Surface density measurements of turbulent premixed flames in a spark-ignition engine and a Bunsen-type burner using planar laser-induced fluorescence

Surface densities of turbulent three-dimensional premised flames determined from planar flame images and flame surface densities in a spark-ignition engine are both reported for the first time. The flame images were obtained using laser-induced fluorescence of biacetyl in the engine and of OH radicals in a Bunsen flame. Images from orthogonal planes have been used to determine the mean orientation angle along the line of intersection of the planes in the engine experiments. Because of the axisymmetric flow field of the Bunsen flame, it has been assumed that the flame front has a symmetric mean orientation behavior about the axis, and the normal to the flame front can be determined from images in one plane. The flame surface density (Sigma) profiles and orientation angle that produced them are presented as a function of the progress variable. The directional cosines for the engine flame are almost identical in both orthogonal planes, indicating the isotropic nature of the flame front. A typical value of 0.7 has been found for the directional cosine for the Bunsen flame and the engine flame at 1200 rpm. At a lower engine speed, the directional cosine has increased slightly. The Bray-Moss-Libby (BML) model coefficients obtained at maximum flame surface density in this work agree with the previously reported results and display an inverse power-law dependence on the integral length scale when normalized by the laminar flame thickness. The BML coefficients obtained from direct numerical simulation (DNS) studies are in qualitative agreement with the present results. In the range of u'/S-L investigated in this work the flame-surface density has not shown much variation.