C. Doute et al., EXPERIMENTAL-STUDY OF THE CHEMICAL-STRUCTURE OF LOW-PRESSURE PREMIXEDN-HEPTANE-O-2-AR AND ISO-OCTANE-O-2-AR FLAMES, Combustion science and technology, 124(1-6), 1997, pp. 249-276
Temperature and species mole fraction profiles have been measured in l
aminar premixed n-heptane/O-2/Ar and iso-octane/O-2/Ar flames. Both fl
ames have been stabilized on a flat-flame burner at low pressure (6.0
kPa), and species identification and concentration measurement have be
en performed by mass spectrometric analyses of samples withdrawn local
ly by molecular beam formation. Temperature profiles were measured by
Pt - Pt 10% Rh thermocouples with corrections of the signals to compen
sate radiative heat losses. A wide range of equivalence ratios extendi
ng to 0.7 up to 2.0 has been considered in order to check how the natu
re of the fuel influences the evolution with this parameter of the spe
cies mole fraction profiles. Mole fraction profiles of reactants, majo
r products (CO2, H2O, CO, H-2), main active species (H, O, OH), and sm
all intermediate species (CH3, CH4, C2H2, C2H4, C2H5) have been obtain
ed with working conditions of the MBMS technique usually adopted to st
udy the structure of small fuel molecules. Care was taken to minimize
fragmentations in the ionization source of the mass spectrometer and s
o be able to derive quantitative measurements for intermediate species
such as large olefins and alkyl radicals involved in the first steps
of the combustion mechanisms of heavy fuels. In this work, C3H6, C4H8,
C5H10 and n-C3H7, 1-C4H9, i-C4H9 have been analyzed. The use of very
low electron energies in the ionization source of the mass spectromete
r limits strongly the accuracy of the mole fraction measurements and a
additional analyses by Gas Chromatography have been performed to contr
ol the MBMS data. Emphasis was put as well on the analyses of the spec
ies involved in the formation of benzene in hydrocarbon flames: C-3(C3
H3, C3H4) and C-4 species (C4H3, C4H4, C4H5) have been analyzed. Compa
rison of the species mole fraction profiles in flames with identical e
quivalence ratios points out two main differences: (i) a marked hierar
chy is observed in n-heptane flames in the concentrations of the inter
mediate olefins with ethylene > propene > l-butene, while the correspo
nding species (ethylene, propene and i-butene) are formed ill very clo
se concentrations in iso-octane flames; (ii) benzene is formed in larg
er amounts in iso-octane flames. Allene and propyne follow the same ev
olution, in agreement with the C-3 mechanism now widely accepted to in
terpret benzene formation in flames.