Radical Overshoot in the Reaction Zone of Methane-Air Diffusion Flames.

摘要

Analysis of the temperature and composition profiles obtained by probing a confined,axisymmetric laminar methane-air diffusion flame suggested that radical concentrations in excess of equilibrium exist in the post-flame zone. Subsequent to hydrocarbon depletion,it was determined that the fast,bimolecular reactions associated with the CO-H sub 2 -O sub 2system were balanced,and that the thermolecular reactions were not. This situation is analogous to that observed in the post-flame gases of premixed hydrocarbon flames. In such flames, a partly equilibrated state prevails in which the radical and stable species remain equilibrated among themselves by rapid bimolecular reactions,while slowly approaching the recombination-dissociation equilibria as a pool. By assuming that the bimolecular reactions associated with the CO-H sub 2 -O sub 2system were equilibrated in the post-flame zone, a partial equilibrium composition which satisfied the measured stoichiometry was determined. The mole number was estimated from the mole number of the sample gas by assuming that each mole of H sub 2 , O sub 2and H sub 2 O formed in the sample probe resulted in the net loss of one mole. The concentrations calculated agreed with the measured concentrations,thereby validating the assumption of a state of partial equilibrium subsequent to hydrocarbon depletion in methane-air diffusion flames. Experimentally determined nitrogen oxide concentration profiles were used to confirm the calculated superequilibrium radical concentrations. The oxygen radical concentrations were used in the extended Zeldovich NO formation rate mechanism. The estimated NO formation rates were integrated to obtain the total mass production rate of NO and compared with the total outlet flux of NO/sub x/determined by integration of experimental NO/sub x/concentration data. Comparison was sufficient to support the existence of free radical concentrations in excess of equilibrium. (ERA citation 03:023293)