Hydrodynamic analysis of a ducted, open centre tidal stream turbine using blade element momentum theory

This paper analyses two different configurations of horizontal axis Tidal Stream Turbines (TSTs) using a Blade Element Momentum Theory (BEMT) model. Initially, a ‘conventional’ three bladed and bare turbine is assessed, comparing against experimental measurements and existing literature. Excellent agreement is seen, increasing confidence in both the implementation of the theory and the applicability of the method. The focus of the paper lies on the analysis of a ducted and open centre turbine. An analytical adjustment to the BEMT model is applied, using empirical expressions detailed in the literature which are devised from Computational Fluid Dynamics (CFD) studies. This is modified to a symmetrical duct profile, calibrating certain geometrical parameters against blade resolved CFD studies of a bi-directional device. The results are validated with a coupled CFD blade element model (RANS BEM), where both models align very closely (within 2%) for most tip speed ratios (TSRs), including the peak power condition. Over predictions are seen at higher TSRs of up to 25% in power and 13% in thrust at TSR = 5, due to model limitations in replicating fully the complex flow interactions around the hub and the open centre. The presented approach benefits from significantly lower computational requirements, several orders of magnitude lower than reported in the RANS-BEM case, allowing practicable engineering assessments of turbine performance and reliability.