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Air thermal conductivity issue

Question asked by Domenic Santoianni on Sep 27, 2008
Latest reply on Oct 11, 2008 by Ian Hogg
I have a project which consists of 2 tubes (tube A and Tube b) of equal lengths. Tube B is smaller than Tube A and is placed inside Tube A concentrically. Cryogenic liquid flows through the inner tube (Tube B). The space between The outer wall of Tube B and Inner wall of Tube A is vacuum (Air at low pressure) with no flow and at room temp initially. Lids are placed at both ends and they are set to insulation material. I put a heat load (heat transfer coefficient) on the outer wall of the outer tube A.

I put boundary conditions on the inlet & outlet of the inner tube B that has cryo fluid flowing through it. And The outer wall bounder condition is set on Tube A.

The model solves nicely with no warnings BUT the vacuum air does not seem to insulate as it should. I am measuring the average surface temperature of the outer tube A as a goal. I change the air pressure between the 2 shafts but I get the same result. No matter what pressure I'm putting for the air, the outer shaft temperature is the same.

what can I do to troubleshoot this problem?

thanks in advance