Will painting a tube black cool it down?

[pkc111]

TL;DR Summary

I am building a Ruben's tube from aluminum tubing (100 mm diameter, 1.2 m long). I expect it to get very hot (say >200C), so for safety in a classroom I want it to be as cool as possible. I would like it to run for about 20 minutes on natural gas. Its raw alumium surface now (grey/silver), should I leave it or paint it black.

My thinking is that a paint coating will slow down heat loss due to an insulation effect, but at the same time speed up radiative heat loss from the outside black paint surface. I am unsure though which effect may be the greatest and therefore the net effect on tube temperature. Many thanks

 

[Baluncore]

If the tube is horizontal, the flames will induce a rising air flow around the outside of the tube. That will cool the tube. I expect little heat from the flame would heat the tube.

Monitor the temperature and see how it goes. The speed of sound in the tube will be a function of temperature and the molecular weight of the fuel gas.

 

[pkc111]

Ive built a prototype with a 50 mm diameter tube horizontal, and it gets super hot after about 5 or so minutes. I am not sure why. I am guessing the its mainly radiative from the flame bases down onto the metal. There are about 70 x 1 inch flames in a row.

 

[Baluncore]

Unless you can add fins to the tube, to increase the heat exchange area, you must find a coating or a surface treatment that will increase the IR radiation from the tube to the air. At the same time you must reduce the energy radiated by the flame from reaching the tube through the new surface treatment.

Maybe you could consider something like a reflective stainless steel shim washer, attached somehow to the tube at the base of each flame.

Heat conduction is significantly less through stainless steel than through aluminium. It is also less through thin sheet materials. If you could use a very thin walled tube you would greatly reduce both heat conduction and storage. Consider a thin wall stainless steel tube that would be hot only close to the flames. Drill the holes with a carbide drill. Polish the tube only close to the holes to reflect incident radiation from the flame.

Look for thin walled, 2" = 50mm stainless steel exhaust tube.

The problem with the choice of a surface coating will be the temperature. For low temperatures you could have used a white PVC paint that reflects light, but transmits IR.

https://www.finishing.com/270/37.shtml

P.S. Here is a supplier of very thin wall stainless steel tube.
https://www.materials.sandvik/en-au...ts/precision-tubes/thin-wall-precision-tubes/

 

[pkc111]

Thank you so much Baluncore that all makes sense!
Unfortunately I have already bought the Aluminium tube 100mm diameter so I will have to make the best of it.
I think I might go polished top near holes and then black matt painted body.