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. Planar laser-induced fluorescence (PLIF) measurements of hydroxyl (OH) and formaldehyde (CH₂O) were performed. The PLIF measurements were compared to results from a two-dimensional numerical simulation with a comprehensive chemical kinetics model, and good qualitative agreement is shown. Previous work has suggested that the formaldehyde fluorescence may be a useful marker for local scalar dissipation rate, which is difficult to measure directly. This relationship is further examined in this work for conditions including local extinction. Formaldehyde width is still a useful marker for local scalar dissipation within a vigorously burning flame. However, the formaldehyde fluorescence signal increases significantly following local extinction, so it cannot be used as a direct marker. A combination of formaldehyde fluorescence width and intensity can still help identify local extinction points and scalar dissipation rates just prior to extinction.