Absolute Mole Fraction Research Articles

Infrared emission spectroscopy in H2/F2 and CH4/F2 premixed flames

AbstractThe infrared emission corresponding to the Δ v=1 transitions of the vibrationally excited HF (1 < =v< =3) has been recorded in rich premixed hydrogen‐fluorine and methanefluorine flames burning at various pressure (1.3–9 torr). The initial hydrogen and fluorine element content was identical in both CH4‐and H2‐ fluorine flames.The rational temperature profiles have been determined as well as in H2/F2 flames than in CH4/F2 ones. The analysis of the experimental data has put forward the occurrence of an equilibrated vibrational population relatively early in the burnt gases region of flames burning at pressures higher than 5 torr. From these evidences, profiles of the relative mole fractions of HF in its vibrationally excited states (1 < =v< =3) have been established in both series of flames. The absolute total concentrations of HF have been deduced by considering the Lewis number equals to one and therefore the absolute mole fraction profiles of the individual vibrationally excited HF (0 < =v< =3) have been determined.Besides the well‐known mechanism of the HF formation in H2/F2 systems it appears clearly that in CH4/F2 flames the final concentration of fluorocarbon derivatives are close to those of fluorhydric acid. This evidence indicates that in rich methane‐fluorine flames the reaction CH3 + F2 → CH2F + HF and subsequent reactions of CH2F are important in addition of the primary attack of methane CH4 + F → CH3 + HF and CH4 + F → CH3F + H.

Hydroxyl and its concentration profile in methane–air flames

Hydroxyl concentration profiles are measured in several atmospheric-pressure premixed laminar flames. The first quantitative comparison of hydroxyl concentration profiles to recent flame models is presented. These models include the influence of hydroxyl on fuel breakdown, on carbon monoxide oxidation, and on other species concentrations. The hydroxyl concentration profiles are measured by laser-induced fluorescence, and absolute mole fraction concentrations are determined by normalizing to laser absorption measurements in similar flames. The 100-microm spatial resolution of laser-induced fluorescence provides the first optical measurements of the flame's reaction zone at atmospheric pressure. The data are consistent with the electronic-excited state of hydroxyl having a negligible influence on the flame's structure.

H and O atom profiles measured by ESR in C 2 hydrocarbon-O 2 flames

The feasibility of measuring labile atom profiles in flames by probe sampling combined with quantitative ESR spectroscopy is demonstrated for the first time. The method relies on using O2 as a reference calibrating gas, the validity of this procedure having been proven elsewhere. In this case the O2 was present as a diluent in the flame gases. The great potentialities of this technique in flame kinetic studies are shown by preliminary examples of absolute mole fraction profiles of H and O atoms measured in 0.1 atm flat flames of C2H5−O2, C2H4−O2, and C2H2−O2. These are the same flames whose major stable species concentration profiles were previously reported at the Seventh Symposium, and data obtained subsequently on their temperature and velocity profiles are also given. The major species profiles were corrected for diffusion and converted to net reaction rate profiles in the usual way. Profiles of OH concentration were computed from the measured CO2 formation rates using a rate constant for CO+OH→CO2+H fitted to a variety of reported data. The striking fact about the data is that the OH and fuel disappearance rate profiles definitely precede the appearance of any measurable H or O in each flame, thus pointing strongly to the exclusive reaction of the fuel with OH rather than with H or O. Since the O2 loss rate coincides with that of the fuel, it seems likely that the O2 reacts with the hydrocarbon fragment left after OH attack, with reaction via H+O2→OH+O occurring only later in the flame. The consequences of these observations are discussed. Rate constants estimated from the data are (cm3 mole−1 sec−1): 5×1012 for C2H6+OH in the range 1300°–1550°K, 1×1013 for C2H4+OH in the range 1250°–1400°K, and 5×1015exp(−1300/RT) for H2CO+OH.