Ansys Fluent solves the mass transport equation for the NO species, taking into account convection, diffusion, production, and consumption of NO and related species. This approach is completely general, being derived from the fundamental principle of mass conservation. The effect of residence time in NOx mechanisms (a Lagrangian reference frame concept) is included through the convection terms in the governing equations written in the Eulerian reference frame. For thermal and prompt NOx mechanisms, only the NO species transport equation is needed:
 | (9–1) |
As discussed in Fuel NOx Formation, the fuel NOx mechanisms are more involved. The tracking of nitrogen-containing intermediate species is important. Ansys Fluent solves a transport equation for the HCN, NH3, or N2O species, in addition to the NO species:
 | (9–2) |
 | (9–3) |
 | (9–4) |
where 
, 
, 
, and 
are mass fractions of HCN, NH3,
N2O, and NO in the gas phase, and 
is the effective diffusion coefficient. The source terms 
, 
, 
, and 
are to be determined next for different NOx
mechanisms.