mass flow rate entering and exiting a cell
 

I'm a chem eng student of turin, italy. i'm trying to write an udf in fluent.. i'd like to know, if possible, how to write the mass flow rate of a component (i work in a multiphase eulerian system) that enters in each single computational cell, and the mass flow rate that exits from the same cell. of course the net flux is zero if thare are no source terms inside the cell. It is nothing but the material balance of each cell and i need it because i have to put a source term in the code that depends from the value of the mass flow rate at each computational cell. thank you in advance, :)

Samir