Numerical Investigations on Cavitation Behavior of Francis Turbine

Abstract

In Francis turbine xed-pitch runner shows a cavitation on the runner blades and draft tube zone, so low net positive suction head may form in Francis turbine, means actual pressure of a liquid may become close to the liquid's vapor pressure at a given temperature. The precession of cavitation shows pressure uctuation in the draft tube which may lead to variation in power output, vibration of the shaft, corrosion and damage to the runner blades. All these factor ultimately may result into decrease in e ciency. So the main aim behind this project is to analyze the cavitation behavior of Francis turbine. The ow analysis inside the hydro turbines is very di cult and expensive through experiments. Hence, computational uid dynamics(CFD) has been used as numerical simulation tool for the analysis ow in Francis turbine. In the present study, ow analysis of Francis turbine is carried out using commercial CFD package 'FLUENT/ANSYS-CFX'. The simulations were carried out under non-cavitationg and cavitating conditions. For turbulence modeling, SST k ! model is used. The cavita- tion analysis was carried out using mixture model which is capable of solving homogeneous ow of water liquid and water vapor. The operating characteristic curves predicted by CFD simulations were compared with the model testing results provided by manufacturer, which shows good agreement. To improve the cavitation performance of Francis turbine. Two techniques are presented which shows decrease in cavitation number and improvement in e fficiency.