EXPERIMENTAL ANALYSIS OR AIRFOIL FOR HIGH ANGLE OF ATTACK

Transcription

1 UNIVERSIDADE FEDERAL DO PARÁ MECHANICAL ENGINEERING DEPARTMENT TURBOMACHINERY GROUP SCIENTIFIC INICIATION MAGAZINE PROJECT IDENTIFICATION Professor: André Luiz Amarante Mesquita Title: Doctor Student: João Paulo da Paz Sêna Funding : CNPq EXPERIMENTAL ANALYSIS OR AIRFOIL FOR HIGH ANGLE OF ATTACK Revista Virtual de Iniciação Acadêmica da UFPA Vol 1, No 2, Julho 2001 Página 1 de 15

2 INDEX 1 Introduction 2. Aerodynamic wing section 3. Aerodynamic balance calibration 4. Calibration procedure 4.1. Balance zero 4.2. Drag calibration 4.3. Lift calibration 5. Aluminum wing section 6. Experimental analysis in NACA 0012 wing section 7. Results 8. Conclusion 9. References Revista Virtual de Iniciação Acadêmica da UFPA Vol 1, No 2, Julho 2001 Página 2 de 15

3 ABSTRACT During a period of operation of wind turbine rotors, and mainly in the occurrence of strong winds, the blades of the rotor can face high angles of attack. For this zone, the data of the aerodynamic coefficients are scarce in literature and normally models to predict these coefficients are used. A detailed experimental analysis of drag and lift coefficients is the object of study of the work. This aspect is very important for energy generation analysis of a wind turbine system in conditions of great variation of the wind speed In this work, was used a low-turbulence intensity wind tunnel of 0.3 x 0.3 m transversal section and 11 m/s of maximum velocity. The measure of the profiles lift and drag coefficients was carried out through an aerodynamic balance. The principle and the methodology of the balance calibration are described in details. The work had been performed using a NACA 0012 profile, for a variation of angle of attack of up to 40 (degrees) and winds speed of 5, 8 and 11 m/s. It is discussed the variation of drag and lift coefficients with the angle of attack. The experimental results for the NACA0012 profile were considered satisfactory, therefore, as already mentioned in the literature, did not have these values for the aerodynamic coefficients in high angle of attacks, normally met up to 16 (degrees), fact considered unsatisfactory for the study of wind turbine rotors.for the development of a small-scale medium velocity windmill system, it is worth considering the use of simplified airfoil shapes rather than the specially cambered forms of NACA airfoils. The low Reynolds number encountered by such a windmill could degrade the performance of sophisticated airfoils to such an extent that a simpler design might prove just as effective. For all the reasons above, tests were conducted on three aluminum cambered plates to determine their lift and drag characteristics, using the same procedures as the one used for the NACA 0012 airfoil. Once again the results were considered satisfactory, when compared to other works as the same nature as ours. Revista Virtual de Iniciação Acadêmica da UFPA Vol 1, No 2, Julho 2001 Página 3 de 15

4 1 Introduction In Amazon there are numbers of communities which use diesel generators as energy suply. However the operational cost, drives us to search for new energy sources. In this context, the wind turbine energy appears as a great alternative of energy supply for these communities. Alves et all (1997 and 1998) says its very important to know that the rational use of wind turbine energy its based in deep study of availability and specifications for optimum systems, economical and technical aproach, for each shown case. This way the GTDEM ( Turbomachinery Group from Mechanical Engineering Department UFPA) develops a research program of wind turbine conversion units. One crutial involvment in this work it is the so called aerodynamics in wind turbine. The object of study of this paper is fully based upon the behavior of rotors blades and windmills. During a period of operation of wind turbine rotors, and mainly in the occurrence of strong winds, the blades of the rotor can face high angles of attack. For this zone, several works (Ostowari, 1984: Rijs, 1990: Neogi, 1995: Siddig, 1992: Pandey, 1988: Viterna, 1981) show that the data of the aerodynamic coefficients are scarce in literature and normally models to predict these coefficients are used. A detailed experimental analysis of the drag and lift coefficients is the object of study of the work. This aspect is very important for energy generation analysis of a wind turbine system in conditions of great variation of the wind speed. This way, this paper shows a new methodology to measure and understand the physical phenomenon based in statistics and mathematical approaches that support the whole procedure of data achievement. For the development of a small-scale medium velocity windmill system, it is worth considering the use of simplified airfoil shapes rather than the specially cambered forms of NACA airfoils. The low Reynolds number encountered by such a windmill could degrade the performance of sophisticated airfoils to such an extent that a simpler design might prove just as effective. For all the reasons above, tests were conducted on three aluminum cambered plates to determine their lift and drag characteristics, using the same procedures as the one used for the NACA 0012 airfoil. Revista Virtual de Iniciação Acadêmica da UFPA Vol 1, No 2, Julho 2001 Página 4 de 15

5 2. Aerodynamic wing section Abbot et all (1959) says that in aviation a wing section its a very important part of the airplane, responsible to hold all the dynamic reactions which during the flight. Nevertheless, a wing section is not only useful in the aeronautical industry, it can be used too in wind turbines to provide electrical energy and water pumping in certain situations. The wing sections have been studied thru the years, specially during the world wars. During world war II, the development of wing sections was so high that these aerodynamic geometries started to be divided in families, like four digit, five digit, and so on and so forth. Most expressive details about the conception of the wing section will be shown during this paper. Figure 1. Wing section components The Figure (1) shows the main components of a wing section, the leading and trailing edges are defined as the forward and rearward extremities, respectively, of the mean line. The chord line is defined as the straight line connecting the leading and trailing edges. Ordinates of the cambered wing sections are obtained by laying off the thickness distributions perpendicular to the mean lines. The abscissas, ordinates, and slopes of the mean line are designated as Xc, Yc and tan θ, respectively. If Xu and Yu represent, respectively, the abscissa and ordinate of a typical point of the upper surface of the wing section and Yt is the ordinate of the symmetrical thickness distribution at chordwise position X, the upper-surface coordinates are given by the following equations: Xu = X Yt sin θ Yu = Yc + Yt cos θ (1) There are some corresponding equations to the lower-surface: Xl = X + Yt sin θ Yl = Yc Yt cos θ (2) The center for the leading-edge radius is found by drawing a line through the end of the chord at the leading edge with a slope equal to the slope of the mean line at that point and laying off a distance from the leading-edge along this line equal to the leading-edge radius. This method of construction causes the cambered wing sections to project slightly forward of the leading-edge point. Revista Virtual de Iniciação Acadêmica da UFPA Vol 1, No 2, Julho 2001 Página 5 de 15

6 Because the slope at the leading-edge is theoretically infinite for the mean lines having a theoretically finite load at the leading-edge, the slope of the radius through the end of the chord for such mean lines is usually taken as the slope of the mean line at x/c equals 0,005. This procedure is justified by the manner in which the slope increases slowly until very small values of x / c are reached. Large values of the slope are thus limited to values of x / c very close to 0 and may be neglected in practical wing-section design. 3. Aerodynamic balance calibration The balance is the equipment used to obtain numerically the values of forces applied into the wing section. To these values become visible another equipment is connected to the balance. The conditionator Fig. (3) translates the electrical pulse in numerical numbers. Each visor indicates a respective value, which is in fact the forces applied in the dynamometers of the balance (D1, D2, D3). According to Fig. (3) the loads applied in the balance can be reproduced in two main forces, drag and lift. To a better understanding of the applied forces in the balance the next item will bring some more details about it. Figure 3. Applied forces in the balance The drag and lift forces obtained are demonstrated in this section, showing how the wind action over the wing section is reproduced in electrical signs and later in digital reading. During the experiment was analyzed that to the drag force the momentum can be equalized, this way. M 1 = Fx. L 3 (3) M 2 = F1. L 1 (4) M 1 = M 2 (5) Revista Virtual de Iniciação Acadêmica da UFPA Vol 1, No 2, Julho 2001 Página 6 de 15

7 Analyzing the Fig. 4 and assuming that L 3 = mm and L 1 = 140 mm we easily can deduce that M 1 = Fx. 462,5 and M 2 = F Using Eq. (5) we can assume that Fx. 462,5 = F1. 140, this way we can find the drag force applied in the wing section. Fx = 0,303.F1 (6) The value of force Fx (drag) can be read directly in the visor F1 of the conditionator because the 0,303 factor is already multiplied in the equipment. To obtain the value of lift forces it is only necessary to ad the results found in the visors F2 and F3. Fz = F2 + F3 4. Calibration procedure (7) The calibration of the balance is a very important step in the experiment process, the procedure should be done over the best conditions as possible. Giving this way a great quality in the results achieved. The equipment is at first installed in a flat platform, cleaned and free of imperfections. This first step looks simple but in deed it is very important, due to the conditions of the experiment. To level the balance was used a measure instrument with 3 seconds of accuracy (square level with microscope). To do that is necessary to obtain the instrument reading in both sides of the balance, trying to leave it as much leveled as possible. The balance it self has four regulators Fig. (4) in each corner to provide the user a good tool to level the quipment according to the necessity. Drag device 2 1 Lift device Level regulator Figure 4. Balance with calibration devices The several other components of the balance should be leveled with the platform. The hard points 1 e 2 as shown in Fig. (4) are responsible to maintain the lift device in a very stable platform during the calibration procedure, so these components have to be straight and leveled with the balance. To achieve this step is used a vertical tracer of 1/150 mm of accuracy. The whole procedure it self consists in comparing the reading of both sides to level them each other. Revista Virtual de Iniciação Acadêmica da UFPA Vol 1, No 2, Julho 2001 Página 7 de 15

8 4.1. Balance zero During the calibration is extremely necessary to take in consideration the balance zero. This procedure will give us the exactly measure of the force, avoiding us to take wrong readings in the experiment process. To obtain this value is necessary at first take out every single device of the balance and them take a reading in the conditionator. It means that, the numerical values found in the conditionator`s visor is not a force applied in the balance but only a digital inertia found in the equipment due to last experiment that were done. Table 1. Values found during balance zero procedure F1 F2 F When the calibration device is set in the balance an extra weight will be added in the process, so its imperative that the forces caused by these devices should be discounted. This new value can be called the calibration device discount (β). Table 2. Values from lift calibration device Table 3. Values from drag calibration device F1 F2 F F1 F2 F Mathematically we can find the discounts caused by these devices. β DRAG = = 0,58 (8) β LIFT = = 2.44 (F2) β LIFT = = 0.46 (F3) β LIFT = 2.9 (9) 4.2. Drag calibration This procedure should be the most simple and efficient as possible, this way a methodology was created to it. To simulate the force over the balance are used several standard weights as seen in Fig. (4), the balance answer is visualized in visor F1 of the conditionator in Newton (N). One by one the weights are fixed in the plate Fig. (4), with a group of standard measure can be plot a graphic between the real force and the read force. With this result, using a graphic approach can be found the calibration equation. It means that, every single force applied into the balance the equation found into the calibration have to be used. When the data is collected it has to be treated mathematically, discounting the balance zero and the calibration device discount. During this paper two calibration were done, one to the NACA 0012 wing section and other to the aluminum wing section. The whole procedure is Revista Virtual de Iniciação Acadêmica da UFPA Vol 1, No 2, Julho 2001 Página 8 de 15

9 valid for both, it means that every time the balance is assembled for a new configuration, a new calibration has to be done. This way two equations were found, respectively one to NACA 0012 wing section and other to the aluminum wing section. Y = x 06 (10) Y = X 5X 2 (11) 4.3. Lift calibration The procedure to calibrate the lift force is the same as used during the drag force calibration, however the plate position is in the middle of the balance Fig. (4). Once again are used standard weights to simulate the force applied into the balance, one by one they are set in the plate and them is read the measure of the force in the conditionator. Using Eq.(7) the lift force can be found. The discounts due to the balance zero and the calibration device have to be done. With a plot between the real force and the applied force is found the calibration equation, respectively to NACA 0012 and aluminum wing section. Y = 1.005x 2 Y = 1.28X 0.12 (12) (13) 5. Aluminum wing section During the experiment was used an aluminum wing section with 0.04 mm of thickness and a curvature obtained geometrically, the method will be shown in this section. To be able to compare the results found, our paper has the same aspect reason (relation between wide and chordwise) as the one used in the Indian Aeronautical Institute. Due to wind tunnel limitations the wing section is 295 mm wide. The aspect reason is 2, this way the chordwise can be found by: R = B / C (14) R = 2, B= 295 C = 148 mm Revista Virtual de Iniciação Acadêmica da UFPA Vol 1, No 2, Julho 2001 Página 9 de 15

10 The next step considers the curvature of the wing section, from the literature we can find a relation between f / c. Using this data f / c = 0.04 we assume that f = Assuming that D = y + F and R = (y+f)/2 Figure 5. Slope ratio Y = (c/2) 2 / F = sin -1 (c/2/r) as θ = 2 AA = θ. R Tabela 4. Aluminum wing section components F/c 0,04 F (mm) 5,92 Y (mm) 925 R (mm) 465,5 θ (rad) 0,32 AA 148,63 With a measure table we can give the right slope to the wing section thru and effort made in the plate. The AA value defines the effort measure, this limit is mm, and the wing section will finally have 0.32 rad ( ) of curvature. 6. Experimental analysis in NACA 0012 wing section The wing section is properly installed in the balance Fig.(6), to this process were used small aluminum devices which are able to connect the wing section with the balance hard points Fig. (4). During the process the angle of attack (AOA) of the wing section varies with a range of 0 0 to 40 0 (degrees), this way it is possible to read the aerodynamic forces applied in the NACA In this paper only the drag and lift forces were analyzed, due to their importance in wind turbine energy studies. The data is not simply read in the condinionator, they are carefully analyzed specially because the forces caused by the wing section weight. Once again the zero has to be analyzed, so one reading is taken when the balance does not have the wing section, and other one considering the wing section weight and the drag caused by the aluminum devices which Revista Virtual de Iniciação Acadêmica da UFPA Vol 1, No 2, Julho 2001 Página 10 de 15

11 are responsible to keep object of study connected in the balance. The distance between the wing section to the hard point Fig. (4) now is different. During the considerations before the value which is read in the balance visor F1 is multiplied by 0.303, so when the drag force is read this value is already included, this way this value is not take in consideration. Figure 6 : Wing section and balance The method to eliminate this factor is simple. At first the distance between the wing section and the hard point should be measure, analyzing Fig. (3) this step can be much more clear. This distance plus is used now to equalize the momentum in the balance. Using the same mathematical process seen in section 3 the new value can be found. To be more specific two equation should be equalized, Eq. (3) and Eq. (4). After eliminating the value in drag forces the calibration equations are used to find the forces and are multiplied by the new value found to the NACA 0012 wing section. To aluminum wing section was not necessary to determine this value. All the coefficients were found using the aerodynamic equations to drag and lift force. The wind tunnel has 30 x 30 cm and the maximum wind speed was 11 m/s. 7. Results Cd = Fd/ ½ ρ v 2 A (15) Cl = Fl/ ½ ρ v 2 A (16) The first result shown are to NACA 0012 wing section, during the process three different air velocities were used over NACA 0012 wing section, in the aluminum wing section was used the highest wind velocity according to the wind tunnel limitations. Revista Virtual de Iniciação Acadêmica da UFPA Vol 1, No 2, Julho 2001 Página 11 de 15

12 Re = 0.6 x Cd Cl Re = 0.93 x Cd 0.40 Cl Revista Virtual de Iniciação Acadêmica da UFPA Vol 1, No 2, Julho 2001 Página 12 de 15

13 Re = 1.11 x Cd Cl The next graphics refer to aluminum wing section Cd Cl Revista Virtual de Iniciação Acadêmica da UFPA Vol 1, No 2, Julho 2001 Página 13 de 15

14 8. Conclusion This paper is able to show us all the physical phenomenom related to the wing section studies for high angle of attack, and most importantly, all the results found were totally experimental. The NACA 0012 and aluminum wing section had a few literatures to compare and the final results show us that ijn comparision they are all acceptble. This conclusion is very important, because to low Reynold numbers (low velocity) the behavior of wing sections satisfy all the needs found in the wind turbine project. The engineering teaching is also very important, comproving that science in this country is not only made in certain spots, but too in other regions around the country. Revista Virtual de Iniciação Acadêmica da UFPA Vol 1, No 2, Julho 2001 Página 14 de 15

15 9. References Alves, A. S. G.,1997, Análise do Desempenho de Rotores Eólicos de Eixo Horizontal, Dissertação de Mestrado, UFPA. Alves, A. S. G., Amarante Mesquita, A. L., and Cruz, D. O. A.,1998, On the Strip Theory for Horizontal-Axis Wind Turbine Design, VII ENCIT, Vol. 2, pp , Rio de Janeiro. Amarante Mesquita, A. L. e Alves, A. S. G., 2000, An Improved Approach for Performance Prediction of HAWT using the Strip Theory, proposed to the Wind Energy Journal. Amarante Mesquita, A. L., Silva, O. F. e Quintana, F. L. P.,1994, "Uma Metodologia para Projetos de Rotores Eólicos de Eixo Horizontal", III Congresso de Engenharia Mecânica Norte-Nordeste, Vol 1, pp , Belém. CNPq Desenvolvimento de Modelos para Análise do Escoamento em Turbomáquinas de Pequeno Porte na Amazônia, CNPq, No /94-5 FUNTEC Sistema Eólico para Bombeamento de Água de Alto Desempenho, Convênio Convênio /97-SECTAM/FUNTEC/UFPA/FADESP. Ostowari, C., and Naik, D.,1984, Post Stall Studies of Untwisted Varying Aspect Ratio Blades with na NACA 4415 Airfoil Section Part I, Wind Engineering, Vol. 8, No. 3. Rijs, R. P. P. and Smulders, P. T.,1990, Blade Element Theory for Performance Analysis of Slow Running Wind Turbines, Wind Engineering, 14(2), pp Neogi, S.,1995 A Modified Flexible Iterative Model for the Performance Evaluation of Slow Wind Turbines for Water Pumping, Wind Engineering, 19(5), pp Siddig, M. H.,1992 An Investigation of the Characteristics of Horizontal-Axis Wind Turbines at Low Tip-Speed Ratios, Wind Engineering, 16(5), pp Pandey, M. M., Pandey, K. P., and Ojha, T. P.,1988 Aerodynamic Characteristics of Cambered Steel Plates in Relation to their Use in Wind Energy Conversion Systems, Wind Engineering, Vol. 12, No. 12. Viterna, L. A., and Corrigan, R. D.,1981 Fixed Pitch Rotor Performance of Large Horizontal Axis Wind Turbines, Proceedings Workshop on Large Horizontal Axis Wind Turbines, NASA CP-2230, pp Abbott, I. H., and Von Doenhoff, A. E.,1959, Theory of Wing Sections, Dover Publications, Inc., N.Y. Revista Virtual de Iniciação Acadêmica da UFPA Vol 1, No 2, Julho 2001 Página 15 de 15