On the dual response and surface recombination in burner-stabilized flames

The classical dual response of burner-stabilized methane/air flames was studied using a steady, one-dimensional premixed flame model. The flame exhibits two flame speeds for either the same heat loss rate to the burner, or the same flame standoff distance. This is in agreement with the experiments of Spalding and Yumlu, who observed two flame speeds for the same heat loss rate to the burner, and of Ferguson and Keck, who observed two flame speeds for a given flame standoff distance. It is, however, clarified that the response is actually unique in that with decreasing flow rate and thereby flame speed, the flame is free to adjust both in its standoff distance as well as the heat loss rate to the burner, thus giving a unique flame response for a given flow rate, which is the proper independent parameter of the present system. The numerical results of flame standoff position and flame temperature were compared with the experimental data of Ferguson and Keck and reasonable agreement was found. The effect of radical recombination at the burner surface on the flame response was also investigated. Heterogeneous recombination of hydrogen radicals, and of hydrogen and methyl radicals, were both considered, and the appropriate boundary conditions at the cold boundary are discussed. Recombination at the burner was found to have no noticeable effect on the flames studied in this work.