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Why are wind turbine blades long and skinny instead of short and fat? - Page 8

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Posted by m II on April 14, 2010, 5:16 pm
 


Bob F wrote:


air

...and that is precisely why the wings on a biplane have to be a certain
minimum distance apart. On models, I'd use the length of the longest
chord as the smallest acceptable spacing between the upper and lower
wing surfaces.

Even with that spacing, there was still most likely conflict between the
air 'envelopes' surrounding the wings.


mike

Posted by daestrom on April 14, 2010, 11:47 pm
 


Bob F wrote:

air

Careful, his '100%' is 100% of the Betz limit, not 100% of all the
energy in the wind.  He's at least got that right.


Actually, there is an interesting characteristic.  On a typical sloop
rig, the foresail is often larger than the triangular area between mast
and fore-stay.  150% is common.  When sailing close to the wind, much of
this sail is 'behind' the main sail.

This works well because the air hitting the face of the foresail is
deflected back along its length and that funnels it right along the back
side of the main sail.  So the speed of the air along the back of the
main sail is increased by this oversized fore sail.  And that generates
a lower pressure region on the back side of the main sail so it produces
more thrust (dare I say 'lift' :-/).


(of course now 'guy' is going to claim that's his whole idea and why
don't designers do this with wind turbines, sigh...)

The thrust is just about perpendicular to the boom (bottom of the sail)
and so is not in the direction of straight 'forward'.  But it decomposes
into two vectors, one in the direction of movement and one sideways.
The keel resists the sideways force and the forward component moves the
boat.

The wind turbine analogy is that 'thrust' is created by the low pressure
that is created by the air flowing over the cambered surface.  This
thrust is at some angle with the axis of rotation.  It decomposes into
two vectors, one perpendicular to the axis, causing the blades to rotate
around, the other pushing the blades in the direction of the wind and
that is resisted by the blade structure and hub.


Exactly right.  He doesn't seem to understand how the blades are
oriented and which 'direction' the 'lift' force is pointing with respect
to the shaft.

daestrom

Posted by daestrom on April 14, 2010, 11:31 pm
 

Energy Guy wrote:

It's no mystery to airplane designers.  There is a large low-pressure
zone created above the wing that extends upward from the wing surface
several feet.

The same is true for the blades of a turbine except the area extends
around in a circle, trailing behind the blade.


An assumption that you keep making but haven't any proof.


You *still* don't get it.  The blade is arranged so the angle of attack
of the incoming air is slightly below 'horizontal' if it were a
conventional wing.  The air impacting the 'bottom' or flat side of the
blade is deflected and that creates some force of rotation.  But the air
traveling over the 'top' or cambered side creates a low pressure region
and that 'pulls' the blade in the same direction.  Both effects are
working together to create a torque on the axle to turn the system.


We're not 'confusing it', we're trying to explain to you that you seem
to have the idea that the wind approaches the 'wing' at some angle other
than the truth.


Different plane wings, for different speeds of flight use some of both
of these phenomenon.


Nonsense.  You're 'vector math' is assuming simple collision/deflection
of the air with a stationary plate.  Like a jet of water impacting a
flat plate.

45 Degrees will get you the most force on the plate, *IF THE PLATE ISN'T
MOVING*.  Start moving the blade at right angles to the incoming air and
the vectors all change.  If the tangential speed is the same as the wind
speed, a 45 degree angle will extract *ZERO* energy from the wind.  This
is because the apparent wind speed to the blade is now sqrt(2) times
higher, but coming in at a 0 degree angle and flowing over both sides of
the plate equally with no deflection anymore.

And this is very important to consider in a turbine blade design unless
you expect your blades will never move.


But just like a wing in level flight, a blade with a shallow angle of
attack can be 'lifted' around the circle of its rotation.  And with
laminar flow over the back side of the blade (where the camber is), you
reduce drag quite a lot.


On the contrary, that is *exactly* the air that is flowing over the
surfaces of the blade.  It is very *real*.


This is 'apparent wind'  Any sailor will tell you that as your boat
picks up speed and adds vectorily to the wind's own speed, you have to
trim the sails to a new angle to maintain optimum performance.

Similarly, the mounting angle of the blade in a turbine has to consider
not only the wind speed but the tangential speed of the blade.  The
combination will have the air meeting the leading edge of the blade at a
shallow angle of attach and flowing over both sides.


No one has made such a suggestion except you, you keep dragging this
'red herring' out and it's nonsense.

and that a wind blade is also pulled forward because of this

Again, no one has suggested it is pulled 'forward', whatever direction
that is.  The blade is pulled *around* the axis of rotation by this lift
force.

If we use 0 degrees as the reference and have it pointing toward the
direction the wind is coming from, and we have an 'apparent wind' on the
blade of 45 degrees because the tangential speed (at right angle to the
wind direction) is the same as wind speed, then the 'lift' force
developed by the camber on the blade is at an angle of 135 degrees.  If
the flat side happens to be angled at 45 degrees to the reference, it
will not deflect the air at all and create no torque.  But a real blade
design would have the flat face at something like 50 to 55 degrees from
the reference direction so it does deflect the air 5-10 degrees.

The 'lift' force can be broken down into two vectors, one 70% in the
same direction as the wind (180 degrees from the reference) and the
other 70% in the direction of rotation.  Not against rotation, in the
same direction of rotation.  So 70% of the 'lift force' is applied to
the machine to help it rotate.  While 70% of the 'lift force' is acting
along the axis of rotation and must be resisted by the strength of
materials.  None of the 'lift force' is working against rotation.


BZZZT, sorry, wrong again.  The orientation of the 'wing' guarantees
that the 'lift' it develops will be at least partially in the direction
of rotation.

Wings don't pull airplanes or

Wings pull airplanes and gliders 'up' and the same forces 'pull' blades
'around' on a turbine.  Your constantly restating these fake straw-man
arguments are a waste of time.

daestrom


Posted by daestrom on April 14, 2010, 10:55 pm
 

Energy Guy wrote:

And I proved that it is *more* than just the blade face area that is
extracting energy.  What's your point?

You think more area will extract more energy?  Perhaps, but more area
will also increase the losses so the net output is not improved.  And
that's why thousands of engineers around the world that design these
things for a living have compromised at three blades and not a larger
number.  Guess they didn't have your 'insight' to revolutionize the
industry.



You've misunderstood my position.  I'm saying that adding more area will
not increase the power output very much more than you already get with
three blades.  That's all I'm saying.

You seem to think that putting more blades on will somehow increase the
power output by a significant margin, but all you have is 'seat of the
pants' idea, no facts to back it up.



Pressure is not measured in square inches.

And the differential pressure across the unit is a big deal.  Study the
Betz limit derivation and you'll find it right in the middle of
everything.  Home fans accelerate just the air column through the fan
and as soon as it exits the fan a large fraction spins off in eddies and
recirculates.  This is because the fan not only increases the dynamic
pressure (caused by the velocity of the air), but also the static pressure.

Commercial ventilation fans used in ductwork develop even more dP across
them.  Have to in order to get air to flow through the duct-work.

Centrifugal pumps are a similar machine and similar design tradeoffs.
You want a lot of flow at low head, go with axial flow (wind-turbine or
fan).  You want a high head but not so much flow, go with a volute
design (centrifugal blower style).  You want even more head, go
multi-stage (jet-engine compressor).

Why is it so hard for you to understand that wind turbines are very low
head, high flow devices and that is a major dictator of their design?


Yes, the camber improves the lift-to-drag ratio by *increasing* lift.
Not reducing drag.  If a flat blade had the same lift as a cambered one
for a given angle of attack, then the Wright brothers and all the
aeronautical engineers since then have wasted a lot of time making
cambered wing designs.

BZZZT. I'm sorry, you're wrong.  But thank you for trying.


So you just stated that the blades would be pull forward just for sake
of arguing.


Which they are, except they have a twist along their length to account
for the change in 'apparent wind' angle caused by the faster tangential
speed as you move further from the axis.


Again you don't seem to understand how the speed of rotation and the
wind speed combine to have the air approaching the blade nearly head on,
almost the same angle as an air plane wing.

Look at a blade up close and you'll see that the wind doesn't 'push' on
the curved side, that would make the thing rotate in the other
direction.  It blows against the 'flat' side (what would be considered
the bottom of an airplane wing) at a very shallow angle and flows over
the back, curved side (top of an airplane wing) in a smooth (ideally
laminar) layer.

daestrom


Posted by Energy Guy on April 15, 2010, 1:14 am
 

daestrom wrote:


The energy available in wind currents is expressed as a function of the
volume of air (a cubic meter) and it's mass (1.23 kg per cubic meter)
and it's velocity (m/s, to the power 3).

The exact formula is 0.5 x air density (kg/m3) x area (m2) x wind speed
(m/s)^3

At a wind speed of 12 meters per second, it takes 0.083 seconds for our
hypothetical 1 cubic meter of wind to pass by (or through) a wind
turbine blade.

The GE 1.5 MW turbine has blades that are 38 meters long, and a typical
rated RPM of about 20 (or .333 rotations per second).  In one second,
the tip of one blade will travel 80 meters (total circumference is 242
meters).  In 0.083 seconds the blade tip will travel 6.64 meters.
Naturally, other sections of the blade closer to the hub will travel a
shorter distance during this time.

This gives the blades an apparent larger area then their actual physical
area.


That if fans with wide blades are efficient at making air move, then the
converse must also be true (that air can efficiently make wide blades
turn).


Can you show mathematically that a thin, long blade is more efficient at
being rotated by an air current compared to a short wide blade?

How do we know that the large wind turbines that we see in current
operation aren't a comprimize that take other factors into consideration
- such as aesthetics, impact on wildlife, humans, extreme weather, etc?

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