Thermal conductivity of a "vacuum"
Thermal conductivity of a "vacuum"
(OP)
How would I calculate the thermal conductivity, k, of a "vacuum", meaning obviously a "thickness" of air that is not at atmospheric pressure? Would I simply proportion down the "k" of air? I other words, if my "vacuum" was 1/15th of an atmosphere, would I say that k=.025 (w/m*k) / 15 = .00166 (w/m*k)?
Plastics Industry





RE: Thermal conductivity of a "vacuum"
RE: Thermal conductivity of a "vacuum"
The thermal conductivity of gases doesn't change much with pressure, but they do with temperature, which affects molecular motion. It varies (and so does sound velocity) with the square root of the absolute temperature.
I suggest you pay a visit to the NIST chemistry webbook looking for nitrogen, for example, and may find that the thermal conductivity at, say, 25 deg C doesn't change much between 1 atm abs and a high vacuum.
RE: Thermal conductivity of a "vacuum"
RE: Thermal conductivity of a "vacuum"
I'm pretty sure a Thermos worke quite well, so well in fact that I've burned my lips with them a few times not thinking coffee would still be hot 8 hours later...
Plastics Industry
RE: Thermal conductivity of a "vacuum"
RE: Thermal conductivity of a "vacuum"
The comments about independence of pressure only applies for a limited range. Obviously, there can be no conduction if there's no gas, so it can't be completely independent over the entire span of pressures.
Read:
http://www.thermco.com/ss119.asp and
http://ww
At sufficiently low pressure, thermal conductivity is directly affected by pressure, otherwise, there are a lot of vacuum pressure meters that shouldn't be working. The pressure stated in the OP is in-between, so there might be some amount of reduction in thermal conductivity.
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: Thermal conductivity of a "vacuum"
I guess the only thing the fiberglass does is disrupt convection?
Plastics Industry
RE: Thermal conductivity of a "vacuum"
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: Thermal conductivity of a "vacuum"
Plastics Industry
RE: Thermal conductivity of a "vacuum"
That's why foamed insulators are so effective, the cells keep the air from circulating.
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: Thermal conductivity of a "vacuum"
Other gas thermophysical properties besides thermal conductivity, k, not much affected by pressure are: absolute viscosity μ, specific heat capacity cp and, as a result also the Prandtl number which is cp.μ/k.
RE: Thermal conductivity of a "vacuum"
Plastics Industry
RE: Thermal conductivity of a "vacuum"
f
http://fluidproperties.nist.gov/thermal.html
I suggest you do some searching there.
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: Thermal conductivity of a "vacuum"
Thanks.
-Plasmech
Mechanical Engineer, Plastics Industry
RE: Thermal conductivity of a "vacuum"
In fact, at that pressure, you should already have a thermocouple gauge that's doing the calculation of pressure from the thermal conductivity of whatever's left in the chamber.
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: Thermal conductivity of a "vacuum"
Visit, among others:
http://www.belljar.net/tcgauge.htm
RE: Thermal conductivity of a "vacuum"
NIST has a calculator for nitrogen:
http:
Hopefully, that comes out correct. The second to the last column is thermal conductivity, which is pretty close to what you had originally proposed.
If the link doesn't work, the values were:
273 0.023989
274 0.024059
275 0.024128
276 0.024197
277 0.024266
278 0.024335
279 0.024404
280 0.024473
281 0.024542
282 0.02461
283 0.024679
284 0.024747
285 0.024815
286 0.024883
287 0.024952
288 0.02502
289 0.025088
290 0.025155
291 0.025223
292 0.025291
293 0.025358
294 0.025426
295 0.025493
296 0.025561
297 0.025628
298 0.025695
299 0.025762
300 0.025829
301 0.025896
302 0.025963
303 0.026029
The pressure wound up at 0.0047 MPa
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: Thermal conductivity of a "vacuum"
I think.
-Plasmech
Mechanical Engineer, Plastics Industry
RE: Thermal conductivity of a "vacuum"
A 20-mm gap will will just get it up to 1 W/m^2-ºC, based on the data above.
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: Thermal conductivity of a "vacuum"
-Plasmech
Mechanical Engineer, Plastics Industry
RE: Thermal conductivity of a "vacuum"
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: Thermal conductivity of a "vacuum"
-Plasmech
Mechanical Engineer, Plastics Industry
RE: Thermal conductivity of a "vacuum"
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: Thermal conductivity of a "vacuum"
do you know why thermal conducticity of a gas does not decrease proportionally with pressure as one (like me) would intuitively think it would?
-Plasmech
Mechanical Engineer, Plastics Industry
RE: Thermal conductivity of a "vacuum"
Grosso modo explanation: from the kinetic theory of gases the thermal conductivity k of a gas at a given temperature is proportional to n.v.λ
where n = number of molecules per unit volume, indeed directly related to pressure
v = mean velocity of molecules
λ = mean free path between collisions proportional to 1/n
Thus, the "density" factor vanishes and k is independent of the pressure of the gas.
RE: Thermal conductivity of a "vacuum"
Are you stating that k=(n)*(n)*(lambda) ?
and that lambda = 1/n?
wouldn't the "n's" cancel out?
-Plasmech
Mechanical Engineer, Plastics Industry
RE: Thermal conductivity of a "vacuum"
k = (somefactor)(n)(v)(lambda)
and that lambda = (someotherfactor)/n
so k = (andnow4somethingcompletelydifferent)v
..and I'd better quit writing equations now.
RE: Thermal conductivity of a "vacuum"
-Plasmech
Mechanical Engineer, Plastics Industry
RE: Thermal conductivity of a "vacuum"
RE: Thermal conductivity of a "vacuum"
"Come, friend Watson, the curtain rings up for the last act" Sherlock Holmes.
The same kinetic theory of ideal gases tells us that vaverage = factor * (T0.5) for any particular (ideal) gas with T being absolute temperature.
n, in fact a "density" depending on pressure, cancels out as btrueblood pointed out leaving
independent of pressure.
It has been claimed that with pressures down to 0.1 torr random collisions indeed follow the above statements.
At higher vacuum levels when the mean free path length becomes of the same order of magnitude as the distance between the walls, intermolecular collisions are less important and ballistic trajectories become preeminent, with pressures again affecting the conductivity.
Nothing clears up a case so much as stating it to another person Sherlock Holmes.
RE: Thermal conductivity of a "vacuum"
-Plasmech
Mechanical Engineer, Plastics Industry