Numerical analysis of the effect of runner-to-basin diameter ratio on the performance of gravitational water vortex turbine in a scroll basin

A gravitational water vortex turbine is a new development suitable for low to ultralow head with medium to low flow that also facilitates aeration of the water and harvesting of power during water transit. Recent investigations have shown that curved blade profiles are more efficient to harness kinetic energy of the vortex. However, understanding of the optimum runner position along the vortex flow field in a scroll basin is still incomplete considering the design parameters such as vortex-blade interaction, the runner-to-basin diameter ratio and its effect on torque and power output. In this regard, a numerical investigation on the performance of such a turbine has been carried out using OpenFOAM. The effect of runner to basin diameter ratio on performance parameters such as effective head, torque, power and efficiency has been characterized after validation of the methodology using analytical model predictions. The results suggest that relative size of a runner strongly influences the rotational speed acquired and torque developed on account of a stronger vortex near the air-core and lower tangential velocity at higher radii. This work demonstrates that either reducing the size of runner blades close to the orifice region or extending to the far-field region can both result in a reduction of the runner performance. The maximum efficiencies predicted are 25.8%, 42.9% and 41.2% for runner-to-basin diameter ratios 0.18, 0.27 and 0.36, respectively. The optimum runner size is observed to be one-fourth of the basin diameter and predictive performance correlations have been developed for power output and efficiency in this regard as a function of the rotational speed and runner diameter. Hence, the outcomes from this study will be helpful to design and predict performance of a gravitational water vortex power plant with scroll basin for a given flow and head at a particular site. Moreover, it can also be used as a pointer towards further development of gravitational water vortex turbine technology.