wind turbine blade design
The use of wind as an energy source has been used for hundreds of years to pump water or grind corn, this equipment is also known as a windmill. In the 19th century, fossil fuels replaced the use of these large, heavy, inefficient windmills. Then, knowledge of aerodynamics and lightweight materials brought back wind turbine technology around the 20th century.
Based on the orientation of the direction of rotation of the axis, these wind turbines are divided into two categories: Horizontal Axis Wind Turbine (HAWT) dan Vertical Axis Wind Turbine (VAWT).
Each configuration has advantages and disadvantages. In general, VAWT development began to decline due to the limitations of the VAWT at low speed operating conditions and the difficulty of controlling the rotor speed, this design also has difficulties in its starting . However, VAWT has the advantage that it does not require additional mechanisms and a large generator can be used because it is not limited by the use of high towers. The development of HAWT is increasingly popular because of the increase in performance and control can be done with pitch and yaw control.
THEORY MAXIMUM EFFICIENCY
High rotor efficiency is desirable to increase the conversion of wind flow energy into mechanical energy of the rotor, surely with affordable production costs. To calculate its efficiency, it is first necessary to define the incoming wind power (potential energy):
Where,
P = Power (watt)
ρ = Air density (kg/m3)
A = Turbin area (m2)
V = Velocity (m/s)
The air flow through the wind turbine will drop its velocity due to the interaction between the air and the turbine, the velocity drop also indicates a change in wind energy into mechanical energy of the rotor. If we want 100% efficiency, the wind speed after passing the turbine must be zero, or stop at all, definitely this is not possible; while it can be calculated using the rotor disc theory that the maximum efficiency that can be achieved theoretically is 59.3%, this efficiency parameter is called the power coefficient Cp, the maximum Cp = 0.593 also known as the Betz limit in wind turbine design.
The actual efficiency of the wind turbine will be reduced due to several factors such as the emergence of wake flow in the blades which reduces the lift on the airfoil, the selection of an airfoil that has low efficiency and the emergence of flow “leakage” at the tip which results in undesirable vortex flow.
To produce rotation (torque) on the wind turbine rotor, two methods are used, namely utilizing drag or using lift from the aerodynamic shape of the blade. Here is a comparison table of the two models:
For the Drag model, the wind turbine blades are intentionally made to block the air flow and are given a certain moment arm about the rotating axis, thereby producing torque to rotate the turbine. Another alternative is to use the aerodynamic lift that occurs on the airfoil rotor then the lift is directed in the direction of the rotation of the rotor and a moment arm is given to the axis of rotation to produce torque. The lift method tends to be more efficient because it does not change the airflow pattern much or produces a lot of wakes. Here are some types of wind turbines along with some of their descriptions:
BLADE HAWT DESIGN
The focus of the discussion in this article is HAWT because of its popularity in the wind turbine industry, this type of turbine is very sensitive to the design of the blade profile and its design. The first thing we must pay attention to in wind turbine design is the Tip Speed Ratio (TSR) parameter, this parameter is a comparison between the tangential speed of the tip blade to the speed of the incoming wind (free stream) which is mathematically written as follows:
Where,
λ = TSR
Ω = Rotation speed (rad/s)
r = Radius (m)
Vw = Wind velocity (m/s)
Aspects such as efficiency, torque, mechanical stress on the blade and noise should be considered in this TSR calculation. Modern wind turbines tend to be designed using a TSR value of around 6-9 because of the above considerations, in general the peak efficiency is at TSR = 7.
BLADE SHAPE CALCULATION
Based on the theory of Blade Element Momentum (BEM), which is the calculation of wind turbine performance based on the cross section or airfoil shape of each section of the blade calculated by dividing it into small elements in 2D. For blades with a design TSR of about 6-9, Betz’s momentum theory provides a fairly good approximation to calculating the blade profile shape with the following equation:
n = number of blades , CL = Airfoil Lift Coefficient, Vr = Resultant of Wind Velocity (m/s), U = Wind velocity (m/s), Uwd = Wind velocity design (m/s), Copt = Optimum Chord Length. The Copt can be plotted against r to produce the optimal “shape” of the blade.
From the above equation it can be seen that the larger the TSR, the smaller the blade size, then the more the number of blades, the smaller the blade size (see changes made to the root and tip to adjust the actual conditions both for installation on the hub and due to structural reasons, this can also be done because the power contribution of the root is also relatively low):
The smaller the blade size will be advantageous in terms of cost, because the material needed will be less, but on the other hand the blade structure will also be weaker. In general, the most optimal choice of blades is 3 pieces.
This approach is quite good for the initial design, but it is a 2D approach so it is not very accurate in considering the emergence of 3D phenomena or the appearance of wakes, tip losses and so on. For more accurate and comprehensive results, analysis using Computational Fluid Dynamics (CFD) simulation is used.
In the end, to analyze aerodynamic performance in 3D and comprehensively, we cannot just rely on the calculations of the approach above. One of the well-known methods in wind turbine design is the use of computational fluid dynamics (CFD) (read more in the introduction to CFD). This method is carried out using a computer to analyze the fluid flow on the wind turbine blade in detail to show 3D flow interactions and other features such as tip vortex, wake and others without simplification.
From the CFD simulation we can also predict the performance of the wind turbine such as calculating power coefficient and torque under various conditions of TSR variation and changes in twist angle, number of blades, airfoil variations, tip model variations and so on.
As for calculating the structural strength of the wind turbine blades, analytical equations alone are not sufficient, due to variations in materials and discontinuous forms of the blade frame and wind turbine tower. The method that is often used for this analysis is using Finite Element Analysis (FEA).
>> CLICK HERE TO DESIGN WIND TURBINE USING CFD METHOD!
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References:
Peter J. Schubel * and Richard J. Crossley, “Wind turbine blade design“. Energies 2012, 5, 3425-3449; doi:10.3390/en5093425
airplane wing design
In principle, an airplane can fly because its wings generate lift force. The principle of the formation of lift can occur because of the “suction” of air upward due to the shape of the airfoil that is designed in such a way. The formation of lift can also be caused by the higher angle of attack resulting in a greater upward reaction, but also followed by an increase in drag, which is termed induced drag.
To determine the size of the wing, the first thing we should understand is the relationship between the surface area of the wing and the lift itself, which is formulated by the equation:
L = 0.5 ρ CL V2 A
Where,
L = Lift force (N)
ρ = Air density (kg/m3)
CL = Lift Coefficient
v = Flight velocity (m/s2)
A = Wing’s area (m2)
From the above equation, it can be seen that the lift is directly proportional to the square of the flight speed. It means that when the aircraft moves twice as fast, the lift will increase fourfold. Then, the larger the cross-sectional area of the wing, the greater the lift will be, but the larger the area will certainly increase the weight of the aircraft, which makes the need for even greater lift. Then, the CL or coefficient of lift is influenced by several factors, namely the angle of attack, shape and Reynolds number which are basically determined when selecting the airfoil.
Theoretically (2D), the value of CL is only determined by the design of the airfoil and multiplied by its cross-sectional area (doesn’t depend on the shape of the cross-section itself). However, in fact the effect of the wing planform itself is very influential on the flow characteristics in 3D so that it affects the CL value, and must be taken into account in the calculation of the area. The parameters that play a role in the design of the airplane wing planform:
ASPECT RATIO
Aspect ratio is the ratio between span and wing chord, meaning that the longer the wing with the same width, the greater the aspect ratio. The aspect ratio selection is important for the mission carried out by the aircraft, a high aspect ratio is used for aircraft that are stable, relatively low speed and require long endurance, because the larger the aspect ratio, the smaller the induced drag. Then, a small aspect ratio is usually used for aircraft with extreme maneuvers, relatively high speed and does not require long endurance, in addition to the agility advantages of short wings (low aspect ratio), strength and structural rigidity are also the reasons why short wings are ideal for extreme maneuvers.
The induced drag referred to in the explanation above is due to the “leakage” effect of the flow from the bottom of the wing to the top of the wing that occurs at the tip of the wing. We know that to produce lift, the underside of the wing has a higher pressure than the top, and we also know that air flows from high pressure to low pressure. Leaking from the bottom to the top at the tip (tip) of this wing produces a swirling flow pattern, otherwise known as a tip vortex.
The equation used to calculate the aspect ratio is as follows:
AR = b/c
AR = b2/A
Where,
AR = Aspect Ratio
b = Wing span (m)
c = average chord (m)
A = Surface area (m2)
TAPER RATIO
The taper ratio is the ratio of the chord tip to the root chord, the thinner the wing shape, the lower the taper ratio. 1 taper ratio means the wings are rectangular. Mathematically defined as follows:
TR = Ctip/Croot
Where,
TR = Taper Ratio
Ctip= Tip Chord (m)
Croot = Root Chord (m)
The taper ratio has an effect on wing weight, the lower the taper ratio, the wing weight can be minimized, and the selection of the taper ratio determines the wing efficiency due to the effect of induced drag. The following is a graph of the relationship between the taper ratio and the induced drag factor:
The calculations above are very dependent on the 3D detail of the wing, so that in the end the final CL and CD values are calculated using experiments or methods that are currently quite widely used are Computational Fluid Dynamics (CFD) because of their relative flexibility, speed, and cost. lower than the wind tunnel experiment.
aeroengineering services is a service under CV. Markom with solutions especially CFD/FEA.