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

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

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

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