Numerical Study of Cavitation in Francis Turbine of a Small Hydro Power Plant

Cavitation is undesirable phenomena and more prone in reaction turbines. It is one of the challenges in any hydro power plant which cause vibration, degradation of performance and the damage to the hydraulic turbine components. Under the present study, an attempt has been made to carry out a numerical analysis to investigate the cavitation effect in a Francis turbine. Three dimensional numerical study approach of unsteady and SST turbulence model are considered for the numerical analysis under multiphase flow such as cavitating flow. The performance parameters and cavitating flow under different operating conditions have been predicted using commercial CFX code. Three different operating conditions under cavitation and without cavitation with part load and overload conditions of the turbine for a plant sigma factor are investigated. The results are presented in the form of efficiency, pressure fluctuation, vortex rope and vapor volume fraction. It has been observed that variation in efficiency and vapor volume fraction is found to be nominal between cavitation and without cavitation conditionsat rated discharge and rated head. Turbine efficiency loss and vapor bubbles formation towards suction side of the runner blade are found to be maximum under overload condition. However, the pressure pulsation has been found maximum under part load condition in the draft tube. The simulation results are found to be in good agreement with model test results for efficiency. The locations of cavitating zone observed wellwith the result of previous studies.