Formaldehyde fluorescence as a marker for scalar dissipation through local extinction

A co-annular, counterflow diffusion burner was used to stabilize a local extinction point off centerline. The extinction point was measured using planar laser-induced fluorescence (PLIF) of hydroxyl and formaldehyde. The PLIF measurements through the local extinction point were compared to results from a two-dimensional numerical simulation with a comprehensive chemical kinetics model. Previous work has indicated that formaldehyde profile width may be a useful marker for scalar dissipation rate. This relationship is tested in this work for conditions prior to, during and after local extinction. Prior to the extinction point, the formaldehyde fluorescence exhibits self-similar behavior with respect to mixture fraction, similar to that in a standard counterflow flame. However, in the extinction region, the formaldehyde signal departs from this behavior, and the formaldehyde profile is largely determined by mixing processes. Formaldehyde width is found to be a useful marker for global scalar dissipation within the selfsimilar flame region; however, it is not sensitive enough to be used as a marker for local scalar dissipation in the presence of local extinction.