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Tracy.. if you want to "boil" your fuel to use as a coolant source, you
need to pressurize the system/plug the vent to prevent the loss of
volatile portions of the gasoline in its vapor state. Your "wet wing"
would become a pressure vessel at that point.
As I noted previously (but was delayed in posting due to server
probs).. using fuel for cooling IS done in jets, but Jet A is much less
volatile, is cold soaked at altitude, and when they LAND, have a much
greater fuel capacity at LANDING than WE do taking off with full fuel.
If I am landing with VFR reserves, I theoretically may not even have
any fuel in the main tanks, only in the sump.. not a whole lot of
effective cooling going on there. Realisticly its not even an option
for us (glass bird.. insulated!).. but in the setting we are in, I
think that it SOUNDS great to try and use fuel as a heat sink/coolant
source.. but in practice its adding a level of complexity for minimal
to NO return.
Dave
Tracy Crook 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.
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.
Another useful factoid: The oil cooler will be giving about a
40 deg delta (oil temp) at normal 13B oil flows and cruise power
settings.
Tracy
<...it will be headed back to the tank where it will quickly be cooled ...>
How quickly? How exactly does does it cool it all that quickly? You
got a heat exchanger in the tank? How about if you're down to your
last 5 gal of fuel in the tank? How fast will fuel cool in your tank
if you have no air - fuel heat exchanger. In my tank it could take
hours.
If Tracy's pump is returning 40 gph to the tank (and, we must infer,
burning about another 15 gph) that's a total of 55 gph through the
pump. Assuming a 25 gal tank (in each wing), that means at the end of
an hour you've got 10 gal left and recirculated 40 through the tank.
The average fuel in the tank over that hour would be 17.5 gal, so
you've run it all through your heat exchanger about 2.3
times. How hot do you reckon it's going to be at the end of that
hour?
Do the math .. it IS "the mother of all vapor locks" ... Jim S.
DaveLeonard wrote:
Bill & Jim, first and most important, we are all in agreement that this is
not worth doing. However, you made a couple of assumptions that are not
quiet accurate. First, the pumps from Tracy will return 40 gal/hr to the
tank (not 15 - I know, I monitor it in flight). Second, there will be a
much higher rise in the fuel temp. Because the fuel would be going so
slowly through the heat exchanger I would EXPECT it to reach the temperature
of the incoming coolant, or about a 100 deg rise. This will not cause "the
mother of all vapor locks" becuase it will be headed back to the tank where
it will quickly be cooled. How quickly? Quickly enough that the temp of
the fuel on its way to the engine will be no warmer than ambient. It
matters not the volume of fuel in the tank (other than contact area). I
expect it to be the temp of the fuel going TO the engine to be at ambient -
and adding no additional risk of vapor lock.
But, the coolant will still only drop about 3 deg, which is not enough to
make it worth it.
Dave Leonard
<... For a 10 degree rise in gasoline temperature you would get a
630/12000 = 0.0525 deg F drop in coolant temperature ...>
OTOH, you'd soon heat your fuel to very near the coolant temp and have
the mother of all vapor locks.
If that's what you're looking for ... Jim S.
William wrote:
Dave,
If you burn 10 g/hr, and are returning 15 g/hr to the tanks, that is
15*6 = 90 #/hr of gasoline, with a specific heat of ~.7Btu/#, that
means you can transfer 90* 0.7 = 63 Btu/hr/degF rise in gasoline
temperature. Your coolant flow is probably about 25 gallons/minute, =
25*60 = 1500 gallons/hour = ~1500*8 = 12000 #/hour. For a 10 degree
rise in gasoline temperature you would get a 630/12000 = 0.0525 deg F
drop in coolant temperature. Doesn't seem worth it.
* note: I used rounded figures for densities and flow rates and heat
capacities. The fact remains that the flow mismatch makes this not do
much.
Bill Schertz
KIS Cruiser # 4045
----- Original Message -----
*From:* DaveLeonard <mailto:daveleonard@cox.net>
*To:* Rotary motors in aircraft <mailto:flyrotary@lancaironline.net>
*Sent:* Sunday, February 06, 2005 11:46 PM
*Subject:* [FlyRotary] Heating the Fuel
In anticipation of the new turbo, I am considering ways to improve
coolant cooling. Besides the obvious more air, more ducting, more
radiator; I have been again toying with the idea of a heat
exchanger. This time coolant to fuel. The plan would be to use a
typical oil/water exchanger but use fuel instead of oil. I would
use the fuel on the way back to the tank, and the coolant after it
has already been cooled by the radiator. The fuel would then go
back to the nice metal tank of the RV.
I am looking for input on the implications of heating the fuel. I
expect it would reach max temps of about 190 (usually a
little cooler) but quickly cool once in the tank. Can the fuel
tolerate that temp without vaporizing? It will probably expand in
the tank but I don't expect that will occur faster than it is used
up. I have no guess as to what temp will become steady state for
the fuel pumped out of the tank. My guess is that it will not be
much warmer than normal, but a slight increase in temp may help
with vaporization.
The last question is how much will it cool the coolant. My hope
is about 10 deg but I doubt it will be quite that much. I know
others have considered using the fuel to cool (Tracy) and I would
appreciate your thoughts.
Dave Leonard
Homepage: http://www.flyrotary.com/
Archive: http://lancaironline.net/lists/flyrotary/List.html
Homepage: http://www.flyrotary.com/
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