A CFD Study on Vortex Control Techniques in Micro-class Francis Hydro Turbine

Abstract

Micro-hydropower stands out as one of the most cost-effective energy technologies for hydroelectricity by tapping local water resources and catering to green energy generation. Francis Turbines are emerging as efficient and better-performing turbines for Micro Hydro Power (MHP) generation schemes. With robust computer technologies and design developments, a Computational Fluid Dynamics (CFD) based study on design, performance evaluation, flow analyses and vortex control techniques are carried out in this research work. A preliminary mathematics to design a 70kW Francis hydro turbine for small scale hydropower plant has been dealt with out of available hydrodynamic parameters of head, discharge and rotational speed of the turbine. The adopted direct method of design procedure is based on the basic principal of fluid dynamics of turbo machineries, turbine design theory and rule of thumb which was later fine-tuned in solid modeling tools. The so designed turbine was numerically analyzed to evaluate its Best Efficiency Point (BEP) and its performance in part-load operating regimes.

A time dependent numerical simulation was then carried out at its full load to study the interaction between rotating and stationary components, and at its partial load to study draft tube surge and flow instability brought about by vortex shedding. Pressure fluctuation, torque variation and level of vibration were the parameters of interest in this analysis. A periodical behavior was observed for pressure distribution and torque variation in runner blades at full load while a distinct vortex rope was observed in draft tube at part load operation as the flow became unstable due to swirl component of the velocity attached with the exiting flow downstream the runner. The swirling flow at the runner outlet generated a corkscrew-shaped vortex resulting in pressure pulsation, fluctuation in torque, axial and radial forces and structure vibration causing the turbine to suffer loss in its performance.

In order to minimize the vortex shedding and the flow instability, three different vortex control techniques viz. Misaligned Guide Vanes (MGVs), hub modification of runner and J-grooves in draft tube have been proposed and evaluated numerically.

Two axisymmetrically located misaligned guide vanes (MGV) in casing did not contribute in controlling the vortex breakdown. The unstable flow was further aggravated by the use of MGVs. As for runner hub modification, two different profiles of runner hub were analyzed and their effects on the flow in the draft tube were examined. The modified hub did alter the flow instability in the draft tube, relatively minimizing the swirl velocity and intensity of the vortex rope however at the expense of some efficiency. This opened an avenue for the design optimization of the modified hub for better flow control and for suppressing the vortex breakdown. Likewise, the use of J-grooves in draft tube turned out to be an effective technique to minimize the swirling flow and recover energy loss in the draft tube. Five different cases of J-grooves, by varying its number and depth, were numerically analyzed. It was inferred that all the cases altered the flow configuration of the draft tube without significant loss in the efficiency at given operating point. However, the level of surge control changed with different number of grooves and their depths. For the given level of swirl and vortex breakdown with base model draft tube, a draft tube with best number of grooves and depth was figured out that helped minimize vortex at larger extent.

Swirling flow in draft tube at off design operating regimes of Francis hydro turbine is a major operation challenge and the aforementioned techniques can be optimized and applied to mitigate the flow instability.