[Lee 小亢]

Hi everyone,i'm new here and I just read "How to ask a question on the forums". I met some problems and wanted to discuss with you. I worked about the mass transfer between liquid and gas. So I created a user define scalar. there are two questions confused me a long time.

The first one is as we know if we define the scalar we need to define the diffusivity of it. So in the fluent, the default munber of diffusivity is 1,what's the mean of 1? The unit of the diffusivity is kg/m·s（default）,but actually the unit of diffusivity is m^2/s, is that mean if we know the actually diffusivity and when we define the diffusivity in fluent we need multiply by density? For example, if one's actually diffusivity is 2.0e-10m^2/s, when we define it in fluent should put in 2.0e-7kg/m·s?

Second, I am confused by how to define the diffusivity. As i said if the actually diffusivity is 2.0e-10m^2/s, but i had saw a UDF exmaple like this:
Fluent uses the follwing turbulent difusion coefficients
Species transport model (includes laminar+turbulent diffusion)
Diff_coeff=D_lam*C_R(c,t)+C_MU_T(c,t)/0.7 with D_lam = laminar coefficient
Non premixed combustion (full turbulent)
Diff_coeff=C_MU_T(c,t)/0.85
what's the relationship between them?(actually diffusivity,laminar coefficient and turbulent diffusion)

thanks a lot~

[annan]

Dear Lee,

When you define your diffusivity in Fluent, you need to multiply it by the density (rho*D)

Hope this will help.
Good luck

[Lee 小亢]

Thank you,sir.
I had understood the first question.
Can you give me some advice about the second question?

[annan]

Dear Lee,

As for your second question, it depends on how you want to model your diffusivity. If your case is laminar and the diffusivity value is known then you do not need to model it as you found in that UDF example. It's really case dependent.

Now, if you are in Laminar flow regime, then your diffusivity is equal to D_lam.
If you are in turbulent flow regime, and a turbulence model is on, then you need to take into account the turbulent diffusivity D_turb too. Your effective diffusivity is then equal to the sum of D_lam and D_turb.
The turbulent diffusivity can be computed from the Schmidt number as the following : D_turb = mu_turbulent / (rho*Schmidt_turb)

( mu_turbulent is C_MU_T(c,t) in your formula )

After that you sum D_turb and D_lam => D_eff = D_turb+D_lam
and you multiply by the density so that your units can be homogeneous.

Hope this will help.

P.S: Women can do CFD too

[Lee 小亢]

first,sorry madam.
second, thank you again!!!
I am really interested in CFD and wanted to learn more about it,but sometimes, it's really a hard time when you met trouble. By the way, can you live me a way of your contact?