Fluid Dynamic Simulations of Wind Turbines. John Abraham, Brian Plourde, Greg Mowry University of St. Thomas

Transcription

1 Fluid Dynamic Simulations of Wind Turbines John Abraham, Brian Plourde, Greg Mowry University of St. Thomas 1

2 Presentation Overview Why vertical-axis turbines? How are they modeled? How much energy can they extract? How do the models compare with experiments? 2

3 Project Description Cellular communication towers need electricity to power the electronics. 3

4 Why Vertical Axis Turbines? The turbine is designed to be attached to an existing tower, reducing installation costs. The positioning on a tower allows the turbine to access high-speed wind. 4

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6 6 The Numerical Model Simulations of Wind Turbines Turbulence was accounted for with the Shear Stress Transport Model (Menter 1994). i k t i k i i x k x k P x u k t k 1 i i i t i i i x x k F x x S x u t , max SF a k a t Shear Stress Transport Equations:

7 Numerical Model The mesh was refined in the vicinity of the blade. Two-dimensional simulations were carried out. A mesh independence study was completed. Simulations with and without side walls were made to allow comparison with wind-tunnel tests. Figure Computational mesh used to define the fluid domain 7

8 Numerical Model Vents were employed on the device to reduce negative drag and thrust loads on the tower. Figure - Image showing the positioning of a vent on the turbine blade. R 1 = 36 inches R 2 = 16 inches R 3 = 8 inches L 1 = 4 inches = 120 degrees Figure - Nomenclature for defining the turbine geometry 8

9 Numerical Model Blockage effects have to be considered when wind-tunnel tests are used. Power Blockage Power tunnel free Estimate of blockage Method 1: From literature, blockage factor ~ 2 Method 2: Velocity scaling ~ 1.95 Method 3: Comparing simulations with and without walls ~1.85 9

10 Results and Discussion Power is calculated from torque and rate of rotation. Power increases with rotation rate. Figure - Numerically simulated power generation for a three-stage rotor positioned within a wind tunnel, wind speed = 8.7 m/s. 10

11 Wind Tunnel Tests Figure - Schematic of the wind tunnel Figure - Photograph of a rotor section 11

12 Results and Discussion Often, results are characterized by the tip-speed ratio. Velocity of Rotor Tip Speed Ratio Wind Speed Tip The simulations slightly overpredict performance compared to experiments. Possibly because of airbypass in experiments (actual rotors only 1.3 meters in height). Figure - Comparison of simulated and experimentally determined power generation corresponding to a wind speed of 8.7 m/s and a three-stage rotor. 12

13 Results and Discussion Simulated maximum power extraction exceeds results from wind-tunnel tests. Wind tunnel results show that performance depends on electrical load. If electrical load is optimized, results will be in better agreement. Figure - Comparison of simulated results with results from wind-tunnel tests. Results are for a single rotor stage. 13

14 Results and Discussion Each image is taken at an offset from the preceding image. The flow patterns, including fluid acceleration around the blade and large vortices released in the downstream section are clearly shown in the figure. Figure - Contour images showing the velocity pattern of flow in the vicinity of the rotor. 14

15 Numerical Model 15

16 Numerical Model 16

17 Preliminary 3D Simulations 17

18 Results and Discussion 18

19 Numerical Model 19

20 Numerical Model 20

21 Numerical Model 21

22 Concluding Remarks A study showing that vertical-axis turbines can be used to power communication towers. Design carried out through numerical simulations. Results are confirmed by experimentation. These devices hold real promise for off-grid applications. 22