A study on performance and flow characteristics of single and double H-type Darrieus turbine for a hydro farm application

Abstract

The wake of a turbine has a significant effect on the performance of downstream turbine for a hydro farm application's energy conversion in a channel flow. The performance and wake structure of the three-bladed H-type Darrieus turbine with NACA 4418 blade profile were measured in an open water channel using torque sensor and Particle Image Velocimetry (PIV) method and computed by Large Eddy Simulation (LES). Firstly, the torque measurements of the single turbine were completed for a wide range of tip speed ratio (TSR) 0.5-1.4. According to the experimental measurement and LES results, the maximum power coefficient values were obtained as CP = 0.146 and CP = 0.163 at TSR = 1.0, respectively. Then, the velocity field, vorticity pattern, and turbulence kinetic energy in the wake region of the single turbine were qualitatively and quantitatively examined with PIV and CFD at TSR = 1.0 in the horizontal plane. The time-averaged flow field contours and spanwise profiles showed that the PIV results are in good agreement with the LES model to determine the complex flow in the wake of the turbine. The turbine's wake structure exhibited asymmetrical velocity distribution along the downstream due to dynamic stall phenomena and rotation effect. The minimum streamwise velocity values were found to be 73% and 82% of the free-stream velocity at x/D = 6 for PIV and LES results, respectively. However, as the flow moves downstream in the wake, it increased up to 84% and 94% of the free-stream velocity at x/D = 11. The stronger peaks of turbulence is measured behind the turbine at x/D = 0.8 and close to the zero at x/D = 11. Lastly, considering the wake characteristics of a single turbine, the effect of co-/counter-rotating, TSR, and streamwise distance on the performance of a downstream turbine, which was placed in the in-line direction of the upstream turbine, is experimentally and numerically investigated for its use as a hydro farm in a open channel flow. The downstream turbine reached the maximum performance at approximately TSR = 1.0-1.1 for all downstream locations and both rotation direction. In both method, the downstream turbine reached approximately 72% performance of the upstream turbine for the co-rotation at x/D = 11 location, however, this value increased to 78% in the counter-rotation. Due to the asymmetrical wake structure, the results showed that a 6% increase in performance of the downstream turbine can be achieved in the counter-rotating arrangement.