Hi,

in general, text books say that the thermal conductivity is independent of pressure (for gases at low density). In which case does pressure have an influence on the thermal conductivity, e.g. of air? Many thanks in advance, Max

- posted
18 years ago

Hi,

in general, text books say that the thermal conductivity is independent of pressure (for gases at low density). In which case does pressure have an influence on the thermal conductivity, e.g. of air? Many thanks in advance, Max

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- Vote on answer
- posted
18 years ago

Thermal conductivity of a gas is pretty much independent of pressure until you get to very low pressures. The question is how you define "low pressure". That is related to the distance between the surfaces you are interested in--if that distance is comparable to (or smaller than) the "mean free path" of the molecules, the pressure is low.

Heat transfer by natural convection between two surfaces depends on more than just conductivity, though. While conductivity is not a function of pressure, the ability of the gas to transfer heat IS a function of pressure. Actually, the only one of the significant properties that is affected by pressure is density, but that's enough--the more mass of gas available, the more heat can be transferred.

-Paul

- Vote on answer
- posted
18 years ago

Somebody else already mentioned the effect at ***extremely*** low densities where continuum assumptions no longer apply.

There is always ***some*** effect of increased pressure/density, it is just very small at low densities (say, near atmospheric pressure). You can get an idea of the magnitude of the effect by going to:

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choosing "Thermophysical Properties of Fluid Systems". You can choose a fluid (maybe nitrogen if your interest is air) and calculate an isotherm as a function of pressure. For example, I get that, for N2 at 300 K, going from atmospheric pressure to about 50 times atmospheric pressure the Th. Cond. increases from about 0.26 to 0.28 W/m*K.
Dr. Allan H. Harvey, Boulder, CO, steamdoc at aol dot com Insert usual disclaimers here ...

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