Assessing degree of convection under near-vacuum
Assessing degree of convection under near-vacuum
(OP)
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/viewth read.cfm?q id=231539& amp;page=1
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





RE: Assessing degree of convection under near-vacuum
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RE: Assessing degree of convection under near-vacuum
I presume you have looked at Nusselt, Grashof and Prandtl numbers.
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RE: Assessing degree of convection under near-vacuum
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...
RE: Assessing degree of convection under near-vacuum
They're unitless numbers.
Patricia Lougheed
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RE: Assessing degree of convection under near-vacuum
It's this uncertainty plus just overall inability to check my work that is inhibiting me from re-working the math myself.
RE: Assessing degree of convection under near-vacuum
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
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RE: Assessing degree of convection under near-vacuum
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
RE: Assessing degree of convection under near-vacuum
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.