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

Abstract

CH and CH₂0 inner cone structures in partially premixed methane/air Bunsen flames are examined with 2-D laser-induced fluorescence (LIF) imaging for stoichiometries 1·36 ≤φ≤ 3·0. Chosen LIF excitation strategies minimize the temperature dependent partition function variation for CH₂0. 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 (φ=2·7). A distinct formaldehyde inner cone structure persists even for very rich O. with an increasing width. A simple ID 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 CH₂0 taken for these perturbed flames reveal the CH₂0 structures are spatially expanded by the inserts.

Keywords:

• NOx formation
• Premixed flames
• Laser diagnostics

Additional information

Notes on contributors

ROBERT J.H. KLEIN-DOUWEL

Dept. of Applied Physics, University of Nijmegen. Toernooiveld, 6525 ED Nijmegen. The Netherlands

JAY B. JEFFRIES

Dept. of Mechanical Engineering, Stanford University. Stanford, CA 94305-3032

GREGORY P. SMITH

Corresponding Author: Gregory P. Smith Molecular Physics Laboratory SRI International 333 Ravenswood Ave. Menlo Park, California 94025 650-859-3496 voice 650-859-6196 fax [email protected]