CFD study on prediction of vortex shedding in draft tube of Francis turbine and vortex control techniques

Abstract

Francis Turbine when operated at part load suffers flow instability due to swirl component of the velocity attached with exiting flow downstream the runner. The swirling flow at runner outlet generates corkscrew-shaped draft tube vortex invites undesirable flow features like pressure pulsation, fluctuation in torque, axial and radial forces and structure vibration causing the turbine. This paper presents the results obtained from the numerical analysis regarding prediction of unsteady vortex behavior at the draft tube and flow instabilities specific to Francis hydro turbines occurring at the part load operating regime, using ANSYS CFX. The RNG k-ε turbulence model is adopted for turbulence treatment and better prediction of the non-cavitating vortex core and its effects. A single part-load operating condition is chosen to analyze the vortex breakdown. To minimize vortex shedding and flow instability, two vortex control techniques viz. Misaligned Guide Vanes (MGVs) and J-grooves in draft tube are evaluated numerically. Two different MGV angles were selected and four different depths of J-grooves were analyzed separately to see their effect on flow features. The use of MGV did not help much in minimizing swirl intensity whereas the use of J-groove showed better swirl control and flow enhancement in the draft tube.