Vibrational Relaxation of Oxygen by Methane, Acetylene, and Ethylene

Abstract

The vibrational relaxation of oxygen containing from ¼% to 2% of CH4, C2H2, and C2H4 has been studied by shock-tube interferometry. From about 450° to 1300°K the vibrational relaxation zone is not affected by chemical reaction, and the observed oxygen relaxation in mixtures containing only small amounts of hydrocarbon may be accounted for by using the standard formula for a binary mixture and assuming the relaxation time in seconds of oxygen dilute in 1 atm of the hydrocarbon to be given by log10τO2–CH4 = 18T—⅓−8.7, log10τO2–C2H2 = 7.5T—⅓−7.3, and log10τO2–C2H4 = −6.92. For higher temperatures, the exothermic reaction may result in a nonlaminar flow behind an aplanar shock, and the relaxation zone then cannot be resolved interferometrically. For still higher temperatures the relaxation zone is again laminar, but the vibrational excitation of O2 is accelerated by concurrent chemical reactions in the induction zone. From the observation at lower temperatures that τO2–CH4<τCH4 and τO2–C2H4<τC2H4 one is led to conclude that at least below 800°K collisions with O2 are more effective in the excitation of CH4 and C2H4 than are self-collisions, and that the excitation of O2 by collision with a hydrocarbon molecule is through a vibrational exchange reaction with the more easily excited hydrocarbon molecule.