----- 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)