Design and Optimization of a Multi-Element Hydrofoil for a Horizontal-Axis Hydrokinetic Turbine

Abstract

Hydrokinetic turbines are devices that harness the power from moving water of rivers, canals, and artificial currents without the construction of a dam. The design optimization of the rotor is the most important stage to maximize the power production. The rotor is designed to convert the kinetic energy of the water current into mechanical rotation energy, which is subsequently converted into electrical energy by an electric generator. The rotor blades are critical components that have a large impact on the performance of the turbine. These elements are designed from traditional hydrodynamic profiles (hydrofoils), to directly interact with the water current. Operational effectiveness of the hydrokinetic turbines depends on their performance, which is measured by using the ratio between the lift coefficient (CL) and the drag coefficient (CD) of the selected hydrofoil. High lift forces at low flow rates are required in the design of the blades; therefore, the use of multi-element hydrofoils is commonly regarded as an adequate solution to achieve this goal. In this study, 2D CFD simulations and multi-objective optimization methodology based on surrogate modelling were conducted to design an appropriate multi-element hydrofoil to be used in a horizontal-axis hydrokinetic turbine. The Eppler 420 hydrofoil was utilized for the design of the multi-element hydrofoil composed of a main element and a flap. The multi-element design selected as the optimal one had a gap of 2.825% of the chord length (C1), an overlap of 8.52 %C1, a flap deflection angle (δ) of 19.765°, a flap chord length (C2) of 42.471 %C1, and an angle of attack (α) of –4°.

Surrogate modelling for high-lift multi-element hydrofoil shape optimization of a hydrokinetic turbine blade

“…In general, the objective of the hydrodynamic shape optimization, for a given operating condition, is to design a hydrofoil that maximizes CL, or minimizes CD, or maximizes the lift-to-drag ratio (CL/CD); which is subjected to several constraints, such as the minimum negative pressure coefficient (min Cpre) for avoiding the cavitation inception, as well as geometrical constraints, among others [1,[3][4][5][13][14].…”

“…Overlap (ovl) is the flap leading-edge position on the xaxis and gap is the flap leading-edge position on the yaxis. In turn, C, which is equal to 0.1773 m in this study, C1 and C2, are the chord length of the blade, the length of the main element and length of flap, respectively [3][4][5]. It is noteworthy that changing the values of the gap and the overlap slightly resulted in a significantly different hydrodynamic performance for the multi-element hydrofoil.…”

“…A main objective of hydrokinetic turbine designers is to maximize the performance and reduce manufacture costs [1][2]. One possible way to reach the objective of maximizing the power coefficient (Cp) is to design highlift systems consisting of a hydrofoil with a flap [3][4]. This geometrical configuration results in the maximization of the lift and the minimization of the flow separation around the hydrofoil [4].…”

“…One possible way to reach the objective of maximizing the power coefficient (Cp) is to design highlift systems consisting of a hydrofoil with a flap [3][4]. This geometrical configuration results in the maximization of the lift and the minimization of the flow separation around the hydrofoil [4]. Commonly, flow separation leads to a loss of the lift, an increase of the drag and, therefore, a reduction of the blade performance [3][4][5].…”

“…This geometrical configuration results in the maximization of the lift and the minimization of the flow separation around the hydrofoil [4]. Commonly, flow separation leads to a loss of the lift, an increase of the drag and, therefore, a reduction of the blade performance [3][4][5].…”