Assessing degree of convection under near-vacuum

[blueandwhiteg3]

I am trying to work out how significant a factor natural convection is under near-vacuum conditions, assuming normal, somewhat moist air.

I am looking at using a vacuum for insulation purposes. Essentially, imagine a thick panel (20-100 cm) with a vacuum space inside for insulating purposes, keeping one side of the panel hot, the other side cold. Air is sufficiently insulating for my needs, but convection is what screws everything up. No, vacuum insulated panels are not suitable nor are foams; I require free, open space for my needs.

I can get pressures down to about 0.012 mbar practically, so I want to assess the degree to which convection is a factor between 0.012 mbar and 0.1 mbar.

At those pressures, the mean free path of air isn't so great that I can be confident that an air space of say 20, 50, 100 cm won't experience convection. However, given the much reduced air mass at these pressures and relatively significant mean free path length, I imagine that the potential for convective thermal conduction

I've tried several times to work this out without any luck. Just getting a general assessment as to the degree which convection is an issue would help a huge amount and could indicate whether this is worth exploring further or not. It would be even better if I could model potential designs and work out thermal conductivity - but that probably requires some serious math, or even fluid dynamics software.

Any suggestions on this issue would be appreciated!

And yes, this is something of a cross-post from another section of this forum. It's just that I am increasingly seeing this as a fluid dynamics problem more than a thermal conduction problem. The other thread is here:

http://www.eng-tips.com/viewthread.cfm?qid=231539&page=1

 

[IRstuff]

Obviously, you're welcome to try your luck in different forums. Nonetheless, given that this forum is "Piping & fluid..." the majority of users here are more into liquids.

TTFN

FAQ731-376

 

[BigInch]

Gas is fluid.

I presume you have looked at Nusselt, Grashof and Prandtl numbers.

**********************
"Pumping systems account for nearly 20% of the world’s energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[blueandwhiteg3]

Gas is a fluid, hence my inquiries here. I do hope I don't offend by sort of cross-posting. Some forums frown on that more heavily than others.

My problem is re-working the Nusselt, Grashof and Prandtl numbers at extremely low pressures. I see the formulas. I don't know the units. And even if I had a good idea, I don't have anything solid to check my work against. Hence, even if I solved it, I'd have no real confidence I solved correctly.

I'd love it if anybody wanted to point me towards a specific resource for solving these and checking, or would assist in calculating them out or whatever.

It just seems odd that this question isn't really clearly answered anywhere online, as far as I can tell...

 

[vpl]

My problem is re-working the Nusselt, Grashof and Prandtl numbers at extremely low pressures. I see the formulas. I don't know the units.

They're unitless numbers.

Patricia Lougheed

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[blueandwhiteg3]

Patricia, I grasp that they are unitless numbers. But the numbers that are input into the equation to create the unitness number DO have units - e.g. pressure, temperature, etc.

It's this uncertainty plus just overall inability to check my work that is inhibiting me from re-working the math myself.

 

[vpl]

blueandwhiteg3

Let's bring this back over to the Thermo forum; I'll copy my post over there and provide you the units for the various inputs.

Patricia Lougheed

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[HamishMcTavish]

Just make sure that you keep the dimensions of the parameters consistent when generating non-dimensional numbers and you should be fine

Try looking at section 1 and section 4 of the Handbook of Heat Transfer by Rohsenow, Hartnett and Cho. The former has quite reasonable explanations of the meanings of the dimensionless numbers relevant to natural convection (Prandtl and Raleigh numbers). Section 4 deals (in part) with natural convection in enclosures which is probably most relevant to what you're doing.

Still not sure what you're trying to do but hopefully that should point you in the right direction.

Regards, HM

No more things should be presumed to exist than are absolutely necessary - William of Occam

 

[blueandwhiteg3]

Patricia, I'm looking forward to your post in the main thread. Thank you for you effort. It is really appreciated. I've spent so much time trying to work out the answer to this particular issue.

As for the issues... I get what the numbers mean, well, at least I think I understand them enough for for my purposes. I just don't get how to re-calculate them at the pressures I desire.