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David_010 January 20, 2011 06:21

Energy balance error with high prandtl numbers.
Hi to all,

I m using the buoyantBoussinesqSimpleFoam (with g=[0 0 0]) solver for a laminar flow inside a duct with constant heat flux at the walls (basically a fixedGradient b.c.). Inside the duct there is a moving device (moving walls).

When I use a low Prandtl number (like with air properties, Pr=0.7) the temperature field seems to be correct, and the energy balance between inlet, outlet and heat flux at the wall (q"*Area=m*cp(To-Ti)) is accomplished. But when I change the properties to a Pr=256 (corresponding to a propylene glycol) the temperature is higher than expected (specially near the heated walls) and therefore the energy balance is wrong.

What could be the problem? The high Pr number leads to a very low thermal diffusivity (DT), and the Laplacian therm could be negligible. I have tried to increase de DT value keeping the Pr constant, but I get the same results.

Thank you very much in advance for your help!


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