EXPERIMENTAL-STUDY OF THE CHEMICAL-STRUCTURE OF LOW-PRESSURE PREMIXEDN-HEPTANE-O-2-AR AND ISO-OCTANE-O-2-AR FLAMES

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and species identification and concentration measurement have be en performed by mass spectrometric analyses of samples withdrawn local ly by molecular beam formation.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.