The Effect of Nitrogen Dilution on Nonfuel Hydrocarbons in Laminar Nonpremixed Flames

Abstract

Centerline and radial profiles of gas temperature and C3 to CI2 hydrocarbon concentrations were measured in nitrogen-diluted methane and ethylene coflowing nonpremixed flames. The methane or ethylene volumetric flowrates were kept constant while the nitrogen flowrate in the fuel mixture was systematically varied. In both series of flames, the lemperatures in the pyrolysis zone were shifted to higher heights in the flame by increases in the overall flame length, and were reduced both by the decrease in the adiabatic flame temperature and by a mechanism that likely involves decreased heat transfer from the flame front to the centerline. The maximum centerline concentrations of every measured non-fuel hydrocarbon monotonically decreased and shifted to higher heights in the flame as dilution was increased. The maximum mole fractions of most linear hydrocarbons were roughly proportional to the hydrocarbon mole fraction in the un reacted fuel mixture, while the maximum mole fractions of aromatic hydrocarbons varied roughly as the square of that quantity. The decrease in the initial methane or ethylene concentration in the fuel appears to be the primary cause of the reductions in the maximum nonfuel hydrocarbon concentrations. For all hydrocarbons the shape of the centerline mole fraction profiles, including the location of the maximum concentration and of the species'disappearance, correlate well with temperature.