Experimental study of nonfuel hydrocarbon concentrations in coflowing partially premixed methane/air flames

Abstract

Centerline measurements have been made of temperature, CH 4, O 2, CO 2, and C2 to C12 nonfuel hydrocarbons in a CH 4/air nonpremixed coflowing flame and in five partially premixed coflowing flames with primary equivalence ratios varying from 12.3 to 2.5. Partial premixing decreases the flame height and thereby compresses all of the profiles towards the burner surface, so a nondimensional vertical coordinate has been developed to account for this and make other effects more apparent. The temperature and major species results show that partial premixing reduces radial heat and mass transfer in the lower part of the flames, and causes an inner rich premixed flame front to form at one-half the height of the outer flame front. Partial premixing increases the mole fractions of the oxygenated hydrocarbons CH 2O and C 2H 2O to hundreds of parts per million, and of C 2H 4O and C 3H 4O to parts per million. The mole fractions of regular hydrocarbons are decreased by partial premixing, in roughly the same proportion as they are reduced by dilution with nitrogen, which suggests that fuel dilution is the primary cause. The decrease in concentrations is progressively greater for larger hydrocarbons. In the flames that exhibit a double flame structure, nonfuel hydrocarbons are formed inside the inner rich premixed flame front, peak at this front, and are completely consumed in the region between the flame fronts.