Partially premixed combustion systems are premixed flames with non-uniform fuel-oxidizer mixtures (equivalence ratios). Such flames include premixed jets discharging into a quiescent atmosphere, lean premixed combustors with diffusion pilot flames and/or cooling air jets, and imperfectly premixed inlets. Ansys Fluent has three types of partially premixed models, namely Chemical Equilibrium, Steady Diffusion Flamelet, and Flamelet Generated Manifold.

The Chemical Equilibrium and Steady Diffusion Flamelet partially-premixed models assume that the premixed flame front is infinitely thin, with unburnt reactants ahead and burnt products behind the flame front. The composition of the burnt products can be modeled assuming chemical equilibrium or with steady laminar diffusion flamelets. The flame-brush is indicated by a value of mean reaction progress between 0 and 1. Note that at a point within the turbulent premixed flame brush, , the fluctuating thin flame spends some time at the unburnt state () and the remaining time at the burnt state (), with a mean reaction progress between zero and one. It should not be interpreted that the instantaneous premixed flame reaction progress is intermediate between the unburnt and burnt state.

The Flamelet Generated Manifold (FGM) model [666] assumes that the thermochemical states in a turbulent flame are similar to the states in a laminar flame, and parameterize these by mixture fraction and reaction progress. Within the laminar flame, reaction progress increases from in the unburnt reactants to in the burnt products, over a nonzero flame thickness. A point within the turbulent flame brush with has contributions from both fluctuating flame fronts, as well as intermediate reaction progress. Ansys Fluent has the option to model the FGM with either premixed or diffusion laminar flames.

For limitations that apply to the partially premixed combustion model, see Limitations in the Fluent User's Guide.