Content 1. Introduction 2. Calculation of the blade angle 3. Reynolds number 4. Testing the blade rotor in the wind turbine 5. Conclusion
Introduction
Wind power is one of the renewable technologies the world is counting on to provide sustainable and non-polluting power. The purpose report discusses how a prototype wind blade is designed, built and tested. The aim is to build a wind blade is built in terms of power extracted from the wind.
Method
F (resultant)
D (drag) L (lift)
Rw U
The diagram above shows an aerofoil-section blade in a free air stream velocity U. Due to rotation of the turbine the blade velocity (at radius) is Rw. The angle of the aerofoil to the plane is θ. The air velocity relative to the blade is W, at an angle α to the blade: is called the angle of incidence. To ensure a good value of the lift coefficient Cl, α should be in the range 5o -12o.
Calculation of the blade angle
Take tip ratio λ=6.3
I choose 6.3 as the tip ratio because I am designing for 2 blade rotor. It needs a low drag coefficient on the blade to oppose the motion of the blade and so to detract from the wind turbine performance.
Wind speed u= 9ms-1
The blade velocity Rw=54
The blade angle θ =?
Tan θ = (u/Rw) θ =9.460 (approximate to whole number) =100
The angle of aerofoil θ=100.
Since we already know the angle θ we need to calculate w which is a constant and since R is the radius of the blade, we need 5 values.
Taking Radius R=100mm
Therefore w= 54/R =54/100 =0.54
Hence, the blade velocity Rw is proportional to the radius, so to keep to α constant the blade will be twisted, so to get the blade angle Φ. The blade angle should be varying with radius.
20mm 40mm 60mm 80mm 100mm
When R=20mm
Tan Φ= (U/Rw) = (9/20*0.54) =39.81- 10 ‘θ’= 29.810
At R=40mm
Tan Φ= (9/40*0.54) =22.62-10 =12.620
At R=60mm
Tan Φ= (9/60*0.54) =15.52-10=5.520
At R=80mm
Tan Φ= (9/80*0.54) =11.77-10 = 1.770
At R=100mm
Tan Φ= (9/100*0.54) =9.46-10 = -0.540
I choose the value for the value of the chord length to be 22.0mm and R because it would be easier to draw using solid works and if I use a small radius and length the blade will not perform well. To design the aerofoil I used the NACA4 standard, developed by the US National Advisory Committee for Aeronautics (NACA).
Reynolds Number
I will begin this section by explaining why we need to calculate the Reynolds number Re. As an object moves through the wind turbine, the air molecule near the object are disturbed and moved around the blade, hence a force is generated. The magnitude of the force depends on the shape of the object, the speed, the mass of the air, thickness. As the blade moves through the wind tunnel there is a layer formed on the surface of the blade called the boundary layer. Thus the Reynolds number expresses the resistant to change forces to the air forces glued to the blade. Therefore the formula for Reynolds number is
Re=ρVL/Ų
Where ρ= air density (1.2kg/m3) v=velocity= (Rw/cos Φ)
L= chord length= 100mm
Ų=Dynamic viscosity of air (1.8* 10-6 Ns/m2)
Substituting the value
Re= {1.2*[(100*0.54)/Cos 0.54]*100*10-3/1.8*10-6} =13200
Testing
In this section I will be discussing the procedure on how to test the blade in the wind turbine. Before testing the blade rotor we have to balance it on a dummy shaft. This is because the vibration at high speeds can destroy the rotor. To balance the blade I used a file on the edges of the blade. After that I weight the blade. Blade without rotor= 7 gram. Next mount the balanced rotor on the test rig in the wind tunnel. A barometer was there to show how fast the blades are going. The barometer measures the pressure inside the wind tunnel, which is the height h in mm. And then, we have to calculate h.
Using the formula
V=2√ {2[p1-p2]}/ρ}
Where v= wind speed (ms-1) ρ=air density (kgm-3) p= pressure
When v= 8 ms-1
8=2√ {2[p1-p2]}/1.2}
64= {2[p1-p2]}/1.2}
2[p1-p2] =76.8
P1-p2=34.8
Substituting the value p1-p2 in the formula p= ρwgh (where p1-p2=p) h=38400/1000*9.81 h=3.91 N/mm-2
When v=9ms-1
9=2√ {2[p1-p2]}/1.2}
81= {2[p1-p2]}/1.2}
2[p1-p2] = 97.2
P1-p2=48.6
Substituting the value p1-p2 in the formula p= ρwgh (where p1-p2=p) h=48600/100*9.81 h=4.95 N/mm-2
When v=10 ms-1
10=2√ {2[p1-p2]}/1.2}
100=2[p1-p2]}/1.2}
2[p1-p2] =120
P1-p2= 60
Substituting the value p1-p2 in the formula p= ρwgh (where p1-p2=p) h=60000/1000*9.81 =6.1N/mm-2 When v=11 ms-1
11=2√ {2[p1-p2]}/1.2}
121=2[p1-p2]}/1.2}
2[p1-p2] =145.6
Substituting the value p1-p2 in the formula p= ρwgh (where p1-p2=p) h=72600/1000*9.81 =7.4 N/mm-2 Below is a picture of the blade rotor in the wind tunnel
Taking a series of air speed we can measure the toque, frequency, power. To measure the torque we have to know M1 which is the balance between M2 and M3 together. The table below shows the measurement of the M1, M2, M3, Torque, frequency (note that to check the frequency we need an oscilloscope measuring the square wave), power, coefficient power Cp and non dimensional speed λ respectively. Picture of the oscilloscope taking measurement
Plotting graph power against time
Plotting the graph Cp against λ
Plotting frequency against torque
Equally important we have to calculate the diameter of the rotor required to generate 1kw when the wind speed is 9m/s.
To calculate the diameter we use the formula Cp=P/(1/2 ρAU3).
1.02=1000/1/2.*1.2*A*93
A=2000/ (1.2*129*1.02)
A=2.24m2
WhereA=πr2
2.24= πr2 r2=2.24/ π r=0.84m Conclusion
This report has discussed how the blade is designed and the calculation of the angles. Furthermore it showed how to test the blade in the wind turbine and the test results. However, in this project it has showed me that design can not spin with air speed of 8m/s and with a torque larger than 0.0429N/m. Also, the design limit was an air speed of 12m/s and a torque of 0.084N/m. If I exceed this limit the blade will brake.
