A flux balance at the surface is required for each gas-phase species in the system. The surface boundary condition for the gas-phase species equation is:

(3–1)

Where _g is the number of gas-phase species in the system, u is the convective velocity normal to the surface, is the diffusive velocity of the species and is the sum of any net external flux imposed at the boundary. Ansys Forte does not currently allow for the possibility of mass deposition onto or surface chemistry at wall boundaries, so such net flux would be related to specified flow velocities only. For a no-slip velocity condition, then, the net flux (including diffusive and convective flux) of each species to the wall must be zero, and the sum of all species fluxes (or the net mass flux of gas) to the surface is also zero.