Flame Structure Studies. III. Gas Sampling in a Low-Pressure Propane-Air Flame

Abstract

A previous investigation of this series showed that the temperature in a very lean, low-pressure propaneair flame continues rising for more than a centimeter downstream from the zone of blue luminosity. This suggested a two-stage combustion process: (I) the hydrocarbon oxidizes rapidly and luminously to carbon monoxide; (II) the carbon monoxide oxidizes more slowly to carbon dioxide. The current gas-sampling study has provided support for this hypothesis. Both water-cooled and uncooled probes have been used to obtain gas samples from the region of the flame just beyond the luminous zone. Tests were made to insure that reaction-quenching in the probe was effective and that flame disturbance was minimized. Gas analyses were performed both mass-spectrometrically and by a vacuum-freezing technique. Most of the samples were taken from a flame burning at 46 mm Hg abs pressure, with a mass air-propane ratio of 30. Substantial proportions of CO, and about 15 as much H2, were found as much as a centimeter downstream from the luminous zone, with probes of much less than a millimeter diam. The CO concentration decayed exponentially with distance; a kinetic analysis yielded a decay constant of 231 reciprocal seconds. (The flame temperature was 1605°K.) The H2/CO ratio was considerably less than that corresponding to water-gas equilibrium in the flame. No traces of hydrocarbons were found beyond the luminous zone.