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[FlyRotary] Re: Performance coatings- bearing damage?
Al, thanks for the input. My tank is not
pressurized (no check valve on the vent) and any vapor bubble formed after
the heat exchanger would flow harmlessly back to the wing tank. As long as
the fuel will not be boiling at 200F at ambient pressure at 18,000 ft (which it
may be, I should look it up) then I should be ok. Todd has a good point
that only the lower side of the wing can be counted on for cooling, but I still
feel it should be adequate.
I was
beginning to fear that the problem would simply be inadequate fuel flow, as
Tracy seems to have found through experience. That small amount of fuel
(about 40 gal/hr) is simply not enough to cause any significant
cooling of the coolant flowing at what? about 20 gal/min? Assuming the
fuel went from 70 to 170F, and that fuel has about the same specific
heat as Evans Coolant. That means that the coolant temp would drop by 3.3
deg. I could turn on the aux pump and bring the return fuel flow up
to 80gal/hr in which case I could see a 6.6 deg drop in coolant temp which
is almost enough to be worth it - but not quite.
It sure would
be nice to make use of all that drag-free surface area of the
wings.
Evans coolant
may just the stuff to design a cooling system around. Simply route the
heated coolant out to the wing tips where it could passively drain
back through shallow bays on the lower surface of the wing and into a sump where
an EWP could pump it back to the engine pump. The bays in the wings would
be very shallow to minimize weight, they would be sealed but need not be
pressurized. Evans is the perfect stuff because it need not be
pressurized, is very non-corrosive
and non-toxic.
Then there
would be no need for a radiator at all. Probably enough capacity
there for a coolant/oil heat exchanger and whala, zero cooling
drag...
Tracy, is it
too late to make your RV-8 with zero cooling drag?
:-)
Dave
Leonard
Dave;
Normally efforts are made to cool
the gasoline to prevent vapor lock and improve performance. My old XJS
has a fuel cooler that is cooled by the Freon from a AC pump.
The average vapor pressure of
gasoline at room temps is already fairly high, like 9 psia (absolute), which
is below atmospheric pressure and not much of a problem. You may want to
look up somewhere what it is at higher pressures, but I’d guess two things,
one is that as it increases directly with temp it is likely go increase the
pressure in the tank much more rapidly than the fuel is fed out of the tank,
and at temps of about 190 the pressure could be quite high. I referred
to average vapor pressure because gasoline is a mix of stuff, some of which
has much higher VP than others. They usually refer to the ‘Reid’ vapor
pressure which is computed by some complex formula. So you also run the
risk of altering the compostion; e.g., the n-Pentane coming off much more
rapidly than the iso-octane or some other stuff.
At those temps, anywhere in the
line upstream from the pump a vapor bubble is almost assured.
Not to discourage
experimentation, but my opinion would be – maybe not the best
idea.
Al
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.
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