For partially premixed turbulent flames, the unburned mixture at certain locations can be leaned out with excess air and therefore the flame may stop propagating due to charge stratification. This section briefly describes how the method in Ansys Forte for modeling such local flame-quenching phenomena.

For laminar flames, the flammability property of an unburned mixture is mainly governed by a balance between the heat transfer to the preheat zone due to chemical heat release and the heat loss from the preheat zone to the unburned mixture, while for turbulent flames, disturbances from turbulence have significant additional influence on the heat transfer balance. In the present combustion model, local flame quenching is modeled by examining whether the local flame condition crosses the border between the thin reaction-zone regime and the broken reaction-zone regime. Local laminar flame thickness will increase with a decrease of laminar flame speed , thus resulting in a proportional increase of the inner layer thickness. If the local laminar flame inner layer thickness is large enough so that the inner layer can be disturbed by the Kolmogorov eddies, the chemical reactions in the inner layer will cease due to excess heat and active species losses to the preheat zone, resulting in local flame quenching. Therefore, a comparison between the inner layer thickness and the Kolmogorov length scale is used as the criterion for local flame quenching, that is, flame quenching occurs when the relation

(7–36)

is satisfied, where is determined based on the turbulence model, and is a model constant with a typical value of 1.0.