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I'm trying to calculate the heat conductivity of a mold-polymer interface. I'm assuming that there's a thin layer of air between the polymer and the mold (gap conductance). According to some literature (Madhusudana, Thermal contact conductance, Springer) the heat transfer coefficient is a function (among other things) of the physical properties of the air in the gap.

I was wondering if I should use a real gas model for my calculations instead of assuming a real gas.

Under which temperature and pressure conditions should I use a real gas model?

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  • $\begingroup$ What makes you so sure that there will be an air layer? Even if a small amount of air were present, wouldn't it be capable of dissolving in the polymer. Do you get an inaccurate answer if you do the calculation assuming no air is present? $\endgroup$ – Chet Miller Aug 24 '18 at 16:46
  • $\begingroup$ How high is "high?" It's accurate enough to use the ideal gas assumption to model the internals of a jet engine, for example. For nitrogen, the difference between the models is negligible until you get to pressures greater than 100 atm, and at 200 atm the difference is still less than 5%. $\endgroup$ – alephzero Aug 24 '18 at 16:48
  • $\begingroup$ @ChesterMiller Several studies make that assumption as well (Polymer–mould interface heat transfer coefficient measurements for polymer processing). The air does not dissolve. Yes you would get a wrong calculation. $\endgroup$ – user3408085 Aug 24 '18 at 16:51
  • $\begingroup$ If you want to determine whether the ideal gas law is OK to use, get a generalized graph of the compressibility factor z as a function of the reduced temperature and reduced pressure. If, for the conditions you are considering, z is close to 1, the ideal gas law is OK to use. There are also generalized plots of thermal conductivity as a function of reduced properties. $\endgroup$ – Chet Miller Aug 24 '18 at 19:58

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