Energy said:
The wind can only interact with the blades themselves - not the area in
between the blades.
The narrower the blades, the less surface area of that interaction.
Not true. This is a common misconception, that only the face of the
blade interacts. As the wind in front of a blade slows/changes
direction, the wind between the blades is also interacting. Just
indirectly. How much this 'in between' air interacts is a function of
the fluid properties, speed, and pressure drop through the blading
(front/back of turbine).
The longer the blades, the higher the centrifical forces along the
blades and more exotic the blade construction (= expensive).
Airplane propeller blades do not even have the same length-to-width
profile as commercial wind plants.
And look at the 4-blade propellers from WW2 bombers - wide and short.
And look at the turbines of jet engines. Total surface area of the
turbine blades is more than half the swept area.
Turbine blading looks that way because there is a much larger pressure
change across each row of blading. As the pressure drop/rise across a
row of blading rises, there is more turbulence between wind directly in
front of a blade and wind flowing between blades. This turbulence means
the air 'between blades' is not able to transfer as much of its energy
to the air 'in front of blades'.
But adding more blades also adds more drag. In a windmill, where it
isn't enclosed, you have very little pressure difference from front to
back of the swept area. So there is a lot less turbulence of the wind
and more energy from 'between blade' air can transfer to 'front of
blade' air. And fewer blades means more of total energy comes to output
shaft and is not wasted on overcoming drag.
If typical winds were a lot higher, and air was a lot denser, a
different design *would* work better. But wind is what it is, so the
tri-blade design is one of the most common.
If wide and fat blades are efficient when it comes to making air move,
then the converse must also be true.
This depends on 'making air move'. Are you trying to get a lot of air
to move 1-2 mph, or a small amount of air 7-10 mph? And just how
efficient do you need the fan to be? The manufacturer doesn't pay
operating costs, they pay only construction cost. And the end user
might rather have a 'stiff breeze' blowing on their face than the whole
house ventilated by one fan.
Larger fans for moving air in ducts have fewer blades, and when a high
differential pressure is needed, often are centrifugal blower types with
many, many blades.
Function decides form.
Wings are "pulled" by lift? And are also "pushed" ?
What drugs are you on?
I think you people are trying to say that a wind-mill blade has the
cross-sectional profile of an airplane wing, and that's why these blades
are long and narrow and somehow incorporate the wing-lift principle as a
form of energy extraction.
Wings must be moved forward by engine thrust in order to generate lift.
Wings do not generate any sort of imaginary force in front of them that
aids their forward motion, and certainly they experience drag forces
that must be overcome by engine thrust in order to keep them moving
forward.
When the blade is turning, the air flows over the surface just like an
aircraft wing. It's a combination of the wind blowing and the blade
turning. That's why the blade has a 'twist' in it. The part out near
the end is traveling around the circle faster because it is further away
from the center. So for a given wind speed, the apparent wind
approaching the blade near the tip is faster but at a much shallower
angle. To maintain a good 'angle of attack', the blade must be twisted.
The lift that is induced by the forward motion of an airplane wing
presumably acts to draw the blades of a wind plant forward (ie - towards
or into the wind) to help it counteract the bending force of the wind
trying to push backwards on it. This "lift" effect would not result in
any additional power extraction from the wind since it acts
perpendicularly to the blade face and not axially in the direction of
rotation.
Nope. 'Lift' does not pull the wing of an airplane directly forward, it
pulls it upward (i.e. it is what 'lifts' the airplane off the ground).
When climbing, the force (perpendicular to the wing) is actually tilted
towards the back of the plane somewhat. So increasing the wing's angle
of attack to the air increases lift but also increase the drag that
tends to slow the plane down. Therefore you have to apply power when
climbing to maintain air speed.
On a windmill blade this force works at an angle that pulls the windmill
blade forward slightly but mostly 'around' in the circle, creating
torque. It's *generally* perpendicular to the line fore-aft through the
wing/blade profile. And since the profile is at a complicated angle to
the rotor shaft, the 'lift' force is also at a compound angle. The
total 'lift' force splits between some portion turning the rotor and the
rest tending to warp the blade (has to be considered in blade
design/strength).
Modern sailboat sails are another good example of this. When going
towards the wind at a 45 degree angle (sailing NE when the wind comes
from due N), the sail bulges out and creates an air foil. When properly
trimmed, the air flows along both sides of the sail cloth with little
turbulence. Some of the thrust generated from the sail comes from
re-directing air flow into a new direction (SSW in my example instead of
due S). And some of the thrust is actually a 'lift' force created by
the air foil. This force is more due east. But the boat's keel won't
let it slide due east so this force is split between south-east against
the keel and north-east making the boat move forward.
daestrom
