----- Original Message -----

From:=20 Mark = Steitle=20

Sent: Sunday, September 30, = 2007 6:28=20 AM

Subject: [FlyRotary] Another = cooling=20 question

ED wrote:

<snip>

Mark, if you really had excess air flowing through your = radiators=20 the coolant would drop more than 4 Deg F. In fact, the more air = flow the=20 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=20 pass through too fast and not pick up as much heat. This didn't = make=20 sense to me. Maybe I wasn't listening closely and missed the = point=20 altogether (wouldn't be the first time).

This is one of the oldest = myths around -=20 that air or coolant will flow too fast to pick up the heat. It = just IS=20 NOT factual. The more mass flow you have, the more heat you = will carry away. It appears that some early experimenters = noted=20 that if you slowed the flow of coolant through a radiator that there = was a=20 greater temperature drop of the fluid than if it flowed through = faster. =20 This apparently gave rise to the myth as you can still find references = to that=20 experiment supporting the slower is better myth. I once had an = debate=20 with a fererent believer in that myth, after about 30 minutes of = getting no=20 where in convincing the individual of the factual side, I resorted to = this=20 line.

"So you claim that slow water = cools better=20 than fast water, the response was "Yes", then I replied "If slower and = slower=20 water cools better and better then stopped water must cool best = -=20 right?" A long silence, then the individual hung up the = phone. =20

The fact is the coolant (in = this example=20 of slowing coolant through the radiator) will indeed lose more heat to = the air=20 - if you slow its flow through the radiator, because that slug of = coolant spends more time exchanging heat with the air. = However,=20 the slower flow also means you are removing less heat from your = engine=20 - which is the real objective.

We know that molecules = of air=20 transport the vast majority of the Heat (there is a very=20 small amount radiated away) in our installations through contact = with the=20 metal of the radiator. The average speed of these = molecules=20 (in air) is approximate the speed of sound (1100 feet/sec at sea = level). =20 So any velocity of the macro airstream in our ducts and cores are=20 insignificant compared to the air molecules velocity. So = speeding up=20 this air flow or slowing it down has no measurable effect on the = frequency at=20 which the molecules contact the metal. Turbulent flow has more = impact=20 than velocity change. Now changing the velocity of = the flow=20 does effect the mass flow through the core and therefore our overall = cooling=20 effectiveness, it just does not effect the "speed" with which the heat = is=20 transfer from metal to air.

What I DO know is that the air is flowing faster through the = water=20 radiator than the oil radiator. (I'm not sure I have the ASI's = hooked up=20 correctly, but they're both hooked up the same). I have a pitot = behind=20 each radiator hooked up to two separate ASI's. In slow cruise, = say=20 125-130 kts, the water radiator ASI will read about 110knts and the = oil ASI=20 will read about 90 kts.

110 kts would give you a = dynamic pressure=20 of approx 7.8 " H20. Now what that is measuring depends on how = your ASIs=20 are hooked up. Since they are differential pressure gauges they = are=20 measuring the difference between the static pressure under your cowl = and what=20 ever reference their static side accesses. If they reference the = ampient=20 outside air pressure (as you static system does) then you are = measuring Cowl=20 pressure relative to ambient. If there static lines are simply = open to=20 the cabin, then since cabin pressure in normally a bit lower than = ambient=20 pressure, it would exaggerate the reading a bit.

But, in any case, it appears = to me that=20 you are measuring localized cowl pressure. If you had a pitot = tube=20 before the core with its static referencing ambient pressure, then it = would be=20 the dynamic pressure (converted to a static pressure increase). = If you=20 had the pitot tube before the core and the static accessing the cowl, = then you=20 would be measuring pressure across your core. If you have the = pitot tube=20 under the cowl and the static referencing ambient you would be = measuring=20 your cowl pressure. So it depends on your static reference as = well as=20 where you have the pitot tube position as to what you are=20 measuring.

The way it was behaving before I opened up the exit, it = appeared=20 that the air from the water radiator was trying to exit=20 backwards through the oil inlet. I say this because of how = high the=20 oil temps were reading. I enlarged the cowl exit, and both=20 the water and oil temps dropped significantly.

I would say your = analysis is=20 correct. Dennis also found that enlarging his exit area = improved=20 the cooling.

The ASI's are referencing the static port for these readings; = should they=20 be referencing cowl or cabin pressure instead? Airspeeds = readings seem=20 awfully high to me.

Referencing the static = port would=20 then give you under the cowl pressure. If you reference the cowl = then=20 you would be measuring the localized dynamic pressure of the air = (greater than existing under the cowl pressure) exiting the = core=20 which I would expect to be small since your duct should have converted = most of=20 the dynamic pressure to a static pressure increase before the=20 core.

Mark

(Going to the airport today to recalibrate temp sensors)