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 (TST) using a Blade Element Momentum Theory (BEMT) model. Initially, a 'conventional' 3 bladed and bare turbine is assessed, comparing predictions of turbine power and thrust forces against experimental measurements and existing literature. Excellent agreement is seen, increasing confidence in both the implementation of the theory into the code and the applicability of the method to represent the turbine physical behaviour. The focus of the paper lies on the analysis of a ducted and open centre turbine, where the BEMT model is adapted to take into account the increased mass flow through a duct (or augmenter). This is based on an analytical framework, where empirical expressions are devised based on Computational Fluid Dynamics (CFD) studies to approximate the inlet efficiency, diffuser efficiency and base pressure. This is applied to a bi-directional ducted case, where calibration of certain duct parameters is performed through modelling the OpenHydro device and comparing results with blade resolved CFD studies. The results are validated with a comparison of this ducted BEMT model to a coupled CFD blade element model (RANS BEM). Both models align very closely for most Tip Speed Ratios, capturing the peak power at optimal conditions. Slight over predictions for thrust and power are seen at higher TSRs, where higher velocities are seen at localised elements around the hub. This is due to the model limitations in fully replicating the complex flow interactions in and around the hub and open centre. The presented approach has the benefit of significantly lower computational requirements, in the order of CPU-minutes, rather than CPU-days required for RANS BEM, allowing practicable engineering assessments of turbine performance and reliability