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

Centerline measurements have been made of temperature, CH4, O-2, CO2, and C 2 to C12 nonfuel hydrocarbons in a CH4/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 no ndimensional vertical coordinate has been developed to account for this and make other effects more apparent. The temperature and major species result s 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 inc reases the mole fractions of the oxygenated hydrocarbons CH2O and C2H2O to hundreds of parts per million, and of C2H4O and C3H4O to parts per million. The mole fractions of regular hydrocarbons are decreased by partial premix ing, in roughly the same proportion as they are reduced by dilution with ni trogen, which suggests that fuel dilution is the primary cause. The decreas e in concentrations is progressively greater for larger hydrocarbons. In th e flames that exhibit a double flame structure, nonfuel hydrocarbons are fo rmed inside the inner rich premixed flame front, peak at this front, and ar e completely consumed in the region between the flame fronts. (C) 1999 by T he Combustion Institute.