FLUENT solves the mass transport equations for the species, taking into account convection, diffusion, production and consumption of and related species. This approach is completely general, being derived from the fundamental principle of mass conservation. The effect of residence time in SOx mechanisms, a Lagrangian reference frame concept, is included through the convection terms in the governing equations written in the Eulerian reference frame. If all fuel sulfur is assumed to convert directly to and the other product and intermediate species and assumed negligible, then only the species transport equation is needed:
As discussed in Section 20.2.3, SOx formation mechanisms are more involved. The tracking of sulfur-containing intermediate species is important. FLUENT solves a transport equation for the , , SO, and SH species in addition to the species:
where , , , , and are mass fractions of , , , SO, and SH in the gas phase. The source terms , , , , and are to be determined depending on the form of fuel sulfur release ( and/or ) and inclusion of , SO and SH in the SOx mechanism.