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Date: Thu, 10 Feb 2005 18:12:15 -0700
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
From: Bill Dube <bdube@al.noaa.gov>
Subject: Re: [FlyRotary] Re: Heating the Fuel
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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.