CH and Formaldehyde Structures in Partially-Premixed Methane/Air Coflow Flames

CH and CH2O inner cone structures in partially premixed methane/air Bunsen flames are examined with 2-D laser-induced fluorescence (LIF) imaging for stoichiometries 1.36 less than or equal to Phi less than or equal to 3.0. Chosen LIF excitation strategies minimize the temperature dependent partition function variation for CH2O, and maintain CH signal strength while eliminating Rayleigh scattering background. The formaldehyde structure appears inside CH in the inner flame cone for moderately fuel rich stoichiometries typical of appliance flames. CH LIF becomes too weak to distinguish from background at richer stoichiometries (Phi=2.7). A distinct formaldehyde inner cone structure persists even for very rich Phi, with an increasing width. A simple I-D model replicates the variation in the relative concentrations of CH and formaldehyde in the inner cone, Predicted absolute CH agrees within a factor of two with the measured value. Exhaust probe measurements show that metal inserts reduce NO and increase CO emissions. LIF images of CH and CH2O taken for these perturbed flames reveal the CH2O structures are spatially expanded by the inserts.