The effect of nitrogen dilution on nonfuel hydrocarbons in laminar nonpremixed flames

Centerline and radial profiles of gas temperature and C3 to C12 hydrocarbon concentrations were measured in nitrogen-diluted methane and ethylene cofl owing nonpremixed flames. The methane or ethylene volumetric flowrates were kept constant while the nitrogen flowrate in the fuel mixture was systemat ically varied. In bath series of flames, the temperatures in the pyrolysis zone were shifted to higher heights in the flame by increases in the overal l flame length, and were reduced both by the decrease in the adiabatic flam e temperature and by a mechanism that likely involves decreased heat transf er from the flame front to the centerline. The maximum centerline concentra tions of every measured non-fuel hydrocarbon monotonically decreased and sh ifted to higher heights in the flame as dilution was increased. The maximum mole fractions of most linear hydrocarbons were roughly proportional to th e hydrocarbon mole fraction in the unreacted fuel mixture, while the maximu m mole fractions of aromatic hydrocarbons varied roughly as the square of t hat quantity. The decrease in the initial methane or ethylene concentration in the fuel appears to be the primary cause of the reductions in the maxim um nonfuel hydrocarbon concentrations. For all hydracarbons the shape of th e centerline mole fraction profiles, including the location of the maximum concentration and of the species' disappearance, correlate well with temper ature.