Experimental Study of Cavitation in a Kaplan Model Turbine

Abstract

The cavitation processes present in a Kaplan model turbine was studied with the aim to identify mechanisms that promote erosive cavitation. The studies were carried out with high-speed filming, video filming and visual observations with stroboscopic light. A periodic pattern of the cavitating tip vortex was observed. The main feature of this pattern is that the cavitating vortex is bent towards the blade surface and transformed into cloud formations. These clouds were collapsing in a manner that suggests this to be an erosive process. The blade periphery, where the cloud collapses was observed, is known as an area likely to sustain damage. It was also found that these cloud formations appears in bands, with a periodicity which corresponds approximately to the spacing of the guide vanes. Bubble clouds and cavitating vortices was found to be shed from the sheet cavities which showed signs of re-entrant jets. The cavities at the blade root seem to be mainly of sheet or travelling bubble type, depending on running condition, some facts indicate the contribution of vortex motion as well.