| Rivers are widely distributed and water resources are abundant in China.As the ecological environment is destroyed,soil erosion is serious,resulting in high sand content in river basins,which seriously threatens the efficiency and service life of hydraulic machinery units along the river basin.On the one hand,the solid particles with kinetic energy constantly impact and cut the surface of flow passage parts,causing the wear phenomenon caused by solid particles;on the other hand,the cavitation formed at the low pressure of fluid breaks in the high pressure area,releasing huge energy and shock wave,causing cavitation damage to the flow-through surface;Under the condition of sand laden water,abrasion and cavitation erosion are often accompanied and influenced each other,so it is difficult to distinguish between them.The characteristics of their damage include cavitation erosion and wear traces.These damage phenomena on the surface of flow passage parts are called abrasion,which includes two different failure processes:abrasion caused by solid particles in water flow and cavitation erosion caused by cavitation bubble collapse.The mechanism is very complicated.In order to study the formation mechanism of cavitation and erosion,based on the design method of water tunnel and wind tunnel,the cavitation test rig in School of mechanical engineering of University of Ljubljana was improved by adding contraction section and diffusion section.In order to meet the test conditions of abrasion test,anti-settlement system,cooling system and venturi nozzle flowmeter were designed.Furthermore,the relationship between cavitation and erosion and the formation mechanism of erosion on the surface of venturi in sand laden water are studied experimentally and numerically.The innovation and main work of this paper are as follows:1.Based on the design method of water tunnel and wind tunnel,the cavitation test rig is improved.The diffusion section,contraction section,venturi nozzle flowmeter,cooling system and anti-settlement system are designed and added.The structure of the test section is redesigned.The discharge coefficient C of the venturi nozzle is calibrated.The relative percentage of the measured discharge coefficient(C=0.952615)and the calculated result(C=0.96111)is 0.9%.2.Based on the principle of reflection,a light path system is designed and built to obtain the pit image.The system is composed of power supply,light source,beam splitter and camera.On the one hand,the light source casts light onto the surface of the specimen through the beam splitter;on the other hand,the light reflected from the surface of the specimen is reflected to the camera through the beam splitter,which solves the problem that the light source is coaxial with the test specimen,which brings great interference to the photographing,so as to realize the photographing of etch pit image.3.Based on LabVIEW platform,combined with the characteristics of cavitation image,a set of image processing program is developed to obtain the number and distribution of erosion pits.The location of erosion pit is in the darkest place in the picture,and the gray value is the smallest;the background interference caused by fluid impact and cavitation erosion and collapse is relatively large,which needs to be removed.Based on the difference of gray value between them,the VI vision image processing function is used.Firstly,the image is segmented by local thresholding,and then the image is processed by morphological processing for multiple times of erode and dilate to remove noise particles.Finally,particle analysis is used to remove the noise particles.The results show that the etch pits are clear and the recognition rate is as high as 98.4%.Based on MATLAB platform,the cavitation image obtained by high-speed camera is calculated by mean value and standard deviation,and the location of cavitation erosion is predicted by image processing.The results show that:compared with the average method,the results obtained by standard deviation method are more accurate in predicting the location distribution of cavitation erosion on the surface of the specimen,which is more consistent with the location of corrosion pits on the surface of the specimen,and can reflect the real structure and distribution of cavitation.4.Based on the improved test rig,based on cavitation number of 1.22,the sand particles with particle size range of 0 mm,0.023 mm,0.046 mm and 0.063 mm were selected as the research objects,and 10 schemes with concentration of 25 kg/m~3,50kg/m~3 and 75kg/m~3were selected to study the formation mechanism of abrasion.The conclusion is as follows:(1)Cavitation erosion and wear have a strong synergistic effect,which is the comprehensive performance of fluid viscosity and particle wear.(2)The ratio of different particle sizes and different concentrations can cause mass loss results consistent with cavitation.5.The numerical analysis of the velocity and Mach number of cavitating flow in a venturi is carried out for the first time,and the effect of liquid viscosity on the maximum length and shedding frequency of cavitation is studied.The results show that the speed of sound in the cavity inside the cavity is the smallest,with a value of 4.32m/s,which is much lower than the propagation speed of sound in water of 340m/s.Most areas in the venturi have a Mach number less than 1,which belongs to subsonic flow.The area with the highest Mach number is located in the cavity core near the throat,and the highest value reaches 7,Belonging to a highly compressible state.With the increase of viscosity,the maximum length of cavitation bubbles decreases;the maximum cavitation cloud shedding frequency is 44.1Hz.When the viscosity is maximum,the minimum shedding frequency is 43.1Hz,and the relative percentage between the two is 2.3%.6.The DPM model is used to simulate and calculate the sand-containing water solid-liquid two-phase flow in the venturi tube,and the McLaury erosion model is used to calculate the solid erosion distribution on the surface of the venturi tube.The different particle diameters(0.023mm,0.046mm,0.063mm),different concentrations(25kg/m~3,50kg/m~3,75kg/m~3)and different viscosities(1.003mPa新,1.16 mPa新,1.24 mPa新,1.46mPa新,2.37 mPa新,2.51 mPa惹,6.40 mPa新)on the surface wear distribution of the specimen,the results show:(1)Under the same particle size condition,as the concentration increases,the surface erosion rate of the specimen gradually increases.The erosioin distribution is mainly on the two sides near the wall of the venturi throat,and the erosion of the specimen in the middle area of the flow direction is not significant.(2)As the particle concentration increases,the average and maximum erosion values gradually increase.Compared with the particle concentration,the impact of particle size on erosion is dominant.7.Considering the influence of particle size,concentration and liquid viscosity on the results,the numerical calculation of cavitation number 1.22 in Venturi tube with sand laden water is carried out,and the following conclusions are obtained:(1)Under the same particle size and the same concentration,the length of the cavity gradually decreases with the increase of the viscosity;the particle size,concentration and liquid viscosity have no significant effect on the shedding frequency of the cavity,and the maximum and minimum cavity shedding frequencies are 43.67 Hz and 42.8 Hz,respectively.(2)Under the same particle diameter,as the particle concentration increases,the particle erosion on the surface of the specimen increases;under the same concentration,as the particle size increases,the erosion rate increases.(3)Under different viscosity conditions,when the particle size is0.023mm,the relative erosion rate increases with the increase of viscosity.When the particle diameter is 0.046 mm,the relative erosion rate decreases slightly with the increase of viscosity.It shows that the smaller the particle size is,the greater the influence of viscosity on the erosion results,and the larger the particle diameter,the effect of viscosity on the results is gradually reduced. |
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