Lewis number effects in premixed turbulent combustion and highly perturbedlaminar flames

Various perturbed laminar flames are numerically simulated by reducing comb ustion chemistry to a single reaction. The following flame configurations a re addressed: expanding spherical, cylindrical, and symmetrical planar flam es; converging spherical flame; expanding cylindrical and symmetrical plana r flames affected by external steady or time-dependent strain rate; steady strained cylindrical and symmetrical planar flames. The results (1) show th at a simple linear relation between the local consumption velocity and flam e stretch rate is valid only for weakly perturbed laminar flames; (2) highl ight the importance of transient effects; and (3) show that, in the case of a small Lewis number, the highest local combustion rate is reached in the expanding spherical flame ignited by the hot pocket of the critical radius. This highest local combustion rate is successfully used to describe the ex tensive Karpov's experimental data base on turbulent burning velocities for mixtures characterized by substantially different values of the Lewis numb er. Discussion of Karpov's experimental data showing the drastic effect of the Lewis number on turbulent burning velocity implies the important role p layed by highly perturbed flamelets in premixed turbulent combustion.