Lab-scale Turbine Research Articles

Numerical modelling of a 1.5 MW tidal turbine in realistic coupled wave–current sea states for the assessment of turbine hub-depth impacts on mechanical loads

This paper considers hub-depth impacts on mechanical loads for a tidal turbine operating in realistic coupled wave–current sea states. A novel medium-fidelity actuator-line CFD model for simulating tidal turbine non-steady hydrodynamic rotor load responses in the presence of turbulence, shear, and surface waves is developed. The model is validated using tank testing data from a lab-scale turbine. The validated model is then upscaled, to a power rating of 1.5 MW, and simulated in realistic wave–current conditions consistent with those of the MeyGen site. Mean torque and thrust are found to increase with turbine hub height, and the presence of waves is shown to increase mean torque and thrust values by up to 22% and 11%, respectively. The effect on standard deviations and maximum values for these variables is more pronounced, with increases of up to 2500% and 1700% in signal standard deviations, and up to 80% and 30% in maximum values for torque and thrust, respectively. The presence of longer period waves is also shown to reduce mean torque levels, while the corresponding standard deviations and maximum values remained relatively unchanged. In such circumstances, the turbine is operating with an undesirable combination of low-power and high-fatigue. Tidal turbine hub loading characteristics and sensitivities, in the context of the operational loads which subsequently enter the drivetrain and turbine support structure, are also analysed. The magnitude of out-of-plane rotor moments are found to increase with the hub height and wave height. Complex flow interactions are shown to play an important role in this context, leading to what is termed “wave-driven moment-type dominance” effects. Overall, both the rotor location and wave composition are found to significantly impact the turbine’s rotor mechanical load response.

Horizontal-axis propeller hydrokinetic turbine optimization by using the response surface methodology: Performance effect of rake and skew angles

The design of a horizontal-axis propeller hydrokinetic turbine (HPHT) depends on several geometric parameters affecting its hydrodynamic efficiency which is measured through the power coefficient (CP). In this study, a 1 kW turbine with 1.6 m of rotor diameter (D) was used as the prototype to know the relationship between the CP and the turbine design parameters, such as the skew (ϕ) and rake (γ) angles. A full-factorial design of experiments, as a response surface methodology technique, and computational fluid dynamics simulation were used to determine the significance of the factors considered and their interaction in the maximization of the response variable (CP). A 3D computational domain in ANSYS Fluent software and the k-ω SST turbulence model were utilized, for the unsteady flow simulations. Under optimal design conditions, i.e., when ϕ and γ were equal to 13.30° and -18.06°, respectively, the highest CP was 0.4571. For these optimal values, a scaled model with 0.24 m of diameter was numerical and experimentally studied and the findings were compared. A good agreement was found between the numerical results regarding the lab-scale turbine and the experimental data for the CP values obtained as a function of the tip speed ratio.

Characteristics and suppression of vibration in cross-flow turbine with a cavity

Abstract Cross-flow turbine is widely used for small hydropower in run-of-river type stations because of its excellent partial load characteristics, easy manufacturing, and low cost. The Banki-Michell turbine is the origin, and various studies have focused on improving the turbine performance. The authors have developed a new cross-flow turbine with a cavity and a guide wall from the outer nozzle wall tip to suppress flow separation on the guide wall and make the casing smaller. However, little researches are focusing on vibration, and it is also essential to get knowledge for suppressing vibration and noise from the turbine aiming to install at higher head sites. Our previous studies showed that high-pressure fluctuation occurs near the outer nozzle tip, and the cavity suppresses the pressure fluctuations, but the suppression mechanism is not fully understood. Therefore, this study experimentally investigates the pressure fluctuation generation mechanism and the cavity’s effect on suppressing the vibration. Furthermore, it aims to suppress the vibration by modifying the nozzle tip shape. The vibration level was measured on a lab-scale turbine and CFD analysis to reveal the generation of pressure fluctuation. As a result, the predominant frequency of the vibration corresponds to the blade passing frequency and the pressure fluctuation at the nozzle tip. The cavity and nozzle tip shape show a significant influence on pressure and vibration characteristics and turbine performance.

Experimental analysis of dual coaxial turbines in skew

Ocean currents are a potentially reliable source of renewable energy, but the complications associated with deploying current energy conversion (CEC) devices in deep water make harvesting that energy a challenge. One promising approach is to use tethered axial-flow CECs composed of one or more pairs of coaxial counter-rotating turbines. However, a dual-rotor system moored in unsteady water by a flexible tether is likely to experience a condition, called skew, where the axis of rotation is not aligned with the direction of flow. A lab-scale turbine was constructed to investigate the effect of skew on fluid power conversion of a coaxial CEC. A semi-empirical model of the power conversion was developed for comparing the results to a recently published analytical model. It was found that the analytical model represents the data better than a simple ad hoc modification to previous models that is often used to estimate the power dynamics. Additionally, the results support the existence of a physical phenomenon – captured by the recent model but not represented in the ad hoc modification – in which the downstream rotor of a coaxial pair is partially within the wake and partially out of the wake of the upstream rotor.