Improved Performance of Waterfall Hydraulic Cross-Flow Turbines with and Without Guide Wall

Abstract

The improved performance of a waterfall cross-flow hydraulic turbine was investigated through combined experimental and numerical analyses to elucidate the effects of key design parameters, including tip-speed ratio, number of blades, and off-axis distance. Additionally, the influence of a circular guide wall on the performance of cross-flow turbine was investigated by installing it along the runner. The numbers of blades studied were 8, 12, 16, 24, and 32, and efficiency contours were evaluated for various combinations of off-axis distances and tip-speed ratios. Peak efficiency of the turbine increased with the number of blades, reaching 70% at 32 blades. Correspondingly, the optimum off-axis distance shifted from 0.35 to 0.48, approaching the blade edge with a minor change in tip-speed ratio from 0.7 to 0.6. The improved efficiency stems from local torque caused by flow across the entire blade due to an appropriate inflow angle at large off-axis distances, where a high blade count suppresses leakage flow as observed in the flow-field visualization. Furthermore, the results of the guide-wall study showed that the efficiency of the cross-flow turbine with a lower number of blades increased when using a circular guide wall along the runner. This was due to the suppression of leakage flow on the blades, even with fewer blades. However, the influence of the guide wall became saturated with an increasing number of blades, approaching the efficiency without a guide wall. The efficiency of the waterfall cross-flow hydraulic turbine after optimization was improved and approached that of a closed-type cross-flow hydraulic turbine, while the feature of easy and low-cost maintenance was retained.