----- Original Message -----
Sent: Monday, February 26, 2007
9:38 PM
Subject: [FlyRotary] Pinched
ducts was : [FlyRotary] Re: cowl openings for water radiators
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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