----- Original Message -----
Sent: Sunday, September 30, 2007 6:28
AM
Subject: [FlyRotary] Another cooling
question
ED wrote:
<snip>
Mark, if you really had excess air flowing through your
radiators the coolant would drop more than 4 Deg F. In fact, the
more air flow the more coolant Delta T you would drop through the
radiator.
<snip>
That's exactly what I HAD thought, until I was told that the air
could pass through too fast and not pick up as much heat. This
didn't make sense to me. Maybe I wasn't listening closely and missed
the point altogether (wouldn't be the first time).
This is one of the oldest myths around
- that air or coolant will flow too fast to pick up the heat. It
just IS NOT factual. The more mass flow you have, the more heat
you will carry away. It appears that some early experimenters
noted that if you slowed the flow of coolant through a radiator that there
was a greater temperature drop of the fluid than if it flowed through
faster. This apparently gave rise to the myth as you can still find
references to that experiment supporting the slower is better myth.
I once had an debate with a fererent believer in that myth, after about 30
minutes of getting no where in convincing the individual of the factual
side, I resorted to this line.
"So you claim that slow water cools
better than fast water, the response was "Yes", then I replied "If slower
and slower water cools better and better then stopped water must
cool best - right?" A long silence, then the individual hung up the
phone.
The fact is the coolant (in this
example of slowing coolant through the radiator) will indeed lose more
heat to the air - if you slow its flow through the radiator, because
that slug of coolant spends more time exchanging heat with the
air. However, the slower flow also means you are removing less
heat from your engine - which is the real objective.
We know that molecules of air
transport the vast majority of the Heat (there is a very
small amount radiated away) in our installations through contact with
the metal of the radiator. The average speed of these
molecules (in air) is approximate the speed of sound (1100 feet/sec at sea
level). So any velocity of the macro airstream in our ducts and
cores are insignificant compared to the air molecules velocity. So
speeding up this air flow or slowing it down has no measurable effect on
the frequency at which the molecules contact the metal. Turbulent
flow has more impact than velocity change. Now changing the
velocity of the flow does effect the mass flow through the core
and therefore our overall cooling effectiveness, it just does not effect
the "speed" with which the heat is transfer from metal to air.
What I DO know is that the air is flowing faster through the water
radiator than the oil radiator. (I'm not sure I have the ASI's
hooked up correctly, but they're both hooked up the same). I have a
pitot behind each radiator hooked up to two separate ASI's. In slow
cruise, say 125-130 kts, the water radiator ASI will read about 110knts
and the oil ASI will read about 90 kts.
110 kts would give you a dynamic
pressure of approx 7.8 " H20. Now what that is measuring depends on
how your ASIs are hooked up. Since they are differential pressure
gauges they are measuring the difference between the static pressure under
your cowl and what ever reference their static side accesses. If
they reference the ampient outside air pressure (as you static system
does) then you are measuring Cowl pressure relative to ambient. If
there static lines are simply open to the cabin, then since cabin pressure
in normally a bit lower than ambient pressure, it would exaggerate the
reading a bit.
But, in any case, it appears to me
that you are measuring localized cowl pressure. If you had a pitot
tube before the core with its static referencing ambient pressure, then it
would be the dynamic pressure (converted to a static pressure
increase). If you had the pitot tube before the core and the static
accessing the cowl, then you would be measuring pressure across your
core. If you have the pitot tube under the cowl and the static
referencing ambient you would be measuring your cowl
pressure. So it depends on your static reference as well as where you
have the pitot tube position as to what you are measuring.
The way it was behaving before I opened up the exit, it
appeared that the air from the water radiator was trying to exit
backwards through the oil inlet. I say this because of how high
the oil temps were reading. I enlarged the cowl exit, and both
the water and oil temps dropped significantly.
I would say your analysis is
correct. Dennis also found that enlarging his exit area
improved the cooling.
The ASI's are referencing the static port for these readings; should
they be referencing cowl or cabin pressure instead? Airspeeds
readings seem awfully high to me.
Referencing the static port
would then give you under the cowl pressure. If you reference the
cowl then you would be measuring the localized dynamic pressure of
the air (greater than existing under the cowl pressure) exiting
the core which I would expect to be small since your duct should have
converted most of the dynamic pressure to a static pressure increase
before the core .
Mark
(Going to the airport today to recalibrate temp sensors)