A little doubt about Water Vapour + Dry air.

Felggv · May 13, 2012, 22:19

Hi everyone,

I'm new to CFX and I'm trying to model and study umidity distribution in a room with the influence of an ar conditioner system.

As read in an example of Yunus' book (thermodynamics), we have the mass of each component of the atmosferic air (dry air + water vapour) but their volume are equal in a closed room, as it says. They're equal because both of them fullfil the room they're in.

What puzzles me is, when I'm editing my domain in CFX, it asks for the volume fraction of each fluid, so, if they occupy the same volume, what should I do? Should I use the mass fraction instead volume fraction?

Another thing I'd like to comment is that I'm not creating a new material that is the mixture of Air at 25ºC + Water Vapour at 25ºC, instead of that, I'm adding two fluids on the domain editor and using the homogeneous model for them. What's the best method for this application? Is there another method that would make my problem easier to solve?

At last, as I'm studying a transient case, is there any problem if I use the models of Water vapour and Air at 25ºC and set my initial temperature at 22ºC?

Sorry about the long text.

Thanks in advance,

Felipe.

ghorrocks · May 14, 2012, 02:05

If it is asking for volume fraction then you have set it up as a multiphase model. What you describe sounds like a multicomponent model, and that uses mass fractions. But also note you may be able to do your model with just air as the only component everywhere and a passive scalar (ie additional variable) to track the humidity. This approach is far simpler, but is difficult to model effects like the variation in gas properties due to its composition. If this is not a significant effect then use additional variables.

The material models Water Vapour at 25C and all the others simply means it takes the material properties for that model at that temperature. There will be a small error in using the material properties of 25C at a model which runs at 22C - but the difference will be a tiny fraction of a percent and I doubt many people will notice it. Unless you are looking for extremely high precision do not worry about it.

Felggv · May 14, 2012, 16:58

Thank you for the answer! I'll try to do what you said.

May 13, 2012, 22:19	A little doubt about Water Vapour + Dry air.	#1
Felggv Member Felipe Gobbi Join Date: Apr 2012 Location: Brazil Posts: 76 Rep Power: 14	Hi everyone, I'm new to CFX and I'm trying to model and study umidity distribution in a room with the influence of an ar conditioner system. As read in an example of Yunus' book (thermodynamics), we have the mass of each component of the atmosferic air (dry air + water vapour) but their volume are equal in a closed room, as it says. They're equal because both of them fullfil the room they're in. What puzzles me is, when I'm editing my domain in CFX, it asks for the volume fraction of each fluid, so, if they occupy the same volume, what should I do? Should I use the mass fraction instead volume fraction? Another thing I'd like to comment is that I'm not creating a new material that is the mixture of Air at 25ºC + Water Vapour at 25ºC, instead of that, I'm adding two fluids on the domain editor and using the homogeneous model for them. What's the best method for this application? Is there another method that would make my problem easier to solve? At last, as I'm studying a transient case, is there any problem if I use the models of Water vapour and Air at 25ºC and set my initial temperature at 22ºC? Sorry about the long text. Thanks in advance, Felipe.

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
mass flow in is not equal to mass flow out	saii	CFX	12	March 19, 2018 05:21
Water subcooled boiling	Attesz	CFX	7	January 5, 2013 03:32
error message	cuteapathy	CFX	14	March 20, 2012 06:45
Constant velocity of the material	Sas	CFX	15	July 13, 2010 08:56
air bubble is disappear increasing time using vof	xujjun	CFX	9	June 9, 2009 07:59

May 14, 2012, 02:05		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,700 Rep Power: 143	If it is asking for volume fraction then you have set it up as a multiphase model. What you describe sounds like a multicomponent model, and that uses mass fractions. But also note you may be able to do your model with just air as the only component everywhere and a passive scalar (ie additional variable) to track the humidity. This approach is far simpler, but is difficult to model effects like the variation in gas properties due to its composition. If this is not a significant effect then use additional variables. The material models Water Vapour at 25C and all the others simply means it takes the material properties for that model at that temperature. There will be a small error in using the material properties of 25C at a model which runs at 22C - but the difference will be a tiny fraction of a percent and I doubt many people will notice it. Unless you are looking for extremely high precision do not worry about it.

May 14, 2012, 16:58		#3
Felggv Member Felipe Gobbi Join Date: Apr 2012 Location: Brazil Posts: 76 Rep Power: 14	Thank you for the answer! I'll try to do what you said.