Experimental evaluation of a shrouded horizontal axis hydrokinetic turbine with pre-swirl stators

Improving the efficiency and power output of hydrokinetic turbines is critical to making them a viable and cost-effective renewable energy solution. Diffusing shrouds have already been shown to improve the performance of horizontal axis, axial-flow turbines. It has been suggested that adding pre-swirl stators to the shroud could alter the inlet flow so as to maximize net tangential force on the turbine blades, thus increasing power output. There is a scarcity of published physical model test data from pre-swirl stator turbines. The present study explores this concept. Four different pre-swirl stator configurations were designed and tested on a shrouded 3-bladed turbine (26.54 cm diameter) in the 36.5 m towing tank at the United States Naval Academy. Three Reynolds numbers (6.07 x 10(4), 8.10 x 10(4), and 1.01 x 10(5) based on stator chord length), corresponding to flow speeds 0.91 m/s, 1.22 m/s, and 1.52 m/s respectively, were tested. At the lowest Reynolds number, all 4 stator configurations improved the maximum C-p by 5-23% compared to the turbine with shroud only. At the middle and highest Reynolds numbers, the stator effect was mostly neutral, with one configuration showing a negative effect at the highest Reynolds number (i.e., lowered the maximum C-p). These results indicate that pre-swirl stators have the potential to improve turbine performance, particularly at lower Reynolds numbers, where the flow is more laminar and the absolute velocities are lower (for a given blade dimension). Pre-swirl stators also appear to reduce the dependence of performance on Reynolds number, flattening the curve and allowing optimal performance over a wider range of operating conditions. Further analytical and computational studies are recommended to explore these findings further, and to develop broader guidance on when pre-swirl stators are advantageous.