Dynamics of Flame Propagation through Layered Methane-Air Mixtures

Abstract

Characteristics of flames propagating in vertical tubes in the direction of steep methane-air concentration gradients of about 1.3 volume-percent methane per cm were investigated using interferometric techniques. The maximum flame velocity for upward and downward propagation in both 2.5- and 5.0-cm diameter tubes occurred in the fuel-rich region; lower flame speeds were observed with the smaller tube diameter at corresponding methane concentrations. Following flame extinguishment in upward propagation, the hot product gas region behind the flame front continued to rise in a predictable fashion as a coherent column into the nonflammable fuel-rich region. This hot column rose at a constant velocity, approximately equal to the flame speed at the time of flame extinguishment. Flame extinguishment limits in the layered mixtures were found not to be strongly dependent on tube diameter. Interferometric techniques developed in this study appear to be useful for the rapid measurements of flammability limits and flame propagation rates.