Cavitation erosion in fluid flow

Selim, Sobeih Mohamed Abbas (1981) Cavitation erosion in fluid flow. University of Southampton, Doctoral Thesis.

Abstract

An experimental study of the erosion rate of cavitating flows in water past three different types of configurations for velocities of 24 to 40 m/s and cavitation number of 0.01 to 0.15 is presented. Within these velocity and cavitation number ranges the damage of each configuration is studied using the weight loss technique and also the surface deformation technique. The effect of flow velocity, cavitation number and cavitating source size on both the weight loss rate and surface roughness rate in the steady state zone is investigated. The investigation showed that the weight loss rate increased as WLR - un, the values of n varied widely depending on the cavitation source shape and size, erosion place and specimen's material. By relating the weight loss to the surface roughness for different configurations, a good correlation between them was obtained. This correlation is independent of the hydrodynamic factors and the source shape and size. A general classification of the cavity mechanics is presented based on the damage patterns, the magnitudes of the damage and the visual observations at similar flow conditions produced by various configurations. Based on the hypothesised bubble energy spectrum suggested by Hamnitt and the physics of the phenomenon of cavitation erosion as currently understood, theoretical models (semi-empirical equations) for predicting erosion rate are presented for two main types of cavitation. The effect of flow velocity and cavitation number on the pitting rate over aluminium foil surfaces are investigated for different configurations. In addition, a theoretical correlation between pitting rate per unit width, the maximum pitting rate per unit area and the cavity length is obtained. An experimental study of the variation of the cavity length with cavitation number for four different configurations is presented and a simple theoretical model is presented for the variation of cavity length with cavitation number produced by a bluff body. The variations of cavitation noise and both the weight loss rate and the pitting rate per unit width for different configurations are compared. The results indicate that there cannot be an exact correlation between erosion and the sound pressure level, and, on the other hand, there is no absolute relation between the sound pressure level and the severity of erosion.

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