> 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

Paul, and Group,
If you read the linked paper near the start of the thread it appears
that they have already made radiators. The radiators produced using
a realitively conventional approach using a wide flat sided tube in direct
contact, (built up), and a tube pushed through a modified block of the
material. Both styles performed better than the same size aluminum fin
radiators. The results were noteable, better than 10%. That is a BIG
difference, so I have to guess that the cost or the toughness isn't as
good or someone would be manufacturing them already!
Bill Jepson


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