Research On Unsteady Aerodynamic Characteristics Of Horizontal Axis Wind Turbine Under Extreme Wind Conditions

Wind energy is environment-friendly and renewable, currently it has become the hot spots of research and investment among the world nations. The main use form of wind energy is wind power generation, wind turbine which is the core component of generating equipment can change wind energy into electrical energy. Wind turbine usually works in the unsteady external environment, wind condition is one of the key factors of operating environment, which affects the aerodynamic performance of wind turbine seriously. The extreme wind condition which is included in wind condition is the critical external design condition of wind turbine, and plays a decisive role in the production safety and service life.Therefore, this paper is based on the standard models of the extreme wind condition, selects the 1.5MW horizontal axis wind turbine as the research object,generates a three-dimensional model and grids, uses Fluent software as the solver,completes the unsteady numerical simulation under the uniform flow and the extreme wind condition by solving navier-stokes equations, gets the influence of wind turbine aerodynamic characteristics under the extreme wind condition by contrasting the blade aerodynamic load and surface flow condition. The main research work and results are as follows:Selecting the extreme continuous gust, the extreme continuous gust with the variation of the wind direction, the extreme operating gust which appears every 1 or50 years three extreme gust wind conditions, writing UDF programs of 10.4m/s wheel reference wind speed, setting them as the inlet boundary condition for each extreme condition after compiling, and obtaining the calculation results in the corresponding operation period. For comparison, setting 10.4m/s uniform flow as the inlet boundary condition for the numerical simulation, and the computation settings are the same as the extreme gust wind conditions except the inlet boundary condition.The change of the external characteristic of the wind turbine and the flow field are very little under the uniform flow condition over a period, the torque basically maintained at 270kN?m, the differential pressure of each section increase along the spanwise of the blade from root to tip, and does not vary with time. The angle of attack has remained constant, and the flow over the blade surface is attached. The normal force and tangential force coefficient cross the section decrease from the root to the tip. The curve of the aerodynamic coefficient-time is approximately a straightline.The wind speed of the extreme continuous gust increases continuously over a period. The torque, the normal force and tangential force coefficient cross the section are similar to the change of time-dependent wind speed during the low speed period.It can be seen that the most area of the blade suction surface is attached flow, and the pressure difference of every section and the angle of attack is small. While there is some difference between the curve of torque and time during the high speed period, as the blade stall, large area flow separation occurs on the blade suction surface, making the torque fluctuating rise slowly. The pressure difference of the airfoil leading edge and angle of attack is increasing all the time with the continuously increase of the wind speed, and the flow separation point is gradually closer to the leading edge. At the same time, the turbulent flow region is extending, and the flow along the spanwise occurs. However, the existence of the detached vortices make the normal force and tangential force coefficient cross the section fluctuating decrease irregularly with the increase of wind speed.The wind speed increases first and then decreases in the operation cycle of the extreme continuous gust with the variation of the wind direction, the torque, the normal force coefficient, the tangential force coefficient and the state of the flow field in the increasing stage of the wind speed are similar to which under the low wind speed of the extreme continuous gust. Similarly, the torque coefficient and aerodynamic coefficient of the section begins to decrease due to the stall when the wind speed is close to the maximum; Flow separation with small scale occurs on the suction side of the blade, the differential pressure and the angle of attack of the incoming flow reach the maximum when the wind speed reaches the maximum; in the decreasing stage of the wind speed, the separation area on the suction side of the blade decreases gradually, but the curves of the torque, the normal force coefficient,the tangential force coefficient can not recover to the same change law of the wind speed curve rapidly.The wind speed fluctuation and variation in the operation cycle of the extreme operating gust, the change law of the wind rotor external characteristics and the fluid field in every moment are all the same as above, the maximum values of the torque,the normal force coefficient and tangential force coefficient of the section appears relatively earlier comparing to that of the wind speed. In the deceleration stage of the incoming flow after the maximum value of wind speed, the separating vortex produced at high wind speed weaken gradually and flowed away by the coming flow,the flow recovers to attached flow and the coefficients recovers, too, the instability of the flow in the process of the turbulent flow converts to the layer flow is much stronger, which makes the reducing process of the normal force coefficient and tangential force coefficient generate irregular fluctuations. The range of the wind speeds of the extreme operating gust which appears every 50 years is much greater,and the performance period is much longer, the variation of the aerodynamic load and the flow state is more obvious than that under extreme operating gust condition which appears once a year.