The steady laminar flamelet model, described in Sections 15.3 and 15.4, models local chemical non-equilibrium due to the straining effect of turbulence. In many combustors the strain is small at the outlet and the steady flamelet model predicts all species, including slow-forming species like NOx, to be near equilibrium, which is often inaccurate. The cause of this inaccuracy is the disparity between the flamelet time-scale, which is the inverse of the scalar dissipation, and the slow-forming species time-scale, which is the residence time since the species started accumulating after mixing in the combustor.
The unsteady laminar flamelet model in FLUENT can predict slow-forming species, such as gaseous pollutants or product yields in liquid reactors, more accurately than the steady laminar flamelet model. Computationally expensive chemical kinetics are reduced to one dimension and the model is significantly faster than the laminar-finite-rate, EDC or PDF Transport models where kinetics are calculated in two or three dimensions. There are two variants of the unsteady laminar flamelet model, namely an Eulerian unsteady flamelet model (described in Section 15.5.1) and a diesel unsteady flamelet model for predicting combustion in compression-ignition engines (described in Section 15.5.2).