MEAN EFFECTS OF STRETCH ON LAMINAR FLAMELETS IN A PREMIXED TURBULENT FLAME

A new approach to modeling the mean effects of stretch on laminar flamelets in a premixed turbulent flame is explored which uses statistical results of direct numerical simulation of the distributions of strain and curvature on a propagating surface. These statistics are combined to estimate the distribution of stretch rates experienced by the flame and hence the mean rate at which the turbulent flame converts reactants to products per unit of flame surface area. The results are presented as a function of the Karlovitz number ranging from zero to unity. The distribution of stretch rates on the flamelet surface reveals that a large proportion (30-50%) of the flame is under compression (i.e., negative stretch rates) at any time. The mean rate of conversion from reactants to products per unit area tends to decreases with increasing Karlovitz number. Certain combinations of modeled parameters lead to a prediction that the mean reaction rate per unit area can be greater than that in an unstretched flame. Using results from a laminar flamelet library for stoichiometric methane:air flames the effect of stretch on the mean rates of reaction is estimated.