Overventilated coflow axisymmetric laminar diffusion flames of methane
, propane, and n-butane were used to study the influence of oxygen add
ition to the fuel side on soot formation. The line-of-sight soot volum
e fractions and the visible flame profiles were measured as a function
of axial location along the centerlines of pure fuel flames, and the
flames in which the fuel was diluted either with oxygen or nitrogen at
selected temperatures of the reactants, to maintain a constant adiaba
tic flame temperature. The relative influences of dilution and direct
chemical interaction effects of oxygen in the fuel gas mixture were qu
antified. It was found that, when allowance was made for the influence
of dilution and thermal effects, the addition of oxygen to the methan
e diffusion flame chemically suppressed soot formation. This suppressi
on was argued to be due to the reduction in acetylene concentration in
the pyrolysis products as the oxygen mole fraction in methane was inc
reased. The chemical influence of oxygen addition to methane decreased
when the adiabatic flame temperature was decreased by decreasing the
temperature of the reactants. In propane and n-butane flames, on the o
ther hand, oxygen addition chemically enhanced soot formation. The deg
ree of enhancement was small for low mole fractions of oxygen, but inc
reased with increasing oxygen. When oxygen is added to the fuel side o
f a diffusion flame, two counteracting chemical effects are expected:
Oxygen promotes fuel pyrolysis and hence production of hydrocarbon rad
icals and H atoms which enhance soot formation. On the other hand, aro
matic radicals and critical aliphatic hydrocarbon radicals are removed
by reactions with molecular oxygen and oxygen atom. The net chemical
influence is the difference of these two counteracting effects.