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Continuously increasing offshore wind turbine scales require rotor designs that maximize power and performance. Downwind rotors offer advantages in lower mass due to reduced potential for tower strike, and is especially true at large scales, e.g., for a 25 MW turbine. In this study, three 25 MW downwind rotors, each with different prescribed lift coefficient distributions were designed (chord, geometry, and twist) and compared to maximize power production at unprecedented scales and Reynolds numbers, including a new approach to optimize rotor tilt and coning based on aeroelastic effects. To achieve this objective the design process was focused on achieving high power coefficients, while maximizing swept area and minimizing blade mass. Maximizing swept area was achieved by prescribing pre-cone and shaft tilt angles to ensure the aeroelastic orientation when the blades point upwards was nearly vertical at nearly rated conditions. Maximizing the power coefficient was achieved by prescribing axial induction factor and lift coefficient distributions which were then used as inputs for an inverse rotor design tool. The resulting rotors were then simulated to compare performance and subsequently optimized for minimum rotor mass. To achieve these goals, a high Reynolds number design space was developed using computational predictions as well as new empirical correlations for flatback airfoil drag and maximum lift. Within this design space, three rotors of small, medium and large chords were considered for clean airfoil conditions (effects of premature transition were also considered but did not significantly modify the design space). The results indicated that the medium chord design provided the best performance, producing the highest power in Region 2 from simulations while resulting in the lowest rotor mass, both of which support minimum LCOE. The methodology developed herein can be used for the design of other extreme-scale (upwind and downwind) turbines.
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Power Estimation of an Experimental Ocean Current Turbine Based on the Conformal Mapping and Blade Element Momentum Theory
Abstract Conformal mapping techniques have been used in many applications in the two-dimensional environments of engineering and physics, especially in the two-dimensional incompressible flow field that was introduced by Prandtl and Tietjens. These methods show reasonable results in the case of comprehensive analysis of the local coefficients of complex airfoils. The mathematical form of conformal mapping always locally preserves angles of the complex functions but it may change the length of the complex model. This research is based on the design of turbine blades as hydrofoils divided into different individual hydrofoils with decreasing thickness from root to tip. The geometric shapes of these hydrofoils come from the original FX77W121 airfoil shape and from interpolating between the FX77W121, FX77W153, and FX77W258 airfoil shapes. The last three digits of this airfoil family approximate the thickness ratio times 1000 (FX77153 => 15.3 % thickness ratio). Of the different airfoil shapes specified for the optimal rotor, there are 23 unique shapes.[15, 16, 17, 18, 19, 20, 21, 22, 24, 25, 28] This study describes the advantage of using at least one complex variable technique of transformation conformal mapping in two dimensions. Conformal mapping techniques are used to form a database for sectional lift and drag coefficients based on turbine blade design to be used in Blade Element Momentum (BEM) theory to predict the performance of a three bladed single rotor horizontal axis ocean current turbine (1.6-meter diameter) by considering the characteristics of the sea-water. In addition, by considering the fact that in the real ocean, the underwater ocean current turbines encounter different velocities, the maximum brake power will be investigated for different incoming current velocities. The conformal mapping technique is used to calculate the local lift coefficients of different hydrofoils with respect to different angles of attack: −180 ≤ AOA ≤ +180. These results will be compared to those from other methods obtained recently by our research group. This method considers the potential flow analysis module that follows a higher-order panel method based on the geometric properties of each hydrofoil cross section. The velocity and pressure fields are obtained directly by the applications of Bernoulli’s principle, then the lift coefficients are calculated from the results of the integration of the pressure field along the hydrofoil surface for any angle of attack. Ultimately, the results of this research will be used for further investigation of the design and construction of a small-scale experimental ocean current turbine to be tested in the towing tank at the University of New Orleans.
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- PAR ID:
- 10339434
- Date Published:
- Journal Name:
- ASME International Mechanical Engineering Congress and Exposition (IMECE)
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Conference Paper:
- https://doi.org/10.1115/IMECE2021-71751
