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Ed, are these calculated or measured numbers? If calculated, what's the formula being used?
This should normally be a tricky business. Cooling/heating formulae normally involve the specific heat and mass of the substance being cooled, the amount of heat transferred, etc. One common formula is:
heat transferred = mass * deltaT * specific heat (1 for water)
So yeah, this seems odd. The bigger the deltaT (in this case, difference between air and radiator temperature) the more heat you can transfer.
There are a bunch of these at:
http://www.fordhamprep.com/gcurran/sho/sho/lessons/lesson210.htm
But a complete formula is complex because it depends on radiator configuration, efficiency, etc. I found a bunch of auto radiator-specific formulae at:
http://www.unb.ca/che/Undergrad/proposed/auto.pdf
but then you're on your own. Or you could ask one of the ACRE guys. =)
-----Original Message-----
From: Ed Anderson [mailto:eanderson@carolina.rr.com]
Sent: Monday, October 20, 2003 10:46 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Cooling Model Enhancement
I finally found a math model that provides the rise in temperature as it
goes through a radiator. The model indicates that for the two evaporator
cores that the air temp should rise between 35F and 38F for 161 HP at 120
MPH burning 15 GPH. Less of a rise if less BTU are being generated.
Here is what the temp rise model gives (all temps in farenheit):
OAT Delta T Exit Radiator
0 34.9 34.9
30 35.59 65.59
60 36.15 96.15
90 36.02 126.02
120 37.48 157.48
I guess I am surprised to find that the delta T is decreasing with
decreasing OAT. The only way I can rationalize the lower delta T at the
lower temps still removing the same amount of heat is that the cold air is
denser and therefore the mass flow is greater and that accounts for carring
away the same amount of heat with a lower Delta T. Would any of your
thermodynamic folks comment on this?
In any case, it appears that some of Todd's delta T figures fall in this
ball park, so this model may not be too far off.
Ed Anderson
RV-6A N494BW Rotary Powered
Matthews, NC
eanderson@carolina.rr.com
>> Homepage: http://www.flyrotary.com/
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