Modeling relative Humidity in CFX
 

Hi..

I am simulating Euler Lagrange flow in a pipe. Air flowing at 4 bar and 200C.
Water is injected at 15C.

Now I want to plot relative humidiy in CFX. How can I calculate it or post process it.

Please help..

regards,
Pradeep

Hi,

As far as I know, you can't do it directly, it's not a default variable built in CFX that you can plot as function of time or whatever, so, you'll need to get your thermodynamics book and find out a way to plot it after you have informations about humidity given by CFX-Post, like average Water Vapour Mass Fraction distribution.

Hope it helped you a bit, in a few moments more people will come and help you.

EDIT: You can access ScienceDirect.com and view an enormous amount of articles about relative humidity, and maybe find some information on how to plot it after you have the desired information given by any CFD software.

Felipe is correct - it is not in CFX by default, so you will have to calculate it from the mass fraction.

Thank you very much Mr. Felipp Gobbi..
I tried as per your instructuions but I could only manage to plot Specific Humidity (Which is ratio of massfrac of vapor to mass frac air). Also I could calculate Vapor pressure (Pv) using Ideal gas laws As I have H2O.Mass concentration (kg/m^3) as one of the variable which is nothing but actual vapor density. ie.( Pv = Rhov / Rv T ) But i am not understanding how can I get saturation pressure of water vapor at a given temp.

If I can get that as a variable...Then I can compute, relative humidity = (Pv/ Psat@T)

I am literally clueless..But it is quite interesteing that Liquid Evaporation model available in CFX computes Saturation Pressure (Antoine Equation). And CFX dosen't has that as a variable for post processing

Hi there,

I'm afraid you'll have to do it manually by using another software with psychrometric info like EES (as suggested by Yunus' thermodynamics book).

Or you can adjust a curve for Pg - T using a thermodynamic table using Excel.

I'm sorry if it won't work, that's what I can suggest for now (gotta read the book again).

Good luck and if you manage to do it, come back to tell us how you did it!

PS: If you're at an University, maybe you have full access to www.sciencedirect.com, try some articles about relative humidity, you may figure out what people did, but I can tell you that you'll have some manual work. If it's not forbidden I can share some articles by attaching them here.

Hello Mr. Felipe Gobbi...

Thank you very much for the reply..If you have some articles with you regarding relative humidity calculation..It will be very useful...Please do post some links..In the mean time I will also search some articles about it.

Regards,
Pradeep

Hey,

I guess they're big to post here, e-mail me and I e-mail you back with the articles!

And I repeat, the articles won't tell you how to do it directly, you may have to figure it out by yourself!

e-mail: felipeggv@gmail.com

Mr. Pradeep,

I've managed to do it with the help of a friend of mine that had already done it.

In CFD Post:

1) Insert> Expression - here we define the Saturation Pressure (Pg).

Definition: (0.61078* exp((17.269*(Temperature - 273.15 [K]))/((Temperature - 273.15[K]) + 237.3[K])))*1000

2) Insert> Expression

Definition: (((Water Vapour at 25 C.Mass Fraction/Air at 25 C.Mass Fraction)*Absolute Pressure)/((0.622 + Water Vapour at 25 C.Mass Fraction/Air at 25 C.Mass Fraction)*Pg))*100

3) Insert> Variable - name it "Relative Humidity" and in expression set it like the expression above (in item 2).

Now you can plot it as contours, charts, tables, etc.

Hi Thank you very much...

Even I have 1 method to compute sat vapor presure.

Psat = 1[bar]*exp^(Antoine A - Antoine B/(T + Antoine C))

http://en.wikipedia.org/wiki/Antoine_equation

The Antoine coefficients can be found in saturation properties of Homogenious binary mixture of Water vapor and liquid water in ANSYS CFX material library (Interphase Mass Transfer).
If Antoine coefficients are being used from different literatures you may have to care for pressure scale.
For Eg. If antoine coefficents are A = 23.7836 , B = 3782. 89 C = (230.15-273.15 ). Then pressure scale has to be replaced by 1 [ Pa].

Relative Humidity can be calculated as you specified or...

Actual Vapor Pressure = H2O.Molar Mass Fraction * Absolute Pressure

R H = Actual Vapor Pressure / Saturation Vapor Pressure

Also I have found few formulations of Sat vap Pressure which can be useful in future.

http://cires.colorado.edu/~voemel/vp.html

@Felipe Gobbi- I tried this code.

1) Insert> Expression - here we define the Saturation Pressure (Pg).

Definition: (0.61078* exp((17.269*(Temperature - 273.15 [K]))/((Temperature - 273.15[K]) + 237.3[K])))*1000

2) Insert> Expression

Definition: (((Water Vapour at 25 C.Mass Fraction/Air at 25 C.Mass Fraction)*Absolute Pressure)/((0.622 + Water Vapour at 25 C.Mass Fraction/Air at 25 C.Mass Fraction)*Pg))*100

3) Insert> Variable - name it "Relative Humidity" and in expression set it like the expression above (in item 2).

Now you can plot it as contours, charts, tables, etc.
I am getting an error that air at 25C.mass fraction does not existing in default domain.
I have added air at 25C in my default domain, but still I get this error. I tried one with formula for Relative humidity using density, still it throws the same error. Do you have any idea as to why it is not accepting the properties of air at 25 C?

Hello Niru,

Look for the naming available in your CFD post..May be you are getting error because of wrong naming..

Regards,
Pradeep

Thank you, this is extremely useful!