Elastic Blade Structure Design Of Large Wind Turbines By Optimization Approach

Due to the effect of aerodynamic force, elastic force and inertial force, the actual operation process of the composite blades for large wind turbines is very likely to have problems such as shaking, shimmy or nonlinear torsional deformation; this deformation, together with the aerodynamic force, may cause unstable aeroelastic vibration, which will lead to blade fracture and result in enormous economic loss and severe social impact. Therefore, an innovative method is put forward to solve these practical engineering problems, and this dissertation is focused on the complex but interesting subject. Being aimed at the typical environmental factors related to wind in the West, further research is needed regarding problems such as the mathematical model of realistic blade loading, the optimized design of aerodynamic and structural coupling of oversized flexible blade, the dynamic characteristics of the blades with the effect of fluid-structure interaction, and the instable mechanism of large wind turbine units, etc.The results were in the following.(1) The distributed wind speed model at hub height(60m) was derived by making an estimate of the wind distribution parameters at standard height and the ones at hub height of wind turbine units from the surveyed wind data at standard height(10m) of a certain measuring wind tower in Jiuquan. During the estimation the vertical wind shear was considered and Newton-Raphson method was used. Avoiding the low accuracy of calculation from the traditional methods, the wind speed in different regions was calculated and the distributed wind speed curves from the distributed wind speed model in the given region were obtained by this model.There was significant difference between the wind condition at standard height and that at hub height(60m), especially for average wind-power density. So was the average wind speed distribution pattern. Compared to the Rayleigh distribution, the two-parameter Weibull distribution has more flexibility and adaptability. And the maximum likelihood was more precise and effective than the least squared method in parameter estimation.(2) The influences of the parameters of the blade airfoil as primary power generation, drag and lift forces over airfoil section, velocity distribution characteristics at different attack angle and Reynolds were studied by both the numerical simulation and the theoretical analysis, Moreover, the parameters consisted of the axial induction factor, normal force coefficient and tangential force coefficient were calculated.The angle values derived from well-known BEM approach had a maximum value at the blade root.The torsional angle near the blade root became heavily predominant. In principle, twist angle was beneficial to the design and manufacture for blades.This results not only increases sensitivity of angle of attack, but also bring difficulties for manufacturing of blade, Furthermore, To ensure power coefficient maximum, Willson model was established, neglecting the effect of the actual output power and constraints of blade chord length. Twist angle was calculated using above model. It showed that the values of aerodynamic chord length and twist angle were larger than BEM method, Bacause the modified momentum model considered the tip and hub loss, the results of the shape parameters was closer to values of real working conditions.(3) The optimization design model of aerodynamic for wind turbine blade was presented. The factors of hub loss, blade width and thickness were considered in this model.The inducing factor was solved by using the iterative method and the related calculation program was compiled. In this case, torque coefficient and engine load could be reduced, when the speed ratio of blade varied from 4.0 to 7.5. In addition, the ability of annual energy production were greatly improved.Values of chord length and thickness distribution were positive merging smoothly.The model on structure optimization mathematical of main girder was established.It revealed that the optimized unidirectional layer thicknesses had little change, the bidirectional fabric thickness and main girder cap in middle blade were both heavily decreased.(4) Lamination design of blade had both multi-objective and multiple constraints characteristics. An aerodynamic and structure coupling optimization model was established by applying a genetic algorithm.The design variables represented by twist angle, chord length, main girder,the leading edge and trailing edge ply thickness. The design constraints used in this work were maximum tip deformation under flapwise bending and wind turbine power output. The objective function was the minimum weight. Meanwhile, the loss of the blade root and tip were considered. The optimized tip chord length could be reduced, which was similar with the actual working conditions. While twist angle in the rigid condition was significantly larger than the other aerodynamic loads, this contribute to area of the power output so as tocompensate for the energy loss from the decrease of the chord length. Hence, the accuracy of the finite element model of composite blade had been dramatically improved using the above approach.(5) The blade design optimization was carried out based on the analysis of the coupling between the physical factors of aeroelasticity and the internal structure of the blade. CFD simulations have been exploited to analyze the aerodynamics of the airfoils. This could solve the problems with optimal blades through loading directly aerodynamic load in the blade finite element model. In addition, The large and complex finite elements mesh was established by the rational finite element calculated model of the structure to analyze the vibration.The second modes was basically behaved in flapwise and the high-order modes was heavily complicated, Consequently optimal blades can avoid resonance. The results indicate that above design approach was a reasonably efficient design.The resulting structure optimization design was more structurally efficient under the west wind condition to solve such problems as the stability of wind turbines.