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How to model moisture within Fluent?

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Old   August 2, 2007, 05:55
Default How to model moisture within Fluent?
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Carlos
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Hello,

Does anybody know how to model moisture within fluent? I understand that the species transport model has to be implemented but it is not clear how to set up the model.

I am looking at the humidity levels within a livestock trailer. I have modeled sheep as large blocks with a surface generating heat. I now need to set up a humidity model. Does anybody know how I would approach this problem? What type of boundary condition would I need to set? I assume it will simply be a wall with a base level of wetness but it is not clear to me.

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

Carlos.
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Old   August 3, 2007, 20:04
Default re: How to model moisture within Fluent?
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AAA
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Hi

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

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Old   August 7, 2007, 08:23
Default re: How to model moisture within Fluent?
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Carlos
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Thanks for your help AAA, this method appears to work fine - much appreciated. Just wondered if you had any idea of typical mass fractions of vapour for my boundary conditions?

When postprocessing I find that contour plots of relative humidity show values of 1000's of %! Clearly the humidity cannot exceed 100% at any given temperature. If you have any ideas please let me know.

Best regards,

Carlos.
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Old   August 7, 2007, 17:03
Default re: How to model moisture within Fluent?
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AAA
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Hi

Relative humidity must never exeed 100%.

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

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

Kind Regards

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Old   August 7, 2007, 18:26
Default re: How to model moisture within Fluent?
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Carlos
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Hi AAA,

Thats great, thanks for your advice. I think I may be having problems because I am specifying the humidity on the inlet AND the outlet, maybe I should leave the outlet alone this time! Your help is much appreciated.

Thanks

Carlos.
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Old   August 7, 2007, 19:03
Default re: How to model moisture within Fluent?
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AAA
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Hi

You still have to specify a "practical" mass fraction for moisture at the inlet to get a reasonable relative humidity for the domain. This is done by knowing the temperature and relative humidity at the inlet and finding the corresponding mass fraction from a Psychrometric chart. Otherwise, you might cross the 100% relative humidity line again.

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Old   August 7, 2007, 20:15
Default re: How to model moisture within Fluent?
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Carlos
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Thats brilliant!

Thanks,

Carlos.
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Old   October 25, 2009, 11:36
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Dear AAA,
i am trying to solve humidity transfer in enclosure, every where isolated except for ceiling..ceiling is cooled..but in Fluent i can see temperature gradient but there is no mass fraction of H2O gradient..it is almost same...i am trying to write UDF but, what is the source term? there is no generation of humidity in my room..how to include natural convection using bossinique model in simple species transport without reaction phenomenon.?please help me..
thank you


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Old   October 2, 2015, 06:31
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Astio Lamar
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Sorry to bring this old topic up!

I f I have 20 oc for temperature, atmospheric pressure and 40% humidity, then from the Psychrometric chart I have the ration of 0.006. Should I specify this value for the H2o mass fraction in the inlet boundary condition?
thanks.
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Old   October 2, 2015, 08:38
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Quote:
Originally Posted by asal View Post
Sorry to bring this old topic up!

I f I have 20 oc for temperature, atmospheric pressure and 40% humidity, then from the Psychrometric chart I have the ration of 0.006. Should I specify this value for the H2o mass fraction in the inlet boundary condition?
thanks.
That is correct! Just make sure that's the dry bulb temperature and relative humidity.
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Old   October 4, 2015, 09:13
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Hello and thanks for your swift reply.
I have simulate the humidity based on H2o mass fraction calculated from Psychrometric chart. the question now is how to post process the the result in CFD-post based. I try different method, but I cannot show the humidity for example at plane.
Thanks in advance for your respond.
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Old   October 4, 2015, 14:47
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What is your problem exactly? Are you unable to plot anything or are your missing "humidity" as a plottable variable? For example can you plot temperature on a plane?

If you need to calculate humidity:

The relative humidity is given by the partial pressure of water to the saturated vapor pressure at the same (dry bulb) temperature.

You can convert your mess fractions into mole fractions and calculate the partial pressure of water and then calculate the partial pressure at the same temperature.
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Old   November 3, 2015, 01:19
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KEER AKSHAY TUSHAR
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what is mass fraction?
is it humidity ratio?
what is the formula for mass fraction?
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Old   November 9, 2015, 00:35
Default Air condensation over the Wings
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M Mohsin Iqbal
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Hi
Sir i am carrying research on Air condensation over the wings.
I am having problem for making settings in the Fluent.
Kindly guide me how can i give prescribed humidity (say 65-70 %) to the computational domain.
Thank You
Looking Forward.
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Old   November 9, 2015, 00:51
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Quote:
Originally Posted by M MOHSIN View Post
Hi
Sir i am carrying research on Air condensation over the wings.
I am having problem for making settings in the Fluent.
Kindly guide me how can i give prescribed humidity (say 65-70 %) to the computational domain.
Thank You
Looking Forward.
Humidity is handled by using the species transport model and adding water vapor as the new species. You need to specify mass fractions. Note the earlier discussion on how to do this.

For water condensation, you need to do a multi-phase simulation.
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Old   November 30, 2015, 03:28
Default Air condensation over the Wings
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Quote:
Originally Posted by LuckyTran View Post
Humidity is handled by using the species transport model and adding water vapor as the new species. You need to specify mass fractions. Note the earlier discussion on how to do this.

For water condensation, you need to do a multi-phase simulation.

sir and simulation will be steady or transient ??????
i want to capture the fog cloud over the wings caused due to the condensation of humidity .
looking forward
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