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Thanks, Thomas
I am going to post a few slides of a cooling briefing I
gave a few years ago. This one out of K&W shows the traditional duct
shape - which is actually about the worst way to design a duct as shown by the
quick formation of eddy's up front in the duct.
Ed
----- Original Message -----
Sent: Wednesday, February 28, 2007 9:16
AM
Subject: [FlyRotary] Re: Pinched ducts
was : [FlyRotary] Re: cowl openings for water radiators
Ed,
Okay, now I understand your
reasoning!!
Well, if it works, you definitely did
something right!!
Anyone out there with access to some CFD
programms to run some tests?
Like:
a) optimal streamline
b) Truncated streamline according to
K&N
c) modifed truncated according to Ed
Difference being that b. would have a way bigger
intake than c.
Bob, I already saw your next answer to this
email:
If you are re-doing your ductwork, I suggest you
pay extra-attention to the to the "after the radiator" area. Over the last
couple of years I read various articles of builders finding success with
creating low pressure areas in the exit area, at least 2 used exhaust
augmentation.
Combined with the best doable intake ducting -
efficiency should result! :)
Thomas
PS: I am specifically interested in your
"packaging", as the BD-4 is my "plan"
----- Original Message -----
Sent: Tuesday, February 27, 2007 2:20
PM
Subject: [FlyRotary] Re: Pinched ducts
was : [FlyRotary] Re: cowl openings for water radiators
I agree, Thomas - if you are forced to truncate
the "streamline duct" you want to start at the core and go toward the
inlet. However, I decided that to see if pinching the duct
before the bell shape would work since I have such a short
distance.
In fact, when I calculated out the opening by
simply extending the streamline duct coordinate out 3-6" from the core - the
opening was 60-75% (I forget exactly) the size of my core. I decided
that would up the cooling drag quite a bit - so decided to try the pinched
shape which restricts the size of the opening to near that of the streamline
duct but then tries to compensate for the short length by speeding up the
boundary layer for great penetration toward the core. Again, my
cooling is fine and I can top out at 200 mph in my draggy RV-6A, so I think
the drag is down a bit.
But, again that is just my theorizing about why I
choose to pinch the curve.
Ed
----- Original Message -----
Sent: Tuesday, February 27, 2007 1:52
PM
Subject: [FlyRotary] Re: Pinched
ducts was : [FlyRotary] Re: cowl openings for water radiators
Hmm,
if I remember right from reading what I
have from K&N - if you do not have the optimal lenght available you
start to cut it from the intake - take radiator and apply the optimum
duct, measure from the radiator towards the intake whatever distance you
have available and cut the duct there....
Dave,
are you a Navy-flyer or a medic??
:)
Thomas
----- Original Message -----
Sent: Tuesday, February 27, 2007
1:34 PM
Subject: [FlyRotary] Re: Pinched
ducts was : [FlyRotary] Re: cowl openings for water radiators
Hummm, Dave, perhaps my understanding of what it
takes to keep the boundary layer attached to the duct wall is
flawed.
From what I believe I
understood regarding airflow in a duct is that the pressure
recovery both aids and hinders the boundary layer's attachment to the
duct wall. The pressure build up (area of slower
molecules) tend to push and keep the boundary layer pushed
against the wall of the duct as it curves out - at the same time it
is slowing down the boundary layer. So its the point of
separation is (at least in part) contingent on how much speed the
boundary layer has enabling it to push how far into the pressure
recovery area - before it ultimately separates. The further the
better is my understanding.
My understanding is that in a duct -
it is the recovery pressure which builds in the expansion
area just before the core. This "high" pressure area
will "push" back on the boundary layer causing it slow and
eventually to separate from the wall. . However, if
you keep the boundary layer speed up it pushes further into the pressure
recovery area following the duct curve before the "back pressure" slows
it enough to cause it to separate.
Also the speed of a molecule in all random
directions is much, much higher than the component imparted by the
airspeed - about 1100 ft/sec at sea level as I recall compared to about
40 ft/sec in the duct. So my interpretation is that (at least in a
duct) its the back pressure of the recovered pressure that causes the
separation - not necessary the curve of the duct alone although that
certainly contributes to the pressure recovery. That being
said,its clear that the three factors (duct curve, expansion
area and separation) really go hand in hand. The
greater the curve the more pressure recovery occurs and the greater the
tendency for separation. The higher the velocity of the
boundary layer the further it can penetrate into the higher pressure
area before being slowed and separation occurs.
There is NO doubt that having a longer duct would
improve the situation. However given I only had 3 -6" my take
was that speeding up the air (and boundary layer energy) would
ensure it penetrated deeper into the bell shape before the pressure
recovery caused separation. But, as I have often stated - I could
be completely wrong about what I think I understand.
You are after-all the Navy flyer and I know
they cram a lot of areo into Navy pilot's heads. Me- I'm a
electrical engineer, so what I know about aerodynamics is what I
have read (and think I understand).
But, regardless these pinched ducts
have provided the best cooling with the smallest opening that
I have achieved - so, Dave, if you stay quite it may not learn the
truth {:>)
Ed
----- Original Message -----
Sent: Tuesday, February 27, 2007
11:46 AM
Subject: [FlyRotary] Re: Pinched
ducts was : [FlyRotary] Re: cowl openings for water radiators
Ed,
Good discussion about streamline ducts. No doubt that they
are superior although I have a slightly different take on what quality
makes them work best. I also agree that it is wall separation
that we are trying to avoid.
But IMHO the important way to get there is "avoiding sharp
turns." I think of the air molecules as little race cars
coming in the duct. The less turning they do, the better.
If they need to turn, the radius of the turn needs to be as large as
possible. And just as important, the turn radius is distributed
so that more of the turn is done after the air has started to slow
down (near the face of the radiator). In other words, the turn
radius is a function of speed. Just like with a car, don't turn
it much before slowing down or it will separate.
With that in mind, see why the "conventional duct" is so
terrible. There is a single sharp turn right at the end of the
straight-a-way. Separation occurs there and the whole plenum
becomes turbulent.
With the bell shaped duct (K&N), it is easy to see why we
need length. The longer the duct, the larger the turn radius can
be throughout the whole distance.
Given our limited space however, there will undoubtedly be a
point where the necessary turn radius becomes too small for the speed
of the air and it separates. But at least get the air to expand
as much as possible before that happens.
With your restriction in the neck you are setting yourself back
before you start the necessary expansion. You have created
less distance over which to average the turning radius, you have
increased the speed - meaning the air can tolerate less of a
turning radius before separating (lower velocities are known to
maintain laminar flow much better than high velocity), and you have
increased the total amount of expansion the streamlines need
to undergo (narrower starting point). So
my guess is a rather dismal effect on cooling compared to
what you could have.
BUT, since your cooling is still adequate I am sure you have made
a very nice overall drag reduction. There is no way a
conventional duct with that amount of area would work well. In
other works, while I am very skeptical that the restriction actually
helped cooling, big kudos to you for absolutely minimizing drag
and duct area while maintaining sufficient cooling.
In fact, seeing as how you have proven that it works I am
considering doing that for my oil and intercooler ducts as they are
currently getting more air than they need...
JMHO
On 2/26/07, Ed
Anderson <eanderson@carolina.rr.com>
wrote:
Actually, there is, Joe. But, you are
going to be sorry you asked {:>).
I spent quite a hit of time studying a
tome (Kuchuman and Weber better know as K&W) on air
cooling of liquid cooled engines written back in the hey day of high
speed mustangs lightenings, spitfires, etc. Sort of the liquid
cooling bible. Chapter 12 (the one of most interest to
us) showed a duct that reportedly had the best pressure recovery
(84% or thereabouts) around for a subsonic duct that they had
found. It was called a "StreamLine Duct" (See attached graph -
the graph a of the top graph shows the shape of the duct (or at
least 1/2 around the center line - sorry for the poor
quality).
After quite a bit of studying and
thinking about what I had read about cooling ducts, it finally
became clear to me that the perhaps top thing that is clearly
detrimental to good cooling is having flow separation in the
duct. Most of the old drawings of a cooling duct shape
followed a sinusoidal shape - rapid expansion right after the
opening. It turns out that "traditional" shape is probably one
of the worst shapes for a cooling duct (the story why is too long to
get into here).
Anyhow, Flow separation leads to eddies
and turbulence which casts a "shadow" of turbulent air on the
cooling core. Like a shadow, the further away from the core
the separation occurs (like near the entrance of the duct) the
larger the shadow it casts on the core area. This
"shadow" adversely interferes with the flow of air
through the core and reduces the effectiveness of the core.
What causes this separation is that as
pressure is recovered by the expansion of the duct, the build up of
the very pressure recover we want - starts to hinder the
boundary layer flow near the wall of the duct. It slows it
down and causes it to lose energy and attachment to the duct
wall. At a certain point the flow separates and starts to
tumble/rotate and depending where (near the duct entrance or near
the core) the flow separates, determines how much of the core area
is adversely affected. So if the boundary layer's energy level
(air speed of its molecules) is maintained at a high level
separation is less likely.
So ideally, you would like to prevent any
separation during pressure recovery. The Streamline Duct is
the so called "Trumpet" duct or "Bell" duct . After the
opening, there is a long section of non-expanding duct followed by a
rapid expansion into the "bell" shape just before the core.
The long non-expanding part of the duct maintains the energy (air
flow) of the boundary layer and separation does not occur until well
into the "bell" shape expansion.
In fact, it happens way up in the corner
of the bell/core interface and affects a very small area of the
core.
For full effectiveness the "Streamline duct"
from K&W needs a length of 12-17". Well, that's way more
distance than I had. So I got to thinking that if keeping the
speed of the air molecules near the duct wall helps prevent boundary
layer separation and the cooling killing eddy of turbulent air -
what could I do with my short 3 - 6" (no jokes you
guys). We all know from Bernoulli that if an area is squeezed
down that the velocity of the air flow increases - right?
So I decided to try to maintain or increase
the energy of the air by pitching down the neck just before it goes
into the bell shape expansions in hopes that the increased energy
will help the boundary layer stay adhered to the duct wall until
well into the corner of the bell shape. So that's the story of
the pinched ducts. There is no question in my mind that this
is not as effective as if I could have had the 16" to build the duct
- but, in this hobby, you work with what you've got - right?
Does it work? Who knows - but I seem to
fly with less opening area than most folks and have no cooling
problems. So that's my 0.02 on the topic - see told you, you
would regret asking {:>).
Ed
----- Original Message -----
Sent: Monday, February 26,
2007 8:53 PM
Subject: [FlyRotary] Re: cowl
openings for water radiators
Ed, is there some particular reason
that you necked the inlet down small, then enlarged it
again. Thankyou for the pictures. JohnD
----- Original Message -----
Sent: Monday, February 26,
2007 3:39 PM
Subject: [FlyRotary] Re:
cowl openings for water radiators
John, don't know if these photos will
help. But, like you I only have between 3 and 6" of duct
distance on the radiators. You should do Ok with 20 sq
inch on each opening with a good diffuser/duct. Attached
are some photos of my current ducts. The openings are 18
sq inches each. I have had one opening down to as little
as 10 square inches - but that was a bit marginal - so opened it
back up. I have a generous exit area for the hot air
including a larger 4" x 12" bottom opening as well as louvers on
each side of the cowl. So you mileage could vary - but
Tracy has essentially the same size opening as well as several
others.
Ed
----- Original Message -----
Sent: Monday, February
26, 2007 12:12 PM
Subject: [FlyRotary] cowl
openings for water radiators
What size openings do I need for
the water radiators? The Wittman Tailwind cowl I
have has postal slots of 3' x 7 3/4" , which is
approx. 22 1/4 sq in. on each side. Sam James for the
160 Lycoming is using 4 3/4' round holes which are 17.6 sq.
inches on each side. My radiators are quite close to the
opening and I plan on making the diffusers trumpet shaped,
will the openings be large enough if I can stay over 20 sq.
inches on each side with a decent trumpet shape.
JohnD hushpowere II
on order - hope to start in 2 weeks if weather cooperates.
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