Publication: Performance study of conventional savonius hydrokinetic turbine with two flat deflectors
Date
2022-03-01
Authors
Salleh, Mohd Badrul
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Abstract
Hydrokinetic turbines (HKT) have great potential for providing sustainable energy. Savonius HKT is considered the best option for low-cost applications due to its simplicity and ability to operate under low flow rates. The deflectors have been shown to improve the low performance of the turbine. However, the use of two deflectors for a conventional Savonius HKT, particularly in flows with low and varying speeds is still lacking. This research aims to improve the performance of the conventional Savonius HKT by incorporating two flat deflectors. The power performance of the turbine was investigated in a wind tunnel with respect to three deflector parameters: angle, longitudinal position, and height. First, the turbine performance was evaluated on 2-bladed and 3-bladed turbine models with 30 combinations of the deflector angles of the advancing and returning blades, 𝛿𝐴 and 𝛿𝑅,
respectively at 7.0 m/s airflow speed (equivalent water flow speed of 0.40 m/s). The better-performing 2-bladed turbine model was then selected for further investigation on 9 combinations of the deflector longitudinal positions of the advancing and returning blades, 𝑋𝐴⁄𝑅 and 𝑋𝑅⁄𝑅, respectively, followed by 3 different deflector heights: 0.5𝐻, 1𝐻, and 1.5𝐻. The experiments on the 2-bladed turbine were then repeated at various Reynolds numbers, 𝑅𝑒 ranging from 9.27 × 104to 1.48 × 105. The flow patterns surrounding the 2-bladed turbine were visualized using smoke generators to provide additional insights into the flow behaviors across various deflector configurations. The wind tunnel results were validated (within the experimental errors) by testing the 2-bladed turbine in a water channel for the same parametric variations of the deflectors with dynamically similar flow conditions. The maximum coefficient of power, 𝐶𝑃𝑚𝑎𝑥 of the 2-bladed turbine model with the two deflectors was 14% higher than that of the 3-bladed turbine. The 2-bladed turbine model exhibited the highest 𝐶𝑃𝑚𝑎𝑥 of 0.210 with 61.54% improvement relative to that of the turbine without the deflectors at the optimal deflector angles of 𝛿𝐴 = 30° and 𝛿𝑅 = 90°. The 𝐶𝑃𝑚𝑎𝑥 was further increased to 0.261 with 100.77% improvement at the optimal deflector longitudinal positions of 𝑋𝐴⁄𝑅 = −0.500 and 𝑋𝑅⁄𝑅 = −1.204, and at the optimal deflector height of 1𝐻. These optimal deflector configurations remain unchanged as 𝑅𝑒 increases, regardless of the increment in the turbine performance. In-depth investigation using flow visualization revealed that the deflector configurations influenced the surrounding flow patterns but were independent of variations in 𝑅𝑒. Optimizing the deflector configurations effectively directed the flow towards the advancing blade while preventing it from impinging on the returning blade, resulting in an increase in the positive net torque and thus the performance of the turbine. Findings from this research demonstrated the efficacy of using two flat deflectors in solving the problem of low power performance of the conventional Savonius turbine.