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At 04:49 PM 2/10/2005 -0500, you wrote:
ALL of this discussion was based on the use of a wet-wing aluminum fuel tank. The tank is the cooler.
Lets assume that the fuel is indeed boiling. This is analogous to piping steam into a cool metal tank. The steam will condense on the walls of the tank (or the fuel in the tank) until the walls of the tank get too hot. The question is how much heat will this tank with 175 MPH air blowing on it dissipate before getting too hot. Bill says laminar air will limit this to a great degree. I don't know.
When I did the rough calculations, found that it would take the entire wing surface, -AND- the cowling surface, -AND- part of the fuselage to cool the engine. You definitely can't cool the entire engine with the just tank surfaces, but you can cool a useful fraction of it.
The tank is aluminum and the fuel sloshes around in it. The whole tank will get warm, not just the lower portion.
Using just the tanks is smarter than attempting to use the entire skin. This is because the air path is short. Thus, you get more "new" air per square foot than you would if you attempted to use more of the wing profile. Once you have heated the boundary layer on the leading edge, the aft part of the wing can't throw much more heat into the already heated air.
Think of the layout for de-icing heaters and you will get the picture. They heat the boundary layer at the leading edge. This takes care of the rest of the airfoil. Come to think of it, heating the fuel slightly might offer a minor de-icing effect. :^)
Back to real world data. I was able to cool my oil about 2 degrees with return fuel cooling during cruise flight at 175 mph. I tried to measure the temperature rise in the tank but the .1 degree resolution of the instrument was too coarse to measure it. (40 minute test duration) In other words, it didn't change significantly. This showed me that the fuel flow needed to be much higher in order to get usable level of cooling. It also showed that the fuel tank was far from it's maximum heat dissipation capabilities.
Sounds like a very useful experiment. I agree that more flow would be better. A better (longer, bigger, turbulent) heat exchanger is also probably needed. If you have sized the heat exchanger correctly, the fuel should exit the heat exchanger at a good fraction of the oil exit temperature.
At 75 Celsius, 10% of the volume, at most, of aviation gasoline will boil.
http://www.chevron.com/prodserv/fuels/bulletin/aviationfuel/9_ag_specsandtest.shtm
Thus, 75 C (or 167 F) would probably be the very hottest you would want the gasoline to exit the heat exchanger.
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