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I'm just a computer programmer, not an aerodynamicist, but I think the whole converting speed to pressure thing is a red-herring that causes more confusion than it solves.
What you're after is flow through the entire heat exchanger. Building big plenums so that the you can stop the air and then accelerate it is one way to get there, but where else in aviation do we attempt to stop the air so that we can start it again?
The first radiators just hung out in the airstream and had terrible performance. The problem was that they created so much turbulence that air went around them instead of through, and some parts of the radiator didn't get used at all. As airplanes got faster, that turbulence would occur even between fins, obstructing the flow, and making some parts of the radiator unused. The point that K&W was making, at least by my reading, was that the air needed to be slowed to be smooth through the WHOLE heat exchanger.
The ducting I have on top of my Corvair engine doesn't look like any others I've seen. Bascially, all I have is a sheet of aluminum that taper down as it goes back. I do not have a "plenum" of any sort, and I do not try to slow the air down. I just make sure that it smoothly spreads out and moves through all the cylinders. When first flying, I was concerned about the CHTs being too low. . . that the cooling might be TOO good, because they stay below 250*F.
I guess my message is that you're not looking for "pressure recovery". That is just one means to reach the end, which is to make sure that ALL of the radiator is radiating.
I could very well be completely off base.
On Wednesday, July 21, 2021, 11:30:45 AM EDT, Marc Wiese cardmarc@charter.net <flyrotary@lancaironline.net> wrote:
As I recall the K&N curves, the duct/plenum is supposed to decrease the speed approaching the radiator (or fins), thusly increasing the pressure, which does the cooling. Not speed, pressure differential. After the fins, the deal is to regain the speed and have an adequate size smooth hot air exit. So a couple of inches of H2O pressure drop across the fins? The P51 arrangement seems to be about the best? Or wing root LE like the Hurricane/Mosquito? M Sent from my iPhone On Jul 21, 2021, at 10:12 AM, Finn Lassen finn.lassen@verizon.net <flyrotary@lancaironline.net> wrote:
Seems what we're missing is a curve
that's the product of these curves.
In other words some kind of bell or parabolic curve with top where
you have max cooling/drag.
Obviously you can push fluid (and air) through a radiator at a
furious rate, but the drag will go up.
So for both fluid and air rates there must be an optimum spot.
Finn
On 7/21/2021 8:42 AM, Stephen Izett stephen.izett@gmail.com wrote:
This graph from Mocal might be helpful. It's for their oil coolers
but the trends may be transferable to water exchangers.
The solid line is Pressure Drop.
The two dotted lines tell the story of two different
oil flow rates/tube.
<Screen Shot 2021-07-21 at 7.58.40 pm.png>
| m/sec |
5 |
10 |
15 |
20 |
25 |
| kmh |
18 |
36 |
54 |
72 |
90 |
| mph |
11.2 |
22.4 |
33.6 |
44.8 |
55.9 |
Increasing the air flow 5 fold from 11 to 56 mph
only increases the heat transfer:
2 fold with an oil flow of 0.02 L/sec/ tube and 2.3
fold by doubling the oil flow rate per tube to 0.04 L/sec/tube
While pressure drop increased 13 fold.
So, diminishing returns from increasing airflow or
fluid flow.
Steve Izett
Charlie,
No, no reference,
just what I have read and also talking to Rad
manufacturers such as BWR in Brisbane. You can
check it out by passing your hand through a naked
flame. Quickly and there is no heat transfer. Pass
slowly and you will see what the argument is. As I
said the truth is there somewhere and as Lyn so
aptly puts it “I could well be wrong”..
Neil.
Hi Neil,
Do you have a reference for that? Slowing a medium
down so it has time to absorb the heat seems to
conflict with physics as I've been led to understand
it.
Charlie
On 7/20/2021 5:01 PM, 12348ung@gmail.com wrote:
Charlie,
Much wisdom out
there, you just have to find the truth! Max
cooling is apparently 30 MPH, so Any faster and it
does not pick up heat before going past. Look at
big trucks, that grill is not only for looks, they
slow the air to get max cooling. If too slow they
have a quite large fan that kicks in to drag air
through at 30 MPH not 100!
As you say, what do I
know – I have seen too many that do not work –
without any degree.
Neil.
Subject: Inlet cooling article
I remember the Laboda
article about enlarging their cooling inlets, but
not many of the details.
This:
The plenum receives air through two
circular air intake ducts behind the propeller and
squeezes it, Bernoulli-style, so that the air
accelerates across the cylinders and between their
fins, carrying the heat back, down and out an
outflow "gate" at the back and bottom of the
engine area, forward of the firewall.
Is contrary to everything I've ever read about
cooling efficiently. Faster relative flow will
always have higher drag, all else being equal.
Accelerating the air even faster than freestream
just sounds crazy. My understanding is that there's
a balancing act between having the room in an a/c to
'recover' (increase) differential pressure across
the heat exchanger (engine fins, in this case), and
causing too much drag from the air going through the
fins too fast (there's aerodynamic drag in the heat
exchanger, just like over the a/c itself). It's
surprising to me that James made the plenum the way
he did. The rest sounds like putting bandaids on
stuff. The next-to-last image, of the final inlet,
shows what appears to be a *much* smaller plenum
inlet than the cowl ring in front of it, and a
rather sharp edged lip where the plenum starts. It
looks like the air would accelerate until it hits
that sharp lip, and immediately go turbulent, which
will kill any pressure recovery and actually slow
flow into the cylinder fins.
Most Lyc plenums I've seen (even the ones James made
for the 4 cyl engines) have significant volume above
the cylinders with smoothly expanding ducts feeding
the plenum. That allows the air to slow in an
organized fashion, which increases *pressure*, which
is what actually makes the air move through the
fins.
But what do I know; I have an Economics degree....
Charlie
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