Fire spread characteristics: through homogeneous and stratified combustile mixtures within tubes

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A nine-foot long, two and a half inches inside diameter plexiglass tube was used in examining the flame propagation, from an open end towards a closed end within both homogeneous and stratified mixtures of methane and air. Relatively uniform flame propagation within homogeneous mixtures in the three main directions (upward, horizontal and downward) was ensured throughout the full length of the flame tube by the proper' choice of the size of the open end where ignition took place. A new technique employing fiber optics was developed for the determination of flame propagation rates in the present set up. This technique was found to be simpler and more convenient than other conventional methods of measuring flame velocity. Flame fronts with a cellular structure were observed in the present study under a variety of conditions. Accordingly, this phenomenon was examined in relation to the factors contributing to it and the possible mechanism of the formation of these cells. Stratified methane-air mixtures were obtained by the interdiffusion for a specified period of two initially homogeneous mixtures having different densities. The transient concentration profiles along the length of the tube were then obtained analytically by finite difference approximation of the theoretical diffusion model due to the density difference derived from the conservation of mass and momentum equations. The numerical solution showed reasonable agreement with results deduced experimentally by ultrasonic transducers. The velocities of accelerating flames within stratified mixtures were found to be approximately the same as those under corresponding local homogeneous conditions. However, the velocities of decelerating flames within stratified mixtures were observed to be generally greater than those under homogeneous conditions. These enhanced flame velocities were correlated with the corresponding local values under homogeneous conditions, local concentration, and local concentration gradient. A discussion is then made of the probable reasons for this enhancement of flame propagation.
Bibliography: p. 156-162.
Badr, O. A. (1978). Fire spread characteristics: through homogeneous and stratified combustile mixtures within tubes (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from doi:10.11575/PRISM/18640