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-   -   species conservation problem (http://www.cfd-online.com/Forums/fluent/42524-species-conservation-problem.html)

H Patel October 10, 2006 17:47

species conservation problem
 
Hi

I am simulating a mixing tank with impeller in stady state.My flow domain invoves two species toluene and styrene.I dont have any inlet or outlet boundary condition. i have only wall boundary conditions.To specify initial condition( two seperate layer) i used initialise and patch panel to specify two separate layers for species.for example i created a cell register by selacting cells in top 15% volume. and patched value of 1 for tol in that register.i swithched off species equation and i solved flow equation for 1st order and then swithched on species equation and both species and flow equations are solved.

My quation is when i used fluent's Report menu to calculate vol weighted avg. for mass fraction of toluene,It was showing 0.15% before solving but after solving it was showing 0.08%.

Why individual species are not conserved?

Any help will be appreciated.

thanks


RoM October 12, 2006 09:25

Re: species conservation problem
 
Define a custom function as "specie_mass_fraction*density" and calculate the volume intergral of this function. This is equal to the total mass of the specie in the domain and should be constant.

RoM

H patel October 13, 2006 10:37

Re: species conservation problem
 
thank you very much for your reply.My question is why vol weighted avg. mass fraction is not conserved.i will appreciate any help

H Patel

RoM October 17, 2006 04:37

Re: species conservation problem
 
Mass fraction is not conservative because its related to density and density will change druing mixing. Just take the following example:

We have two volumes: Volume1 is 1m3 and filled with specie1 which has a density of 1kg/m3. Volume2 is 2m3 and filed with specie2 which has a density of 2kg/m3. The volume averaged mass fraction of specie 1 bevor mixing is 1/3.

After mixing we have the same density in both volumes (5/3 kg/m3). So the new mass fraction of specie 1 is 1/5 for each volume and ofcourse volume averaged too.


bevor mixing
|--------------------------------
| V1=1m3, roh_1=1 kg/m3, x_1=1 |
| x_2=0 |
|--------------------------------
| |
| V2=2m3, roh_2=2 kg/m3, x_1=0 |
| x_2=1 |
|-------------------------------|

after mixing
|--------------------------------
| roh=5/3 kg/m3 , x_1=1/5 |
| x_2=4/5 |
|--------------------------------
| |
| roh=5/3 kg/m3 , x_1=1/5 |
| x_2=4/5 |
|-------------------------------|


RoM


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