| Hydraulic machinery is the core equipment for water energy conversion and water resources regulation.It is widely used in hydraulic engineering,power industry and other fields.Solid particle erosion is one of the important reasons for the efficiency reduction and failure of equipment and components.The erosion prediction method with the fixed boundary can only obtain the erosion characteristics of the initial boundary,but cannot obtain the dynamic response of the flow field after the wall is damaged.A numerical prediction method of the progressive erosion based on the dynamic boundary was proposed for flow-passage walls of hydraulic machinery.And the multiphase flow characteristics and erosion evolution characteristics caused by the evolution of the erosion morphology of the walls were carried out.It provides reference for optimal design and erosion protection of equipment and components of hydraulic machinery The main work and research conclusions are as follows.(1)A numerical prediction method of the progressive erosion based on the dynamic boundary was proposed for the flow-passage walls of hydraulic machinery.The selection principles of the mesh size of the flow-passage walls and the mesh movement time step,as well as the principle of mesh movement and mesh smoothing were considered.In the process of multiphase flow solution,the particle dynamics parameters were captured,as well as the multiparameter erosion model,particle rebound model and the mathematical model of the erosion depth were coupled to establish a numerical prediction method for erosion morphology.This method can be applied to solve steady-state flow field and transient flow field.The average error of erosion depth prediction compared with the experimental value is 12.73%,and the prediction accuracy is improved by 28% compared with the fixed boundary prediction method.(2)The selection principle of spatiotemporal parameters affecting the prediction accuracy of erosion depth was studied.The selection of the initial mesh movement time step should satisfy the percentage of its ratio to the total prediction time is not more than 1%.In the erosion prediction process,the mesh movement distance should not be greater than 1.02μm(Threshold value of mesh movement distance)under the unit mesh movement time step.Moreover,the mesh size of the flow-passage wall of the erosion part should satisfy the percentage of its ratio to the corresponding geometric size is not to be greater than 1.2%.(3)The influence of particle impact parameters on the erosion evolution characteristics was studied using the progressive erosion prediction method based on the dynamic boundary.a)The fluid velocity in the erosion pit is lower than the fluid velocity at the edge of the erosion pit,while the appearance of the erosion pit increases the impact angle of the particle.With the increase of the particle impact parameter,the area with the largest increase in erosion depth is also the area where the particle impact angle increases significantly over time.While,the outer region of the maximum erosion depth is the region where the particle impact velocity decreases most significantly with time,and this change range is directly related to the variation in the erosion depth.b)There is a power function relationship between the particle impact parameter and the erosion rate.After considering the erosion evolution of the walls,the influence of the particle impact parameter on the erosion prediction results is that the particle impact velocity is greater than the particle volume fraction,and the particle volume fraction is greater than the particle incident angle.When the particle impact parameters that have a serious impact on the wall evolution are studied,the prediction method that considers the wall erosion evolution should be used to avoid the error caused by the erosion prediction method with the fixed boundary.(4)The influence of particle physical parameters on the erosion evolution characteristics was studied using the progressive erosion prediction method based on the dynamic boundary.There is an Exp Assoc function relationship between the particle diameter and the erosion rate.The erosion rate increases with the increase of particle diameter.But when the particle diameter increases to 160μm,the increase amplitude of erosion depth decreases,which is closely related to the evolution of erosion.In the four sphericity conditions(0.58,0.76,0.86 and 1),the erosion caused by non-spherical particles is larger than that of spherical particles.There is a Lorentz function relationship between the particle sphericity and the relative erosion rate.When the sphericity is 0.76,the erosion rate is the largest,that is,the erosion caused by semicircular particles is about 1.2 times that of sharp particles or round particles.(5)The erosion mechanism of the typical flow-passage wall of hydraulic machinery is carried out using the progressive erosion prediction method based on the dynamic boundary.and the applicability of this method is verified.a)For the erosion problem of flow around airfoil,the most severely erosion region is in the leading edge of the guide vane.In the early stage of erosion,the erosion on the upper and lower sides of the leading edge of the guide vane is more serious than that in the middle area.As the evolution of erosion,there are obvious erosion pits in the upper section near the head cover and the middle section of the leading edge of the guide vane.b)For the erosion of the curved channel wall,the erosion in the 90° curved channel is mainly concentrated near the end(50°～90°)of the curved channel.And after 20,000 hours,the average erosion rate of the outer side(large radius side)of the curved channel is about 2 times that of the inner side(small radius side).The erosion in the 45° curved channel is focus on the end(30° ～ 45°)of the curved channel.Under the same working conditions,the erosion of a 90°curved channel is about three times that of a 45° curved channel.c)For the erosion of doublesuction pump,as the increase of erosion time and the changes of blade surface morphology,the head and the efficiency loss rate roughly show a linear increase trend,and the increase slopes are 0.00109 and 0.0027,respectively.And the particles continue to accumulate to the back of the blade,the particle velocity shows an increasing trend from the hub to the rim,and the average erosion rate and particle impact mass of blade increased.The above indicating that the subsequent erosion is still on the blade trailing edge. |
PDF Full Text Request