[surya.madugula]

If we want to specify the humid air properties in a multi phase flow problem both the phases being air one of which being humid air. How can this be done?....any suggestions on this?

like in the case of mach number we specify indirectly varying the velocity value..in the same way can any thing of this sort can be done.

[me3840]

'humid air' isn't really a phase is it? You'd just have air and water vapor.

What do you mean by properties? There are methods of calculating mass/volume fractions based on %humidity.

[ijas]

you have to calculate weighted average properties.. means first of all you have to calculate mass fraction of air and water vapour.. by using that you can calculate mixture properties ... e.g density of mixture = (density of air ) x (mass fraction of Air) + (density of water vapor) x (mass fraction of water vapor)

[surya.madugula]

me3840
Yes it can be a phase also. If into a system dry air and humid air are sent and have a two way coupling between them then it can be treated as a phase. That is what i meant by phase.

The representation of humid air in terms of density is what i thought of earlier based on the % oh humidity as you said but that will not be a proper assumption because it is not only dependent of % of humidity but also temperature and mean sea level.

Ijas

The way you have explained must work by using VOF method. will give a try in that way and get back.

[ijas]

yes..but i did not say you should calculate with percentage of humidity... you have to see the sp.humidity.. then fraction of water vapour will be=[w/(1 + w)] (w - sp.humidity).
mass fraction is different from R.H
...

[surya.madugula]

humid air as a phase is possible is guess. but if we want to specify a region/boundary with humid conditions for say in terms of RH % in that region. Can it be done in Star CCM or an other tool?

[me3840]

The weighted averages of properties will automatically be calculated for the continuum if you use the multi component gas model.

The only method I could see for computing relative humidity if you want to have a 'humid air' component is through field functions. You can easily make the air properties depend on a field function that takes % RH as input.

[surya.madugula]

Yes that seems to be the only way to define. Will get back and let you know whats happening.

Thanks for your help
cheers!!

[yorelchr]

Hello,

I know that this post is not new, but I encounter the same problem, so...
I have to run simulation with the following conditions:

- first inlet : air at 293.15K and 30% of humidity
- second inlet and ambient: air at 293.15K and 50% of humidity.

Is it ok if I use the multicomponent gas model? H2O and Air.

So for defining the air continuum I put the mass fraction of H2O and the one of Air at ambient conditions, and at the different inlets I put the corresponding values?
For the outlet I put calculated values on the total mixture.

Is it enough?

Thank you for any suggestion and comments.

[surya.madugula]

hi..

Even I heard that the multi component gas can be used but I wonder how the humidity values of 30% and 50% have been specified there. Can you please elaborate the same.
In solving the HVAC related problems then the humidity can be specified directly not in simple air flow problem.

Surya

[me3840]

Once again, there are standard methods of calculating mass/volume fractions of water vapor based on percent humidity. Google them. You can easily implement them as a field function if you want.

[yorelchr]

Hi, thank you for your replies.


For the mass fractions, I found the Psychrometric Chart on the net.
Why using a field function, the multicomponent gas is not ok? how can I do with Field Function (I'm working with Starccm+ since few weeks...and still have a lot to learn!!!!)

Thank you!!!

[me3840]

I'm saying if you want to input your inlet/initial/outlet mass/volume fractions as % humidity instead of directly, you could use a field function so you don't have to hand calculate it every time.

[yorelchr]

ah ok, thank you.
so in the way I'm doing, I use multicomponent gas, then VOF and then I put the values of mass fractions. What is the difference between composite and continue? I mean, for example, in the inlet, I put the 2 mass fractions (one for air, one for H2O), but I can choose composite or continue...what should I do?
Is there anything else to do after that?
thank you again!

[me3840]

I don't know what you mean by "continue". Continue is not an option. I think you mean constant.

Composite allows you to specify each species independently. For example if Species 1 was constant and 2 and 3 depend on one another or another function, one would need to specify each species in this manner. If your species don't change or are not very complex, it's easier to specify via an inline vector through constant.

[yorelchr]

Sorry, I meant constant and not continue.
Thank you for your explanation, I feel better since I chose constant and my simulations are running...
Thanks a lot!!!!

[farr24]

hii
is there some one how can help me to use moist air defined by Species model in multiphase model ?
thx

[Sandy7]

I am doing something similar. My goal is to find out the mass fraction of water in indoor air; to achieve this I am using Multi-component gas model.. (Gas components includes H2O and Air).

I have defined the following properties..

continua/physics1/Initial conditions/species mass fraction [0.012, 0.988]

For each vent:

Regions/vent1/species mass fraction [0.012, 0.988]
similar to the other two vents.

After convergence its giving some results on plane but the mass fraction value is almost same every where in the house. (screenshot attached).. These results are skeptical to me.. did anyone has this issue as well?

Can someone explain how to use field functions?.. for entering the mass fraction of H2O.

Thanks,
Sandy