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Moisture modeling -PLEASE HELP!

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August 2, 2007, 06:55 Moisture modeling - PLEASE HELP! #1

CarlosGuest

Posts: n/a

Hello,

Does anybody know how to model moisture within fluent? Iunderstand that the species transport model has to beimplemented but it is not clear how to set up the model.

I am looking at the humidity levels within a livestock trailer. Ihave modeled sheep as large blocks with a surface generatingheat. I now need to set up a humidity model. Does anybodyknow how I would approach this problem? What type ofboundary condition would I need to set? I assume it willsimply be a wall with a base level of wetness but it is notclear to me.

If anybody can help that would be much appreciated. All thebest,

Carlos.

August 3, 2007, 21:04 Re: Moisture modeling - PLEASE HELP! #2

AAAGuest

Posts: n/a

Hi

Use the Species model and define the properties of the"mixture species" in the materials window to have H2O andAir (H2O must be the top one). Choose "volume-wheighted"for density and "mass-weighted" for thermal conductivity andviscousity. Then specify the moisture mass fraction for inletsto the domain (this is the humidity ratio in the psycrometricchart). Specify a "constant" mass fraction value for the bodyof the sheep.


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CarlosGuest

Posts: n/a

Thanks for your help AAA, this method appears to work fine -much appreciated. Just wondered if you had any idea oftypical mass fractions of vapour for my boundary conditions?

When postprocessing I find that contour plots of relativehumidity show values of 1000's of %! Clearly the humiditycannot exceed 100% at any given temperature. If you haveany ideas please let me know.

Best regards,

Carlos.


AAAGuest

Posts: n/a

Hi

Relative humidity must never exeed 100%.

Have you heard about a Psychrometric chart? This chart isusually used by HVAC engineers (please find one) to findproperties of air such as humidity ratio (mass fraction) byknowing the temperature and relative humidity, for example,among others. This will enable you to specify the massfraction at the inlet and also predict the relative humidity atthe outlet.

Now if you don't know any HVAC enngineers, kindly send methe temperature and relative humidity you are using for theinlet boundary and I will give you the values for the massfraction. Still, if you can find some one to show you the chartand explain it to you, this will be better.

Kind Regards


CarlosGuest

Posts: n/a

Hi AAA,

Thats great, thanks for your advice. I think I may be havingproblems because I am specifying the humidity on the inletAND the outlet, maybe I should leave the outlet alone thistime! Your help is much appreciated.

Thanks



Carlos.


AAAGuest

Posts: n/a

Hi

You still have to specify a "practical" mass fraction formoisture at the inlet to get a reasonable relative humidity forthe domain. This is done by knowing the temperature andrelative humidity at the inlet and finding the correspondingmass fraction from a Psychrometric chart. Otherwise, youmight cross the 100% relative humidity line again.

Regards


CarlosGuest

Posts: n/a

Thats brilliant!

Thanks,

Carlos.

October 25, 2009, 12:36 #8

deniseNew Member

Join Date: Oct 2009Posts: 13Rep Power: 3

Dear AAA,i am trying to solve humidity transfer in enclosure, everywhere isolated except for ceiling..ceiling is cooled..but inFluent i can see temperature gradient but there is no massfraction of H2O gradient..it is almost same...i am trying towrite UDF but, what is the source term? there is nogeneration of humidity in my room..how to include naturalconvection using bossinique model in simple species transportwithout reaction phenomenon.?please help me..thank you



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