Study On Cavitation Control Of Bionic Jet Hole In Axial Flow Pump Blades

The phenomenon of cavitation was first discovered by people in 1893,and has been the focus of attention by domestic and foreign scholars.The high strength micro jet produced in the collapse of cavitation will seriously damage the metal surface,which has a great impact on the strength and service life of hydraulic machinery.In the past hundred years,people have experienced several damage accidents caused by cavitation in large water cons ervancy projects,and the loss is difficult to measure.Therefore,it is of great significance for the research of cavitation generation mechanism and improvement methods.The axial flow pump has the characteristics of low head and large flow rate.The working environment is complex and cavitation is easy to occur.However,the research on cavitation control of axial-flow pump is less.In this paper,the cavitation mechanism of axial-flow pump is analyzed by numerical simulation,and the cavitation control method is explored to provide reference for engineering application.This paper firstly introduces the occurrence,development and classification of cavitation phenomena.There are two main theories of cavitation occurrence,namely the saturated vapor pressure hypothesis and the "cavitation nuclear theory".In the analysis process,the saturated vapor pressure hypothesis is mainly combined to analyze the cavitation process.The research and development of cavitation control methods by scholars at home and abroad are summarized.The cavitation parameters of hydraulic machinery are explained,and the calculation formula of cavitation margin of horizontal axial flow pump is deduced.On this basis,different cavitation stages are selected to analyze the model.This paper introduces the development process,advantages and disadvantages of several commonly used turbulence models and cavitation models.Combined with the previous comparative study,SST k-? turbulence model and ZGB cavitation model are selected,and the turbulent viscosity is modified.The bionics structure is advanced and perfect,which broadens the way of thinking for mechanical design.Through understanding the drag reduction effect of the non smooth surface of soil biology,it is found that the surface back hole structure of earthworm can improve the structure of the near wall boundary layer and optimize the flow field by spraying the body cavity liquid to form a lubricating layer.Based on the biological morphology and characteristics of the earthworm back hole,the bionic jet hole was arranged on the surface of the axial pump blade to explore its effect on the cavitation control of the axial pump.This paper analyzes the characteristics of the cavitation flow field in the axial flow pump,and compares the influence of the position parameters and diameter of the bionic structure on the cavitation process.It mainly includes the following research contents:(1)Take a 350ZQ-125-30 submersible axial flow pump as a model,analyze its cavitation characteristics by numerical calculation method.The accuracy of the numerical calculation is verified by comparing the simulation results of the ext ernal characteristics of the model pump with the experimental values.Through the analysis of the flow field characteristics in the impeller with different cavitation allowance,it is found that cavitation bubbles first appear at the low pressure position near the flange at the inlet of the blade,and expand to the outlet and the flange at the same time.As the degree of cavitation deepened,the middle and rear parts of the blade are seriously eroded by cavitation.On the blade surface,there is a reverse pressure gradient at the end of the cavitation development,so there is a vortex structure,accompanied by a small range of backjet,which indicates that the flow pattern at the edge of the blade is not stable,and the backjet impacts the surface fluid of the blade,causing the cavitation to fall off,which greatly affects the work ability of the impeller.(2)Combined with the original model cavitation occurrence position,a jet hole with a hole diameter of 6mm is arranged on the blade rim side,and the cho rd direction positions are taken as 0.2L,0.35 L and 0.5L respectively.After the jet holes are arranged,due to the pressure difference between the two surfaces of the blades,the high-pressure fluid on the working face flows through the jet holes to the l ow pressure side,which can compensate for the low pressure flow on the back side and effectively control the development of cavitation.There are transverse jet and small-scale backflow in the flow side of the jet hole,causing dist urbance to the convection field;the arrangement of the jet holes closer to the blade inlet has a good effect on the primary control of cavitation.The solution where the jet holes are located in the middle of the blade has the smallest cavitation volume fraction,more uniform distribution of the flow field and small pressure fluctuation in the stage of severe cavitation.(3)According to the analysis of the position coefficient,th e jet hole diameter is studied.The cavitation characteristics of the modified impeller were analyzed under the four schemes of the hole diameter of 4mm,6mm,8mm and 10 mm,which were compared with the original model.At all stages of cavitation development,the bubble volume of the four schemes is reduced compared to the original model;near the jet hole,there is a small vortex at the intersection of the fluids,but it has little effect on the overall flow.Compared with the four schemes,when the pore diameter is 6mm,the cavitation volume fraction decreases the most,and the cavitation control effect is the most obvious.