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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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