Performance and flow characteristics of guide vaneless bulb turbine for ultralow head micro hydropower application

The growing demand for renewable energy and the current focus on intermittent sources, such as wind and solar, have made it imperative to increase hydropower output for grid resilience. This work focuses on the design, development and characterisation of a bulb turbine for microhydropower generation. To achieve the requirements of reduced cost and complexity while retaining a flat efficiency curve, this work proposes a novel bulb turbine without the guide vane. Design is followed by experiments and numerical simulations. The results show that a maximum efficiency of 74.1% is achieved with a flat efficiency-flow rate curve across the entire flow rate range studied (0.75≤QQbep≤1.3). Loss increases with increasing flow rate. A shaft sleeve with a diameter equal to that of the runner hub improves pressure recovery in the draft tube. The influence of the turbine runner geometry on flow phenomena, such as vortex formation and breakdown, and the generation of synchronous and asynchronous pressure fluctuations, is highlighted through a study of the flow field. A rotating vortex core, with a normalised precession frequency varying between 0.2 and 0.48 over the flow range studied, is observed for both design and overload conditions, but not at part-load.