Recombination of Methyl Radicals. 2. Global Fits of the Rate Coefficient

Abstract

The temperature-and pressure-dependent behavior of the cross section for optical absorption by the methyl radical is carefully considered, so we may define a criterion for selecting and correcting measurements of the rate coefficient for the recombination of methyl radicals, CH 3 + CH 3 f C 2 H 6 . The low-temperature data of Slagle et al., Hippler et al., and Walter et al. and the high-temperature data of Glänzer et al., Hwang et al., and our latest results (previous paper in this issue) are used to define a data set which contains 217 points. Subsets of isothermal data show that the temperature dependence of the high-pressure rate coefficient may be described by the simple exponential function A ∞ exp{-T(K)/T ∞ }. Four different formulations for the pressuredependent behavior in the falloff region are used for the global fits: (1) the asymmetric Lorentzian broadening function of Gilbert et al.; (2) the Gaussian broadening function of Wang and Frenklach; (3) the empirical "a equation" introduced by Gardiner; (4) the extension of Lindemann's expression suggested by Oref. All formulations reproduce the data, but Oref's "J equation" produces the least correlation between the best-fit parameters, the least uncertainty in these parameters, and the smallest uncertainty in the predictions. These results are k ∞ (T) ) 8.78 × 10 -11 exp{-T(K)/723} cm 3 s -1 , k 0 (T) ) 9.04 × 10 -27 cm 6 s -1 , and J exp (T) ) {exp[T(K)/268] -1} 2 .

High-temperature ethane and propane decomposition

“…High-pressure rate coefficient for C 2 H 6 fi CH 3 + CH 3 (700-1924 K). Symbols, current RRKM results (listed on figure); solid line, fit to current RRKM results; dashed line, Klippenstein and Harding [19]; dotted line, Hessler and Ogren [17]; and dot-dashed line, Wagner and Wardlaw [13]. Fig.…”

“…Previous high-temperature shock tube measurements have utilized different techniques: gas chromatography in a single-pulse shock tube [1], H-atom atomic resonance absorption spectroscopy (ARAS) [2,3], IR absorption of ethane at 3.39 lm [4], xenon lamp absorption of CH 3 [5], laser-schlieren [6,7], and laser absorption of CH 3 [8][9][10][11]. Reaction (1) has also been the subject of many theoretical studies of varying levels of complexity [12][13][14][15][16][17][18][19][20]. At low temperatures (<1000 K) the theoretical studies agree with each other and with the experimental data quite well, due in a large part to the excellent low-scatter 1540-7489/$ -see front matter Ó 2004 The Combustion Institute.…”

Experimental measurements and kinetic modeling of CH4/O2 and CH4/C2H6/O2 conversion at high pressure

“…h Enhanced third-body efficiencies: Ar = 0.7, H 2 = 2, H 2 O = 6, CH 4 = 3, CO = 1.5, CO 2 = 2, C 2 H 6 = 3. i Enhanced third-body efficiencies: H 2 = 2, H 2 O = 5, CO = 2, CO 2 = 3. dependence of the high-pressure limit. Shock tube experiments [90][91][92], supported by theoretical studies [93,94], indicate a slightly negative temperature dependence, whereas other data [95][96][97], including the preferred expression from Baulch et al [76], advocate a temperature independent high-pressure limit.…”

The use of Monte Carlo simulations to evaluate kinetic data and analytic approximations

“…Pawlowska, Gar-812 HESSLER Figure 9 Natural logarithm of the rate coefficient data for the eight higher temperature points of the high-pressure limit of methyl-methyl association given in Table III diner, and Oref have reviewed the major approaches used to solve this problem [24]. Recently [25], we tested all of these and the new equation of Prezhdo with a set of 217 measured rate coefficients for the methyl-methyl associative reaction [26,27]. The results for the traditional analysis are given in Table VI.…”