Study On Cavitation Erosion Mechanism Of Curved Specimen Surface Exposed To High-pressure Submerged Waterjet

Cavitation erosion has been frequently encountered in fluid machinery and fluids engineering.Such a phenomenon adversely influences the efficiency of fluid machinery and even results in the damage of fluid-wetted components.Heretofore,the mechanism of cavitation erosion has not been well understood.The study on cavitation inception and cavitation evolution,and the exploration of cavitation erosion mechanism,helps to mitigate the adverse effect of cavitation erosion,as well as to open the possibility of utilizing cavitation.In practical applications,curved surfaces are common for the wall of flow passage.Therefore,it is of significance to carry out a study on cavitation erosion of curve surfaces,which is expected to replenish cavitation erosion principles and to provide a sound reference for the design and operation of fluid machinery.In this dissertation,both experimental and numerical methods were used to study cavitation erosion of metallic materials as cavitation was produced through the submerged waterjet.The investigation of cavitation erosion was extended from flat to curved surface.The specimen surfaces with three radii of curvature,7.0 mm,9.0 mm and 10.0 mm were designed to explain the effect of surface curvature on cavitation erosion.Major works and the conclusions drawn from the study are as follows:(1)Experiments of cavitation erosion were performed based on a waterjet cavitation erosion experiment rig,which was built according to the ASTM G134 standard.Pure water was selected as the liquid medium.Effects of the standoff distance,material and liquid medium temperature were inspected.The eroded surfaces were observed and analyzed to explain the influence of operation parameters on cavitation erosion.Experimental results show that the standoff distance has a significant effect on cavitation erosion,the optimal standoff distance of the constructed test rig is 14.0 mm.The resistance to cavitation erosion varies considerably with the materials selected.SUS304 possesses the highest resistance to cavitation erosion among the four materials.Cavitation erosion is intensified as the liquid medium temperature increases.At a liquid temperature of 45°C,the highest cumulative mass loss is attained and cavitation erosion is the severest.At the initial stage of cavitation erosion,the central area is explicit and less eroded.As the cavitation erosion progresses,such a central area vanishes gradually.(2)Three curved specimens with different surface curvatures were tested on the waterjet cavitation erosion experiment rig.The mass loss of the specimens was measured.The cumulative erosion rate was calculated.Three-dimensional morphology of the eroded surfaces was observed.The surface roughness of the eroded specimen was obtained.Cavitation erosion characteristics of the three curved specimens were comparatively analyzed.The influence of surface curvature on the development of cavitation erosion was described and discussed.The results indicate that the resistance to cavitation erosion varies with the surface curvature.Nevertheless,there is no monotonous relationship between the curvature radius and the resistance to cavitation erosion.The largest curvature radius is associated with the highest mass loss.At the initial stage of cavitation erosion,the impacted surface can be divided into central area(less eroded)and a ring-type eroded area.As cavitation erosion proceeds,the demarcation between the two zones is absent.Among the three curved surfaces,the surface with the smallest curvature radius is featured by the largest diameter of the eroded area.The roughness of the surface with the largest curvature radius is the highest.Exposed to continuous impact of the cavitating waterjet,material is removed from the curved surface in a layer-by-layer pattern.(3)Computational fluid dynamics(CFD)technique was used to simulate cavitating flows in the test chamber and near the specimen surface.The geometric model used in the simulation was parallel with the experimental model.Both steady and unsteady numerical simulations were conducted to study the occurrence,development and collapse of cavitation in the test chamber.The flow information obtained numerically was expected to be considered in combination with the experimental results.The results show that an annular cavitation zone is formed at the nozzle exit,then the cavitation zone migrates with the main stream to the target specimen surface.High velocity occurs near the waterjet axis,and the interaction between the high-velocity liquid and surrounding water is weak.In the radial direction,velocity decreases rapidly,giving rise to high velocity gradients,enabling the occurrence of cavitation.As the waterjet stream arrives at the specimen surface,an annual area on the specimen surface,corresponding to the annular cavitation zone in the waterjet stream,is subject to cavitation erosion,which is consistent with the experimental results.Regarding thespecimen with the curvature radius of 7.0 mm,the surface area covered by cavitation is the largest;therefore,the risk of cavitation erosion is the highest.The tangential velocity at the specimen surface is symmetrical with respect to the waterjet axis.At the same radial position,the highest tangential velocity is associated with the surface with the curvature radius of 9.0 mm,which is exposed to the lowest risk of cavitation erosion.In the experiment,the resistance to cavitation erosion of this specimen is the highest among the three curved surfaces.