Research On The Effects Of Silt Mean Diameter And Silt Concentration Towards Cavitation Flow

Cavitation can induce the noise and vibration easily and make the performance of machineries become very low,one kind of complex multiphase turbulent flow.According to the shape and morphology of cavitation bubbles,cavitation includes five different types.It is composed by sheet cavitation,cloud cavitation,bubble cavitation,supercavitation and vortex cavitation.Based on the hydrodynamics factors,structure of cavitation bubbles and the essential conditions for the formation of cavitation bubbles,cavitation is made up by travelling cavitation,fixed cavitation,vortex cavitation and vibratory cavitation.For the evolution process of cavitation,it includes cavitation inception,cavitation development and cavitation bubbles collapse.For the cavitation inception,it has a closed relation with the number of cavitation nuclei,effect of the viscosity,properties of the media and roughness of the surface and properties of the medium.For the cavitation development,it is influenced by the cavitation number,distribution of the static pressure,characteristic of the medium and the boundary layer.During the cavitation bubbles collapse,the jet with high speed can make huge damage towards the materials.Cavitation flow is one kind of two phases flow,including the water-vapor and water-liquid.For the cavitation occurring in the silt-laden flow,it is one three phases flow,including the silt particles,water-liquid and water-vapor.Considering the effect of noncondensable gas,the cavitation flow includes three diverse states and four various phases: water-vapor,water-liquid,noncondensable gas and silt particles.In the silt-laden cavitation flow,mainly through the variation of silt mean diameters and volume fractions of silt particles,silt particles have a closed relation with the evolution of cavitation.For silt particle with different silt mean diameters and volume fractions,it has three distinct effect forms towards the evolution of cavitation.For certain silt mean diameter and volume fraction,silt particles could promote or inhibit the development of cavitation.And compared with the cavitation occurring in the pure water,silt particles may have little effect on the evolution of cavitation.In the paper,based on the 2D nozzle and 3D centrifugal pump,effect of silt particles towards the cavitation flow was studied.In this paper,the primary and chief research contents and the academic achievements gained are as follows:1.Features of turbulence,methods to represent turbulence,governing equations of turbulence,applied scope and characteristics of turbulent models and features of various cavitation models were introduced in detail.Governing equations applied for the cavitation in the pure water was modified to build the new governing equations to make a preparation to simulate the cavitation in the silt-laden flow.2.In the 2D nozzle,the Mixture model,standard k-? turbulent model and Schnerr&Sauer cavitation model were employed to analyze the cavitation occurring in the silt-laden flow.The silt mean diameters were 0.013 mm,0.020 mm,0.025 mm,0.030 mm,0.035 mm,0.040 mm,0.050 mm,0.060 mm,0.070 mm,0.076 mm,0.082 mm respectively.Volume fraction of silt particles varied from 1.0% to 10%.Results gained indicated that when the volume fraction of silt particles varied from 1.0% to 10%,for silt particles with silt mean diameter being 0.013 mm,0.020 mm,0.025 mm and 0.030 mm,the cavitation number decreased constantly.With the same variation scope of the volume fraction of silt particles,when the silt mean diameters were 0.035 mm and 0.040 mm,the cavitation number decreased with slight fluctuation.But when the volume fraction was 10%,the cavitation number was larger than it with the volume fraction being 9.0%.For silt particles with silt mean diameter being 0.050 mm and 0.060 mm,shape of the variation of cavitation number looked like a “W”.When the silt mean diameters changed from 0.070 mm to 0.082 mm,the cavitation number fell down firstly,then grew up and dropped lastly.When the volume fraction changed from 1.0% to 10%,silt particles promoted the evolution of cavitation firstly and then inhibited the development of cavitation.With silt mean diameter becoming larger,scope corresponding with silt particles promoting the evolution of cavitation became smaller and scope corresponding with silt particles inhibiting the development of cavitation became larger.3.For the 3D centrifugal pump,via the Mixture model,standard k-? turbulent model and Schnerr&Sauer cavitation model,effect of silt particles on the cavitation was studied.The silt mean diameters were 0.005 mm,0.010 mm and 0.015 mm.Volume fractions of silt particles were 0.5%,1.0% and 1.5%.Cavitation flow in silt-laden water with different silt mean diameter and volume fraction was simulated and curves of cavitation performance in the pure water and silt-laden water was made.From the curves,effect of silt particles was manifest.The calculated results indicated that for the centrifugal pump,when the volume fraction was 1.0% and silt mean diameter changed from 0.005 mm to 0.015 mm,silt particles promoted the development of cavitation and then inhibited the evolution of cavitation.When silt mean diameter was 0.010 mm,compared cavitation occurring in the pure water,silt particles had little effect on the development of cavitation.So,0.010 mm was regard as the critical diameter.With silt mean diameter being 0.010 mm and volume fraction varying from 0.5% to 1.5%,silt particles promoted the evolution of cavitation when the volume fraction was 0.5%,silt particles had little effect on the process of cavitation when the volume fraction was 1.0% and silt particles inhibited the development of cavitation when the volume fraction was 1.5%.For silt particles promoting the development of cavitation,distribution of water-vapor with volume fraction being 0.1 was larger than it in the pure water.When silt particles had little effect on the cavitation flow,the distribution was similar.During silt particles inhibiting the evolution of cavitation,the distribution was smaller than it in the pure water.When cavitation developed fully in silt-laden water,head calculated in the silt-laden water was lower than it in the pure water.As silt mean diameter and volume fraction increased gradually,the head decreased constantly.