Al Gietzen wrote:
Attached are photos of my air flow test setup to measure flow distributions
and pressure drops. It is shown here with the scoop and custom Ron Davis
radiator. This is one of two radiators for my 20B, this one goes in the
cowling of my Velocity pusher, and another of different configuration is in
the wing root. This is an "armpit" scoop (under the wing at the fuselage),
and has a diffuser ratio a bit over 4 to 1.
I measured flow and flow distribution with a small handheld anemometer from
Radioshack; a very cool little instrument that gives digital readout of air
speed (see at their website); pressure measurements with water manometer.
The output from the centrifugal blower unfortunately turned out to be about
half my design point for the rad, but it gave me some very useful
information on placing some vanes internal to the scoop/diffuser in order to
get a good flow distribution through the rad.
The max flow rate from the blower through the rad was 1300 cfm. The design
point for the installation is 2600 cfm. The radiator is a two row 1" tubes
with 16 fins per inch and a core thickness of about 2 ½ ". The core is 16
x 11.5 for an area of 184 sq in. The measured pressure drop across the core
at this flow rate was only 0.4 in water. I think the pressure drop goes
roughly as the cube of flow rate, so at design point flow, the drop would
be about 3.2" water. This compares favorably with Ron Davis' computed value
of 3.7".
Note the length of duct from the blower to the scoop. The output from a
centrifugal blower is very turbulent, so if you want meaningful measurement
of flow distribution, be sure to use a length of duct to help straighten
things out.
Now; about that gentle breeze:
As you can see the pressure difference across the right radiator
is 5.11 - 3.61 is only 1.5 inches of water or about .055 psi
which is hardly enough to generate a gentle evening breeze.
In effect this radiator is not functioning. Well hardly at all.
I'd say the 1.5" drop across Tracy's radiator gives a lot more than a gentle
breeze. That's enough pressure to produce significant flow, especially
through a fairly open core as I believe the air conditioning cores are.
That's enough for about 2000 cfm through my 16 fins/in radiator. That's a
pretty strong wind - or I guess one could say - it's a lot of hot air.
I've also done some measurements on my custom oil cooler. More on that
later.
Al Gietzen
As I recall Tracy's AC cores are 3.5 inches thick and not 2.5. As I also recall,
but I could be wrong, the flow per square inch is inversely proportional
to the square of the thickness. 1/6.25 = .16 while 1/12.25 = .081 or
about half the flow per square inch of frontal are. Due to the thicker core.
Now as to the volume or CFM of air going through you must realize
Tracy's rads are only 10 by 9 inches for a frontal area of 90 square inches
while your rad is 184 square inches or twice the total flow for the same pressure
drop if the thicknesses were the same which they are not. They are much thicker.
Also the fin spacing looks much closer on the AC cores than it does on a real water
cooling radiator. My guess is Tracy's AC heat exchanger cores are flowing one
quarter of the air that your rads are at the same pressure drop.
Also the left AC core has twice the pressure drop so it should be really
pumping air.
We also need to measure some real world water and air temperatures going in and
out of the rads.
Paul Lamar
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