Numerical Investigations on Part Load Performance of Francis Turbine

Abstract

To meet the variable demand of electricity, it is very common that the Francis turbine tend to be operated at part load conditions. In Francis turbine xed-pitch runner shows a strong swirl at the outlet of the runner at part load operating conditions. As the incoming swirling ow decelerates in the draft tube, a hydrodynamic instability arises which leads to formation of strong helical vortex, rotating like a whirling rope, in the centre of the draft tube. The precession of the vortex rope causes pressure

uctuation in the draft tube which can lead to variation in power output, vibration of the shaft and damage to the runner blade. All these factors ultimately results into decrease in e ciency at part load operations. So the main aim behind this project is to analyze the e ciency of Francis turbine at part load condition. The ow analysis inside the hydro turbine is very di cult and expensive through experiments. Hence, computational uid dynamics(CFD) has been used as numerical simulation tool for the analysis of ow in Francis turbine. In the present study, ow analysis of Francis turbine is carried out using commercial CFD package 'FLUENT'. The simulations were carried out at six operating points with three di erent turbulence models. It is found that standard k- model is more appropriate for the ow simulation in hydro turbines. The unequal ow distribution in stay vane passage has been found at lower than rated discharge. The regions of secondary ow, back ow, vortices and low pressure are observed in the draft tube at part load operating conditions. The operating characteristic curves predicted by the numerical simulation are compared with model testing results provided by manufacturer and similar trend is observed. Two Techniques to improve the part load performance are demonstrated.