Mitigation of Rotating Vortex Rope in a Hydro Turbine using a Guide Vane System

Abstract: Using renewable energy, such as water or wind, to produce electricity has been proven highly effective in Sweden. The ability of these renewable resources to make clean output energy counters the adversities caused by non-renewable resources. Amongst these, hydro turbines are one of the most preferred methods for generating electricity from water. Today, the hydro-turbines are designed to run at their peak efficiency only at a single operating condition called the best efficiency point (BEP). However, the energy consumption increase necessitates the hydraulic turbine’s flexible operation. The issue of pressure pulsations in the draft tube of hydro-turbines, observed at part load operating conditions, has been unresolved for many years. These pressure pulsations are related to the rotating vortex rope (RVR) seen at part load operation and adversely affect the hydro turbine’s lifespan and performance. Several techniques have been investigated in the past to reduce the pressure pulsations in the draft tube of a hydro turbine at part load operation and enhance the operational flexibility of the turbine. However, there is no universal solution. The current study presents a proof-of-concept of mitigation techniques to suppress pressure pulsations in a mixed-flow reaction turbine. The technique involves implementing a variable guide vane system in the draft tube of the Francis-99 model turbine. Guide vanes are mechanical devices that can direct the flow in the desired direction. Therefore, they can suppress the pressure pulsations by re-directing the swirling flows in the draft tube at part load. The current research involved both numerical and experimental investigations. The guide vane system was initially designed using numerical methods. The geometry of the guide vanes, including the number of guide vanes, the chord length, the span, and the location of the guide vanes in the draft tube, is designed through numerical simulations. The results show that a set of three guide vane systems with a chord length of 86% runner radius and a leading-edge span of 30% runner radius is an ideal design that mitigates RVR when placed at some distance from the runner in the draft tube. For the experimental proof of concept, the guide vanes were tested on the Francis-99 model turbine at the Norwegian University of Science and Technology, Trondheim, Norway. During experiments, the guide vanes can rotate up to ±45°. There were six guide vane orientations selected between ±45°. The results indicate that the guide vanes can mitigate the pressure pulsations with a marginal efficiency loss in the turbine at part load operational regimes. However, there are slight efficiency improvements at BEP and high load.