Study of vortex generators on Francis turbine runner for variable speed operation

Abstract

In the present context, the use of energy from renewable sources is evermore growing to meet the goal of being carbon neutral and reduce the impact on climate change. The development and installation of intermittent renewable sources like wind and solar is boosted all around the world. As the name suggests, these energy sources cannot sustain the demand-supply curves due to their dependence on natural phenomena. So, hydropower must be used to regulate this flow of energy and bring stability to the grid. To accomplish such a task, hydropower plants must grow through abnormal operating procedures that can inflict severe consequences on their performance and structural stability. Several noble ideas have been pitched in and researched on in the recent decade to limit and avoid these scenarios. In case of most popular hydraulic turbine like Francis, the concept of variable speed operation has been introduced which can counter some of the complications brought by off-design conditions. However, the design and implementation of Francis runner with variable speed capability requires intensive research works and financial investment. As identified, from many recent investigations in off-design operations of Francis turbine, the main problem is separation of flow on runner blades at the inlet. The flow separation has been dealt effectively in other fields like aerodynamics and marine hydrodynamics with the use of vortex generators, which are simple passive devices attached on the working surfaces. These passive devices have proven to be very effective even in conditions with adverse pressure gradients. So, this research mainly focused on exploring the possibility of use of these devices on Francis runner blades with analytical, numerical and experimental methods. The vortex generators mainly function by drawing energy from mainstream flow and passing it on to the boundary layer which eventually help to sustain optimal flow under adverse operating conditions. These devices can enhance wall shear stress at boundary layers and turbulence kinetic energy in the flow which has been presented in Paper II. The efficiency and power output in the turbine have been significantly improved at offdesign conditions as well which has been presented in Paper I and Paper III. The performance of vortex generators depends heavily on the size, shape and design which should be thoroughly analyzed and curated properly which has been accomplished in this study. However, the procedures followed in this study have some shortcomings and should be avoided for the future research. The numerical results show a bit of over prediction on the improvement of performance as contrary to the experimental results. So, the turbulence models, boundary conditions and analysis settings should be chosen with utmost caution to have results within acceptable levels of uncertainty. The use of vortex generators on the runner blades in the experimental analysis resulted in vortex generators falling off during the analysis which resulted some offsets in the expected outcomes. Therefore, it recommended to select adhesives with good durability in the dry as well wet conditions and extensive preliminary stress tests should be conducted so prolonged experimental analysis can be conducted. Overall, the study is an excellent step towards improving performance of Francis turbine at the off-design conditions and with some technical adjustment, the technology can be implemented on prototypes as well.