Wind Energy Wind Power
Saturday, February 26, 2011
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THE MEANING OF L / C
L / C is the relationship between lift force (L) and the push or resistance (C) or between the coefficients L C and C D respective .
We have seen that, thanks to the movement of the thrust of the drive motors, an aircraft which moves at a speed v suffer a buoyancy
L = r A. C L v 2 / 2
And it will be necessary to provide a boost to counter the resistance that opposes the motion
; ; D = r v AC D 2 / 2
L C and C D known as coefficients of lift and thrust are specific to each airfoil, obtained from tables which express the values \u200b\u200bof the perpendicular forces (L) and the same direction (D) to travel v with respect to the angle α forming this direction with the cord that connects the leading edge and the leakage of the profile used.
the graph of any profile we see that the relationship L / D passes through a maximum for a defined angle α ( abscissa)
What is the meaning? Maximum weight can raise a given thrust, with a defined number of motor fuel or turbine.
In conclusion, the maximum L / D translates into better performance.
Let us follow the reasoning now a planner rather than an airplane. The only energy that holds the glider in flight is the height at which he dragged a trailer half times the weight of the glider plus the pilot. Example: If the glider weighs 120 kg and the driver 80, at 3500 meters altitude is 3500 x 200 = 700,000 Kgrm equivalent to ¼ liter of gasoline. As in the previous case, try to keep flying with the maximum L / D means staying longer in the air until they can find another energy supply thermal buoyancy. Let us now see what happens with the propeller of a wind turbine. The torque is produced as we have seen, as a result of the projection of the lift and thrust on the direction of travel, ie on the tangent of the circle that forms each of the points of the blade in its rotation around axis. The buoyant force is the sum of these projections, that, although positive, will be able to pack the propeller thus increasing the tangential velocity, which consists vectorially with wind speed at every moment give a new angle of attack. Unless a device, enables the blade position, we find that our intention to maintain the angle of attack for best L / D will be fruitless. The device can "create" a new wheelseat angle θ keeping α = α or where α or is the angle of attack that gives maximum L / D for each profile selected, it is what is called "pitch control" or pitch. "With the propeller stopped, v = ω . R = 0 and angle would
α = arc tg u / v , where u is the wind speed and
Α = arc tg u / 0 = arc tg ¥ = 90 or
If the profile asks an α or = 3, it is obvious that the reference setting should be the difference, ie
; Θ = 90 - α or = 90 -3 = 87
As it starts, v will be zero at the pitch delay the angle θ . Suppose you have reached the same speed as the wind
α = arc tg u / v = arc tg a = 45 °
in this case should be θ = 42 or
Thus each increase in speed corresponds a decrease of θ , always keeping α = 3 with which the propeller will speed regime very short time because the time required (reaction) is much lower than available (action) and is only necessary to overcome friction and inertia as power generation will only occur when we reach the synchronous speed of the network, Once docked, or during the excitement, being a standalone system. Figure 1 shows the value of time and power in relation to tangential velocity of blade tip for a NACA 6712 profile whose relationship L / D = 18.5 for an attack angle α or =- 1 º.
Just shows that for the maximum power corresponding tangential velocity is 74 m / s were used as data for a wind of 6 m / s, we obtain the wheelseat angle θ = 5.6 º which as expected agrees with that obtained by derivation of the power and equalization to zero. Recall that, according to the simplified formula, the power is,
derived and equated to zero gives the solution L x = 2 / 3. L / C = 18,5.2 / 3
And knowing that l x = v / u = 1 / tg ( θ + α ) shows that θ = 5.6 º . Please refer to the note * Aerodynamics-General theoretical considerations. Note that as the variable is the angle of reference setting and not the attack, whose value is fixed, like the L / D, referral, whether for power or torque, is strictly accurate, Unlike the case in which the power and the time corresponding to the formula of fixed reference setting system, in which L / D and Cd varies with the angle of attack and the results are only approximate and purely academic. * We also recommend addressing the mechanical balance to help you understand well the difference between torque and power in relation to the operating point of a mechanical system and particularly one for the more complex, ie your application with the wind machines.
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