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Offshore Wind Turbines
Published in Srinivasan Chandrasekaran, Faisal Khan, Rouzbeh Abbassi, Wave Energy Devices, 2022
Srinivasan Chandrasekaran, Faisal Khan, Rouzbeh Abbassi
The blade element theory is based on the aerofoil theory; forces on the blade depend on the flow conditions and the two-dimensional, sectional wing data. The axial and rotational forces are calculated using the lift and drag of the blade section. Flow over an aero-foil leads to pressure distribution over its surface. The lift force caused by the unequal pressure distribution on the upper and lower surfaces of the aerofoil remains perpendicular to the incoming airflow direction while the drag is parallel. Drag shall exist due to the viscous and friction forces. At the same time, separation at the trailing edge creates unequal pressure distribution on the aerofoil surfaces, one facing towards and the other facing away from the incoming flow. Aerofoil characteristic data are required to calculate the lift and drag forces over the blade element and function the relative velocity over the aerofoil. The velocity of a rotor blade element has two components: one along with the flow and the other tangential. The resultant flow velocity, acting over the aerofoil, is the sum of these two velocity components. Fig. 5.17 shows the lift and drag components of an aerofoil.
Implementing varying blade profile and reynolds number in BEMT code
Published in C. Guedes Soares, Developments in Renewable Energies Offshore, 2020
I. Evans, M. Togneri, T. Lake, R. Gwenter, I. Masters, G. Pinon, M. Slama
The blade element theory divides the rotor blade into two-dimensional elements along its length. There is no fluid interaction between the elements and thus the loads on the blades can be assumed to rely solely on the lift and drag characteristics of the blade shape. Figure 1 is a diagram showing velocities and forces for a blade element at radius r relative to the blade chord line. θ, α, and ϕ represent combined pitch and twist of the blade, angle of attack of the blade from the resultant flow, and inclination of the resultant flow respectively. dL and dD are the element lift and drag forces respectively whilst V is the resultant flow. Axial force and torque for the rotor blade are found by resolving the lift and drag forces.
Wind Energy Conversion Systems
Published in Radian Belu, Fundamentals and Source Characteristics of Renewable Energy Systems, 2019
The turbine aerodynamics describes the forces developed on a wind turbine by the airflow. The two major approaches to derive aerodynamic models for wind turbines are the actuator disc theory and the blade element theory. The former explains in a simple manner the energy extraction process. Also, it provides a theoretical upper-bound to the energy conversion efficiency. The latter studies the forces produced by the airflow on a blade element. This theory is more suitable to explain some aerodynamic phenomena such as stall, as well as to study the aerodynamic loads. This model is based on the momentum theory. The turbine is regarded as an actuator disc, which is a generic device that extracts energy from the wind. Consider the actuator disc is immersed in an airflow, which can be regarded as incompressible. Since the actuator disc extracts part of the kinetic energy of the wind, the upstream wind speed V is necessarily greater than the downstream speed V−∞. Consequently, for the stream tube just enclosing the disc, the upstream cross-sectional area A∞ is smaller than the disc area AD, which in turn is smaller than the downstream cross-sectional area A−∞. This is because, by definition, the mass flow rate must be the same everywhere within the tube. From the expression for kinetic energy in flowing air follows the power contained in the wind through an area A with wind velocity v1 is expressed as: () PW=0.5ρAv13
Aerodynamic analysis and modeling of coaxial ducted fan aircraft with the ceiling effect
Published in Engineering Applications of Computational Fluid Mechanics, 2021
Tianfu Ai, Wei Fan, Bin Xu, Changle Xiang, Yu Zhang, Zhiran Zhao
The existence of the ceiling will also affect the calculation of the blade element theory of the upper and lower rotors. The blade element theory of an unconstrained environment assumes that the inflow is an axial flow, and only the flow along with the rotation and axial directions of the rotor should be considered. However, as impacted by the existence of the ceiling, the airflow into the upper rotor deflects significantly, which will lead to the radial velocity, and the radial flow should be considered additionally. Complying with the derivation of momentum theory, the radial velocity should be considered directly in the blade element theory calculation of the upper rotor, while the inflow of the lower rotor in the wake area of the upper rotor can be considered the axial flow.
Design, electromechanical simulation, and control of a variable speed stall-regulated PMSG-based wind turbine
Published in International Journal of Green Energy, 2019
Ebrahim Mohammadi, Roohollah Fadaeinedjad, Hamid Reza Naji
The HARP_Opt tool utilizes a multi-objective genetic algorithm (GA) and BEM theory to design the optimal blades for variable- and fixed-speed WTs considering different control strategies. The code employs GA to maximize the annual energy production (AEP), where the maximum power point tracking is combined with this objective. Moreover, it can be run as a multi-objective tool to minimize the blade mass, as well. To predict the rotor performance metrics, this tool utilizes the WT_Perf (Buhl 2004), a code to evaluate the performance of HAWTs based on blade shape, and aerodynamic characteristics including drag and lift forces. The BEM theory is a combination of blade element and momentum theories; the blade element theory considers that the blade is divided into small elements operating independent of the surrounding ones and act as two-dimensional airfoils and the aerodynamic forces are computed considering the local flow around them. The total exerted forces and momentums on the blades are obtained by summing the elemental forces.
Structural design optimization of a wind turbine blade using the genetic algorithm
Published in Engineering Optimization, 2022
A wind turbine is a complex engineering object, which is subjected to highly fluctuating and time-varying loads. As the wind turbine rotor continues to grow in its size, in terms of its rated power and the load to withstand, obtaining its optimal design has become more critical. For the design and performance estimation of the blade, many researchers have used the blade element momentum theory (BEMT), which was originally developed in the form of Froude’s BEMT, later refined by Glauert (1976) to analyse and assess the performance of the wind turbine in combination with other relevant theories, including the axial momentum and blade element theory (Karam and Mahdy 2001; Lanzafame and Messina 2007; Jerson, João, and André 2011; Kim et al.2011; Patrick, Chen, and Choi 2016). Fatehi et al. (2019) successfully demonstrated the improvement in the aerodynamic performance of the wind turbine blade by the cavity shape optimization method. Ozge and Ismail (2013) introduced the aerodynamic shape optimization methodology based on the genetic algorithm and BEMT. Zhu, Shen, and Sørensen (2014) presented an integrated method of designing the aerofoil families of large wind turbine blades using the BEMT. Krishnil et al. (2018) designed and optimized the aerofoils for a 20 kW wind turbine using the multi-objective genetic algorithm. Vincent et al. (2017) adopted the BEMT and the blade span to estimate the aerodynamic performance of the turbine and to optimize the horizontal axis wind turbine blade. Bai, Chen, and Wang (2016) developed a small-scale horizontal-axis wind turbine suitable for the local wind conditions of Tainan, Taiwan, with the use of the BEMT.