L. Zalotai

Effect of the uncertainty of kinetic and thermodynamic data on methane flame simulation results

A method for assessing and comparing the impact of uncertainties in both kinetic and thermodynamic parameters on the predictions of combustion chemistry models has been developed. Kinetic, thermodynamic and overall uncertainty parameters are defined, which allow tracking the sources of uncertainties for a particular model result. The method was applied to premixed laminar methane-air flames using the Leeds Methane Oxidation Mechanism (K. J. Hughes et al., Int. J. Chem. Kinet., 2001, 33, 513). Heat of formation and rate coefficient data for species and elementary reactions, respectively, related to methane combustion were collected from several recent reviews and critically assessed error limits were assigned to them. Local rate coefficient sensitivities and heat of formation sensitivities were calculated for lean (φ = 0.62), stoichiometric (φ = 1.00) and rich (φ = 1.20) laminar atmospheric premixed methane-air flames. Uncertainties of flame velocity, maximum flame temperature and also the value and location of maximum concentration of radicals H, O, OH, CH and CH2 were obtained from the sensitivities and the uncertainties of thermodynamic and chemical kinetic data. The uncertainty of the calculated flame velocity is typically 2–5 cm s−1. Maximum flame temperature and concentration of H, O, and OH can be calculated accurately, while there is high uncertainty in the calculated maximum concentration of CH and CH2. The calculations have revealed that the uncertainty of the calculated flame velocity is caused mainly by errors of the input rate coefficients. This is the case also for the calculated concentration of CH and CH2. The uncertainty of the location of concentration maxima is also of kinetics origin and it is caused by the very same rate coefficients that affect flame velocity. Uncertainty of maximum adiabatic flame temperature and maximum concentration of H, O and OH originates mainly from errors of the input heat of formation data. In order to obtain good simulation results for methane flames, accurate heats of formation are required in particular for radicals OH, CH2(S), CH2, CH2OH, HCCO and CH2HCO. Simulation results could be improved by better knowledge of the reaction rate parameters for the reactions O2 + H = OH + O, O2 + H + M = HO2 + M, CO + OH = CO2 + H, H + CH3(+M) = CH4(+M), CH3 + OH = CH2(S) + H2O, C2H2 + OH = C2H + H2O and C2H2 + CH = C2H + CH2. This conclusion is somewhat surprising since at least the first three reactions are among the most frequently studied ones in chemical kinetics. The calculations demonstrate that all simulation results of chemical kinetic modelling studies should be accompanied by uncertainty information (e.g. standard deviation) for the model outputs to indicate which results are well supported by the model and which ones are merely nominal values that were obtained using the selected set of input parameters.

Kinetics and energy transfer in the thermal decomposition of 2-methyloxetane and 3-methyloxetane

The thermal decomposition of 2-methyloxetane and 3-methyloxetane has been studied between 660 and 760 K in the pressure range 0.01–3 kPa. In the pressure-independent range, rate coefficient expressions log(k∞1/s–1)=(14.53 ± 0.12)–(249.2 ± 2.2 kJ mol–1)/2.303RT and log(k∞2/s–1)=(15.67 ± 0.17)–(269.8 ± 3.3 kJ mol–1)/2.303RT were determined for 2-methyloxetane decomposition into C3H6+ HCHO (k1) and C2H4+ CH3CHO (k2), respectively, while for 3-methyloxetane decomposition into C3H6+ HCHO (k3) the following kinetic parameters were obtained: log(k∞3/s–1)=(15.38 ± 0.27)–(258.7 ± 3.7 kJ mol–1)/2.303RT. The pressure dependence of the homogeneous decomposition rate and the efficiency of the gas-phase collisional energy transfer have been studied at 743 K. A value of 〈ΔE〉d= 1500 ± 300 cm–1 was extracted from the investigation of the pressure dependence of the two-channel decomposition of 2-methyloxetane. Finally, the efficiency of the surface–gas energy transfer has been studied by the ‘variable encounter method’ in the range 750–1100 K. At 750 K the average energy transferred per collision with the wall was determined to be 2600 cm–1 for both methyloxetanes; however, 〈ΔE′〉 decreased considerably with increasing temperature. The results on the collision efficiencies were discussed and compared with literature data for related molecules.

Surface-gas energy transfer in the oxetane decomposition system

The method called the variable encounter method was applied in the oxetane decomposition system for the study of vibrational energy transfer between gas molecules and the surface. The average probability of reaction per collision was derived from the experimental data and compared with theoretical calculations based on various energy transfer probability models. The Gaussian model fits the data well. The average down stepsize was found to be 3100 cm−1 at 750 K and it decreased to 2200 cm−1 at 1100 K.AbstractТак называемый метод изменяющихся столкновений применяли для исследования передачи колебательной энергии между молекулами газа и поверхностью в разложении оксетана. Среднюю вероятность реакции на одно столкновение получали из экспериментальных данных и сравнивали с теоретическими расчетами, исходящими из моделей вероятностей передачи различных энергий. Гауссовское распределение хорошо согласуется с экспериментапьными данными. Средняя нижняя величина ступени равна 3100 cm−1 при 750 К и уменьшается до 2200 cm−1 при 1100 К.

Collisional energy transfer in the thermal decomposition of oxetane

The first order rate coefficient for the thermal decomposition of oxetane and oxetane-d2 has been investigated at two temperatures as a function of pressure. Gas phase collisional relaxation results are obtained by using RRKM theory and various energy tranfer probability models.AbstractКонстанту скорости первого порядка для термического разложения оксетана и оксетана-d2 измеряли при двух температурах в зависимости от давления. Данные релаксации за счет столкновений в газовой фазе получены, исходя из теории RRKM и на основе различнх моделей вероятностей передачи энергии.

Collisional energy transfer in the two-channel decomposition of 1,1,2,2-tetrafluorocyclobutane and 1-methyl-2,2,3,3-tetrafluorocyclobutane, II. Gas/wall collisions

The two-channel thermal decomposition of 1,1,2,2-tetrafluorocyclobutane (TFCB) and 1-methyl-2,2,3,3-tetrafluorocyclobutane (MTFCB) have been studied in the temperature range of 730–805 K at pressures varied from 1.1 Pa up to 4.6 kPa. In the pressure independent range, Arrhenius expressions were obtained for TFCB decomposition into 2 CH2CF2 (k1) and C2H4+C2F4 (k2), respectively. The same kinetic equations were determined for the decomposition of MTFCB into C3H4F2+C2H2F2 (k3) and C3H6+C2F4(k4). From the study of the pressure dependence of the homogeneous decomposition rates, the average downward energy transfer values of 1800±200 cm−1 and 1600±200 cm−1 were obtained for the TFCB and MTFCB molecules, respectively.

Kinetics of reactions of chlorine atom/arene complexes withn-heptane

Room temperature rate constants have been determined for reactions ofn-hepatane with Cl/benzene (k=6×107 dm3 mol−1 s−1), Cl/toluene (k=1×107 dm3 mol−1 s−1) and Cl/m-xylene (k=1.7×106 dm3 mol−1 s−1) complexes, respectively, in carbon tetrachloride, using the laser flash photolysis of nitrogen trichloride as a chlorine atom source.

Collisional energy transfer in the decomposition of 2-methyloxetane and 3-methyloxetane, II. Gas/wall collisions

The pressure dependence of the unimolecular rate constants for the thermal decomposition of 2-methyloxetane and 3-methyloxetane has been studied. The average energy transferred downward in gas-gas collision was determined by the application of RRKM theory and a stepladder model of energy transfer.AbstractИсследовали зависимость мономолекулярных констант скоростей термического разложения 2-метилоксетана и 3-метилоксетана. Среднюю энергию, переносимую на нижележащие уровни при газ-газовых столкновениях, определяли с помощью РРКМ теории и на основе лестничной модели передачи энергии.