Effects of stretch on the local structure of preely propagating premixed low-turbulent flames with various lewis numbers

Abstract

An experimental investigation of the flame response to strain rate in the case of unsteady premixed low-turbulent flames is presented. In order to point out the fundamental aspects of the mutual interaction between combustion and turbulence, measurements of local flame properties (curvature, displacement speed) and tangential strain rate were performed under varying conditions of Lewis number and turbulence. Three different mixtures (methane/air, propane/air, and hydrogen/air) were successively spark ignited in a vertical wind tunnel. The expanding flame freely propagated in a grid-generated decaying turbulent flow. An advanced field imaging technique coupling high-speed laser tomography and cross-correlation particle image velocimetry (PIV) was used to measure the temporal evolution of local flame stretch exerted by the turbulent cold flow. Local flame curvature and local displacement speed were calculated from flame-front contours. Curvature probability density functions (PDFs) were negatively skewed, especially for nonunity Lewis numbers, and displacement speed distributions underlined the influence of local stretch and thermodiffusive effects on flame-speed variations. Tangential strain rate was determined by using the velocity field in the neighborhood of the flame front and appears to be independent of the Lewis numbers. A strong correlation between local flame curvature and tangential strain rate was demonstrated, underlining the cold flow effects on the local flame structure. The influences of turbulence and Lewis number were evaluated and compared with numerical simulations. Then, local flame stretch distributions were determined versus time, indicating that a significant proportion of the flame was under compression.