Evaluation of a LES cavitation model to simulate the vortex rope phenomenon

Abstract

The new trend on the turbine development is to extend the range of operation, both at part load and at full load conditions. However, it is well known that the dynamic of the flow can considerably change leading tounstable conditions. In the worst case, the flow variation can damage significantly the whole power plant.The operation of a turbine at part load or at full load conditions changes the axial and tangential velocity distributions at the inlet of the draft tube inducing the development of the well-known vortex rope. The volume of the vortex rope is changing over time resulting in a pressure variation that can be felt throughout the power plant. It is thus primordial to predict accurately the dynamical behavior of the vortex rope in order to avoid that the Eigen frequency of the vortex rope reach the Eigen frequency of the hydraulic system. Therefore, the challenge is to predict accurately the behavior of the flow field, which requires to take into account the cavitation phenomenon. The present paper focuses first, on a non-cavitating flow to evaluate the accuracy of a LES computation of the vortex rope phenomenon. The computed flow selected is the one in a test model Francis turbine operated at part load condition （65% BEP）. Second, a cavitating computation is performed to evaluate the performance of the Okita cavitation model implemented in an in-house LES code. The computed flow selected is the flow around a Clark-Y11.7% for two different Angle Of Attack: 2 and 8 degrees.