----- Original Message -----
Sent: Tuesday, February 27, 2007 1:34
PM
Subject: [FlyRotary] Re: Pinched ducts
was : [FlyRotary] Re: cowl openings for water radiators
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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