The Blade Element Momentum Theory (BEMT) is one of the most common and widely used methods in horizontal axis wind and tidal turbine performance prediction. It mainly relies on two-dimensional (2-D) hydrodynamic, lift and drag, coefficients which are used to calculate the hydrodynamic forces within the blade element
section of the model. These coefficients are greatly affected by both Reynolds numbers and turbulence transition
parameter (Ncrit). The performance sensitivity of scale models tidal current turbines (TCTs) to the change of
Reynolds numbers and Ncrit parameter was assessed. Initially, the BEMT was validated with experimental
measurements of two scale models of horizontal axis tidal current turbines. A very good agreement was seen,
thus, confidence in both the implementation of the theory and the applicability of the method was built up. Lift
and drag coefficients were calculated from XFoil through the use of different sets of Reynolds numbers at
different Ncrit values. The best BEMT validations with experimental data were seen when the hydrodynamic
coefficients were calculated at two sets of Reynolds numbers considered at optimal performance. The first set was
calculated at 75% of the blade span while the second set was calculated at every local elemental position of the
blade. For facilitation, reduction of time and effort, Reynolds numbers at 75% of the blade span is considered the
best choice. Furthermore, the BEMT model results showed better agreements with experimental data when lift
and drag coefficients were calculated at low values of Ncrit parameter. The predicted tidal turbine performances
of previous experimental data by the present BEM model were compared with those derived from the academic
SERG-Tidal BEM code. The present BEM model, based on the proper selection of both Reynolds number and Ncrit
parameter, showed an obvious superiority over SERG-Tidal model. |
