Inception of Cavitation From a Solid Surface

Abstract

This study of cavitation inception from a solid surface includes the presentation of an empirical model for the nuclei, a completely new design for experimental test equipment, a rigorous analysis of liquid conditions within the test section, and quantitative test data. The results prove that it is possible to reduce the effective size of cavitation nuclei, thereby making the interface more resistant to cavitation. The empirical model for cavitation nuclei provides the mechanism for bubble formation from gas-filled, submicroscopic crevices in a solid surface. The size of these nuclei may be reduced by forcing the liquid into these crevices. If the liquid-solid interface is hydrophilic, the pressure treatment is permanent, but the liquid will pull out of a hydrophobic interface thus providing only a temporary reduction of nuclei dimensions. The newly designed experimental equipment for cavitation inception studies has been demonstrated to produce cavitation under controlled conditions. The test section resembles a 180-deg journal bearing constructed in three slices. The middle slice is slid away from the journal to produce a diverging wedge which induces a tensile stress in the liquid. The inception stage of cavitation is detected by an acoustical probe. The temperature and pressure distributions in the thin film are calculated numerically by successively solving the energy and generalized Reynolds equations. A strongly implicit iteration algorithm is employed to solve the large set of simultaneous algebraic equations resulting from placing the nonlinear elliptic partial differential equations in finite difference form. Experimental data from test runs with petroleum oil, glycerin, and a glycerin-soap mixture showed that stress required for inception of cavitation could be determined with a repeatability of ±1.4 × 104 Pa (±2 psi). A linear relationship was experimentally determined to exist between the magnitude of pressure forcing oil into the crevices of a steel surface and a capacity of the oil-steel interface to withstand a tensile stress.