The spectral radiance growth, flame temperatures, and flammability behavior of large-scale, spherical, combustion waves

Abstract

The absolute spectral radiances of the infrared emissions from spherical combustion waves in methane-air mixtures were measured as a function of methane concentration, nitrogen dilution, and CF 3 Br addition. Data were obtained for the spectral range 1.7 to 4.8 μm, in a 3.66 meter (12 ft.) diameter spherical vessel, from the time of ignition to complete combustion. Radiance growth patterns for the H 2 O and CO 2 bands are resolvable into an early stage of emissivity growth and a subsequent stage of adiabatic compression. The absolute radiance of the CO 2 band peak near 4.4 μm is used to obtain measured flame temperatures, which are compared with calculated adiabatic values. Measured temperatures for the more rapidly propagating methane-air and nitrogen-diluted mixtures are nearly identical to adiabatic values. The radiance data are spectrally integrated to obtain broad-band emissivities for emission paths up to ( p H 2 O + P CO 2 ) L ≅ 7 meter atm, and temperatures as high as 2550°K. For methane-air mixtures diluted with 2% to 4% Halon, flame temperatures are significantly lower than adiabatic values during the early stages of inhibited propagation. This is followed by a rapid stage of uninhibited combustion which generates near-adiabatic pressure rises. This anomalous propagation behavior is attributable to the effects of adiabatic compression and/or turbulence. Two possible mechanisms for the reduction in Halon effectiveness are selective diffusional demixing of the CF 3 Br molecule and changes in the relative significance of pressure-dependent kinetic pathways. Low-frequency radiance oscillations are observed for all near-limit mixtures, and some general estimates of large-scale propagation behavior are made in terms of a buoyancy-induced Reynolds number.