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In a message dated 11/29/2002 9:13:30 PM Central Standard Time, lors01@msn.com writes:
I'm not 100% sure what is going on but I think you would be surprised how
much the 50-50 coolant expands with heat. I flight tested the new bottle
yesterday and verified that on cooldown the system was completely topped
off
with coolant (no air at top of fill port.) This was never the case before.
This also accounts for another anomaly I've noticed over the years. It
seemed like the water cooling was slightly better on the first flight after
topping off the system. I was never sure because the next flight was
usually on a different day, different temperature, humidity, etc. Now that
I've been paying close attention I have verified that only a few ounces
worth of air in the system makes a noticible degredation in cooling
performance (about 5 degrees worth). Not sure why this is happening
either but I'm happy with the results. With more experimentation the answer
will probably come out.
Tracy Crook
tcrook@rotaryaviation.com
www.rotaryaviation.com
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This is just a theory, but I think that a possible explanation for the lower performance when there is just "a little" air in the system is that the water pump essentially 'emulsifies' the air into the water, creating many 'very' small bubbles. These don't separate from the liquid phase very fast, and so they circulate with the water.
If that is the case, then the coolant that is circulating is no longer 100% liquid, so its heat capacity is no longer that of 100% liquid, and the temperature rise across the engine must be a little higher to carry away the same amount of heat.
Lets say the total capacity is ~1 gallon (engine + two cores + 5/8" hose), this is 128 ounces. Tracy refers to a 'few' ounces having an effect, if we use 5 ounces, then that is 5/128 = ~4% air. This would lower the heat capacity of the liquid by 4%, and cause a corresonding rise in the required delta-T across the engine to remove the heat. Also, on the radiator side, I would expect a decrease in effectiveness, so the inlet temperature to the engine would be a little higher also (can't calculate the magnitude, but no more than 4%).
Bottom line -- keep all air out of the system.
Bill Schertz
KIS Cruiser #4045
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