Research On The Influence Of Leading Edge Shape On The Cavitation Field In The Tip Region And Cavitation Performance Of Axial Pump

Axial flow pump has the advantages of simple structure,large flow rate and high efficiency,and has played an important role in flood control and drainage,large-scale water transfer.Due to the tip gap between the impeller rim and the runner chamber,the axial flow pump is often accompanied by a TLV structure during operation,and it is easy to induce complex flow structures such as TLV cavitation,resulting in the working capacity of the impeller is reduced,and in severe cases,hydraulic excitation and targeted noise will occur,which will seriously affect the operating stability of the unit.The curved and swept blades have the characteristics of anti-cavitation and vibration reduction and noise reduction.Therefore,it is a very important research method to study the shape of the leading edge of the axial flow pump blade and improve the cavitation performance of the axial flow pump.In this paper,the hydraulic model of an axial flow pump with n_s=700 is used as the prototype impeller.The prototype impeller is redesigned by the method of circular forward sweep and backward sweep,and the forward and backward sweep angles impeller of 20°and 40°are obtained respectively.Based on numerical calculation and visual experiment,the cavitation characteristics and transient characteristics of the tip area of the axial flow pump are studied,and the influence of the forward and backward sweep of the axial flow pump blade on the cavitation flow field of the blade tip area and cavitation performance is discussed.It provides a reference for the design and cavitation characteristics study of forward swept axial flow pump and backward swept axial flow pump.The main research contents and results are as follows:(1)The axial flow pump model of the prototype impeller was used as the research object,the external characteristics、cavitation characteristics and the transient process of the tip cavitation flow were analyzed by numerical calculation,and compared with the experimental results.The results show that the external characteristic curve and cavitation characteristic curve obtained by numerical calculation and the experimental error are less than 5%,and the transient process of cavitation flow in the tip region captured by numerical calculation and visual experiment is in good agreement,which verifies the accuracy of numerical calculation in this paper.(2)Based on the numerical calculation results,the cavitation flow field characteristics in the tip area and pressure pulsation characteristics of the axial flow pump were analyzed.The results show that the cavitation in the tip area develops from angular vortex cavitation and TLV cavitation to triangular cavitation cloud structure.The swirling strength and vorticity of TLV vortex core first increase and then decrease.In the early stage of cavitation,the amplitude of the dominant frequency of pressure pulsation decreases gradually from the inlet to the outlet of the impeller.In the late stage of cavitation,the amplitude of the dominant frequency of pressure pulsation in the middle of the impeller is larger than that of the inlet and outlet of the impeller due to the interference of the cavitation structure to the flow field.In the whole process of cavitation development,the amplitude of the main frequency of the pressure pulsation in the tip region is the highest,and it gradually decreases along the direction of the hub.(3)By analyzing the macroscopic energy characteristics,pressure distribution,cavitation volume fraction and cavitation flow field in the tip area of forward sweep 20°and40°impeller,and compared with the prototype impeller.The results show that the critical cavitation number of the pump device decreases with the increase of the blade forward sweep angle.The blade forward sweep reduces the range of the low pressure area on the suction surface and the cavitation volume fraction in the tip area,and The larger the blade swept angle,the more obvious the effect of suppressing the primary cavitation in the tip area,the blade forward swept also reduces the low-velocity region and the high-turbulent kinetic energy region in the tip region,and suppresses the back jet phenomenon at the tail of the cavitation structure.By comparing and analyzing the dynamic characteristics of the tip cavitation vortex and pressure pulsation characteristics,it is found that the tip leakage of the forward sweep 20°impeller is 56.2%lower than that of the prototype impeller,and the forward sweep 40°impeller is 56.7%lower than that of the prototype impeller.The Swirling Strength of the TLV vortex core of the forward swept impeller is lower than that of the prototype impeller.With the intensification of cavitation,the Swirling Strength of the TLV vortex core of the forward swept 20°impeller first increases and then decreases,while the Swirling Strength of the TLV vortex core of the forward swept 40°impeller is only Severe cavitation conditions are slightly reduced.The high vorticity region of the TLV vortex core and the runner chamber wall is entrained with each other,and the vorticity decreases in severe cavitation conditions,the high vorticity region of the tip region of the forward swept impeller is significantly smaller than that of the prototype.impeller.The area of negative axial velocity in the tip region due to leakage flow decreases with the increase of the blade forward sweep angle.In the early stage of cavitation,the blade forward sweep reduces the amplitude of the dominant frequency of the impeller inlet pressure pulsation.In the severe cavitation stage,the blade forward sweep reduces the amplitude of the main frequency of the pressure pulsation in the tip region,and the interference of cavitation on the flow field in the tip region is reduced.(4)The cavitation flow field in the tip region and cavitation performance of backward sweep 20°and 40°impeller were systematically studied.The results show that the critical cavitation number of the swept impeller is lower than that of the prototype impeller,and the critical cavitation number of the backward swept impeller with the same angle is higher than the forward swept impeller,it shows that the anti-cavitation performance of the backward swept impeller with the same angle is lower than that of the forward swept impeller.The blade backward sweep not only reduces the cavitation area and cavitation volume fraction of the suction surface、inhibits the development of the triangular cavitation cloud in the tip region to the impeller outlet,but also reduces the pressure difference between the pressure surface and the suction surface in the tip region,thereby reducing the tip leakage,the tip leakage of the backward swept impeller with the same angle is higher than that of the forward swept impeller,indicating that the suppressing effect of the backward swept impeller on the tip leakage is not as good as that of the forward swept impeller.By analyzing the dynamic characteristics of the tip cavitation vortex and pressure pulsation characteristics,it is found that the TLV vortex core vortex and Swirling Strength decrease with the increase of the blade backward swept angle,and the TLV scale increases with the increase of the blade backward swept angle,so the negative axial velocity region in the tip region of the backward swept impeller is larger than that of the prototype impeller.As the degree of cavitation intensifies,the Swirling Strength and vorticity of TLV vortex core of the backward swept impeller first increase and then decrease,and the high vortex area and high Swirling Strength area in the tip area are more concentrated.The blade backward sweep reduces the amplitude of the dominant frequency of pressure pulsation at the impeller inlet and the tip region,and reduces the interference of cavitation on the flow field at the impeller inlet and tip region.