Research On The Unsteady Characteristics Of Cavitation Impact Energy On Centrifugal Pump Blade Surface

Hydraulic machinery is prone to cavitation during operation due to the particularity of the working conditions.The negative effects of cavitation restrict the development of hydraulic machinery in the direction of high efficiency,stability and energy saving.Cavitation in hydraulic machinery such as pumps,turbine and propellers are complicated.Severe cavitation will produce many negative effects such as cavitation noise,mechanical vibration and material damage.It is of great significance to study the formation mechanism of cavitation erosion and predict the law of cavitation process on solid surface.This thesis uses a modified turbulence model and numerically simulates the cavitation shedding mechanism and pulsation characteristics of the cavitation unsteady evolution process based on the cavitation dynamics equations.The solution results are compared with experiments to quantify different cavitation impact energy.The main research contents and innovation results of this thesis are as follows:(1)The research on cavitation and cavitation erosion is summarized in detail.The basic theory of cavitation dynamics is sorted out.The mathematical models used in current numerical simulation and the mechanism of unsteady cavitation evolution is summarized.(2)Aiming at the problem of turbulence model commonly used in engineering to predict the viscosity transition in the fluid domain,this thesis modified the original turbulence based on the numerical method of Filter-Based Density Correction model and considered the influence of turbulent kinetic energy on the saturated vapor pressure during the cavitation unsteady process.The applicability of the modified turbulence model is evaluated and the results show that the modified method can accurately capture the unsteady behavior of cavitation.(3)The unsteady evolution of the re-entrant can be divided into three stages:(I)the growth of attached cavities;(II)the development of attached cavities and the transmission of re-entrant;(III)the formation of cloud cavitation and collapsed.At the same time,the unsteady process of the cavitation structure on the hydrofoil surface is accompanied by local pressure changes.The size of the cavitation group and the distance from the wall directly affect the pressure amplitude of the airfoil surface.During the stage of cloud cavitation collapse,the amplitude of the airfoil surface pressure is the maximum value of the evolution period.(4)The unsteady evolution of the shock wave can be divided into four stages:(I)growth of attached cavitation bubbles;(II)attached cavities rolled up;(III)generation and transmission of shock waves;(IV)attached cavitation exfoliation and the large-scale cloud cavitation collapse.The significant difference between the development of shock wave and the development of re-entrant is the form of cavitation collapse.During the action of shock waves on cavities,the cavities are non-uniform,multi-scale discontinuous and the local cavitation volume fraction,sound velocity,and cavitation structure will change drastically after the shock wave.(5)A mathematical model of cavitation impact energy was proposed to quantify the damage effect of the cavitation bubble collapse,collapse and structural changes on the surface of the solid wall.And The impact energy of different forms of cavitation on the surface of the solid wall is predicted.The cavitation shock energy mainly comes from the change of local pressure and the volume fraction of the cavities.The collapse of the cavities and the drastic change between phases are the main reasons for the generation of cavitation shock energy.(6)The modified turbulence model was used to simulate the cavitation of the centrifugal pump and its internal flow structure during the unsteady evolution.The cavitation numerical simulation of three typical working conditions of the centrifugal pump was performed and the simulation results were compared with the experiments.At the same time,the cavitation impact load on the impeller surface caused by the unsteady behavior of cavities in the impeller flow channel was analyzed.The cavitation impact energy was quantified and the high-risk area of the centrifugal pump blade surface cavitation was predicted.