Flame Structure Studies of Neat and NH3-Doped H2/N20/Ar Flames by Laser-Induced Fluorescence, Mass Spectrometry, and Modeling

A combined experimental and modeling study of neat and NH3-doped (3>=1), 30-Torr flames is reported. The major species concentrations are measured by molecular beam mass spectrometry (MB/MS), whereas the minor species OH, NH, and O-atom concentrations are measured by laser-induced fluorescence (LEF). The species NO is measured both by LJF and MB/MS, and 02 by MB/MS. The flame temperatures are measured both by OH and NH LIF and by thin wire-thermometry. The flames are modeled with PREMK using the temperature profiles and several detailed chemical mechanisms as input. The mechanisms include the GRI 2.11, SSLA, and their derivatives. The SSLA mechanism was developed previously in our laboratory from a critical literature review. Calculations using all the mechanisms predict fairly well the profiles of the major species for both neat and doped flames. However, both the SSLA and GRI 2.11 calculations fail to predict the postflame 02 concentration in the neat flame, the drop in the 02 concentration with the addition of NH3, and the NH3 decay in the doped flame. Sensitivity analyses suggest refinements to the SSLA and GRI 2.11 mechanisms. The experimental results are predicted rather well using a modified SSLA mechanism in which the NH+NO=N20+H reaction rate is decreased and the N20+M=N2+0+M reaction rate and/or H20 third body efficiency is increased to the limit of their uncertainty. Rate analyses performed on the modeled calculations reveal the reactions important to NO, 02, NH, OH, and O-atom production and consumption and NH3 consumption. These reactions are presented and discussed.