Optimized structural design of concentrated wind energy device based on CFD numerical simulation

Abstract: The structure of the wind concentrator will directly affect the performance of wind-concentrating turbine. In this paper, to optimize the structure, the CFD software was used. The structure was optimized by adding a new conical tube behind the diffuser of the original model, and the influence of both the generatrix length (d) and the deflection angle (β) of the conical tube on the concentrator was also analyzed. First, through the CAD software, the optimized model of a wind concentrator was built and a cylinder (diameter: 20 m; length: 30 m, coaxial with the model) was created. With the help of Boolean subtraction, the fluid field model was obtained by subtracting the optimized model from the cylinder. The patch conforming algorithm was used to mesh the fluid field and the mesh type was tetrahedral. Twenty inflation layers were divided in the thickness range of 1 cm marked on the optimization model. In the simulation, SST k-ω turbulence model was adopted, energy equation was used and heat exchange was considered. The air velocity, temperature, density, pressure, viscosity, thermal conductivity, constant pressure specific heat capacity, flow rate, turbulent kinetic energy k value and specific dissipation rate ω value were 10.83 m/s, 296.75 K, 1.044 kg/m3, 88 800 Pa, 1.85×10-5 kg/(m·s), 0.026 22 W/(m·K), 1 013 J/(kg·K), 3 552.048 kg/s, 0.165 382 m2/s2, 11.786 s-1, respectively. The inlet boundary was the mass flow inlet, and the velocity direction was perpendicular to the inlet boundary. Both the thermal boundary conditions of the wall of the concentrator and the shell of the fluid field were at fixed temperature, with a value of 296.75 K. Pressure outlet was used as the outlet boundary. The results of flow field calculation show that the model was optimal when the generatrix length and the deflection angle of the conical tube were 0.4D and 50° respectively. The concentrating performance of the optimized model was determined by the vortex behind the conical tube and the flow separation on the inner surface of the tube. The existence of vortices rendered the optimized model better than the original one. However, if the intensity of the vortices was too high, the air ejected from the outer edge of the conical tube would be attracted to the inner wall of the tube and would affect the radial diffusion of the air up-flow at the outlet of the diffuser, thus reducing the originally superior concentrating performance. Meanwhile, the flow separation also reduced the concentration performance. Conversely, if a strong vortex appeared behind the conical tube, and the flow separation near the inner surface of the tube was not so strong, the optimal condition will be reached. At this time, the generatrix length was 0.4D and the deflection angle was 50°. Besides, when the deflection angle was 50° and the length of generatrix was extended, the vortex intensity behind the conical tube would increase, and the concentrating performance of the concentrator would improve. However, the flow separation was easy to occur near the inner wall of the conical tube, thus reducing the concentrating performance. But when the generatrix length was 0.3D, flow separation appeared. When the generatrix length was 0.4D, the flow separation was very slight. And when the generatrix length was extended to 0.5D, strong flow separation occurred, dampening the enhancement of the concentrating performance. Compared with 0.5D, When the generatrix length was 0.6D, the flow separation was slighter. So, for cost-effective consideration, when the deflection angle was 50°, 0.4D was the best generatrix length.