Aerodynamic Performance Prediction And Optimization Design Of Horizontal Axis Wind Turbines

As one of the most important key parts of horizontal axis wind turbines, therotors not only extract energy from wind but also transfer loads to other parts. Theaerodynamic performance of the rotors has a main influence on the quantity of thepower output and the reliability of almost all parts of wind turbines. Theaerodynamics of a wind turbine is extremely complicated. Such problems include thechallenges in understanding and predicting the unsteady blade air-loads and rotorperformance under complicated environmental effects that may affect the air-loads ona wind turbine, as well as the predicting the dynamic stresses and aero-elasticresponse of the blades due to the increasing size of the wind turbines. It has greatvalue not only in engineering applications but also in theoretical research to setupreliable, precision and efficient tools to predict the aerodynamic performance ofHAWTs under both steady and unsteady conditions.In this thesis models are established for the purpose of predicting theaerodynamic performance of HAWTs under both steady and unsteady flow conditions,by which the influence of3D geometry effects and the aerodynamic responses subjectto the complicated effects such as wind shear, tower shadow, pitching motion andindividual pitch control are studied. Following are the main work and findings of thisresearch project:1. A lifting surface method based on a relaxation free wake model is developed and aerodynamic performance of HAWTs under steady flow condition is studied. The relaxation free wake model can capture the main characters of the distortional wake such as the expansion ratio under both axial flow and yawed flow condition and the skew angle of the wake under yawed flow condition.2. The numerical predictions based on the lifting surface method with the relaxation free wake model under rated and low load working conditions show good agreement against the experiment data. The effects of dihedral and sweep3-D shape on blade aerodynamic performance are studied. The aerodynamic results analysis show that the3-D shape has a major influence along the blade due to the changing the relative postion between the blade and the tip vortex.3. A lifting surface method based on a time marching free wake model is developed. A new backward difference scheme is deduced. The linear stability analysis shown that the new scheme is stable for all values of time discretization. The aerodynamic performance of HAWTs under unsteady flow condition such as pitching motion, tower shadow, wind shear and individual pitch control are studied.4. The numerical predictions based on the lifting surface method with the time marching free wake model under pitching step motion show excellent agreement with the measured results. There are two stages during the entire dynamic inflow process. The first stage, the sudden change of the angle of attack due to the blade pitch motion leads to the instantaneous change of the torque which has a slight delay compared to the pitch motion. At this stage the shed vortex plays an important role and its influence has a very short time scale in the order of c/Ωr. The second stage is the recovery stage. The second stage is the induced velocity caused by the shed vortex go wake and the induced velocity caused by trailed vortex take the main part of the whole induced velocity. At this stage the trailed vortex is a mixture of the ‘new’ and ‘old’ vortex and an end with the ‘old’ vortex has travelled away from the rotor and the ‘new’ vortex takes the dominant influence to the induced velocity. Then the rotor goes into the new equilibrium. This stage takes place with a longer time scale in the order of.5. The influence of the tower shadow to both upwind and downwind rotor are studied. It is shown an impulsive response of the blade loading when the blade is passing through the tower shadow region, the reason of which is that the ‘sharp’ velocity gradient at the tower shadow region. Compaered to the unyawed flow condition the tower shadow has a wider influence region to the rotor in yawed flow.6. The aerodynamic performace of the wind turbine working under wind shear flow is studied which is shown that the wind shear flow has a periodic influence to the rotor. Then the individual pitch control technic is used to improve the performance of the wind turbine under sheared flow condition. It is an effective active control system that not only aims at good quality power control but also focuses on the reduction of fatigue relevant loads on the turbine component caused by wind shear, tower shadow and turbulence.7. A general procedure of multi-objective optimization of wind turbine blades with sweep and dihedral3-D shape is proposed in this thesis. A600W type small wind turbine is designed which takes the AEP, starting performance and blade mass as objectives. Two objectives the AEP and thrust of the rotor are optimized bases on the NREL Phase VI rotor blade using the NSGA-II algorithm as the optimization algorithm which can provide the Pareto optimal front of conflicting objectives.