No abstract

A reinvestigation of the absolute rate constants of the metathesis reactions C2H5• + HBr → C2H6 + Br• (R1) and C2H5• + HI → C2H6 + I• (R2) has been performed and led to the following Arrhenius expressions: k1 = 3.69(±0.95) × 10–11 exp(−10.62(±0.66)/RT), k2 = 1.20(±0.38) × 10–11 exp(−7.12(±1.059)/RT) in the temperature range 293–623 K (A/cm3 molecule–1 s–1, Ea/kJ mol–1). The study has been performed using a Knudsen reactor coupled to single-photon (VUV) photoionization mass spectrometer (SPIMS). Hydrocarbon free radicals have been generated externally before admission into the Knudsen reactor according to two different chemical schemes, enabling the generation of thermalized C2H5• free radicals. A minor correction to k1 and k2 for the wall loss of C2H5• (kw) has been applied throughout the temperature range. The obtained results are consistent regarding both the disappearance of C2H5• and the formation of closed shell products (n-C4H10, C2H4, C2H6), indicating that the chemical mechanism is largely understood and complete. Thermochemical parameters for C2H5• free radical resulting from the present kinetic measurements are discussed and point toward a slightly lower value for the standard heat of formation ΔfH298°(C2H5•) compared to some presently recommended values. On the basis of the present results and suitable data on the reverse reaction taken from the literature, we recommend ΔfH298°(C2H5•) = 117.3 ± 3.1 kJ/mol resulting from an average of “third law” evaluations using S298°(C2H5•) = 242.9 ± 4.6 J/K mol. The present work yields a standard heat of formation in satisfactory agreement with the results obtained by W. Tsang (ΔfH298°(C2H5•) = 119 ± 2 kJ/mol) despite using two very different experimental techniques.

Experimental mole fractions profiles of stable species and radicals are reported for a stoichiometric ethanol laminar premixed flame C 2 H 5 OH/O 2 /Ar burning at 50 mbar. Different mechanisms are tested by comparison of model predictions with experimental results. The results show that some ethanol mechanisms predict reasonably well reactants and products profiles but significant differences for many intermediate species remain.

A reinvestigation of the absolute rate constant of the metathesis reactions t-C4H9• + HBr → i-C4H10 + Br• (1) and t-C4H9• + HI → i-C4H10 + I• (2) was performed thanks to a recently developed apparatus consisting of a Knudsen reactor coupled to detection based on single-photon (VUV) photoionization mass spectrometry (SPIMS). It enables the generation of thermalized hydrocarbon free radicals owing to a source upstream of and external to the Knudsen reactor. The following Arrhenius expressions were obtained: k1 = 5.6(±1.4) × 10(–12) exp(−6.76(±0.94)/(RT)) and k2 = 2.0(±0.6) × 10(–11) exp(−8.48(±0.94)/(RT)) with R = 8.314 J mol(–1) K(–1) over the range 293 to 623 K. The mass balance of the reaction system based on closed shell product detection (CSPD) was checked in order to ensure the accuracy of the used reaction mechanism and as an independent check of k1 and k2. The wall-loss rate constants of the t-butyl free radical, kw(C4H9), were measured and found to be low compared with the corresponding escape rate constant, ke(C4H9), for effusion of t-C4H9• out of the Knudsen reactor. On the basis of the present results, the free radical standard heat of formation ΔfH298°(t-C4H9•) = 44.3 ± 1.7 kJ mol(–1) was obtained when combined with the kinetics of the inverse halogenation reaction taken from the literature and using S298°(t-C4H9•) = 322.2 J K(–1) mol(–1) following a “Third Law” evaluation method. The standard enthalpy for t-butyl free radical is consistent for both the bromination and iodination reactions within the stated uncertainties.

The main objectives of this research consist in achieving both experimental and numerical studies of the combustion of several flames using acetaldehyde as a fuel. Experimental mole fraction profiles of chemical species (stable, radical, and intermediates) have been measured in three CH 3 CHO/O 2 /Ar flat premixed flames stabilized at low pressure (50 mbar) and with equivalence ratios equal to 0.75, 1, and 1.25, respectively. The experimental setup used to determine the structure of one-dimensional laminar premixed flames consists of a molecular beam mass spectrometer system (MBMS) combined with electron impact ionization (EI). The reaction mechanisms proposed by Yasunaga et al. (2007) and by Marinov (1999) are tested by comparison of model predictions with experimental results. The results show that modeling predicts reactants and products mole fraction profiles reasonably well, but significant differences for many intermediate species remain. In order to improve the predictions for these intermediate species, several improvements on the Marinov's mechanism are suggested. They ensure a reasonably good modeling of the acetaldehyde flame structures.