Charles P. Lazzara

Molecular beam mass spectrometry applied to determining the kinetics of reactions in flames II. A critique of rate coefficient determinations

The microstructure of low-pressure methane-oxygen-argon flames was investigated using modulated molecular beam-mass spectrometry. Profiles of radical and stable species concentration, temperature, and area expansion ratio were used to calculate rate coefficients as a function of temperature for certain elementary reactions occurring in flames, namely, H + O/sub 2/ ..-->.. OH + O, H + CH/sub 4/ ..-->.. CH/sub 3/ + H/sub 2/, CO + OH ..-->.. CO/sub 2/ + H, CH/sub 3/ + O ..-->.. H/sub 2/CO + H, and H + CF/sub 3/Br ..-->.. HBr + CF/sub 3/. The profiles were modified (computationally) to simulate the effect of various perturbations and errors possible in sampling and analysis, and the effect on the rate coefficients is discussed. Detailed consideration is given to data reduction techniques, temperature profile-composition profile alignment, and the possible temperature dependence of mass spectral fragmentation. The rate coefficients are not dramatically sensitive to the imposed perturbations, although the results depend upon the nature of the reaction in question. Rate coefficients determined for high activation energy reactions and for reactions singularly responsible for the chemical behavior of a given stable species are in agreement with values determined by other techniques. Flame structure studies in which all significant radicalmorexa0» and stable species are measured by a single technique are judged to be viable sources of high-temperature rate data for elementary reactions, where such reactions have been identified.«xa0less

Flame-structure studies of CF3Br-inhibited methane flames

Low-pressure flat flames have been probed with a molecular-beam sampling system coupledto a modulated-beam mass spectrometer. The purpose of the study is to determine and compare the microstructure of normal CH 4 -O 2 -Ar flames, and those inhibited by CF 3 Br, in order to provide some basis for an understanding of how inhibition is effected in this system. The experimental facility is new and is described in some detail. Composition profiles of slightly lean, uninhibited flames were obtained for the major stable species (CH 4 , O 2 , CO, CO 2 , H 2 O) and H 2 and H 2 CO. Sampling at low electron energies gave, with appropriate corrections, a profile for the 17 + peak which is assigned to hydroxyl radical. Profiles were also determined for a CH 4 -O 2 -Ar flame containing 0.3% CF 3 Br. In addition to the usual flame species, the inhibitor, its products, and intermediates associated with its reactions could be followed. In particular, HBr and Br atoms could be followed; hydrogen fluoride and carbonyl fluoride were also observed. The inhibited flame is characterized by an early disappearance of inhibitor and correspondingly early appearance of products due to its reaction. Carbonyl fluoride and HBr behave like intermediates. Hydrogen fluoride and bromine atoms are the primary equilibrium flame products to be associated with the inhibitor. The immediate appearance of fluorinated species suggests a short lifetime for the CF 3 radical in this system, and the observation of carbonyl fluoride suggests that this inhibitor is effective, at least in part, by virtue of its interference with the oxygen-containing species of the normal chain mechanism.

Flame structure studies of CF3Br-Inhibited methane flames: II. Kinetics and mechanisms

Composition profiles for atomic, radical, and stable species, as well as temperature and area expansion ratio profiles, have been determined for a nearly stoichiometric CH4−O2−Ar flame and for one to which 0.3% CF3Br inhibitor had been added. Net reaction rate profiles were calculated for all the observed species. For the normal flame, these and the mole fraction profiles gave rate coefficient information about the elementary reactions in the methane flame, viz., C H 3 + O → 4 H 2 C O + H , k 4 = 1.05 × 10 14 c m 3 m o l e − 1 sec u2061 − 1 for 1350≤T≤1750°K. Comparison between the inhibited and normal flame showed that [H] and [CH3] were significantly reduced at the lower temperatures in the inhibited flame even though in the hot gas region the [H], [OH], and [O] were the same in both flames. The CF3Br disappears very early in the flame, relative to the fuel, and the reaction primarily responsible for its disappearance is H + C F 3 B r → 7 H B r + C F 3 where k7 is found to be 2.2×1014 exp (−9460/RT), 700–1550°K. Reaction, of the inhibitor with methyl radicals provides for the relatively small amounts of CH3Br observed, but CH3+Br2→CH3Br+Br must also occur. The HBr formed reacts rapidly with H atoms to form H2 and Br, but the reaction is soon “balanced” in the flame as demonstrated by calculation of the equilibrium constant at various temperatures. The fluorocarbon fragment produced in reaction (7) also reacts rapidly, in part with methyl radicals to give the observed elimination product CH2CF2. The magnitude of the net reaction rate for both HF and F2CO early in the flame indicates that these, too, are formed by rapid reactions involving CF3. Later in the flame, above ∼1400°K, F2CO is formed from the reaction C H 2 C F 2 + O → 18 F 2 C O + C H 2 and k18∼1.5×1013 at 1600°K. The rather slow decay of carbonyl fluoride is attributed to reaction with H atoms, and the sequence F2CO+H→HF+FCO and FCO+H→HF+CO plus reaction (6) provides an additional radical recombination route in the inhibited flame.

Flame spread over horizontal surfaces of polymethylmethacrylate

The U.S. Bureau of Mines conducted a large-scale study to determine the horizontal flame spread rates over the top surface of polymethylmethacrylate (PMMA) sheets. This study is part of the Bureaus program to gain a better understanding of the fire propagation of combustible materials in ventilated mine entries. The experiments were performed in a fire gallery at airflows of 0.80, 1.4, and 3.8 m s −1 with samples 9.75 m long by 1.07 m wide by 6.35 mm thick. The PMMA sheets were placed on top of 12.5-mm-thick calcium silicate boards and the upstream edge ignited with a 1 liter heptane tray fire. Pyrolysis front spread rates were calculated from temperature measurements obtained from thermocouples mounted with their beads about 1 mm above the PMMA surface and spaced along the centerline. Gas temperatures and average downstream concentrations of CO, CO 2 , and O 2 were also measured and peak heat release rates estimated. The experimentally measured pyrolysis front rates of 6±2 cm s −1 were essentially independent of airflow, and are an order of magnitude higher than those of large-scale experiments reported by other investigators whose sample lengths were too limited. These largescale spread rates are also about three orders of magnitude greater than those measured in small-scale studies where laminar flow and diffusion control the pyrolysis front rate. The rapid pyrolysis front rates reported here give good agreement with theoretical predictions based on a radiation dominated heat transfer mechanism from an optically thick flame plume, a realistic ignition temperature and physical properties for the PMMA solid.