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 $\frac{1}{5}$ 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.