Emission of Vacuum Ultraviolet Radiation from the Acetylene-Oxygen and the Methane-Oxygen Reactions in Shock Waves

Abstract

The C2H2–O2 and CH4–O2 reactions were studied by observing the emission of vacuum ultraviolet radiation from shock waves in gas mixtures containing 85 to 99% Ar and with 2×10−6≤[O2]≤60×10−6 moles/l. In both reactions after an induction period of length ti, the emission intensity rises exponentially with a time constant τ and then decreases about as rapidly. For the C2H2–O2 reaction, emission predominantly in the wavelength range 1500<λ<1700 A was observed. It was estimated that one photon was emitted in this range per 3×104 C2H2 molecules passing through the reaction zone at 1840°K. The integrated emission intensity depends on the temperature as if the process producing it has an activation energy of 15 kcal/mole. In the range of conditions 0.65≤[O2]/[C2H2]≤3.6 and 1300≤T≤2200°K it was found that log10([O2]ti) (mole sec/l)=−10.57+17 100/4.58T. The data for τ from mixtures with 0.65≤[O2]/[C2H2]≤7.7 fit the expression log10([O2]τ) (mole sec/l)=−11.48+17 100/4.58T with a standard deviation of a factor of 1.4 in τ. In the first approximation the values of [O2]ti and [O2]τ are functions of temperature only. In lean mixtures, however, [O2]τ and especially [O2]ti are larger than the average. These results are explained in terms of a branching chain mechanism in which the reaction H+O2=OH+O plays the dominant role in controlling the rate. The CH4–O2 reaction was studied in the range of conditions 0.5≤[O2]/[CH4]≤4.5 and 1800≤T≤2700°K. The emission intensities are an order of magnitude smaller and more strongly temperature dependent, τ is a factor of 2 or 3 larger, and ti is a factor of 20 larger at the low temperatures and a factor of 4 larger at the high temperatures than for the C2H2–O2 reaction. The kinetic implications of these findings are discussed.