Numerical study of ducted turbines in bi-directional tidal flows

Abstract

Installing a turbine in a duct generally increases the flow through the rotor. If there is flow confinement due to limited water depth, the presence of lateral boundaries, or the presence of adjacent turbines, a further flow enhancement through the rotor will be achieved. Extensive studies of ducted turbines in uni-directional flow have shown that rotor-based power coefficient (Formula presented.) can exceed the Betz-Joukowsky limit (BJL) for bare turbine, whereas the exceedance of the BJL is rarely reported for duct-area-based power coefficient (Formula presented.). In this work, CFD studies have been carried out for several flanged duct turbines with different duct geometries under bi-directional flow conditions. The actuator disc approach is used to simulate the rotor. The results show that a flanged duct turbine with an inlet-arc flap and a curved brim can achieve a maximum value of (Formula presented.) close to the BJL for flow from either direction while a symmetric duct turbine- gives maximum (Formula presented.) of about 60% of the BJL. The effect of the flow confinement on (Formula presented.) shows (Formula presented.) increases with blockage ratio (Formula presented.) and a little variation in the relative performance of ducted and  bare turbines. (Formula presented.). The findings pave the way for the realization of turbines with a flanged duct.