Flame structure and flame reaction kinetics. VII. Reactions of traces of heavy water, deuterium and carbon dioxide added to rich hydrogen + nitrogen + oxygen flames

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Abstract
The overall reactions of traces of heavy water, deuterium and carbon dioxide added to some fuel-rich hydrogen + nitrogen + oxygen flames have been examined both experimentally and theoretically. The mechanism and most of the controlling rate constants for the flames both without and with the additives are now sufficiently firmly established for computation of the detailed flame profiles to be possible in all cases. By comparison of the computed profiles with experiment it is possible to deduce information about the reactions: OH + H2 ⇌ H2O + H, (i) OH + HD ⇌ HOD + H, (iDa) OH + CO ⇌ CO2 + H. (xxi) At 1050 K the ratios k1/k1Da = 2.8 and k1/k21 = 11.3 were found, both with an accuracy of ± 15%. Assuming A1/A1Da = 2.4 (the ratio of collision frequencies) led to a mean k1/k1Da = 2.4 exp (+ 155/T); while combination of the observed ratio k1/k21 with results of other investigators led to k1/k21 = (120 ± 15)exp{ — (2400 ± 50)/T}. It was also possible to relate the absolute rate constants for the above reactions fairly precisely with the hydrogen atom recombination rate constant k15, so that for a known value of the latter, the former could be derived with an accuracy of ± 10 to 15%. For k15 = 4.5 x 1015 for all molecules, the values found for the three rate constants were k1 = (2.7 ± 0.4) x 1012, k1Da = (9.6 ± 0.5 ) x 1011 and k21 = (2.4 ± 0.12) x 1011, all at 1050 K. Comparison of k1 and k21 with results of other investigators at comparable temperatures suggested that both may be slightly high. Lack of knowledge of the precise form of the relation between Ig k; and 1/T makes a closer evaluation difficult; but on the basis of a linear relation between 300 K and the flame temperature, a reduction of both k1 and k21 by about 10% would improve the agreement with much other work. By comparison with previous results showing the effect of variation of k15 on the H atom concentration profile (Dixon-Lewis 1970), this would suggest an optimum value of k15 of around 3.0 x 1015 cm6 mol-2 s-1 in the flame reaction zone. Combination of the observed values of k1 and k21 at 1050 K with those of other investigators led to the expressions lgk1 = 13.57 ± 0.15 — (2770 ± 100)/2.303T, lgk21 = 11.49 ± 0.15 — (370 ± 100)/2.303T.

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