An unsteady laminar flamelet model for non-premixed combustion

While it is widely recognized that unsteady effects are important in turbulent diffusion flames, few simple models take unsteadiness into account. We show that this can be done with a laminar flamelet model based on steady strained flamelet libraries. In this approach, unsteady effects are included in an ‘equivalent’ strain rate calculated from the strain rate history. This full unsteady laminar flamelet assumption (FULFA) model is tested in a counterflow diffusion flame subjected to an oscillating or pulsed strain rate. Comparisons between the FULFA model and the numerical simulations show good agreement. An extinction diagram is also proposed for the prediction of extinction conditions of unsteady counterflow flames, and good agreement is obtained between predicted and computed extinction points.