CFD BASED IMPACT ANALYSIS OF CAVITATION IN FRANCIS TURBINE

Abstract

Reaction turbines basically Francis turbines and Propeller/.Kaplan turbines are suitable for medium and low head hydro power sites. The management of the small hydropower plants for achieving higher efficiency of hydro turbines with, time is an important factor, but the turbines show declined performance after few years of operation as they get severely damaged due to various reasons. One of the main reasons is erosive wear of the turbines due to cavitation. However reaction turbines are more prone to cavitation especially for Francis turbine where a zone in the operating range is seriously affected by cavitation and considered as forbidden zone. The phenomenon which manifests itself in the pitting of the metallic surfaces of turbine parts is known as cavitation because of the formation of cavities. Thoma's cavitation factor `o (sigma), which can be used for determining the region where cavitation takes place in reaction turbines, varies by varying the suction head. Computational fluid dynamics analysis of cavitation in reaction turbines could be cost effective solution for an extensive analysis. A computational fluid dynamics analysis of cavitation has been carried out in order to identify the different zones and impact of cavitation in Francis turbines by varying the critical parameter responsible for cavitation. Different values of cavitation factor are considered as the main parameter. Numerical simulation of 3.14 MW Francis turbine using commercial Navier-Stokes code called `FLUENT' has been carried out. In first step the creations of 3-dimensional computational model of Francis turbine in GAMBIT and then numerical simulation in carried out by using FLUENT. The simulation is carried out at different operating conditions by varying the values cavitation factor. It is found that, a low pressure zone is developed near the inlet portion of the draft tube and inlet and outlet of the runner which causes pressure fluctuation in the draft tube and runner which may lead to variation in power output, vibration of the shaft, damage to the runner blade and hence ultimately results in decrease in efficiency. It is also found that, due to acceleration of the flow in the draft tube, pressure in the bend portion goes below the vapor pressure of water and hence this region is prone to cavitation