[souza.emer]

Hello everyone

I want to use the Kinetic Theory model for Mass Diffusivity and use its value, for each species, in a UDF to calculate the reaction rate. My question is, which macro should I use? Is it one of those two?

C_DIFF_L(c,t); Laminar species diffusivity

C_DIFF_EFF(c,t,i); Effective species diffusivity

If not, how can I call it?

Thanks for the attention.

[obscureed]

Hi there,

The macros that you mention are for accessing values, not setting them. For defining, look at DEFINE_DIFFUSIVITY. If you want to include the effects of turbulence on diffusion (which, in a turbulent situation, you definitely do), you need to add this to the diffusivity of a User-Defined Scalar. (The manual's example makes this clear.) For a species, I *think* turbulence effects are added separately -- this is what the Theory Guide appears to say, but I don't remember checking in detail.

But then, there is a built-in version of kinetic theory for diffusivity, and this uses the molecular masses, species by species. (In fact, it seems to calculate it for each species-species pair, which does not seem possible in the UDF. Hmm.)

Good luck!
Ed

[souza.emer]

Hello

Yeah, I know that Macros are used for accessing the values, that's why I'm in doubt because I'm not sure of how I could use the value of diffusivity calculated by the Kinect Theory when I set this previously at the materials menu in Fluent.
I do not want to use the macro to modify the current diffusivity coefficient calculated by Kinect Theory theory, I just want to use the value which Fluent calculates.

I think you thought that I wanted to modify it, which I do not want to, yet.

Do you know how I can do this? Or make any suggestions?

Best

[rewol]

You can use DEFINE_NET_REACTION_RATE macro to calculate reaction rates and inside the code you can utilize C_DIFF_L(c,t) or C_DIFF_EFF(c,t,i) to access mass diffusivities of the species.

[obscureed]

Hi Souza.Emer,

Yes, you are correct that I assumed you wanted to define the values. Sorry, this is a more frequent request than accessing values in UDF (because you can access the values in normal postprocessing if you want).

For accessing the built-in values, without any turbulent influence, C_DIFF_L is what you want. But check the manual for syntax -- C_DIFF_L(c,t) is not correct.

You can do this accessing inside several types of UDF -- rewol mentions one, though that is specific to the kinds of model that will accept it. DEFINE_ON_DEMAND is a general option: first look up a domain pointer, then find some cell or face zones, and loop through some faces or zones.

Good luck!
Ed

[Weiqiang Liu]

hello, can I ask a question under this thread?
I checked the syntax in manual and found the correct syntax is C_DIFF_L(c,t,i,j). I am wondering is the diffusion coefficient I am accessing with this macro is binary diffusion coefficient? what if I want to access mixture averaged diffusion coefficient or multi-component diffusion coefficient？