Prediction of incipient cavitation in micro tubular propeller turbine

Abstract

Cavitation is a phenomenon that cavities or bubbles form in a liquid when static pressure falls below the vapor pressure. It can damage the machine, hence it is recommended to be avoided in the design and operation of the machine. In this study, the cavitation characteristics were numerically investigated with a change in the flow rate of a micro tubular propeller turbine. Here, the rotational speed was set to the value corresponding to the best efficiency point for each flow rate. A multiphase flow analysis incorporating water and water vapor was conducted, using the Zwart-Gerber-Belamri (ZGB) model as the cavitation model. Results show that the tip clearance is the first region where the cavitation starts to occur as the flow rate increases. At this time, the Thoma number was found to be 1.178. The second region where the cavitation occurs with further increases in the flow rate is in the vicinity of the leading edge. It turned out that the leading edge cavitation occurs when the Thoma number is below 1.111. As the tip clearance increases, the range of the flow rate that cavitation does not occur increases, but it reduces turbine performance. Therefore, the selection of appropriate tip clearance given the operating flow rate range and performance needs to be considered.