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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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