Ignition and flame propagation within stratified methane-air mixtures formed by convective diffusion

Abstract

The concentration changes with time and space of methane following its release into air, under the combined effects of molecular diffusion and natural convection were examined. A smooth, cylindrical, vertical flame tube of 116.0 inches (2.946 m) long and 2.5 inches (63.5 mm) inside diameter was used. The tube was separated into two sections by a plane shutter: a lower small fuel section and an upper large air section. Fuel stratification, caused mostly by convective diffusion, was initiated following the opening of the shutter. The concentration profile as a function of diffusion time at specified regions of the tube was determined by measuring the velocity of sound in the medium using a specially developed “ultra-sonic gas analyzer”. To predict the experimental concentration profile in such a system an eddy diffusivity that is a function of both the diffusion time and the local concentration gradient is needed. The range of diffusion time within which a flammable zone was developed within the stratified mixtures was determined at various locations employing a periodic spark source, for downward flame propagation. It was observed that the flammability limits of downward flame propagation within stratified methane-air mixtures were wider than those obtained for the corresponding homogeneous quiescent mixtures. Flame speeds of both accelerating and decelerating flames within stratified methane-air mixtures caused by the natural convection process were found to be higher than those encountered under homogeneous conditions at the corresponding methane concentration. Some possible reasons for the enhanced flame speeds are discussed.