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Old   March 21, 2007, 16:54
Default buyoancy and operating density
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This is a question on the use of the operating density. I have a buyoancy driven flow in an open chemney with heated walls. I always used the incompressible ideal gas density for the air and an operating density equal to the volume average of the density over the whole chemney. However, I just tried to unselect the operating density option and instead of going up, the major part of the flow goes down. The result is therefore very different. Could anyone please explain me which option I should choose ?

Thank you !

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Old   March 21, 2007, 23:04
Default Re: buyoancy and operating density
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As a matter of realistic case, in this scenario, the flow should go upwards to the atmosphere unless there is recirculation region which generally happens in confined regions with natural convection ( Ref: covective heat transfer, kays & crawford).

I suggest u try this option: define gravity in the negetive Z (if Z is along lenght of chimney and ur chimney c/s is parpendicular to XY plane) as -9.81 and try both select/unselect operating density. the resulting flow shoule be upwards if there is no reciculation or u should find a recirculation region between the chimney walls.

Hope this helps, Regards Rizwan
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Old   March 24, 2007, 08:22
Default Re: buyoancy and operating density
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I used to have the same probleme. With Incompressible Ideal Gas, if you don"t specifie operating denstity, fluent used the mean value computed on the whole domain. The operating density for non isotherm problem, is then less than the value of the air at ambiant temperature. So fluent computes a buoancy even if the fluid is at ambiant température beacause the difference between density at ambiant temperature and operating density is not null. Yoou have to specifie the operating density at the value at inlet or ambiant temperature.
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