Research On The Effects Of Silt Mean Diameters And Silt Concentrations On Transient Cavitation Of Hydrofoil And Centrifugal Pump

Cavitation phenomenon is often found in all kinds of hydrodynamic systems,such as pump,turbine,propeller,hydrofoil,submerged body and so on.Cavitation problems involve complicated flow phenomena such as multiphase flow,turbulence,compressible flow and supersonic flow.It can cause a series of pro blems in decline of machine performance,stability of unit operation,vibration,noise,cavitation erosion and abrasion etc.Most rivers in China contain sediment,the existence of silt particles make the probability of occurrence of cavitation increase gr eatly,and the pressure pulsation induced by the transient cavitation in the silt-laden flow makes the vibration,noise,and cavitation erosion aggravated prominently in hydraulic machinery.To provide some reference for solving problems of cavitation and cavitation erosion in hydraulic machinery under the condition of silt-laden flow,the unsteady cavitation flow in hydraulic machinery is urgently needed.To investigate the effects of silt diameter and silt concentration on the transient cavitation performance of hydrofoil and centrifugal pump,this paper simulates the transient cavitation flow of two-dimensional NACA0015 hydrofoil and centrifugal pump in the pure water and the silt-laden flow.The main research contents and results are as follows:Taking the two-dimensional NACA0015 hydrofoil as the research object,based on the Schnerr-Sauer cavitation model,the RNG k-? turbulence model and the Mixture model,the numerical simulation of the unsteady cavitation of the hydrofoil in the pure water and the silt-laden flow was carried out,respectively.The slit diameter were 0.01 mm,0.05 mm and 0.15 mm,respectively,and th e simulated silt concentration were 5%,10% and 20%,respectively.The results show that:When silt concentration is 10% and slit diameter are 0.01 mm,0.05 mm and 0.15 mm,the fluctuation amplitude of lift and drag coefficient curve in the silt-laden flow is significantly stronger than that of pure water,the average lift coefficient decrease obviously.With the increase of silt diameter,the average lift coefficient increases gradually,and the lift-drag ratio increases first and then decreases but is smaller than that of pure water.The frequency of cavity shedding is 20 Hz in the pure water,when slit diameter is 0.01 mm,0.05 mm and 0.15 mm,the frequency is 16.7Hz,16.7Hz and 20 Hz,respectively,with the increase of silt diameter,the corresponding Strouhal number decreases first and then increases.When silt diameter is 0.05 mm and slit concentration are 5%,10% and 20%,the periodic oscillation in silt-laden flow is more intense.With the increase of silt concentration,the lift-drag ratio decrease gradually.With the increase of slit concentration,the frequency of cavity shedding gradually decreased,and the Strouhal number decreased gradually.Taking a three-dimensional centrifugal pump as the research object,the Zwart-Gerber-Belamri cavitation model and the SST k-? turbulence model were used to numerically calculate the transient cavitation of centrifugal pump in the pure water and the silt-laden flow respectively.The silt diameters were 0.005 mm,0.025 mm and 0.05 mm.Silt concentrations were 1%,5% and 10%.Numer ical simulation results show that:When silt concentration is 5% and slit diameter are 0.005 mm,0.025 mm and 0.05 mm,when cavitation is fully developed,the volume of cavity in silt-laden water is larger than that of pure water in a complete cycle.The cavi ties continue to attenuating in pure water,the cavities in silt-laden water show a tendency of attenuation before increase and then attenuate,and the attenuation rate is higher than that of pure water.On the condition of different silt diameter,the amp litude of each point decreases with the increase of frequency.The main frequency of the pressure pulsation is the impeller frequency.The point with the largest frequency amplitude of each is near the tongue of volute,followed by the point close to the i mpeller outlet.The pressure pulsation amplitude of each monitoring point increases sharply when the cavitation number decreases.When silt diameter is 5% and slit concentration are 1%,5% and 10%,under the condition of cavitation conception,the volume c hange rate of cavities is relatively stable when silt concentration is 5%,the change trend of cavity volume is basically the same as that of pure water.At the same time,the change of cavity volume is consistent on the condition of 1% and 10%.When cavita tion is fully developed,the local vortex at the impeller outlet is more serious.The head decreases gradually with the increase of silt concentration.In a cycle,the higher silt concentration,the more severe the attenuation of the cavity volume.The pre ssure pulsation at each monitoring point is mainly concentrated in the low frequency region below 1500 Hz on the condition of different silt concentration.The attenuation of main frequency multiple frequencies of the points near the middle of impeller pass age and impeller outlet are slower.The main frequency amplitude of pressure pulsation at impeller outlet and the tongue of volute in silt-laden flow is lower than that of pure water.The pressure pulsation at each point increases with the decrease of cavi tation number,the increase of pulsation in impeller frequency is most obvious.