IGNITION OF METHANE FLAMES IN OXYGEN NEAR INERT SURFACES - EFFECTS OFCOMPOSITION, PRESSURE, PREHEAT, AND RESIDENCE TIME

Ignition behavior of premixed methane-oxygen mixtures in stagnation fl ow near a heated inert surface was examined using the GRI reaction mec hanism [1]. The effects of pressure (1-100 atm), preheat (298-773 K), and residence time (3-250 ms) on ignition temperature were investigate d for the full range of fuel-to-oxygen ratios. A minimum in ignition t emperature with composition occurred at about a 15% methane in oxygen feed (phi = 0.3) and was not affected strongly by pressure, preheat, o r residence time. At atmospheric pressure, thermal feedback from the h eat of reaction was a prerequisite for ignition at all fuel-to-oxygen ratios. However, at 50 atm, thermal feedback was only necessary for ig nitions of mixtures leaner than 15% methane. For mixtures richer than 15% methane at 50 atm, ignitions due to chain branching preceded therm al ignitions. These chain-branching ignitions were unaffected when the thermal feedback was computationally turned off. Moreover, the fuel-r ich ignitability limit increased from 55% methane at atmospheric press ure to 66% methane at 50 atm, while the fuel lean ignitability limit a t 7% methane was not affected significantly by pressure. Reaction path analysis before ignition showed that for high-temperature ignitions, methane consumption for fuel-lean mixtures was by OH and O radicals, w hile for fuel-rich mixtures, it was by H and OH radicals. The main sou rce of OH radicals for these ignitions was by the reaction of H and O- 2. For low-temperature ignitions, methane was predominantly consumed b y OH radicals before ignition regardless of feed composition, and the main source of OH radicals was by reactions involving HO2 and H2O2. (C ) 1997 by The Combustion Institute.