Ambient temperature for radiation?

DaveStupple

Hello all,

Probably one for Victor, but thoughts from anyone welcome.

How does the ambient temperature affect a radiation constraint? To the best of my limited understanding, radiation flux is proportional to T^4 and E (T = temperature of the surface, E = emissivity).

In my model it made little difference what ambient temperature I chose, but that is probably because there is a forced convection that swamps any radiation effect. As the surface would be in vacuum, I guess 0 K is the correct ambient.

I will probably leave off the radiation altogether, but I thought I should try it.

Dave

Victor

Yep, it's proportional to those things but Mecway also includes incident radiation from the ambient if you specify an ambient temperature. Normal heat flux out of the surface is

Q = σE(T^4 - T0^4)

T = temperature of the surface

T0 = ambient temperature

σ = Stefan-Boltzmann constant

E = emissivity

If you use T0=0 K then it's like radiating into outer space or other zero temperature environment.

σ is tiny which might help to hide the effect, as well as the power of 4 of course.

DaveStupple

OK, I can see why the effect could be so small. In another situation, where radiation might be significant, how would you approach the ambient temperature for an object in a vacuum chamber? Is there just a relatively constant background radiation from all the objects and walls, and this is no different whether in air or in vacuum?

Victor

I'm just studying this more now so beware I'm not too sure of myself...

I think you can ignore air over short distances since it doesn't absorb much radiation at typical thermal radiation wavelengths.

In a vacuum chamber where there's radiation going in both directions between the object inside and the interior walls, you have to modify the emissivity value in Mecway to be a function of the emissivities of both surfaces, as in eqn 19.3 here  I suppose that's only if the object is big enough that it's surface is effectively parallel to the interior walls. If the object is much smaller than the chamber, I imagine the chamber would act more like a blackbody cavity since a lot of radiation it emits would end up being absorbed by itself.

That's if the chamber walls are at a uniform temperature and all have the same emissivity. Some FEA software can model radiation between more than one arbitrary surface at different temperatures and emissivities but Mecway currently only considers one surface and assumes the other one is a blackbody uniformly at the ambient temperature.

DaveStupple

Thanks for taking the time, Victor. I will have to think about this and look at the link in more detail.