how to create a temperature dependant fluid

Amy · October 17, 2009, 10:52

Hello,

i'm trying to model a fluid that changes properties temperature dependant. this fluid (air or better exhaust) is flowing through a pipe and getting cooled by water on the outside.
to find out the pressure-drop of the tube it's important to have changing properties in density. so i guess "air ideal gas" is not the right fluid to use in this case.
i've read the documentation and found sth. about "real gas", but could not figure out how to set this up.
Can you please tell me how to do this for a multiflow like exhaust (N2, O2,...) and how to creat this multiflow-fluid. or it would be ok, if CFX provides air with real gas properties as default.

i'd appreciate every help. thanks!

Amy

ghorrocks · October 18, 2009, 08:04

Ideal gas probably is OK for most purposes. This way density can change as a function of temperature and pressure and that sounds like what you want. You can also set the exhaust up to be a multicomponent mixture N2, O2 CO2 etc and do the ideal gas model on the mixture.

Only go to real gas stuff if the ideal gas assumption is inadequate. Unless you are looking for super accuracy I suspect ideal gas will be fine for exhaust flows. I have done a lot of exhaust modelling in the past and it was all using ideal gas models and it worked fine.

Amy · October 18, 2009, 09:44

Thanks a lot! That's the information I needed. Is it possible to have a look at the function for ideal air?

Amy

Amy · October 18, 2009, 10:26

How can I use the ideal gas model for that multicomponent fluid? I can't find an opportunity to do that in the domain settings

I have another issue... Is it possible to measure the average temperature of a domain on a plane or a boundary in CFD-Post? I have several tubes and want to view the avg temperature of the air at the end of the tubes.

Amy

ghorrocks · October 18, 2009, 20:30

Quote:

Is it possible to have a look at the function for ideal air?

PV=mRT. This is pretty basic stuff.

Quote:

How can I use the ideal gas model for that multicomponent fluid?

Refer to the documentation.

Quote:

Is it possible to measure the average temperature of a domain on a plane or a boundary in CFD-Post?

Yes. Again, this is pretty basic stuff, check the tutorials for how to do this.

October 17, 2009, 10:52	how to create a temperature dependant fluid	#1
Amy New Member Join Date: Oct 2009 Posts: 3 Rep Power: 17	Hello, i'm trying to model a fluid that changes properties temperature dependant. this fluid (air or better exhaust) is flowing through a pipe and getting cooled by water on the outside. to find out the pressure-drop of the tube it's important to have changing properties in density. so i guess "air ideal gas" is not the right fluid to use in this case. i've read the documentation and found sth. about "real gas", but could not figure out how to set this up. Can you please tell me how to do this for a multiflow like exhaust (N2, O2,...) and how to creat this multiflow-fluid. or it would be ok, if CFX provides air with real gas properties as default. i'd appreciate every help. thanks! Amy

October 18, 2009, 09:44		#3
Amy New Member Join Date: Oct 2009 Posts: 3 Rep Power: 17	Thanks a lot! That's the information I needed. Is it possible to have a look at the function for ideal air? Amy Last edited by Amy; October 18, 2009 at 10:26.

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October 18, 2009, 08:04		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,852 Rep Power: 144	Ideal gas probably is OK for most purposes. This way density can change as a function of temperature and pressure and that sounds like what you want. You can also set the exhaust up to be a multicomponent mixture N2, O2 CO2 etc and do the ideal gas model on the mixture. Only go to real gas stuff if the ideal gas assumption is inadequate. Unless you are looking for super accuracy I suspect ideal gas will be fine for exhaust flows. I have done a lot of exhaust modelling in the past and it was all using ideal gas models and it worked fine.

October 18, 2009, 10:26		#4
Amy New Member Join Date: Oct 2009 Posts: 3 Rep Power: 17	How can I use the ideal gas model for that multicomponent fluid? I can't find an opportunity to do that in the domain settings I have another issue... Is it possible to measure the average temperature of a domain on a plane or a boundary in CFD-Post? I have several tubes and want to view the avg temperature of the air at the end of the tubes. Amy