Experimental and Computational Study of the Archimedes Screw Turbine

Abstract

The Archimedes Screw hydrokinetic turbine (AST) is garnering considerable interest because of its potential applicability in harvesting wave and tidal energy. The turbine is well suited to bi-directional flows, low-velocity flows, and shallow watercourses. Understanding its performance characteristics and energy conversion mechanisms will be fundamental in determining the optimal geometric properties, which will, in turn, expedite its use in onshore and offshore renewable energy systems. Because the AST is a reasonably new hydro-technology, very little literature is available on its design and performance optimization. This study experimentally and computationally investigates the torque and power generation of the AST. Laboratory scale turbine models with one, two, and three flights (blades) were tested in a water channel to measure torque and angular velocity at different flow velocities and varying inclination angles (β) of the turbine. From experimental results, a maximum coefficient of performance (CP ) of 0.41 was obtained at a tip speed ratio (λ) of 0.52 at a flow velocity (U∞) of 0.45 m/s for a turbine with two flights. In the case of the 3-flight turbine, the highest value of CP was also obtained at 30° (0.40 at λ = 0.53). For the 1-flight turbine, a maximum CP of 0.23 was obtained at a β of 28° and λ of 0.30. The results also showed a time-varying fluctuation in the torque, which reduced in magnitude with an increase in number of flights. The ripple was found to occur once-per-revolution and not once-per-flight, irrespective of the number of flights. Particle Image Velocimetry (PIV) results showed no evidence of a strong frequency component in the flow and Proper Orthogonal Decomposition (POD) analysis showed that the contributions of the low-order modes to the turbulent kinetic energy were low (less than 60% for the first six modes). The CFD study showed that the torque generation on the turbine is dominantly pressure-driven, and the viscous component of the torque is small compared to the pressure component. It also suggested that free-surface effects are important in the turbine’s operation and have appreciable implications for added mass effects on the turbine.