Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx> At 05:22 AM 1/21/2006, you wrote:
Original poster: "Barton B. Anderson" <bartb@xxxxxxxxxxxxxxxx> Hi Jim,Surface area is certainly important for cooling, but mass is also a major factor. A tube has little mass and as a result, transfers heat very quickly (heats up quickly, cools quickly). It also has the added surface area inside the tube for heat dissipation. A solid round stock will heat much slower and only has the outer surface to dissipate heat, but, the mass of the object has more "mass" to transfer within itself. This lowers the overall heat the object will need to dissipate and will require less cooling on the outer surface.
Until the solid bar reaches equilibrium. Then, you've got half the radiating/conduction surface to get heat out of (assuming air can move through the inside of the tube).
Of course, something else to think about is that in either case, you're nowhere near melting the copper. If, at steady state, the temperature (relative to the air) is double, you'll reject twice as much heat to the air. Since we're talking about a few hundred degrees here, that's pretty likely.
I have to say, I just favor the tubes because they are cheaper and use less material.
There probably is a minimum wall thickness that will work well. At some point, the wall gets too thin to carry the heat away from the point where the spark is to somewhere else.
Optimizing it is another one of those "solving Poisson's equation" problems. Another finite element analysis problem using diffusion of heat instead of charge. There's probably an analytical solution from M. Fourier of transform fame. The problem's not much different from heat dissipation in cannon barrels.