----- 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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