Hello Amine

In actual I've to solve the following equation, but there is not any direct provision for solving in terms of concentration.

" alt=""> where

*C = concentration, Source = -kb.C*

But species transport module uses the following equation which uses **massfraction** and **density** rather than **concentration**. and the rest terms are same as of above equaiton.

So, I converted concentration into massfraction, as I already know the density of mixture and then gave the input values for simulation.

**One another option:**

Can I use "*User Defined Scalar*" for directly solving the 1st equation which contains **concentration** terms ?

As in my problem, the fluid domain is also porous. So, I also have to give porosity to the fluid domain.

**If yes, can I give porosity for the flow domain in cell zone conditions for the equation to be solved by **

**"**

*User Defined Scalar*" ?- And also do I need to give the velocity which is represented by
**'u'**in the equation - in the UDS code or in the boundary conditions ? - And also the term
**'D'**which represents diffusivity, where can I define it - in the material properties or in the UDS code ?

Thanks and regards

mettusagar