High-Temperature Problems

[markfh11q]

Greetings. I'm just hailing from SpudTech real quick, because I've got a question or two concerning very high temperatures.

I've been thinking over the designs for an electrothermal gun for a little while now. The basic principle is this:

A sample of steel wool of known mass is connected to two electrodes inside a combustion chamber. In this case, electrode #1 will be a stainless spring. Electrode #2 will be the walls of the chamber, (insulated from #1 by epoxy).

The chamber is sealed off with a burst disk and filled with compressed air until enough oxygen is present to completely oxidize the steel wool. I still haven't done the stoichiometry to see if I'll need to enrich the atmosphere in the chamber as far as O₂ concentration, but I'm gonna get to that when I have the time, so don't bother, (I'm not one of those people who goes "how muhc psi do i need for a pad of steehl wul?" ;) )

The problem I have is not with the overall design, it's just with electrode #2, (i.e. the inside of the chamber). It will be made out of steel pipe, so corrosion from the violent reaction will eventually make it less conductive. May mean more heat, but I'm more interested in a quicker reaction, not heat.

Does anybody know of some sort of conductive epoxy-type coating to paint the inside of the chamber with? All I know of is epoxy with some aluminum powder in it, but I've had problems with coatings like this chipping before. I guess I could take the plunge and buy some high-temp stainless tubing and weld some fittings on there for the compressed air and union, but that would just be ridiculous.

Anybody have any ideas?

[LibertyOrDeath]

I don't know much about spudguns or what would be the best conductive epoxy for what you're doing, but there seems to be a wide variety of such epoxies out there. Just searching on Yahoo for "conductive epoxy" should turn up a lot.

How about something like this, for example? http://www.mgchemicals.com/products/8331.html

[megalomania]

Perhaps you could insert a sacrificial sleeve of metal inside the steel tube to be replaced as needed. You could use something as simple as aluminum foil, or something more durable like a thin sheet of copper, tin, aluminum, brass, or steel sheet.

[markfh11q]

LibertyOrDeath, the maximum temp. on that epoxy is rated at 194 F. The temperature inside of this thing will exceed that by a huge margin. The adiabatic values would be extremely high, (rough mental calculation), and the reaction is quick enough that the real-world values won't very much.

Megalomania, I guess it's partly my fault for not explaining in the above post, but the reaction is going to be initiated by dumping one or two drum capacitors through the electrodes. If I could find a sleeve that fit well enough, it'd be a good idea, but it arc otherwise. You'd need a pretty thick sleeve, (high mass), as well to prevent heat transfer to the walls of the chamber, which would cause oxidation as well.

If I can find a high temperature conductive coating I'll use it, but otherwise I might just have to order a 316 stainless steel nipple from McMaster or some such.

[LibertyOrDeath]

Oops, you're right about the 194 F -- I was focusing on the electrical conductivity and overlooked that figure. Yeah, that's a wee bit low for a combustion chamber, even though it probably refers to continuous operating temperature rather than fairly quick flashes.

If you can't find a suitable conductive coating, then I suspect that something along the lines of what mega said might be your best bet. If a protective metal sleeve is inserted into the steel tube, are you sure there'd be arcing if there's sufficient electrical contact between the sleeve and the tube? I haven't played around with this sort of high-voltage setup, but I would think that arcing would only happen if there wasn't a good conductive path between the two tubes.

As for the oxidation of the steel, what about coating it with a conductive epoxy first -- even if that epoxy isn't rated high enough to be exposed directly in the chamber -- and then using the protective liner on top of the epoxy? That might keep the temperature of the epoxy within reason while helping limit oxidation or other damage to the steel tube. (I'm not suggesting that the liner be glued in there, of course, but letting the epoxy cure first and then inserting the sacrificial liner.) So in other words, some kind of multi-layer solution might do the trick.

Sorry if any of this is ignorant, but like I said, I have no experience with making or even using spudguns. Nevertheless, the problem you pose is interesting, so I thought I'd give it a shot.

[markfh11q]

Not at all. I'm sort of out on a limb with this extreme temperatures as well. Max I've dealt with is absolutely no more than around 2200 F for a split second, (propane in air), in a combustion chamber, and that rusts steel, (at least in my hybrid cannon). In that one, I don't have to have a conductive chamber, though.

I've looked at the 316 stainless and it's reasonable, (around 7 bucks for a 3/4" by 2" nipple). I can then just use normal black steel fittings after that.

Thanks for the suggestions.

[wolfy9005]

http://powerlabs.org/electrothermal.htm

would this help?