Numerical and Experimental Study of Mass Transfer Through Cavitation in Turbomachinery

Abstract

The vapour generation in a liquid can be caused by two different mechanisms: following a heat input, thus an increase in temperature at constant pressure, which is well known as the boiling phenomenon, or, at constant temperature, a decrease of pressure, which corresponds to the cavitation phenomenon. When the liquid pressure decreases below the saturation pressure, some liquid undergoes a phase change, from liquid to vapour. The saturation pressure, pv, is a fluid property which depends strongly on the fluid temperature. The cavitation phenomenon is manifested, in the fluid flow, by the formation of bubbles, regions of vapour or vapour eddies. The cavitation phenomenon frequently occurs in hydraulic machines operating under low pressure conditions. The cavitation phenomenon causes several undesirable effects on this type of machines, for example: the noise generated by the mass transfer between the phases, the efficiency loss of the hydraulic machines, and the erosion of certain elements caused by the vapour bubbles collapses near walls. Additionally, it should be mentioned the flow instabilities caused by the vapour appearance, such as alternate blade cavitation and rotating blade cavitation (Campos-Amezcua et al., 2009). The formation of cavitating structures in the hydraulic machines, their geometry and more generally, their static and dynamic properties, depend on several parameters (Bakir et al., 2003), such as: • Geometrical conditions: profile, camber, thickness, incidence, and leading edge shape of the blades, as well as the walls roughness. • Local flow conditions: pressure, velocities, turbulence, the existence of gas microbubbles dissolved in the flow. • Fluid properties: saturation pressure, density, dynamic viscosity and surface tension. This chapter presents an analysis of the cavitating flows on three axial inducers. These studies include numerical analyses at a range of flow rates and cavitation numbers, which were validated with experimental tests (Campos-Amezcua et al., 2009; Mejri et al., 2006). The obtained results can be summarized of the following way: