Thank you for your reply. However, I didn't get the answer to my question after going through the suggested chapter. The only thing related to the question I found was that the boundary condition Im asking about is the default at the walls. Let me rephrase my question. Some problems at the inlet don't provide a specific species mass fraction (also called Dirichlet boundary condition) but they provide a specific mass flux (or Neumann boundary conditions). In this case, how can I add this mass flux in fluent since at the species option under velocity inlet there is only an option to add species mass fraction only?