Modelling and optimisation of a Kaplan turbine — A comprehensive theoretical and CFD study

Undoubtedly, the whole world has been forced to increase the use of clean/renewable energy sources particularly due to the alarming environmental concerns that have been caused by fossil fuels based emissions/pollution and also with the predicted depletion of those fossil fuel reserves. Over the last few decades, the global fossil fuel consumption has increased significantly (i.e., 136,761 TWh in 2019 which is a 45% increase since 2000) with the rapid industrialisation of many economies and also with the increase of the energy intensiveness of the domestic lifestyle. Hence, switching to renewable energy sources is a current key global priority to restore the balance of nature. Of the renewable energy reserves, hydropower has been one of the major sources which date back to 1770s. The efficiency of hydropower turbines is highly affected by the performance of their runner wheel as it serves as the main unit that is responsible for extracting the energy from the flowing water. Hence, this research aims to investigate the design and optimisation approaches for a runner wheel of a Kaplan turbine. Initially, a detailed theoretical design was carried out for defining the main features of the runner and it gave a theoretical efficiency of around 94%. Usually, theoretical designs are based on numerous simplifying assumptions and hence these designs might provide an approximate solution only. This fact was established as the CFD model implemented for the exact same theoretical design achieved only an efficiency of 59.98%. Hence, CFD was used to further analyse and also to modify the theoretically proposed design where the effects of the inlet/outlet tangential velocities of the blades, pressure distribution across the runner, and the number of blades were evaluated and modified to improve the energy extraction capacity of the runner. The CFD results showed that the original theoretically designed runner could be optimised to reach an efficiency of 93.01% with carefully performed modifications. Overall, the inlet and outlet angles of the blades presented a significant impact on the Kaplan turbine's power generation performance. Eventually, a comprehensive comparison of the theoretical, CFD-theoretical and CFD-optimised designs are presented with possible recommendations for improving the power generation efficiency of a Kaplan turbine.