Fuel Composition Effects on Flame Stretch in Turbulent Premixed Combustion: Numerical Analysis of Flame-Vortex Interaction and Formulation of a New Efficiency Function

Direct numerical simulations (DNS) of flame-vortex interaction are performed to study the effect of fuel composition on premixed flames wrinkling. Flame stretches are deduced from the DNS and compared with phenomenological functions from the literature for a range of vortex characteristics. Such functions, which are used as an input in most flamelet models for premixed combustion, depend on the turbulent scale parameters and the laminar flame properties. Therefore, all mixture related effects are supposed to be described by these latter quantities. However, such an assumption can be restrictive when considering various fuels and especially low Lewis number mixtures. In these conditions, thermo-diffusive instabilities can occur, leading to an increase of the flame wrinkling, which is not correctly described by common stretch models. To overcome this issue, DNS performed in this work integrate a description of chemistry processes through a new 4-steps kinetic scheme dedicated to the combustion of C H (4)- H (2) and C (3) H (8)- H (2) blends. It is shown that fuel composition effects can at first order be described through the mixture Lewis number, but also that the asymptotic behavior of current stretch functions are not relevant at large turbulent length scales. A new efficiency function for the flame stretch is thus finally proposed.