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Vibrational Fatigue Analysis of NACA 63215 Small Horizontal Axis Wind Turbine blade
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 5 (2018) 6665–6674
www.materialstoday.com/proceedings
IMME17
Vibrational Fatigue Analysis of NACA 63215 Small Horizontal Axis Wind Turbine blade Senthil Kumar. Ma*, Krishnan. A.Sb and Vijayanandh. Ra* a
Department of Aeronautical Engineering, Kumaraguru College of Technology, b Department of Mechanical Engineering, Coimbatore Institute of Technology, Coimbatore, Tamilnadu- India
Abstract Wind turbines are critical in structural behaviour, which are characteristically using the wind in order to produce power. In wind machines, blades are considered to be an important component because of its critical profile at different sections, weight, and the structural parameters with relatively high amplitude and high frequency. Life-cycle estimation of wind turbines is crucial to develop their design and maintenance process, since they should have more lifespan with minimum foreign object disruptions as well as low probabilities of failures. Wind turbine is eco friendly technology; it should provide a high lifespan of its whole set up by reduces the major failure factors. Due to the effect of aerodynamic loads acts in the wind turbine blades may cause to fail at unpredictably high an amount, which creates the wind turbine to make fatigue analysis as important factors in its performance. Fatigue life and its analysis of each rotating component is one of the major factors of concern due to the terrible failures that can result from it. In this paper, the fatigue behaviour of wind turbine blade in response to different frequencies has established to the level that the prediction of working lifespan is fitting an essential part of the design process also compare the suitability of a wind turbine blade with different composite materials such as Kevlar, Glass Fiber Reinforced Plastic (GFRP) and Carbon Fiber Reinforced Plastic (CFRP) by simulates the displacement and principal stress using numerical method. The reference component of this paper is modeled by using CATIA. A numerical model of the blade was created using ANSYS Workbench 16.2 in order to estimate the typical mode shapes occurring within the blade based on a wind profile and mass approximating the location where these blades are expected to vibrate. Also, fatigue life of wind turbine blade analyzed for three composite materials and the results are compared in order to find out the optimum material body. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Emerging Trends in Materials and Manufacturing Engineering (IMME17). Keywords: Composite materials, Fatigue life, Frequency, Loads, Wind turbine blade.
* Corresponding author. Tel.: +91 9791639490; fax: +0422-2669406. E-mail address: [email protected], [email protected] 2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Emerging Trends in Materials and Manufacturing Engineering (IMME17).
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Senthil Kumar et. al./ Materials Today: Proceedings 5 (2018) 6665–6674
1. Introduction Wind turbines translate the kinetic energy in the wind into power, which can be used for specific tasks or else with the help of generator this mechanical power can convert into electricity. The electricity is sent via electrical cables to homes, businesses, schools, and so on. Advantages of using wind turbines are electricity generation without any environmental pollution, fast installation, commissioning, and low maintenance. Its main disadvantage is the transitory nature of an air flow [1], which is conquered with the help of high aerodynamic efficient components, by utilizing as much as energy from the wind during the limited period of time that it flows strongly. Nowadays, almost all wind turbines used to generate electricity are wind turbines with the horizontal axis of rotation because rotor speed and power output can be efficiently handled by pitching the rotor blades about their longitudinal axis, rotor blade pitching operation provides effective protection against high rotational speed and maximum aerodynamic forces, rotor blade profile can be aerodynamically optimized compared to vertical axis wind turbine, technological lead in the development of propeller design is an influential factor [2]. 2. Problem Description Modern wind turbines are structurally crucial equipments, which are typically used to generate electrical power from the moving air fluid. Large rotating wind machines specify that their rotary apparatus were failing at unpredicted high rates, which guided the wind turbine community to develop fatigue analysis capabilities. Hence the prediction of lifespan is becoming an essential parts of the design process and wind turbine blade material selection. To attain the uppermost achievable power from the wind turbine under precise atmospheric conditions, modifying two parameters are indispensable. The first one is design of wind turbine blade and its optimization then the next one is a change of the mechanical and fatigue properties of the wind turbine blade materials [3]. The factors involved in the selection of materials for wind turbine components are high stiffness to maintain optimal aerodynamic performance, low density to reduce gravity forces, and long fatigue life to reduce material degradation. Nowadays blades of the horizontal axis are now entirely made up of composite materials have lower weight and good stiffness, also providing good resistance to the static, dynamic, and fatigue loadings. Designing of the composite blades to get the ultimate energy from the air flow is a vital task. Admittedly, modification of the shape of blade changes the stiffness and strength, but it may control aerodynamic efficiency of the wind turbine. The design of the composite blade depends upon the strength to weight ratio, selection of airfoil, load estimation on the blade and stiffness to weight ratios [4]. Therefore, specifying the finest shape of the blade and most favorable composite material properties are the significant and intricate problem. This paper deals the design, materials selection and structural analysis of small horizontal axis wind turbine blade in order to estimate the fatigue life with the effect of vibrations. With the help of the complete literature survey the fatigue life estimation of this composite blade divided into two major parts: first one is stress analysis of the wind turbine blade and the next one is optimization of the wind turbine blade to withstand maximum fatigue life. 3. Design of Small Horizontal Axis Wind Turbine Blade 3.1 Principle of Wind Turbine Blade design In wind turbine manufacturing process, geometry of the blade and its supporting parameters takes important position because its control directly the increment of the electrical power generation. The most important part in designing of a wind turbine is the blade profile at various sections. The rotational force and performance of the blade is highly depends on airfoil performance and its perpendicular force. NACA63215 airfoil was selected for this analysis [5]. Physical as well as mathematical models are the representation of a blade, which is processed by the workstation. Commonly, a model intended for a numerical simulation, in which comprises of mathematical model creation, finite element model creation, application of boundary conditions and finally numerical solution. Here the wind turbine blade is modeled by CATIA and then imported to ANSYS Workbench for numerical simulation. Figure 1 shows the CAD model of our wind turbine blade.
Senthil Kumar et. al./ Materials Today: Proceedings 5 (2018) 6665–6674
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Fig. 1. CAD model of Wind turbine blade
3.2 Wind turbine blade materials and its selection In the early windmills, wood was used and revealed to be a good material due to its reasonable strength-toweight ratio, low density, but it’s not applicable for very large rotor blades due to their low stiffness and also difficult to obtain in reproducible and high quality, which is a requirement for stable and economical manufacturing of rotor blades [6]. Metals were a popular choice because they can be easily manufactured, give a high degree of reliability and yield a low-cost blade. Aluminum was only implemented in testing situations because it was found to have a lower fatigue level than steel. Older style wind turbines were designed with heavier steel blades or nickel alloy steels which have higher inertia [6, 7]. The purpose of nickel alloy is lessened alteration in heat treatment and lowers the critical temperatures of steel. Alloy steel was thought to be a most advantageous choice for blade production but was soon discarded due to its high weight and low fatigue level [7]. Nowadays, composites have become the most used blade material, which have good mechanical, thermal and chemical properties and ability to fit the material to the maximum loads by combine high strength with high stiffness. The optimal design of the wind turbine blades is today is an essential task and needs optimization of mechanical and fatigue properties, performance, and financial wealth because it's subjected to significant loads, so the materials used have to support the structural stress, so composite material is one of the perfect options to handle the practical problems. 3.2.1 Composite Material Composite material is a material, which comprises of strong fibers surrounded by a matrix. The matrix and fibers efficiently joined in composite manufacturing process for the purpose of increase their properties such as high stiffness to weight ratio, high load withstanding capabilities, etc. The principal reinforcements using the wind turbines are Glass; Kevlar- very light; Carbon; Boron - high modulus; Silicon carbide - high-temperature resistance [8]; Carbon fibers have the high modulus of elasticity with longest breaking length, and their stiffness is high. Glass fibers are amorphous with isotropic properties. Commonly, glass fibers combined with carbon fibers for the purpose of areas more prone to stress. Most glass-reinforced products are made with E-glass, which has good electrical and mechanical properties and high heat resistance. Nowadays, manufacturers prefer epoxy resins, which are more costly but present higher strength characteristics and higher fatigue resistance [9]. The main objective of this paper is to select the best suitable composite material for a wind turbine with high lifetime and efficiency. This selection is based on the numerical simulation results such as total deformation, principal stress, natural frequencies effect, forced vibration impact, and vibrational fatigue acts on the wind turbine blades among Kevlar, GFRP, and CFRP. 4. Numerical Analysis Here the finite element method is used for the estimation of structural analysis, thermal analysis, pre-stressed modal analysis, harmonic analysis of wind turbine blade. ANSYS Workbench 16.2 is a finite element tool, which is used to calculate the stresses, total deformation, mode shapes, etc., as shown in figure 2. The wind turbine blade is well thought-out as a cantilever and as the thickness of the wind turbine blade is very less than the length and the
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cross section of the blade the plane stress condition is assumed. For that, the solid element which has a 6 degree of freedom is used. The finite element model is defined in terms of the chord length (c), the length of the blade (l) and the twist angle (Ф). The total number of nodes generated is 26874, while the total number of elements generated is 26615 with the orthogonal quality of 0.9622 out of 1.
Fig. 2. Finite element model of wind turbine blade
The centrifugal force, the gravity force, the aerodynamic normal force and tangential forces are applied to the wind turbine blade. The basic requirement of wind turbine materials is to withstand the aerodynamic loadings with low density; the composite is the only solution to overcome this problem [10]. The mechanical properties of different composite materials are shown in table 1. Table 1. Mechanical properties of different composite materials Material name Density (kg / m3) Poisson’s ratio Young’s Modulus (GPa) Kevlar 1470 0.33 75 GFRP 1800 0.25 54 CFRP 1600 0.10 70
Bulk Modulus (GPa) 73.529 36.000 29.160
Shear Modulus (GPa) 28.195 21.600 31.818
Figure 3 shows the pressure contour of the wind turbine blade which has the rotational speed of 41.888 rad/s for air speed of 10 m/s. Figure 4 shows the different loads acting on the wind turbine blade. The different load's acting on the blade profiles are rotational speed, the aerodynamic load which import from ANSYS and remote displacement applied on the hub blade connection region.
Fig. 3. Pressure contour
Fig. 4. Blade with all boundary conditions
4.1. Deformation and Principal stress results 4.1.1. Deformation and Principal stress of Kevlar Figure 5 to 8 shows the total deformation, maximum principal elastic strain and principal stresses of wind turbine blade for Kevlar properties. The equivalent stress induced in the blade is 1127 MPa and the total displacement of a blade deformed at the free end is 75.501 mm.
Senthil Kumar et. al./ Materials Today: Proceedings 5 (2018) 6665–6674
Fig. 5. Total deformation
Fig. 6. Maximum Principal elastic strain
Fig. 7. Minimum Principal stress
Fig. 8. Maximum Principal stress
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4.1.2. Deformation and Principal stress of GFRP Figure 9 to 12 shows the total deformation, maximum principal elastic strain and principal stresses results of wind turbine blade for GFRP properties. The equivalent stress induced in the blade is 1081.1 MPa and the total displacement of a blade deformed at the free end is 107.18 mm.
Fig. 9. Total deformation
Fig. 10. Maximum Principal elastic strain
Fig. 11. Minimum Principal stress
Fig. 12. Maximum Principal stress
4.1.3. Deformation and Principal stress of CFRP Figure 13 to 16 shows the total deformation, maximum principal elastic strain and principal stresses results of wind turbine blade for CFRP properties. The equivalent stress induced in the blade is 1061.3 MPa and the total displacement of a blade deformed at the free end is 84.105 mm.
Fig. 13. Total deformation
Fig. 14. Maximum Principal elastic strain
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Fig. 15 Minimum Principal stress
Fig. 16. Maximum Principal stress
The numerical simulation result suggests that the maximum stress creates on the hub to blade connection regions and the maximum deformation occurred in the blade tip. Reduction of sharp edges, increase the structural members, and selection of suitable materials are the important factors for stress reduction in the small horizontal axis wind turbine in order to increase the high lifespan. From the principal stresses and deformation results clearly shows the breaking region in the blade so the designer must concentrate on the design of the hub to blade connection region. GFRP and CFRP are best for aerodynamic stress to withstand capability in small axis wind turbine blade because of its high strength to weight ratio and Kevlar is best for low deflection and more stability because of its high stiffness to weight ratio. 4.2. Fatigue Life The external load acting on the wind turbine components is fluctuating nature in mean as well as the amplitude and there is progressive crack to the mechanical apparatus which results in much more in advance breakdown of parts well before its yield strength. Such mode of loads is called fatigue loads and the failure is termed as fatigue failure. The actual loads that contribute to fatigue of a wind turbine originate from a variety of sources, which are high aerodynamic stable forces, intermittent loads with the effect of rotations and gravity, wind speed variational fatigue loads, external gust loads, and the vibrational natural frequencies induced in wind turbine blade. The numerical simulation of the fatigue stresses is used to find out the fatigue life of a wind turbine blade and its relations [10]. Length wise distribution of life estimation and a safety factor of wind turbine blade have been plotted with the help of principal stresses for the given boundary condition. Life and safety factors are important factors to be considered in wind turbine blades design, performance, production, and efficiency. 4.2.1
Safety factor and life estimation of Kevlar
Figure 17 and 18 shows the lengthwise variation for the safety factor and life of wind turbine blade madeup of Kevlar. The safety factor range of small horizontal axis wind turbine blade is 2 to 5 and it can withstand 10000000 cycles with a low probability of failures.
Fig. 17. Safety factor variation along the blade
4.2.2
Fig. 18. Life estimation throughout the blade
Safety factor and Life estimation of GFRP Figure 19 and 20 shows the lengthwise variation for the safety factor and life of wind turbine blade madeup of GFRP. The safety factor range of small horizontal axis wind turbine blade is 2 to 5 and it can withstand 1000000000 cycles with a low probability of failures.
Senthil Kumar et. al./ Materials Today: Proceedings 5 (2018) 6665–6674
Fig. 19. Safety factor variation along the blade
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Fig. 20. Life estimation throughout the blade
4.2.3
Safety factor and Life estimation of CFRP Figure 21 and 22 shows the values of safety factor and life of wind turbine blade made-up of CFRP along its length. The safety factor range of small horizontal axis wind turbine blade is 2 to 5 and it can withstand 100000000 cycles with a low probability of failures.
Fig. 21. Safety factor variation along the blade
Fig. 22. Life estimation throughout the blade
4.3 Optimization with Fatigue and Failure Constraints Fatigue phenomena of the wind turbine blades depends upon the randomness in the load spectra due to wind nature, natural frequencies vibrations, insufficient maintenance process, continuous operation under different environments. These reasons are create structural design constraints that must be considered in the blade optimization process. As for the design against fatigue, loads are defined for all input conditions and then summed and weighted by the relative frequency of occurrence [11]. In this section, wish to include both static and fatigue failures in the optimization scheme and evaluate its effect on the optimal design variables. The modal analysis of wind turbine blade for different materials has been analyzed and the mode shapes are listed in Table 2. 4.3.1 Natural Frequencies Results Table. 2. Mode shapes for different composite materials Different mode shapes – Frequency (Hz) Sl No Kevlar GFRP 1 14.567 11.064
CFRP 13.235
2
28.966
22.053
26.469
3
88.240
67.193
80.602
4
142.50
108.47
130.17
5
177.81
135.62
163.42
6
240.28
183.27
220.46
4.3.2 Pre-stressed free vibration Results The process of reduction of vibration is a better way to attain the most advantageous design of blade structure and it provides high stiffness, which creates the natural frequency of the blade should be separated from the harmonic vibration associated with wind turbine sub components resonances. Therefore, mode separation constraint was set up to examine the first six natural frequencies and is separated from each other by more than ±10% of its natural frequency. Moreover to convene the requirements of the blade structural parameters, optimization of high displacements of the blade at the tip would have to be carried out with a limiting boundary conditions and acceptable stress should be within the range. The mode shapes, pre-stressed vibrations, forced vibrations of wind turbine blade has been plotted for the purpose of lifespan estimation process [12].
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4.3.2.1
Pre-stressed free vibration of Kevlar Figures 23 to 28 show the total deformation of wind turbine blade made-up of Kevlar for the given boundary condition at different frequencies.
Fig. 23. Total deformation at 14.735 Hz
Fig. 24. Total deformation at 28. 854 Hz
Fig. 25. Total deformation at 90. 445 Hz
Fig. 26. Total deformation at 143. 94 Hz
Fig. 27. Total deformation at 167. 43 Hz
Fig. 28. Total deformation at 225. 76 Hz
4.3.2.2 Pre-stressed free vibration of GFRP Figures 29 to 34 show the total deformation of wind turbine blade made-up of GFRP for the given boundary condition at different frequencies.
Fig. 29. Total deformation at 11. 191 Hz
Fig. 30. Total deformation at 22.059 Hz
Fig. 31. Total deformation at 69. 542 Hz
Fig. 32. Total deformation at 107. 62 Hz
Fig. 33. Total deformation at 120. 69 Hz
Fig. 34. Total deformation at 163.31 Hz
4.3.2.3
Pre-stressed free vibration of CFRP Figures 35 to 40 show the total deformation of wind turbine blade made-up of CFRP for the given boundary condition at different frequencies. The pre-stressed modal analysis was made for a blade with thickness of 1 mm. Materials with a lower density such as Kevlar, CFRP have higher natural frequencies, pre-stressed vibrations,
Senthil Kumar et. al./ Materials Today: Proceedings 5 (2018) 6665–6674
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and bigger deflection. In the case of GFRP have low natural frequencies, pre-stressed vibrations, and lesser deflection because of its mechanical properties.
Fig. 35. Total deformation at 13. 374 Hz
Fig. 38. Total deformation at 131. 77 Hz
Fig. 36. Total deformation at 26. 244 Hz
Fig. 39. Total deformation at 153. 16 Hz
Fig. 37. Total deformation at 82. 702 Hz
Fig. 40. Total deformation at 208. 20 Hz
4.3.3 Forced Vibration Results The aerodynamic load plays a vital role in the forced vibration of wind turbine blade. In this paper the vibration analysis has been done based on the natural frequencies range of different composite materials. The deformation response of different frequencies for different composite materials is listed in Table 3. Table. 3. Forced vibration results with 10 intervals Different forced vibration variation – Frequency (Hz) Sl No Kevlar GFRP 1 36.5 29
CFRP 34
2
63.0
48
3
89.5
67
82
4
116.0
86
106
5
142.5
105
130
6
169.0
124
154
7
195.5
143
178
8
222.0
162
202
9
248.5
181
226
10
275.0
200
250
58
4.3.3.1 Amplitude variation of composite materials
Fig. 41. Amplitude variation for Kevlar
Fig. 42. Amplitude variation for GFRP
Fig. 43. Amplitude variation for CFRP
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Figures 41 to 43 show the variation of amplitude with respect to natural frequencies range for different composite materials. The vibration creates on CFRP and Kevlar is higher and the lengthwise deflection level in the blades is in critical range because of its low density and stiffness. The probabilities of failure are high in CFRP as well as in Kevlar due to amplitudes of CFRP and Kevlar are higher. The vibration creates in the GFRP is less and occurred low amount of deflection due to its high density so the probability of success is high in the GFRP. 5. Conclusion The numerical simulation of wind turbine blade has been analyzed with the help of ANSYS Workbench 16.2. Principal stress induced along the blade and deformation created in different composite materials has been plotted. The stresses induced in the composite materials are within the acceptable limit. The pre-stressed modal analysis and harmonic analysis of wind turbine blade for different composite materials have been analyzed. The importance of vibrational parameters estimation is to increase the life and safety factor of wind turbine blades by avoiding the estimated frequencies. The GFRP have lower natural frequency range, pre-stressed vibration range and low amount of deflection – frequency response compared to CFRP and Kevlar. The fatigue life of GFRP is greater than or equal to 1000000000 cycles and the average safety factor value of throughout the blade is 4 with a low probability of failures. Hence the major structural and vibrational parameters result concludes that GFRP material is safer and secure for the small horizontal axis wind turbine blade. References [1] [2]
Ahmad Reza Ghasemi and Masood Mohandes, Intech open publisher, http://dx.doi.org/10.5772/63446, pp. 1 – 25. Sowdager Moin Ahmed, International Journal Of Innovative Research & Development, ISSN 2278 – 0211 (Online), February, 2015 Vol 4 Issue 2, Pp 275 – 302. [3] Ricardo Emanuel da Rocha Teixeira, University of Porto, pp 1 – 126. [4] Herbert J. Sutherland, On the Fatigue Analysis of Wind Turbines, Sandia National Laboratories P.O. Box 5800, Albuquerque, New Mexico 87185-0708, Printed in 1999, pp 1 – 133. [5] T Westphal and R P L Nijssen, Journal of Physics: IOP Conference Series, 2014, doi:10.1088/1742-6596/555/1/012107, pp 1 – 11. [6] Christoph W and Kensche, Third International Conference on Fatigue of Composites, 13 – 15 September, 2004, Kyoto, Japan, pp 1 – 21. [7] Mahmood M. Shokrieh and Roham Rafiee, Composite Structures 74 (2006), ISSN: 0263-8223, pp 332–342. [8] Patricio Andres Lillo Gallardo,University of Victoria, 2011, pp 1 – 84. [9] Konstantinos C. Bacharoudis, Stochastic analysis of structures made of composite materials, PhD thesis, defended in public on July 15th 2014 at University of Patras, pp 1 – 189. [10] Neelabh Gupta, Structural Study and Parametric Analysis on Fatigue Damage of a Composite Rotor Blade, Master of Science Thesis for the MS in Sustainable Process and Energy Technology at Faculty of Mechanical, Maritime and Materials Engineering, Delft University of Technology, pp 1 – 85. [11] David A Hensher , Fiber-Reinforced-Plastic (FRP) Reinforcement for Concrete Structures: Properties and Applications, Elsevier publication, 2016, ISBN - 148329143X, 9781483291437. [12] M Senthil Kumar, S Vijayarangan, Journal of scientific and industrial research, Vol 66, February 2006, page no 128 – 134.
ScienceDirect Materials Today: Proceedings 5 (2018) 6665–6674
www.materialstoday.com/proceedings
IMME17
Vibrational Fatigue Analysis of NACA 63215 Small Horizontal Axis Wind Turbine blade Senthil Kumar. Ma*, Krishnan. A.Sb and Vijayanandh. Ra* a
Department of Aeronautical Engineering, Kumaraguru College of Technology, b Department of Mechanical Engineering, Coimbatore Institute of Technology, Coimbatore, Tamilnadu- India
Abstract Wind turbines are critical in structural behaviour, which are characteristically using the wind in order to produce power. In wind machines, blades are considered to be an important component because of its critical profile at different sections, weight, and the structural parameters with relatively high amplitude and high frequency. Life-cycle estimation of wind turbines is crucial to develop their design and maintenance process, since they should have more lifespan with minimum foreign object disruptions as well as low probabilities of failures. Wind turbine is eco friendly technology; it should provide a high lifespan of its whole set up by reduces the major failure factors. Due to the effect of aerodynamic loads acts in the wind turbine blades may cause to fail at unpredictably high an amount, which creates the wind turbine to make fatigue analysis as important factors in its performance. Fatigue life and its analysis of each rotating component is one of the major factors of concern due to the terrible failures that can result from it. In this paper, the fatigue behaviour of wind turbine blade in response to different frequencies has established to the level that the prediction of working lifespan is fitting an essential part of the design process also compare the suitability of a wind turbine blade with different composite materials such as Kevlar, Glass Fiber Reinforced Plastic (GFRP) and Carbon Fiber Reinforced Plastic (CFRP) by simulates the displacement and principal stress using numerical method. The reference component of this paper is modeled by using CATIA. A numerical model of the blade was created using ANSYS Workbench 16.2 in order to estimate the typical mode shapes occurring within the blade based on a wind profile and mass approximating the location where these blades are expected to vibrate. Also, fatigue life of wind turbine blade analyzed for three composite materials and the results are compared in order to find out the optimum material body. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Emerging Trends in Materials and Manufacturing Engineering (IMME17). Keywords: Composite materials, Fatigue life, Frequency, Loads, Wind turbine blade.
* Corresponding author. Tel.: +91 9791639490; fax: +0422-2669406. E-mail address: [email protected], [email protected] 2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Emerging Trends in Materials and Manufacturing Engineering (IMME17).
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Senthil Kumar et. al./ Materials Today: Proceedings 5 (2018) 6665–6674
1. Introduction Wind turbines translate the kinetic energy in the wind into power, which can be used for specific tasks or else with the help of generator this mechanical power can convert into electricity. The electricity is sent via electrical cables to homes, businesses, schools, and so on. Advantages of using wind turbines are electricity generation without any environmental pollution, fast installation, commissioning, and low maintenance. Its main disadvantage is the transitory nature of an air flow [1], which is conquered with the help of high aerodynamic efficient components, by utilizing as much as energy from the wind during the limited period of time that it flows strongly. Nowadays, almost all wind turbines used to generate electricity are wind turbines with the horizontal axis of rotation because rotor speed and power output can be efficiently handled by pitching the rotor blades about their longitudinal axis, rotor blade pitching operation provides effective protection against high rotational speed and maximum aerodynamic forces, rotor blade profile can be aerodynamically optimized compared to vertical axis wind turbine, technological lead in the development of propeller design is an influential factor [2]. 2. Problem Description Modern wind turbines are structurally crucial equipments, which are typically used to generate electrical power from the moving air fluid. Large rotating wind machines specify that their rotary apparatus were failing at unpredicted high rates, which guided the wind turbine community to develop fatigue analysis capabilities. Hence the prediction of lifespan is becoming an essential parts of the design process and wind turbine blade material selection. To attain the uppermost achievable power from the wind turbine under precise atmospheric conditions, modifying two parameters are indispensable. The first one is design of wind turbine blade and its optimization then the next one is a change of the mechanical and fatigue properties of the wind turbine blade materials [3]. The factors involved in the selection of materials for wind turbine components are high stiffness to maintain optimal aerodynamic performance, low density to reduce gravity forces, and long fatigue life to reduce material degradation. Nowadays blades of the horizontal axis are now entirely made up of composite materials have lower weight and good stiffness, also providing good resistance to the static, dynamic, and fatigue loadings. Designing of the composite blades to get the ultimate energy from the air flow is a vital task. Admittedly, modification of the shape of blade changes the stiffness and strength, but it may control aerodynamic efficiency of the wind turbine. The design of the composite blade depends upon the strength to weight ratio, selection of airfoil, load estimation on the blade and stiffness to weight ratios [4]. Therefore, specifying the finest shape of the blade and most favorable composite material properties are the significant and intricate problem. This paper deals the design, materials selection and structural analysis of small horizontal axis wind turbine blade in order to estimate the fatigue life with the effect of vibrations. With the help of the complete literature survey the fatigue life estimation of this composite blade divided into two major parts: first one is stress analysis of the wind turbine blade and the next one is optimization of the wind turbine blade to withstand maximum fatigue life. 3. Design of Small Horizontal Axis Wind Turbine Blade 3.1 Principle of Wind Turbine Blade design In wind turbine manufacturing process, geometry of the blade and its supporting parameters takes important position because its control directly the increment of the electrical power generation. The most important part in designing of a wind turbine is the blade profile at various sections. The rotational force and performance of the blade is highly depends on airfoil performance and its perpendicular force. NACA63215 airfoil was selected for this analysis [5]. Physical as well as mathematical models are the representation of a blade, which is processed by the workstation. Commonly, a model intended for a numerical simulation, in which comprises of mathematical model creation, finite element model creation, application of boundary conditions and finally numerical solution. Here the wind turbine blade is modeled by CATIA and then imported to ANSYS Workbench for numerical simulation. Figure 1 shows the CAD model of our wind turbine blade.
Senthil Kumar et. al./ Materials Today: Proceedings 5 (2018) 6665–6674
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Fig. 1. CAD model of Wind turbine blade
3.2 Wind turbine blade materials and its selection In the early windmills, wood was used and revealed to be a good material due to its reasonable strength-toweight ratio, low density, but it’s not applicable for very large rotor blades due to their low stiffness and also difficult to obtain in reproducible and high quality, which is a requirement for stable and economical manufacturing of rotor blades [6]. Metals were a popular choice because they can be easily manufactured, give a high degree of reliability and yield a low-cost blade. Aluminum was only implemented in testing situations because it was found to have a lower fatigue level than steel. Older style wind turbines were designed with heavier steel blades or nickel alloy steels which have higher inertia [6, 7]. The purpose of nickel alloy is lessened alteration in heat treatment and lowers the critical temperatures of steel. Alloy steel was thought to be a most advantageous choice for blade production but was soon discarded due to its high weight and low fatigue level [7]. Nowadays, composites have become the most used blade material, which have good mechanical, thermal and chemical properties and ability to fit the material to the maximum loads by combine high strength with high stiffness. The optimal design of the wind turbine blades is today is an essential task and needs optimization of mechanical and fatigue properties, performance, and financial wealth because it's subjected to significant loads, so the materials used have to support the structural stress, so composite material is one of the perfect options to handle the practical problems. 3.2.1 Composite Material Composite material is a material, which comprises of strong fibers surrounded by a matrix. The matrix and fibers efficiently joined in composite manufacturing process for the purpose of increase their properties such as high stiffness to weight ratio, high load withstanding capabilities, etc. The principal reinforcements using the wind turbines are Glass; Kevlar- very light; Carbon; Boron - high modulus; Silicon carbide - high-temperature resistance [8]; Carbon fibers have the high modulus of elasticity with longest breaking length, and their stiffness is high. Glass fibers are amorphous with isotropic properties. Commonly, glass fibers combined with carbon fibers for the purpose of areas more prone to stress. Most glass-reinforced products are made with E-glass, which has good electrical and mechanical properties and high heat resistance. Nowadays, manufacturers prefer epoxy resins, which are more costly but present higher strength characteristics and higher fatigue resistance [9]. The main objective of this paper is to select the best suitable composite material for a wind turbine with high lifetime and efficiency. This selection is based on the numerical simulation results such as total deformation, principal stress, natural frequencies effect, forced vibration impact, and vibrational fatigue acts on the wind turbine blades among Kevlar, GFRP, and CFRP. 4. Numerical Analysis Here the finite element method is used for the estimation of structural analysis, thermal analysis, pre-stressed modal analysis, harmonic analysis of wind turbine blade. ANSYS Workbench 16.2 is a finite element tool, which is used to calculate the stresses, total deformation, mode shapes, etc., as shown in figure 2. The wind turbine blade is well thought-out as a cantilever and as the thickness of the wind turbine blade is very less than the length and the
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Senthil Kumar et. al./ Materials Today: Proceedings 5 (2018) 6665–6674
cross section of the blade the plane stress condition is assumed. For that, the solid element which has a 6 degree of freedom is used. The finite element model is defined in terms of the chord length (c), the length of the blade (l) and the twist angle (Ф). The total number of nodes generated is 26874, while the total number of elements generated is 26615 with the orthogonal quality of 0.9622 out of 1.
Fig. 2. Finite element model of wind turbine blade
The centrifugal force, the gravity force, the aerodynamic normal force and tangential forces are applied to the wind turbine blade. The basic requirement of wind turbine materials is to withstand the aerodynamic loadings with low density; the composite is the only solution to overcome this problem [10]. The mechanical properties of different composite materials are shown in table 1. Table 1. Mechanical properties of different composite materials Material name Density (kg / m3) Poisson’s ratio Young’s Modulus (GPa) Kevlar 1470 0.33 75 GFRP 1800 0.25 54 CFRP 1600 0.10 70
Bulk Modulus (GPa) 73.529 36.000 29.160
Shear Modulus (GPa) 28.195 21.600 31.818
Figure 3 shows the pressure contour of the wind turbine blade which has the rotational speed of 41.888 rad/s for air speed of 10 m/s. Figure 4 shows the different loads acting on the wind turbine blade. The different load's acting on the blade profiles are rotational speed, the aerodynamic load which import from ANSYS and remote displacement applied on the hub blade connection region.
Fig. 3. Pressure contour
Fig. 4. Blade with all boundary conditions
4.1. Deformation and Principal stress results 4.1.1. Deformation and Principal stress of Kevlar Figure 5 to 8 shows the total deformation, maximum principal elastic strain and principal stresses of wind turbine blade for Kevlar properties. The equivalent stress induced in the blade is 1127 MPa and the total displacement of a blade deformed at the free end is 75.501 mm.
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Fig. 5. Total deformation
Fig. 6. Maximum Principal elastic strain
Fig. 7. Minimum Principal stress
Fig. 8. Maximum Principal stress
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4.1.2. Deformation and Principal stress of GFRP Figure 9 to 12 shows the total deformation, maximum principal elastic strain and principal stresses results of wind turbine blade for GFRP properties. The equivalent stress induced in the blade is 1081.1 MPa and the total displacement of a blade deformed at the free end is 107.18 mm.
Fig. 9. Total deformation
Fig. 10. Maximum Principal elastic strain
Fig. 11. Minimum Principal stress
Fig. 12. Maximum Principal stress
4.1.3. Deformation and Principal stress of CFRP Figure 13 to 16 shows the total deformation, maximum principal elastic strain and principal stresses results of wind turbine blade for CFRP properties. The equivalent stress induced in the blade is 1061.3 MPa and the total displacement of a blade deformed at the free end is 84.105 mm.
Fig. 13. Total deformation
Fig. 14. Maximum Principal elastic strain
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Fig. 15 Minimum Principal stress
Fig. 16. Maximum Principal stress
The numerical simulation result suggests that the maximum stress creates on the hub to blade connection regions and the maximum deformation occurred in the blade tip. Reduction of sharp edges, increase the structural members, and selection of suitable materials are the important factors for stress reduction in the small horizontal axis wind turbine in order to increase the high lifespan. From the principal stresses and deformation results clearly shows the breaking region in the blade so the designer must concentrate on the design of the hub to blade connection region. GFRP and CFRP are best for aerodynamic stress to withstand capability in small axis wind turbine blade because of its high strength to weight ratio and Kevlar is best for low deflection and more stability because of its high stiffness to weight ratio. 4.2. Fatigue Life The external load acting on the wind turbine components is fluctuating nature in mean as well as the amplitude and there is progressive crack to the mechanical apparatus which results in much more in advance breakdown of parts well before its yield strength. Such mode of loads is called fatigue loads and the failure is termed as fatigue failure. The actual loads that contribute to fatigue of a wind turbine originate from a variety of sources, which are high aerodynamic stable forces, intermittent loads with the effect of rotations and gravity, wind speed variational fatigue loads, external gust loads, and the vibrational natural frequencies induced in wind turbine blade. The numerical simulation of the fatigue stresses is used to find out the fatigue life of a wind turbine blade and its relations [10]. Length wise distribution of life estimation and a safety factor of wind turbine blade have been plotted with the help of principal stresses for the given boundary condition. Life and safety factors are important factors to be considered in wind turbine blades design, performance, production, and efficiency. 4.2.1
Safety factor and life estimation of Kevlar
Figure 17 and 18 shows the lengthwise variation for the safety factor and life of wind turbine blade madeup of Kevlar. The safety factor range of small horizontal axis wind turbine blade is 2 to 5 and it can withstand 10000000 cycles with a low probability of failures.
Fig. 17. Safety factor variation along the blade
4.2.2
Fig. 18. Life estimation throughout the blade
Safety factor and Life estimation of GFRP Figure 19 and 20 shows the lengthwise variation for the safety factor and life of wind turbine blade madeup of GFRP. The safety factor range of small horizontal axis wind turbine blade is 2 to 5 and it can withstand 1000000000 cycles with a low probability of failures.
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Fig. 19. Safety factor variation along the blade
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Fig. 20. Life estimation throughout the blade
4.2.3
Safety factor and Life estimation of CFRP Figure 21 and 22 shows the values of safety factor and life of wind turbine blade made-up of CFRP along its length. The safety factor range of small horizontal axis wind turbine blade is 2 to 5 and it can withstand 100000000 cycles with a low probability of failures.
Fig. 21. Safety factor variation along the blade
Fig. 22. Life estimation throughout the blade
4.3 Optimization with Fatigue and Failure Constraints Fatigue phenomena of the wind turbine blades depends upon the randomness in the load spectra due to wind nature, natural frequencies vibrations, insufficient maintenance process, continuous operation under different environments. These reasons are create structural design constraints that must be considered in the blade optimization process. As for the design against fatigue, loads are defined for all input conditions and then summed and weighted by the relative frequency of occurrence [11]. In this section, wish to include both static and fatigue failures in the optimization scheme and evaluate its effect on the optimal design variables. The modal analysis of wind turbine blade for different materials has been analyzed and the mode shapes are listed in Table 2. 4.3.1 Natural Frequencies Results Table. 2. Mode shapes for different composite materials Different mode shapes – Frequency (Hz) Sl No Kevlar GFRP 1 14.567 11.064
CFRP 13.235
2
28.966
22.053
26.469
3
88.240
67.193
80.602
4
142.50
108.47
130.17
5
177.81
135.62
163.42
6
240.28
183.27
220.46
4.3.2 Pre-stressed free vibration Results The process of reduction of vibration is a better way to attain the most advantageous design of blade structure and it provides high stiffness, which creates the natural frequency of the blade should be separated from the harmonic vibration associated with wind turbine sub components resonances. Therefore, mode separation constraint was set up to examine the first six natural frequencies and is separated from each other by more than ±10% of its natural frequency. Moreover to convene the requirements of the blade structural parameters, optimization of high displacements of the blade at the tip would have to be carried out with a limiting boundary conditions and acceptable stress should be within the range. The mode shapes, pre-stressed vibrations, forced vibrations of wind turbine blade has been plotted for the purpose of lifespan estimation process [12].
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4.3.2.1
Pre-stressed free vibration of Kevlar Figures 23 to 28 show the total deformation of wind turbine blade made-up of Kevlar for the given boundary condition at different frequencies.
Fig. 23. Total deformation at 14.735 Hz
Fig. 24. Total deformation at 28. 854 Hz
Fig. 25. Total deformation at 90. 445 Hz
Fig. 26. Total deformation at 143. 94 Hz
Fig. 27. Total deformation at 167. 43 Hz
Fig. 28. Total deformation at 225. 76 Hz
4.3.2.2 Pre-stressed free vibration of GFRP Figures 29 to 34 show the total deformation of wind turbine blade made-up of GFRP for the given boundary condition at different frequencies.
Fig. 29. Total deformation at 11. 191 Hz
Fig. 30. Total deformation at 22.059 Hz
Fig. 31. Total deformation at 69. 542 Hz
Fig. 32. Total deformation at 107. 62 Hz
Fig. 33. Total deformation at 120. 69 Hz
Fig. 34. Total deformation at 163.31 Hz
4.3.2.3
Pre-stressed free vibration of CFRP Figures 35 to 40 show the total deformation of wind turbine blade made-up of CFRP for the given boundary condition at different frequencies. The pre-stressed modal analysis was made for a blade with thickness of 1 mm. Materials with a lower density such as Kevlar, CFRP have higher natural frequencies, pre-stressed vibrations,
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and bigger deflection. In the case of GFRP have low natural frequencies, pre-stressed vibrations, and lesser deflection because of its mechanical properties.
Fig. 35. Total deformation at 13. 374 Hz
Fig. 38. Total deformation at 131. 77 Hz
Fig. 36. Total deformation at 26. 244 Hz
Fig. 39. Total deformation at 153. 16 Hz
Fig. 37. Total deformation at 82. 702 Hz
Fig. 40. Total deformation at 208. 20 Hz
4.3.3 Forced Vibration Results The aerodynamic load plays a vital role in the forced vibration of wind turbine blade. In this paper the vibration analysis has been done based on the natural frequencies range of different composite materials. The deformation response of different frequencies for different composite materials is listed in Table 3. Table. 3. Forced vibration results with 10 intervals Different forced vibration variation – Frequency (Hz) Sl No Kevlar GFRP 1 36.5 29
CFRP 34
2
63.0
48
3
89.5
67
82
4
116.0
86
106
5
142.5
105
130
6
169.0
124
154
7
195.5
143
178
8
222.0
162
202
9
248.5
181
226
10
275.0
200
250
58
4.3.3.1 Amplitude variation of composite materials
Fig. 41. Amplitude variation for Kevlar
Fig. 42. Amplitude variation for GFRP
Fig. 43. Amplitude variation for CFRP
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Figures 41 to 43 show the variation of amplitude with respect to natural frequencies range for different composite materials. The vibration creates on CFRP and Kevlar is higher and the lengthwise deflection level in the blades is in critical range because of its low density and stiffness. The probabilities of failure are high in CFRP as well as in Kevlar due to amplitudes of CFRP and Kevlar are higher. The vibration creates in the GFRP is less and occurred low amount of deflection due to its high density so the probability of success is high in the GFRP. 5. Conclusion The numerical simulation of wind turbine blade has been analyzed with the help of ANSYS Workbench 16.2. Principal stress induced along the blade and deformation created in different composite materials has been plotted. The stresses induced in the composite materials are within the acceptable limit. The pre-stressed modal analysis and harmonic analysis of wind turbine blade for different composite materials have been analyzed. The importance of vibrational parameters estimation is to increase the life and safety factor of wind turbine blades by avoiding the estimated frequencies. The GFRP have lower natural frequency range, pre-stressed vibration range and low amount of deflection – frequency response compared to CFRP and Kevlar. The fatigue life of GFRP is greater than or equal to 1000000000 cycles and the average safety factor value of throughout the blade is 4 with a low probability of failures. Hence the major structural and vibrational parameters result concludes that GFRP material is safer and secure for the small horizontal axis wind turbine blade. References [1] [2]
Ahmad Reza Ghasemi and Masood Mohandes, Intech open publisher, http://dx.doi.org/10.5772/63446, pp. 1 – 25. Sowdager Moin Ahmed, International Journal Of Innovative Research & Development, ISSN 2278 – 0211 (Online), February, 2015 Vol 4 Issue 2, Pp 275 – 302. [3] Ricardo Emanuel da Rocha Teixeira, University of Porto, pp 1 – 126. [4] Herbert J. Sutherland, On the Fatigue Analysis of Wind Turbines, Sandia National Laboratories P.O. Box 5800, Albuquerque, New Mexico 87185-0708, Printed in 1999, pp 1 – 133. [5] T Westphal and R P L Nijssen, Journal of Physics: IOP Conference Series, 2014, doi:10.1088/1742-6596/555/1/012107, pp 1 – 11. [6] Christoph W and Kensche, Third International Conference on Fatigue of Composites, 13 – 15 September, 2004, Kyoto, Japan, pp 1 – 21. [7] Mahmood M. Shokrieh and Roham Rafiee, Composite Structures 74 (2006), ISSN: 0263-8223, pp 332–342. [8] Patricio Andres Lillo Gallardo,University of Victoria, 2011, pp 1 – 84. [9] Konstantinos C. Bacharoudis, Stochastic analysis of structures made of composite materials, PhD thesis, defended in public on July 15th 2014 at University of Patras, pp 1 – 189. [10] Neelabh Gupta, Structural Study and Parametric Analysis on Fatigue Damage of a Composite Rotor Blade, Master of Science Thesis for the MS in Sustainable Process and Energy Technology at Faculty of Mechanical, Maritime and Materials Engineering, Delft University of Technology, pp 1 – 85. [11] David A Hensher , Fiber-Reinforced-Plastic (FRP) Reinforcement for Concrete Structures: Properties and Applications, Elsevier publication, 2016, ISBN - 148329143X, 9781483291437. [12] M Senthil Kumar, S Vijayarangan, Journal of scientific and industrial research, Vol 66, February 2006, page no 128 – 134.