The present research aims to experimentally study the dynamic characteristics of a rotating bubbly flow in the vaneless gap of a reversible pump-turbine operating in condenser mode by performing high speed image acquisition. Image processing is developed to detect and track the bubbles in the flow for estimating their size and shape and for measuring the velocity profiles. The influence of the gauge pressure is investigated by performing the experiments at five gauge pressure conditions between 2 bar and 17 bar.

The experimental investigation evidences the formation of an air-water ring in the vaneless gap between the impeller blades and the closed guide vanes. The air-water ring is characterized by a rotating bubbly flow due to the air-water mixing. Bubbles have an oblate spheroid shape whose size and velocity depend on the gauge pressure. The velocity is mainly tangential and a periodic fluctuation of both velocity components is recorded owing to the geometrical shape of the guide vanes walls.