Is a compressed fluid better at heat transfer

[walkersea]

Is a compressed fluid better at transferring heat?

As an example, a metal bottle container with a central internal heat source filled with air at atmospheric pressure. The heat source surface temperature increases (imagine it is a PSU that is generating heat as a by-product of electricity being passed through it). The air in the bottle is heated over time, and the bottle tube temperature is measured at some temperature higher than the ambient air outside the bottle.

If dry compressed air (or nitrogen?) was used to raise the internal pressure in the bottle to say 50psi (3.5bar), the bottle sealed and then the heat source activated, would the time taken for the tube to reach the same temperature as previously be reduced - i.e. is the compressed air better at transferring the heat?

Thanks.

 

[SirLotsaWatts]

If you're trying to cool the source, then the denser the fluid the better. A vacuum is the worst (radiation only), and a liquid is the best, right?

But if you're trying to heat the bottle, then a denser (higher pressure) fluid might slow that process down by absorbing heat itself.

Finally, I imagine there might be a little finesse with natural convection: the heat transfer from the source might not monotonically increase with gas pressure since you might enter different regimes or circulation patterns.

 

[IRstuff]

If you're only counting on natural convection, then the simpler answer is move the air more.

You can more than 5 times the natural convection if you get the air moving at 10 m/s

TTFN

FAQ731-376

 

[IRstuff]

Also, bear in mind that compressing the air is going to make it warmer, in addition to the compressor adding to the general system heat load.

TTFN

FAQ731-376

 

[walkersea]

Thanks, guys.

I'm applying this to the cooling of a PSU board in a subsea bottle. We are experimenting with fans and now filling with nitrogen to transfer the heat from the source to the endcaps and bottle to then radiate away (it operates in seawater).

Direct heat transfer is tricky because the PSU is mounted on a shock isolated chassis.

Regards, Greg