Essay Example on Cavitation erosion of hydraulic machines

As the speed of hydraulic machines increases these machines are more prone to cavitation erosion that reduces the machine performance and the lifetime Cavitation erosion is defined as the formation and subsequent collapse of cavities or bubbles that contain vapor or mixture of vapor and gas within a liquid 1 The collapsing of the bubble will produce highly localized large amplitude shock waves and or microjets that hit the solid surface in contact with the liquid Accumulation of this loading due to repeated collapses causes local surface damage The early features of the damage are pit formation and surface deformation These features rapidly develop and lead to a loss in the surface material This process of development is still controversial or ambiguous A vibratory cavitation erosion test is widely used in studying the erosion mechanisms 2 and factors influencing the erosion damage such as liquid properties test conditions It is well recognized that the rate of erosion is time dependent where this rate increases from trivial values reaches a maximum then decreases and levels off to a steady value 3 Therefore cavitation erosion rate of a material can be divided into four stages according to ASTM standards G 32 1 an incubation period an acceleration period a steady state period and attenuation period 

The specimens were in form of flat surfaced discs of 15 9 mm in diameter and 11 mm in height with threaded sections for attachment to the cavitation vibratory horn The initial roughness of the samples is recognized to be of great importance in the development of damage features such as the creation of pits 4 the facilitation of crack initiation 5 the development of fracture and the increased rate of linear particle removal 5 The samples were polished with grade 3000 dry emery paper Samples were submerged 1000 ml open glass beaker containing 700 ml of tap water Since the test liquid temperature significantly affects the degree of cavitation erosion the test water temperature in the beaker was kept constant using the temperature controlled water bath as shown in Fig 1 The test water temperature was 22 1 C Two specimens were tested at successive intervals of 5 10 15 45 min Such short experimental time i e elapsed time is within 5 min was scheduled to reduce other damage effect on the surface such as corrosion