On the effect of flow rate on pressure fluctuations and cavitation characteristics in a novel bulb turbine

Abstract Bulb turbines, having low excavation cost, are promising solutions for harnessing energy from low/ultra-low head run-of-the-river hydropower sources. A novel bulb turbine with no guide vanes, having a uniform efficiency over a range of discharges, is tested. Unsteady numerical simulations are carried out for this bulb turbine over a range of ± 30% of the design flow rate ( Q d ) of 0.25 m 3 /s. Using a normalised Q-criterion to quantify the intensity of swirling flow inside the draft tube, the strong presence of hub and tip vortices is detected. The strength of these vortices increases with an increase in the flow rate. The amplitude and pattern of pressure fluctuations, monitored at different planes along the longitudinal direction, confirm the presence of vortex structures. Frequency analysis of pressure fluctuations shows that the blade passage frequency of the runner is the dominant frequency. With a flow field exhibiting significant fluctuations, a cavitation study is required to characterise the turbine. A turbine operating at various flow rate conditions is susceptible to encountering cavitation, especially at off-design conditions. The current study reports the effect of partial cavitation on the turbine performance at different discharges and assesses the suitability of the novel design from a cavitation perspective.