Applied Mechanics and Materials Vols. 110-116 (2012) pp 1589-1593 Online available since 2011/Oct/24 at www.scientific.net © (2012) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMM.110-116.1589
Wind Tunnel Experiment for Low Wind Speed Wind Turbine Blade Mohd Hafiz Mohd Noh 1, a, Ahmad Hussein Abdul Hamid 2,b ,Helmi Rashid 3,c , Wirachman Wisnoe 4, Mohd Syahmi Nasir 5 1
Faculty of Mechanical Engineering, UiTM Shah Alam, Selangor, Malaysia
2,3,4,5
Faculty of Mechanical Engineering, UiTM Shah Alam, Selangor, Malaysia
a
[email protected],
[email protected],
[email protected]
Keywords: Wind tunnel experiment, aerodynamics study, NACA four-digit series, lift coefficient, angle of attack
Abstract. Environment and green energy awareness are two main factors why this study has been carried out. This research is focused on aerodynamics study for airfoil structure modification based on NACA 0044 and NACA 0063 by using wind tunnel experiment. Aerodynamic characteristics such as lift coefficient, CL, drag coefficient, CD, lift to drag ratio and cell relative velocity has been investigated in this study. CFD simulation has been carried out at the early stage of the investigation (for NACA 0044 and NACA 0063), and a new airfoil profile had been created (0044-63) by modified the chord length and the location of maximum thickness of the airfoil by using the modified NACA Four-Digit Series. Wind tunnel experiment has been take place for three different wind speeds from 25m/s, 35m/s and 45m/s at various angles of attack from 0o to 40o with 5o incremental for the respective airfoil. The results show that the modified 0044-63 produced the better lift coefficient and this airfoil has been fabricated and tested in the wind tunnel experiment in order to validate the CFD result. This paper reports the result of aerodynamics characteristics for respective new airfoil and it shows that at angle of attack between 5 o to 15 o, this airfoil produced good lift to drag ratio value. Also, by modified the location of maximum thickness 30% to the trailing edge give the increment of lift to drag ratio produced approximately 15% and at the same time, give insignificant changes to the drag coefficient value. Introduction One of the major constraints for wind turbine development is the wind speed and for low wind speed country like Malaysia, the average wind speed only exceeds at maximum value of 3 m/s [9]. Therefore, it is necessary to design wind turbine which capable to rotate at low wind speed. Wind energy promises an almost infinite supply of clean and renewable energy. Governments around the world have recognized its potential and as in developed countries, subsidies and market incentives have supported the early development of wind power as an environmentally friendly alternative to conventional fossil fuels. In these countries, wind energy represents a practical solution for cash strapped governments who cannot afford to extend power lines to every village. Over the past two decades, the growth of wind energy has been phenomenal. Yet despite its rapid growth, the penetration of wind energy in the electricity market is limited. Innovation has driven down costs, but relative to conventional sources of utility-scale power wind energy remains costly. J. Larson [11] has studied and analyzes the effects of varying the cord length and amount of taper of a turbine blade on the overall wind turbine performance. It found that by increasing the chord length or decreasing the amount of taper can increase the power captured from the wind but at the cost of an increased thrust force. Meanwhile, the amount of power generated proportional to the resulting torque does not vary. From here, it can be say that modification on the origin airfoil structure can give the positive impact to the power produced. C. Thumthae [8] has focused her study on the optimal angle of attack for untwisted blade wind turbine. Based on her findings, untwisted blade type is useful for small and medium wind turbines and it has been suggested that the design angle of attack of a wind turbine blade should be searched for All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 202.58.86.23, Universiti Teknologi MARA, Shah Alam, Malaysia-26/02/14,03:55:01)
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iteratively by starting the search at the point of maximum lift to drag ratio. Through the blade element theory, it can be seen that power output of a wind turbine depends both on lift and lift to drag ratio, and not only on lift to drag ratio alone. This suggests that the optimal angle of attack might be somewhere between the point of maximum lift to drag ratio and the point of maximum lift. Overall, optimization of lift to drag ratio and lift force will lead for the optimum value of angle of attack, which it’s related to this paper. Besides cord length and amount of taper, it also important to look into the potential of maximum thickness location modification. Adjusting the maximum thickness will lead to the changes of aerodynamics centre and this paper is focus on this particular approached. This paper present the aerodynamics study for three selected airfoil profiles (NACA0044, NACA0063 and 0044-63) which had been developed by CATIA and the simulations performed in STAR-CCM. The study focus on the lift coefficient, drag coefficient, pressure coefficient contour, cell relative velocity, angle of attack and by adjusting the chord length and the location of maximum thickness of NACA 0044 and NACA 0063, a new airfoil profile has been created called 0044-63. The final design (0044-63) has been fabricated and followed by wind tunnel experiment as the result validation[10]. Literature Review A.T.A Fadzil [2] has been running his BWB model by using the similar apparatus (wind tunnel) with this experiment and after tabulated the result, it can observed that as the particular model running below than 25m/s speed, the results gained were not constant. This is important since the same apparatus has been used in this study, so based on this constrain, we decided not to run at the velocity below than 25m/s. This fact also supported by M. Zaidie [3] in his research that proposed the same outcome when running the wind tunnel testing at below 25m/s velocity. For CFD analysis assumptions of a horizontal axis wind turbine (HAWT) blade, the atmospheric to be turbulence because it can causes important fluctuating aerodynamic forces on the wind turbines. Turbulence can be defined as an irregular motion of fluid that appears when fluid flow past soil surfaces or when streams of fluid flow past or over each other [1]. J. Larson [11] has studied and analyzes the effects of varying the cord length and amount of taper of a turbine blade on the overall wind turbine performance. E387 airfoil profile was chosen from a set of six recommended airfoils for small wind turbines by the National Renewable Energy Laboratory. Furthermore, the author also studies on the design parameters include the shape, number, length, twist, Tip-Speed-Ratio of the blades, the air viscosity, and the wind speed. The results show that the chord length and the amount of taper for the blades on the wind turbine can contribute a significant effect on the overall performance. Increased the chord length or decreased the amount of taper can increase the power captured from the wind but at the cost of an increased thrust force. C. Thumthae [8] has focused her study on into the optimal angle of attack for untwisted blade wind turbine. Based on her findings, twisted blade for wind turbine has proved to be superior to the untwisted blade due to its full utilization of blade area to produce lift at low drag while providing a good starting ability. It has been suggested that the design angle of attack of a wind turbine blade should be searched for iteratively by starting the search at the point of maximum lift to drag ratio. Through the blade element theory, it can be seen that output power of a wind turbine depends both on lift and lift to drag ratio, and not only on lift to drag ratio alone. This suggests that the optimal angle of attack might be somewhere between the point of maximum lift to drag ratio and the point of maximum lift. Furthermore, the effects of camber and thickness have also been analyzed and it was found that camber introduces a significant increase in lift while minor effects are observed from introducing the thickness. S.K. Najid [9] on her journal “Analyzing the East Coast Malaysia Wind Speed Data” has reviewed on wind turbine development around Malaysia and reported that, a 150kW wind turbine generator Malaysia is located at Pulau Layang-Layang, Sabah and two of 100 kW wind turbines at Pulau Perhentian Kecil, Terengganu as part of unique solarwind-diesel hybrid power generation
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system. Also on her research, it shows that the climate of Malaysia is dictated by the Northeast and Southwest Monsoon. As its location is at the equator, the wind speed over the region is generally low. It is observed that the strongest wind occurs on the East Coast of Peninsular Malaysia with 3.5 - 4 m/s during the Northeast Monsoon. Based on these findings, it can be concluded that there is possibility of wind turbine development in Malaysia and also, the average wind speed at Malaysia is around 3m/s – 4m/s respectively. Methodology Based on the CFD result which has been conducted at the early stage of this study, new airfoil 0044-63 has give the best result for overall aerodynamics analysis compared with NACA 0044 and NACA 0063 airfoil. So, the airfoil modified 44-63 has been finalize as the chosen final design and undergo the fabrication process. After fabrication process completed, the model undergoes finishing and polishing process and the completed final testing model shown in Figure 1. Wind tunnel testing take place at various speeds and the dimension of airfoil being fabricate is smaller than testing section on the wind tunnel. The speed of 25 m/s, 30 m/s, 35 m/s and 40 m/s has been selected and it ran at different angle of attack from 0 o until 40 o with incremental 5o. This range has been chosen reflected from the CFD result which has been simulated at the earlier stage of this study. It is agreed that 25 m/s is not considered as low wind speed (respect to the objective of this study), but based on the result produced from previous research which used the same apparatus (wind tunnel) suggest that if this study conducted at wind velocity below than 25 m/s, the results will be not very convenience [2,3]. TABLE 1.
WIND TUNNEL SPECIFICATION
Test cross section area Overall length Overall height Maximum speed Number of blades Maximum power available
500 mm x 500 mm 8050 mm 1900 mm 50 m/s 9 pieces 25 HP
Fig. 1 Final model for testing So based on this constraint, the minimum value of wind speed is set to be 25 m/s and the comparison with the CFD simulation result will be made by using Reynolds number similarities. For this study, wind speed of 25 m/s is going to similarities with the speed of 3 m/s from CFD result and by using Reynolds number similarities, the drag coefficient value gained from this experiment will ratio than the comparison can easily made. The specifications of the wind tunnel are summarizes in Table 1. Result and Discussion The model was experimenting at different wind speed in order to demonstrate the relation of lift and drag coefficient at dissimilar speed. Figure 2 shows the result of lift coefficient against angle of attack.
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Fig. 2, Graph of Lift Coefficient, CL VS Angle of Attacks
Fig. 3, Graph of Drag Coefficient, CD VS Angle of Attacks
Based on this result, we can observe that as angle of attack increases, the values of lift coefficient also increase. The graph clearly show that lift coefficient produced by 40m/s are higher compared with 25m/s and all the velocity recorded the similar trend of lift coefficient increment and the stall angle can be observed at 38.5o which the lift coefficient experienced to be decreased after it reached at highest value. This also proved that the CFD analysis which has been conducted at earlier stage of this study is acceptable. The results also revealed that the chosen airfoil has produced lift at very low wind speed and again, this experimental value proved the CFD analysis earlier. Besides lift coefficient, another aerodynamics behavior that also contributes in producing good wind turbine rotor design is drag coefficient. In actual of wind turbine blade development, the maker is always tried to minimize the drag force in order to make the lift more dominance. This criterion is quietly mandatory in very low wind speed wind turbine application. The result of drag coefficient is presented in Figure 3. It was observed that, the drag coefficient is increased as angle of attack and velocity increased. The result shows that the maximum drag coefficient value observed to be at 40 o and this value consistently increase as the angle of attack move to positive angle. This result also relatively conflict with requirement of maximizing the lift coefficient to compensate the drag coefficient produced. All the velocity relatively produce similar trend of drag increment and the data also give sense that we cant design a wind turbine blade with higher angle of attack since this will tend to produce higher drag coefficient in other hand. As a solution, in this particular case we have to look into lift to drag ratio so that we can figured out how much the dominance of lift coefficient compared to drag coefficient. Furthermore, this approached is going to give the sense which angle of attack had to be choose as design point. Generally, high lift to drag ratio means that the lift is more dominant and this give the sense that this will compensate the drag produce on that particular model. The decision of angle of attack in this study also depend on the result of this ratio. Figure 4shows the result of lift to drag ratio with the variety of angle of attack and wind speed. It can be observed that, at wind speed of 25 m/s, the maximum lift to drag ratio achieved at the angle of attack of 15 o and this is similar to the wind speed of 35 m/s. For the speed of 30 m/s, the maximum lift to drag ratio is at 5 o and for 40 m/s, the maximum ratio end up at 10 o. It also can figure out that, the lift to drag ratio are decreased when the angle of attack more than 15 o and reflected from this outcome, it is not advisable to design at angle of attack higher than 15 o. So based on this finding, this wind turbine blade should be installed between 5 o to 15 o.
Fig. 4, Graph of Lift to Drag Ratio, CL / CD VS Angle of Attacks
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Conclusion Experimental work was conducted to evaluate the aerodynamics characteristics of the chosen airfoil from CFD analysis. Those characteristics were based upon obtained values of lift coefficient, drag coefficient and lift to drag ratio. In summary, the conclusions are listed as below. 1. Lift coefficient is influenced by the velocity and angle of attack. Higher velocity is contributed for higher lift coefficient. Also the values of lift coefficient are increased when the angle of attack increased and reached at the maximum value at 35 o. 2. The drag coefficient also influenced by the velocity and angle of attack. All velocity produced relatively same trend of increment. 3. Lift to drag ratio shows the maximum value reach at 10 o angle of attack with the velocity of 40 m/s. So based on this result, the design point for angle of attack should be at between 5 o to 15 o. Acknowledgement The authors greatly acknowledge the Research Management Institute (RMI) University Technology MARA Malaysia for the financial support under Dana Kecemerlangan, Computer Lab and Flight Technology Testing Centre (FTTC), Faculty of Mechanical Engineering, Universiti Technology MARA (UiTM), Malaysia for all the facilities. References [1] E. Hau, “Wind Turbines Fundamentals, Technologies, Applications, Economics”, 2nd Edition, Springer, 2006. [2] A. T. A Fadzil and W. Wisnoe, “Wind Tunnel Test of BWB Aircraft at Loitering Phase”, UiTM Thesis, 2009. [3] W. Zaidie and W. Wisnoe, “Experimental Aerodynamics of BWB UAV with Elevator Deflecting Angle +5 o to -5 o “UiTM Thesis, 2009. [4] Charles L. Ladson, “Development of a Computer Programme to Obtain Ordinates for NACA 4-Digit, 4-Digit Modified, 5-Digit and 16-Series Airfoils”, NASA Langley Research Center, 1995. [5] Russell M. Cumming, “Computational Challenges in High Angle of Attack Flow Prediction”, Aerospace Engineering Department, California Polythenic State University, 2003. [6] Jeffrey Keith Jepson, “Enhancements to the Inverse Design of Low-Speed Natural-Laminar-Flow Airfoils”, Degree of Master of Science Thesis, North Carolina State University, 2003 [7] Matt M. Hejazi, “CFD Analysis of a Horizontal Axis Wind Turbine (HAWT) Blade”, Department of Mechanical Engineering, Florida Atlantic University, 2008. [8] Chalothorn Thumthae, “Optimal Angle of Attack for Untwisted Blade Wind Turbine”, School of Mechanical Engineering, Suranaree University of Technology, 2008. [9] S.K. Najid, “Analyzing the East Coast Malaysia Wind Speed Data”, International Journal of Energy and Environment, 2008. [10] M. Hafiz and A. Hussien, “ Numerical Investigation for Low Wind Speed Wind Turbine Blade (for 3 m/s)”, ICAME2010 proceeding paper,2010, ISBN: 978-967-363-386-5. [11] John Larson (2008), “Trade Study: The Effect of Cord Length and Taper on Wind Turbine Blade Design”, Group C4: Turbinator Technologies.
Mechanical and Aerospace Engineering, ICMAE2011 10.4028/www.scientific.net/AMM.110-116
Wind Tunnel Experiment for Low Wind Speed Wind Turbine Blade 10.4028/www.scientific.net/AMM.110-116.1589
