> >>> I found this document about carbon foam radiators.
 > >>>
 > >>> http://www1.eere.energy.gov/hydrogenandfuelcells/pdfs/nn0123v.pdf
 > >>>
 > >>>
 > >>> Jeffrey Martin
 > >>>
 > >>
 > >>
 > >> That's pretty interesting. I've seen carbon foam before but I never
 > >> thought
 > >> of this application. Is this going anywhere? The things you have to be
 > >> careful about are that the stuff is brittle and carbon next to
aluminum
 > >> makes a nice battery.
 > >>
 > >> Steve Brazil
 > >
 > > The really interesting part is that, if the foam is available, you can
 > > cut a custom shape, then just drive your coolant tubes through it,
 > > instead of having to go through the fab process for metal fins.
 > >
 > > Keith Wood
 >
 >
 >
 > Keith, I don't think so. One of the issues facing the carbon foam matrix
 > with metal tubes running through it with the fluid is the contact between
 > the carbon foam and the tubes. It is likely to be point contact, (many of
 > them) and the heat has to flow through those contacts to get into the
foam
 > and let it act like the fines. Remember that the fins in a radiator are
 > brazed or welded onto the tubes, giving intimate contact. I don't
know how
 > ORNL made the contact between the foam and the tubes, but I have to
believe
 > that it is more sophisticated than just shoving it through the foam.
 >
 >
 > Bill Schertz
 > KIS Cruiser #4045
 > N343BS (reserved)


I don't have any experience with this either, but I'd guess that you
could braze some of that wrap-around fin material onto the tubes before
pushing them through.  Imagine a bunch of 1" long fins oriented radially
from the tubes--that would add a lot of surface area for contact with
the foam.

One other thing to think about:  Ice.  Maybe the airplane is just
sitting on the ramp and it is raining/freezing:  That carbon matrix will
hold a brick of frozen water that might add a lot of weight.  Will
expanding ice break up the foam?

The idea is interesting and I hope somebody pursues it, but it probably
won't be me.  Experimental airplane, one-off engine setup--I don't think I
want to add another variable to the mix.  But it would make for some
interesting car-top wind tunnel testing.   Paul, have you still got the
rig?  Anybody know where to get some of this carbon foam and what it costs?

Mark Waldron

Liquid cooling systems function by several steps. The heat is transferred from the
engine metal to the fluid coolant. The effectiveness of that transfer is controlled
by a coef., the area of contact and the speed of the coolant flowing over the
metal surface. The higher the speed of the coolant the better. The coolant is then
piped to the heat exchanger and the process is reversed. Heat is transfered from
the coolant to the tubes also controlled by a coef. , the contact area of the tubes
and the speed the coolant flows over the metal.

In this carbon  design that has not changed. The tubes are best shaped as a wide
thin oval to maximize the area of contact between coolant and metal. Therefore you
still need the same number of tubes. Then the heat must be transferred out of the
metal tubes to the air fins. In this case it is a porous carbon foam. If the coefficient
of the transfer is not as good as a brazed aluminum joint there will be no
advantage. As Bill Schertz pointed out this is a series of point contacts.
Obviously not as good as a brazed large area joint.

Perhaps I am not putting this very well. Perhaps Bill Schertz will explain it better.
Frankly I kind doubt this porous carbon foam idea has any real advantage when it comes to liquid cooled engine "radiators".
Paul Lamar

Paul,
You explained it well.  Heat flow is like current flow in a series
electric circuit.  There is a thermal "resistance", defined by the heat
transfer coefficient, at each material interface.  It is going to take a
lot of R&D money to conquer the corrosion problems between carbon foam
and  metal core tubes.  My  guess is, that we will not see wide use of
the carbon foam in a radiator until it can be mated to a high thermal
conductivity plastic core.  I agree with Mark;  we have enough
relatively untested variables to work with already.  I vote to let the
automotive industry develop an all plastic and carbon radiator that
weighs less than a pound.  Then we can test one in an airplane.

Aubrey Thompson

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