tip area of the mixed-flow pump,the load distribution on the blade surface,the spatial structure of the leakage vortex and the change of the structure of the leakage vortex are studied under different blade numbers.The study found that as the number of blades increases,the cavitation volume decreases and the primary cavitation is delayed.In the tip area of the three-blade impeller,flaky adhesion cavitation first appeared.With the increase of the speed,the tip scraped vortex cavitation first,and then the tip leakage vortex cavitation occurred.The cavitation area of the blade tip area becomes larger,forming a triangular bubble cloud.After the acceleration is completed,it can be found that the secondary leakage vortex cavitation begins in the blade tip area,and then combines with the previous triangular cavitation cloud cluster,and finally the triangular cavitation cloud cluster becomes larger.The cavitation area on the surface of the blade first appeared in the acceleration phase,starting from the front edge of the back,and gradually spread to the middle of the blade.After the acceleration was completed,the cavitation area decreased,and then the cavitation area appeared at the rim?3?A test-bed for transient cavitation characteristics of start-up process of mixed flow pump was set up to measure the external characteristics of steady-state performance,transient pressure fluctuation at the inlet and outlet,and high-speed photography.According to the test results of pressure pulsation,the continuous wavelet analysis can be used to obtain the changes of frequency components with time,which can better reflect the time-frequency characteristics of the transient cavitation process.When the inlet pressure is small,for the time-frequency spectrum analysis of the inlet monitoring point,it is found that the main frequency of the three blade in the acceleration stage is lower than 10Hz,and the main frequency has a linearly increased frequency band.After the acceleration,the leaf frequency component still exists,and the axial frequency component weakens.In the process of start-up,the four lobes are mainly low frequency components.However,the intensity of the shaft frequency increases when the rotating speed is stable.For the analysis of the time-frequency spectrum of the exit measuring point,it is found that the low-frequency component is the main component of the three four five blades.After the rotation speed is stable,the axial frequency strength is significantly weakened,and the static and dynamic propagation is blocked.When the start-up time is long,for the four blades inlet monitoring point,it is found that the characteristic frequency will appear during the acceleration process,and the unit may generate vibration.The main frequency of the outlet monitoring point is low frequency during the start-up process,and the five blade inlet and outlet monitoring points also find that two characteristic frequencies appear in the same step during the start-up process,and then increase linearly to the shaft frequency and 50Hz frequency at the same time.Under the condition of small flow,the main frequency of the three blades inlet monitoring point is low frequency during the start-up process.After the speed is stable,the main frequency is shaft frequency and the secondary frequency is blade frequency.For the four blades inlet monitoring point,the main frequency of the whole period is the shaft frequency,and for the outlet monitoring point,the low frequency occupies the main frequency.After the rotation speed is stable,the main frequency is about 40 Hz.For the inlet and outlet monitoring points of the five blades,during the acceleration process,the main frequency is low frequency.After the rotation speed is stable,the main frequency at the inlet is shaft frequency,and there is no strong and stable frequency component at the outlet.?4?Through the analysis of the results of high-speed photography,it is found that the cavitation phenomenon in the top region of the three blade during the start-up process,the leading edge blowing up cavitation appears first,then the gap cavitation and leakage cavitation appear one after another.At the end of start-up,there will be cavitation clouds,which are mainly composed of two parts,one is the leading edge blowing up vortex cavitation,the other is the similar triangle cavitation cloud,and the triangle cavitation cloud mainly consists of leakage vortex cavitation,gap cavitation and shear layer cavitation.With the decrease of cavitation number,the occurrence time of vertical cavitation vortex is advanced.Under the same cavitation number,with the decrease of flow rate,the cavitation intensity in the pump decreases.When the pump works at 0.6Q_opt,the cavitation area did not increase at the end of start-up,but there was a return vortex cavitation at the inlet.When the number of blades increased,it was found that the initial cavitation was delayed in the tip region,and when the acceleration was completed,there was vertical cavitation vortex shedding in the top region of five leaves.When the inlet pressure is reduced,the initial cavitation time of three kinds of blade numbers is advanced,the cavitation situation becomes serious,the leakage vortex cavitation phenomenon of the five blade is obvious,the shedding vortex phenomenon of the four blade at the tail of the cavitation cloud,and the vertical cavitation vortex cloud cluster of the five blades becomes larger.In summary,it is found that during the pump starting process,the initial cavitation time was delayed and the internal volume of the five-blade impeller is the smallest.Therefore,by increasing the number of blades,cavitation can be suppressed during the start-up.