An Experimental Study of Freely Propagating Premixed Flames at Various Lewis Numbers

Abstract

Several freely-propagating premixed turbulent flames are experimentally obtained in a decaying isotropic turbulent flow for various Lewis numbers. The high speed laser sheet tomography technique is applied to visualize the temporal evolution of the spatial properties of the flame fronts and to study the influence of the Lewis number (Le) on the flame front structures at fixed u′/SL 0. This experimental study is also performed to highlight the transient effect over time of the flame front geometry of turbulent premixed flames under varying turbulence conditions and fuel/air mixtures. From the tomographic recordings, the flame surface properties and mean radii are calculated at each stage of flame development and allow the mean global stretch and the mean burning velocity at each step of the flame propagation to be determined. As expected from asymptotic theory, the effects of preferential-diffusion/stretch interactions for low turbulent conditions can be modeled according to a linear correlation. For Le∼l, the mean burning velocity decreases slightly with the mean flame stretch, and the Markstein lengths are independent of the turbulent conditions. For Le∼l.6 and Le∼0.3, the mean burning velocity evolution with mean stretch shows an opposite trend both for laminar and turbulent flames. The linear dependence between <Sc>and <K> predicted for weak stretch in laminar flames appears to be valid for turbulent premixed flames, except for highly turbulent hydrogen/air flames u′/SL 0 = 1.89. For small values of Lewis numbers and/or high values of u′/SL 0, the weakly stretched approach associated with the asymptotic theory is insufficient for modeling premixed turbulent combustion.