Learning Forum

[chand101]

Hello all

I am trying to simulate the condensation of water vapor from the air using eulerian mixture and condensation model . Below is screenshot of the domain and physics I have chosen

I dont understand why RH% determined by fluent is greater than 100%. Have i put in some absurd values anywhere here?

B.Cs

Inlet: Air with 80% RH, velcotiy = 1m/s

Volume fractions of water: 0.0315 based on calculated mass fraction of water vapor for 80% RH using RH formula

Outlet: Gauge pressure: 0 Pa

Backflow volume fraction of water vapor: 0

Walls of tube: 283K

Side walls: adiabatic

 

Multiphysics interaction

Materials defined - one phase is mixture of air and water vapor .other phase is water in liquid only 

 

Condensation evaporation model setting

 

Results

 

[Amine Ben Hadj Ali]

the phase change model is wrong. You require partial pressure or concentration driven or you adjust the saturation temperature so that it matches rh equal to 100%

[chand101]

Hi

Thanks for the prompt reply. I forgot to mention in above model, I also specified mass fraction of water vapor at inlet based on required RH%. Did you mean this by phase change models is incorrect or should i choose a different multiphase model ? 

I changed temperature during initialization to achieve required RH%. Is this correct?

 

[Amine Ben Hadj Ali]

I mean the phase change: either you code your phase change, or you adjust temperature to match RH = 100 in each cell (UDF) or use species mass transfer apporach. REad the manual about that.

[chand101]

So do you mean to say that I need to create the condensation such that both materials are a species material? Also i just read the manual on this. They prescribe saturation temperature of 373K. Since this is evaporation saturation temperature and there is no evaporation in my domain, I changed my sat temp to 283K . Is this the mistake you are talking about? 

[Amine Ben Hadj Ali]

The saturation temperature is not constant. water vapor will condense or evaporate whenver RH=1 is hitten and you need to understand what does RH mean.

[chand101]

I understand what you are saying. There is a dew point temperature that is dependent on vapor saturation pressure which depends on RH. How do i define RH as an input in UDF when fluent calculates the value as output ? Also I have no idea how to use local mesh values to compute sat temperature. Could you kindly guide me on this?

[Amine Ben Hadj Ali]

Yes.
You can use DPM model to model condensation there is an example in the customization manual. The UDFs examples are helpful (how to calculate RH) even for doing Eulerian run. Actually phase change should occur when RH =1. At that level partial pressure of water should be equal to vapor pressure. From that you deduce the temperature via inverse Antoine.

[chand101]

Can you send me the link to this manual where example UDFs and  DPM model example are present ?I searched every where in the internet regarding this and Im not able to find any useful info 

[Rob]

Click on Help, you're looking for the Fluent Customization Manual. 

[Amine Ben Hadj Ali]

How to access the ANSYS Online Help

 

[chand101]

Hey The Link doesn't work for me. Looks like it's broken To modify  saturation temperature as function of vapor pressure, could I use the same physics I showed above but specify the saturation temperature in Lee model as function of vapor pressure using two UDF functions, one to calculate for vapor pressure and other to calculate for saturation temperature as function of vapor pressure? Will this be correct approach instead of using DPM ?

[Amine Ben Hadj Ali]

yes it might work. As you will need to do that anyway you can define your own RH based mass transfer using mass transfer udfs.

[chand101]

Hello Amine 

Ok. Thanks for letting me know

It is possible to do a filtration simulation in fluent for mixture of air and water vapor where I want the air to stay on one part of thin membrane but vapor alone passes through the membrane? Do you have any tutorial for this ? 

[chand101]

Hi. 

 

Did you mean evaporating cooling liquid spray tutorial in customization manual ? Where can i find examples of defining udf ? i cant find anything there. They just ask to upload a precompiled UDF file . 

[Amine Ben Hadj Ali]

Udf manual does contain a detailed example for modeling condensation search after dpm switch or law there.