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 mixed inlets.
The partially premixed model in FLUENT is a simple combination of the non-premixed model (Chapter 15) and the premixed model (Chapter 16). The premixed reaction-progress variable, , determines the position of the flame front. Behind the flame front ( ), the mixture is burnt and the equilibrium or laminar flamelet mixture fraction solution is used. Ahead of the flame front ( ), the species mass fractions, temperature, and density are calculated from the mixed but unburnt mixture fraction. Within the flame ( ), a linear combination of the unburnt and burnt mixtures is used.