Measurements of flamelet orientations in premixed flames with positive and negative Markstein numbers

Measurements of instantaneous flamelet surface orientation of preferential diffusion stable, ethylene/ air and unstable, methane/air flames are presented for three different values of normalized turbulence intensity, u ' /S-L(o), where S-L(o) is the unstretched laminar flame speed. Instantaneous flamelet orientations were measured in three dimensions via crossed-plane imaging. Repeated measurements were used to estimate probability density functions (PDFs) of flamelet orientation. A one parameter fit of the PDF of flamelet orientations was applied that has been applied previously to data from V-flames and from spark ignition engine flames. The fits of the PDF were in good agreement with measurements. The fit parameter, zeta, which describes the breadth of the distribution of flamelet orientations, was found to be systematically larger for stable flames than for unstable flames at similar values of u ' /S-L(o). The degree of wrinkling of the flamelet is described quantitatively by the flamelet surface density, <(<Sigma>)over bar>, averaged across the flame brush, and did not vary for stable and unstable flames, contrary to expectation.; data from the current work are in good agreement with data from previous experiments. A <c > constant contour image is presented to five an indication of the shape and width of the turbulent flame brush. Normals to jc) constant surfaces, N-<c >, were measured for the first time in V-flames and were found to be colinear with the mean of the flamelet normals, <N >.