----- 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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