In a diffusion flame, at the molecular level, fuel and oxidizer diffuse into the reaction zone. Here they encounter high temperatures and radical species, and ignite. More heat and radicals are generated in the reaction zone, and some diffuse out. In near-equilibrium flames, the reaction rate is much faster than the diffusion rate. However, as the flame is stretched and strained by the turbulence, species and temperature gradients increase, and radicals and heat diffuse more quickly out of the flame. The species have less time to reach chemical equilibrium, and the degree of local non-equilibrium increases.
The steady laminar flamelet model is suited to predict chemical non-equilibrium due to aerodynamic straining of the flame by the turbulence. The chemistry, however, is assumed to respond rapidly to this strain, so as the strain relaxes to zero, the chemistry tends to equilibrium.
When the chemical time-scale is comparable to the fluid mixing time-scale, the species can be considered to be in global chemical non-equilibrium. Such cases include NOx formation and low-temperature CO oxidation. The steady laminar flamelet model is not suitable for such slow-chemistry flames. Instead, you can model slow chemistry using one of the following: