Fluid property dependencies
 

I read from 'CFX' manual that 'for a General Fluid, density and specific heat capacity can depend on pressure, temperature and any defined additional variables.' I would like to model the system which the 'real' water is a working fluid. The temperature of the system is changed significantly. So I would like to allow the density of water to change as temperature change. I can't find any example or tutorial for this kind of problem from the manual or the CFX forum. Could anyone suggestion me how to deal with this? Thank you very much.

Atit

Re: Fluid property dependencies
 

what sort of functional dependence of density on temperature did you have in mind? If you know what you want then it could be as easy as entering an expression for density in CFX-Pre.

Neale

Re: Fluid property dependencies
 

Actually I am not sure yet. I found, from textbooks, the density of water in table form or express implicitly in non-linear equation form, that I have no idea how to enter it in CFX. Could you provide me the suggestion please? Thank you very much.

Atit

Re: Fluid property dependencies
 

Hi, Atit,

There is a Density Dependence and Temperature Dependence option when you set the density.

Good luck,

Jeniffer

Re: Fluid property dependencies
 

Hi Atit,

You can find a lot of info (including references!) on http://webbook.nist.gov/chemistry/

The easiest way you can start with is to push the data you have got in tables through some regression, and find a fit for your temperature region. Then you can go for variable density of water, and add the function of density on temperature/pressure into the ccl. (how to do this is described in tutorial.) This way you will see, if you are getting the effect you want and you can search for more precise and correct way of doing it.

good luck matej

Re: Fluid property dependencies
 

Thank you for your very useful information. After I follow your suggestion, I got this problem. I create the expression for density that depend on temperature. In 'Properties' form of 'Water at RTP Modified'. I enter that expression for 'Density', and tick 'Temperature' checkbox at the 'Density depends on' part in 'Density Dependency' form. Following that, the 'Set Buoyancy properties' checkbox in 'Properties' form is gray, and I have to enter the range of temperature. Then I go to 'Fluid Models' form. Since my problem is about natural convection in water, I choose 'Buoyant' in the 'Buoyancy Model'. However when I press 'Apply' , I got the error message say that 'The thermal expansivity has not been set. Please change the fluid so that it has these properties, or chage the domain models.' I solve the above problem by leaving the checkbox blank, that mean I enter only the CEL expression for density. Then the 'Set Buoyancy properties' checkbox is not gray and there is no temperature range to be enter in this time. After entering the expansivity value, everything seems to be OK. However the result from 'Solver' is likely to be wrong. Any suggestion? Thank you very much.

Re: Fluid property dependencies
 

I'm not any expert in buoyancy matters, but I guess you can either set the density dependency and then set the grav. force in momentum equation, OR you can use the buoynancy model in cfx, which switch on the boussinesq approximation where you need to set the thermal expasivity and the grav. force is set automaticly by cfx. You should not try to use all of it at one run :o).

Are you really sure, that the results with buoynancy model are wrong?

matej

Re: Fluid property dependencies
 

Dear Matej, How do we set the gravity force in momentum equations? Thank you very much.

Atit

Re: Fluid property dependencies
 

You need to add the source into the momentum equation. That is described in Physical models of Solver modelling p.28

the source itself is defined for example in Sovler theory on p. 26. where the buoynancy model is described.

From a technical point of view: You create Subdomain, and you prescribe a source in the subdomain. If you want a source in the whole geo, you prescribe the subdomain for the whole geometry. weverything in panels in cfx5pre.

final hint - it is a good idea to run a buoynancy problem in double precision!

matej

Re: Fluid property dependencies
 

Atit,

Thermal expansivity is only required if density is a constant and you are running a buoyancy model. In this case the flow solver uses the Bousinessque model which adds a source term to the momentum equation which is something like g*beta*(T-Tref) where T is the fluid temperature.

When density is varying with temperature, as in your case, the Bousinessque model no longer applies and the flow solver will switch to using the full bouyancy model which adds a source term to the momentum equation in the form (rho - rho_ref)*g. CFX-Pre is not smart enough to automatically detect that your density is a function of temperature so it asks you to specify it instead. Sounds like a bug in Pre really. The flow solver will do the right thing, have no worries.

Neale