Effect of wind sand on flow around airfoil of wind turbine and its aerodynamic performance

Abstract: As a kind of clean and environment-friendly energy, wind power has been developed rapidly in recent years. With top wind power capacity, Northwest region is also a region with frequent dust storm incidents. Obviously, when wind turbine works under sand-wind conditions, its aerodynamics performance will be affected severely, and the flow structure around airfoil will be affected by the function of particles in the air. Wind turbine blades will even be eroded due to the impact of particles. Therefore, it is urgent to study the effect of sand-wind flow on wind turbine. The effect of particles on the flow structure around airfoil and its aerodynamic performance has been studied in this paper. In order to capture the detail of the flow structure around airfoil in various dusty environments with different particle diameters, a delayed detached eddy simulation (DDES) method, which is a hybrid Reynolds average Navier-Stokes equation and large eddy simulation (RANS-LES), and the discrete phase model were used for the simulation of the flow around NREL S809 airfoil. The flow structure around airfoil, mass distribution of particles and its aerodynamic performance in each dusty environment were compared. Both aerodynamic performance and flow structure were affected by particle, and the influences would be different when the angle of attack or particle diameter was different. When the angle of attack was small (6.1°, there was no flow detachment), the particles did not affect the flow around airfoil obviously, but the lift coefficient was decreased. With the increasing of particle diameter, lift coefficient of S809 decreased first and then went up, and turning point was 20 μm (3.9% lower than the lift coefficient obtained in clear air). But the lift coefficient could not be completely recovered, and it was still smaller than the lift coefficient in clear air when the particle diameter was 150 μm. When the angle of attack became larger (8.2°, flow detachment occurred), particles had a great influence on the flow structure around airfoil, such as the advancing of the flow separation point and the appearing of the flow along spanwise direction, and these influences were also affected by particle diameter. With particle diameter increasing, the flow structure became chaotic first and then gradually recovered. When the particle diameter was 20 μm, particles had the most effects on the aerodynamics performance of airfoil and the flow around it. This was due to the dramatic momentum exchange between particle and gas, that was, a large number of particles were rolled into wake area and broke the flow structure around airfoil, which affected the lift coefficient greatly. When the particle diameter was smaller than 20 μm, the particles would flow closely with air due to its weak inertia force. On the contrary, the particles would move independently of air flow, because of its strong inertia force when the particle diameter was larger than 50 μm. The flow structure recovered gradually with the increasing of particle diameter when particles diameter was larger than 20 μm. However, the aerodynamic performance of airfoil could not recover completely, only close to the lift coefficient in clear air, and the minimum value of lift coefficient was also obtained when the particle diameter was 20 μm (7.9% lower than the lift coefficient in clear air). In summary, the particles in the air will have a great influence on both the flow around airfoil and the aerodynamic performance of airfoil, especially in the area where the flow separates. When the particle diameter is smaller than 20 μm, the particle diameter increase will strengthen the effect, which reaches the climax when the particle diameter is 20 μm. Under this condition, if there is no flow detachment in clear air (the attack angle is small), the influence of particles would be weak, which can be ignored. If there exists flow separation phenomenon in clear air (the attack angle is large), then the influence would be strong, and it would aggravate the flow separation, causing the flow to transform into three-dimensional flow. After that, further increase of the particle diameter would weaken the effects of the particles, and when the particle diameter is 150 μm, both the flow structure and the lift coefficient are very close to that in clear air.