Influence of the Size of Cylindrical Geometry in the Nozzle Outlet on Aggressive Intensity of Cavitating Jet

摘要

A geometry of a nozzle outlet greatly influences an aggressive intensity of a cavitating jet. A nozzle with a cylindrical space at a nozzle outlet was reported to have an optimum ratio between d, D and L under injection pressures of 15 MPa and 30 MPa. d is the nozzle throat diameter, D is the cylindrical space diameter and L is the cylindrical space length. However, the optimum ratio under other injection pressures is not clear. Also, it is important to clarify characteristics of cavitation clouds periodically shedding from the nozzle for understanding the aggressive intensity of the cavitating jet. In this paper, in order to investigate an influence of the cylindrical space size at the nozzle outlet on the aggressive intensity of the cavitating jet, an erosion test of aluminum alloy JIS A1050P was carried out under a condition at injection pressure of 40 MPa. Also, the cavitating jet was shot with a high-speed video camera to understand frequency characteristics of cavitation clouds. In the range of 17 ≤ D/d ≤ 31 and 10 ≤ L/d ≤ 31, it was found that large D and L stabilize the shedding frequency of cavitation clouds and improve a straightness of the clouds. The increase in D and L enlarges a flow channel of filling flows from outside to inside of the cylindrical space, which occurs when the cavitation clouds are shedding. As a result, it is suggested that a flow field for clouds generation is stabilized and a resistance to clouds shedding is reduced. Also, increasing D and L increased a mass loss per unit area. Since an erosion area decreased with the increase of D and L, it is considered that an impact density of cavitation bubbles increased.