Effect of Flow and Fluid Structures on the Performance of Vertical River Hydrokinetic Turbines

Abstract

Field and laboratory measurements characterize the performance of vertical axis hydrokinetic turbines operating in uniform and non-uniform inflow conditions for river applications. High sampling frequency velocity measurements, taken at 200 Hz upstream of a stopped and operating 25-kW H-type vertical axis hydrokinetic turbine in the Winnipeg River, show the existence of large eddies with an order of magnitude of the turbine’s diameter. Scaling laws allow modeling river conditions in the laboratory for more detailed investigations.

A small-scale, 30 cm diameter, squirrel-cage vertical turbine designed, manufactured and equipped with a torque and position sensors is investigated for the detail behavior of the turbine subjected to different inflow conditions in a laboratory setting to study the effect of flow and fluid structures. The adjustable design of the laboratory turbine enables operations with different solidities, 0.33 and 0.67, and preset pitch angles, 0°, ±2.5°, ±5° and ±10°. Tests are first performed with uniform inflow condition to measure the sensitivity of the turbine to solidity, preset pitch angle, free-surface, and Reynolds number to obtain the optimum operating conditions. During the free-surface testing a novel dimensionless coefficient, clearance coefficient, is introduced that relates the change in turbine efficiency with change in the free-surface height. High-speed imaging at 500 fps of semi-submerged blades visualizes the vortex-shedding pattern behind the blades and air entrainment. High-speed imaging results of large eddy pattern behind the vertical turbine are consistent with theory and measurements.

Subsequently, cylinders of different diameters create non-uniform inflow conditions in the water tunnel by placing them at different longitudinal and lateral locations upstream of the model turbine. Thus, the effects of non-uniform inflow generated under controlled settings shows the impact of eddies and wake on the turbine performance. High sampling frequency measurements of torque and position at 683 Hz enables investigating the impact of flow variations on turbine performance in the frequency domain. These results are also useful for fatigue analysis.

Finally, entrained air bubbles in the flow—in river and laboratory settings—affect turbulence quantities, as measured using an acoustic Doppler velocimeter, and successfully addressed by implementing a new hybrid filter developed for this application.