| The cavitation characteristic is an important characteristic of large vertical axial flow pump devices.The cavitation generated during the operation of the pump can induce cavitation,vibration,and seriously affect the safe and stable operation of the pump device.Therefore,conducting research on it is of great significance.The critical cavitation margin in cavitation characteristics is the most important parameter to characterize cavitation characteristics and also the basis for determining the center elevation of the pump impeller.At present,the criterion for determining the critical cavitation margin is only based on changes in the external characteristics of the pump device,that is,1%decrease in pump device efficiency or(2+K/2)%decrease in head(K is the type number).However,a large number of experimental studies have found that if judged according to this criterion,the suction surface of the blade has already covered a large number of bubbles,and cavitation has developed quite seriously.How to provide a reasonable criterion to determine the critical cavitation margin is an urgent problem to be solved in cavitation research.This article adopts a combination of numerical simulation calculation and experimental research method,taking a large vertical axial flow pump device from the Yangtze River to the Huaihe River as the research object,to study the spatiotemporal evolution of cavitation under different operating conditions,and proposes a new criterion for determining the critical cavitation margin based on the ratio of cavitation area;The mathematical model reveals the influence of the number and chord length of water pump blades on cavitation characteristics;The pressure pulsation and radial force of the water pump under cavitation were obtained through numerical simulation,and the influence of different cavitation allowances on the stability of the unit was studied.The main research content and conclusions are as follows:(1)Construct a three-dimensional solid model of a large vertical axial flow pump unit based on The turbulence model SST-ω and Rayleigh-Plesset equation are used to obtain the external characteristics and cavitation characteristics of the pump unit.Compared with the experimental results,it shows that the digital-analog external characteristics are basically in agreement with the experimental results.The curve of the digital-analog cavitation allowance is located under the experimental curve,and its variation law is consistent with the experimental curve,the maximum relative error between them is less than 6%.(2)The spatial evolution of blade cavitation under 5 different operating conditions(0.8Qd、0.9 Qd、1.0 Qd、1.075 Qd、1.15Qd)was studied by numerical simulation,and the feasibility of determining critical cavitation margin by void area ratio was explored.The results show that the cavitation starts at the inlet edge of the suction surface of the blade when it is at small flow rate and migrates to the outlet with the increase of flow rate.As the cavitation margin NPSHa decreases,the cavitation on the suction surface of the blade diffuses along the rim towards the outlet and the hub,and the distribution core area shifts towards the outlet of the blade.According to the evolution law of void distribution,under different flow conditions,when the void margin is less than 4.0m,the void area ratio of suction surface of blade is close to 60%.The bigger the flow rate,the bigger the gradient of void area ratio with the change of void margin.The efficiency of pump unit increases first and then decreases with the change of void area ratio.The larger the flow rate,the greater the gradient of efficiency decrease.Under the design flow(1.0 Qd)condition,when the void area ratio approaches 53.3%,the growth rate of the area ratio slows down and the efficiency begins to decrease.The development state of the void and the external characteristics of the pump unit have changed significantly,reflecting that the cavitation begins to enter the next stage of development.At this time,the corresponding voiding margin is more suitable as the critical voiding margin.Therefore,it is feasible to use the void area ratio on suction surface of blade as a new criterion for critical cavitation.(3)The digital-analog calculation explores the effect of different blade numbers(the number of blades is 3,4,5)and chord length changes(10%increase in chord length)on the cavitation evolution of pump blades at design flow(Qd).The results of comparative analysis show that increasing the number of vanes can improve the cavitation performance of the pump,reduce the entropic production of the pump section,and decrease the gradient of the vane void distribution area with the variation of the cavitation allowance,but the pump efficiency is significantly reduced.Increasing chord length of blade(10%increase in chord length)can also improve cavitation performance,evolution law of void distribution area on suction surface is basically the same,but pump head increases and high efficiency area shifts to large flow rate.(4)Pressure fluctuation characteristics in different areas and radial force variation of pumps under five different cavitation allowances were studied by unsteady three-dimensional turbulence numerical simulation.According to the time-domain characteristics of pressure fluctuation and the digital-analog results of frequency characteristics based on Fast Fourier Transform(FFT),the following conclusions can be drawn:Cavitation affects the impeller area differently,the main frequency amplitude of pressure fluctuation at the inlet and outlet of impeller increases first and then decreases,while the middle of blade chord rises monotonously;Cavitation has no influence on pressure fluctuation frequency.The main frequency of pressure fluctuation at impeller inlet and outlet is blade frequency 4 f0(f0=21.6Hz)and the main frequency at middle of blade chord is blade frequency f0.The radial force amplitude of the pump decreases first and then increases with the variation of cavitation allowance.When the cavitation margin is greater than 4.24m,it experiences 4 peaks and troughs in a cycle.However,when the cavitation margin is less than 4.24m,the original regularity disappears.The pressure fluctuation characteristics and radial force variation in different areas of the above pumps correspond to the growth law of the distribution area of the cavitation,which further demonstrates the feasibility of the new criterion for determining the critical cavitation allowance based on the area ratio of the cavitation. |
