An examination of the partial equilibration hypothesis and radical recombination in 1/20 atm methane flames

Abstract

Slightly lean CH4−O2−Ar flames at 0.042 atm with and without small amounts of the inhibitor CF3Br were probed using molecular beam-mass spectrometric techniques. All the principal radical and stable species concentration profiles and temperature profiles were measured in the primary and part of the secondary zone of these flames. Ratios of product concentrations to reactant concentrations were formed and compared with the equilibrium constant for the following reactions: H + O2 = OH + O (1) O + H2 = OH + H (2) OH + H2 = H2O + H (3) OH + OH = H2O + O (4) CO + OH = CO2 + H (5) Br + HBr = Br2 + H (6) H + HBr = H2 + Br (7) in order to determine to what extent they may be considered balanced in various regions of the flame. Although, with the possible exception of reaction (4), none of these reactions are completely balanced until the maximum flame temperature is reached, the order in which the first five reactions become partially equilibrated is (4), (2) and (3), (5), (1). Once these reactions become equilibrated, their balance is maintained as the temperature and radical concentrations fall off in the secondary reaction zone. Reaction (7), in inhibited flames, is balanced at relatively low temperatures, comparable to (4); reaction (6) was found not to balance in either the secondary or primary reaction zones. The radical decay in the secondary reaction zone is analyzed using the concept of pool decay. The important recombination reactions were judged to be H + OH + M → H2O + M and H + O2 + M → HO2 + M. Assuming equal rate coefficients and chaperon efficiences for these three reactions, rate coefficients of 2 × 1016 cm6 mole−2 sec−1 are calculated for M = the burnt gas mixture of Ar, H2O, CO2 and 5 × 1015 cm6 mole−2 sec−1 for M = Ar, both for T = 1750 ± 150 K.