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-   -   mixture template (http://www.cfd-online.com/Forums/fluent/33681-mixture-template.html)

co2 May 9, 2004 12:30

mixture template
 
hi all,

I am using fluent 6.1.22 I am using species transport In my mixturre template i have co2,o2,n2 I set the mixture first - I set density formulation to be ideal gas When I go then to indivisual fluids, it appears that I can not set the density formulation for the indivisual fluids -- I have to change the box Mixture from "mixture template" to "none" to get all the options including density -- I was wondering if doing that is correct -- or when you use mixture template, you can set density formulation only for mixture?

please let me know if you know anything about this issue.

zwdi May 9, 2004 12:57

Re: mixture template
 
Hi

I can set individual fluid density in Fluent.

Zwdi

co2 May 10, 2004 11:21

Re: mixture template
 
ok, what about fluent 6.0.20?

For mixture template I set incompressible ideal gas. In the mixture I have co2 + o2 + N2 now when i go to indivisual fluids, i dont see any option for density formulation -- I see only cp and molecular weight !

do you know what wrong i am doing?

MN May 10, 2004 13:16

Re: mixture template
 
Check section 7.2.7 of the user's guide. If you specify ideal gas (compressible or incompressible) for the mixture, the individual properties of the component species do not come into play. If you use a mixing rule (the volume-weighted-mixing-law or a user-defined-mixing-law), then you will need to specify the individual species. In all other cases, they aren't needed so become unavailable to set.

co2 May 10, 2004 18:48

Re: mixture template
 
my case has been running ok for mixture template modeled as ideal gas.

But when I switch to volume-weighted-mixing-law and use rho = fcn(T) for individual densities I get the following message during every converged solution:

reversed flow in 115 faces on pressure-outlet 14. turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 2719 cells

do you know what is going wrong ?

co2 May 11, 2004 08:28

Re: mixture template
 
it appears that modeling the mixture template is ideal gas is appropriate -- I dont need to worry about individual density formulations for fluids. The R value for the mixture will be calculated from the R values of individual gases and mass fractions -- which is what I want -- Please let me know if I am in correct

MN May 11, 2004 11:47

Re: mixture template
 
Actually, just using the ideal gas (or incompressible ideal gas) formulation will mean that the molar density of the gas will be calculated the cell temperature and pressure as from the ideal gas law (PV=nRT, or n/V = P/RT), without any direct dependancy on the composition of the gas. The mass density can be obtained with knowledge of the gas composition (This is all internal to FLUENT). No direct calculation of the indivdiual species densities are necessary by this method, meaning you do save some calculation steps in the process.

So yes, this method is correct give what you'd like FLUENT to do with individual species.

(Effectively, you could set the mixture to ideal-gas-mixing-law, then have each species set to ideal gas, but you'll get the same answer, but with a lot more calculation steps, so this is much more efficient.)

co2 May 11, 2004 15:07

Re: mixture template
 
MN:

Thanks for the enlightenment! --

- CO2


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