In this paper, measurements and computations are performed to study the performance of a 45-deg twisted Savonius rotor with a modified profile, at various overlap ratios (δ), aspect ratios (AR), and wind velocity (V). A free air jet test rig is used to carry out the experiments, while three-dimensional unsteady Reynolds-averaged Navier–Stokes (URANS) equations are used, in conjunction with the renormalization group (RNG) k–ɛ turbulence model, to perform the computations. The present experimental results successfully verify the simulation predictions obtained by the selected turbulence model. The RNG k–ɛ turbulence model has been chosen based on previous tests performed and published by the authors. Furthermore, both torque coefficient (CT) and power coefficient (CP) are numerically predicted at various tip speed ratios (λ) for overlap ratios (δ) ranging from 0.0 to 0.5, aspect ratios (AR) ranging from 0.75 to 3, and wind velocity values ranging from 4 to 18 m/s. Unlike the conventional rotor, the present twisted rotor with a modified profile produces significant performance improvement in the case of modified rotor without overlapping (δ = 0.0). Moreover, the peaks of CT and CP of the twisted rotor with the modified profile are enhanced with the increase in the aspect ratio. However, the percentage increase is noticed to be insignificant for AR greater than two. The maximum power coefficient (CPmax) for the twisted rotor with the modified profile and optimized design is 0.305 at a wind velocity of 6 m/s, with a performance gain of 75.3% compared to the conventional Savonius wind rotor which has CPmax=0.174.
Although using a multi-stage rotor of Savonius vertical-axis wind turbine enhances the self-starting ability, it reduces the power coefficient. To improve power coefficient, the influence of varying the stage aspect ratio is investigated. Therefore, two-, three-, and four-stage Savonius rotors at stage aspect ratios ranging from 0.5 to 1.5 with increments of 0.25 are considered. To determine performance parameters such as coefficients of torque, power, and thrust, a comprehensive three-dimensional unsteady incompressible turbulent flow model using Reynolds-Averaged Navier-Stokes (RANS) equations along with k-ω shear stress transport turbulence model is developed. The developed numerical model is validated utilizing the available experimental results. Moreover, a novel assessment technique relying on flow field characteristics such as pressure distribution in conjunction with streamlines around the proposed multi-stage Savonius rotor with various stage aspect ratios is carried out. The contribution of each stage on the performance of the whole rotor is computed and presented. The findings of the current study illustrate that utilizing a multi-stage rotor with stage aspect ratio equal to or greater than 1.0 significantly enhances the output power. By rising the stage aspect ratio within the range of 0.5 to 1.5, the peak coefficient of power boosts from 0.163 to 0.213 for a two-stage rotor, and from 0.183 to 0.23 for a four-stage rotor. In addition, three-stage rotors with stage aspect ratio ranging from 0.5 to 1.5, shows increased average static coefficient of torque from 0.196 to 0.272 with positive values at whole rotation angles. This improves the self-starting abilities of the multi-stage rotor and makes it suitable in areas where the wind is intermittent and very low. Furthermore, raising the stage aspect ratio from 0.5 to 1.5 significantly mitigates the oscillations of both torque and thrust coefficients throughout the entire cycle for all multi-stages. This lowers the mechanical vibrations and noise emission during operation conditions. Accordingly, multi-stage Savonius rotors with stage aspect ratio equal to or greater than 1.0 are highly recommended for practical applications.
Wind energy comprises one of several renewable resources of energy engineered to contain the global energy crisis. Although horizontal axis wind turbines (HAWTs) have proven to be effective in low turbulence and steady wind conditions, vertical axis wind turbines (VAWTs) potentially have the advantage in highly variable and turbulent regions. The Savonius vertical axis wind turbine has several advantages such as simple design, low manufacturing costs, low operating wind speed, low noise, and Omni-directional capability. However, the Savonius rotor requires further design optimization to improve its aerodynamic performance before becoming competitive with other turbine designs.
Thus, the main objective of the current study is to numerically investigate the aerodynamic performance of a multistage Savonius rotor to enhance the power coefficient and the ability of self-starting. In the current study, one-, two-, three-, and four-stage Savonius rotors with twisted blades are investigated. In a two-stage rotor, one single-stage rotor is mounted over another single-stage with a phase angle of 90°. In a three-stage rotor, the three single-stage rotors are mounted one above the other with a phase angle of 60° relative to one another while with a phase angle of 45° for the four stage-rotor. The blades of the studied Savonius rotor are twisted with a twist angle (φ) of 45°.
This is the first contribution to understand how multi-stages influence the aerodynamic performance of the twisted-bladed Savonius rotor. Moreover, variations of torque and power coefficients are computed for all the studied rotors with various numbers of stages. The developed numerical model is simulated using ANSYS Fluent and validated using the available experimental and numerical results. Results showed that the coefficients of torque (CT) and power (CP) increase with rising the number of stages. Increasing the number of stages from 1 to 2 significantly increases the CT and CP of the rotor. However, with a further increase in the number of stages to 3 and 4 stages, both the CT and CP remains almost the same as the rotor with 2 stages. The maximum coefficient of torque (CT, max) and power (CP, max) for a two-stage rotor are 0.42 and 0.253, respectively. The gain in the coefficient of power obtained by using the two-stage Savonius rotor with twisted blades is 53.5% compared to the conventional single-stage which has a coefficient of power 0.165 at a wind velocity of 6 m/s. Moreover, using multi-stages and twisted blades significantly smooth the variations in the generated torque and produce positive values at all rotor angles resulted in improving the self-starting ability of the Savonius rotor.
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