Numerical study on Unstable Hydraulic Factors of Kaplan turbine with semi-spiral case at large flow rate conditions

Abstract

The operation of semi-spiral case Kaplan turbines at large flow rate conditions is restricted by factors such as severe cavitation and severe vibration, which limit the maximum output of the unit. In order to explore the main hydraulic factors that cause the poor operating stability of the unit at large flow rate conditions, and provide a reference for the operation and design of the turbine, the numerical simulation method is adopted to conduct the performance study of an semi-spiral case Kaplan turbine at different operating conditions, by comparing and analysing the results at the optimal conditions and large flow rate conditions, the unstable hydraulic factors at large flow rate conditions is obtained. The research results show that: at large flow rate conditions, the uneven flow distribution in the semi-spiral case cause vortices between the guide vanes, lead to the large fluctuations of blade torque, and generate unstable back-flow and vortices in the draft tube. These phenomena combined with the various gap flows in the runner, make the internal flow of the Kaplan turbine at large flow rate conditions deteriorate, cavitation increase, efficiency decrease, and cause severe vibration, thereby limiting the stable operation of Kaplan turbine at large flow rate conditions.