Anomalous blow-off behavior of laminar inverted flames of ultra-lean hydrogen-methane-air mixtures

An experimental study of rod-stabilized laminar inverted flames of ultra-lean hydrogen-methane-air mixtures has been performed. For mixtures with high hydrogen content, anomalous stabilization and blow-off behavior has been observed. Flames in those mixtures could be stabilized at equivalence ratios below the lean flammability limit for a zero-stretch planar flame. Stabilization of such flames was possible only when the mixture velocity exceeded some critical value. Flames were blown off when the mixture velocity was reduced below this value. The stand-off distance above the flame holder for those flames decreased and heat transfer from the flame base to the flame holder became more intense when the mixture velocity was increased. This is opposite to the regular behavior of inverted flames. These observed unusual phenomena were attributed to the combination of a strong flame stretch and preferential diffusion effects and to the negative value of the Markstein length in mixtures with high hydrogen content. According to the suggested explanation, increasing the velocity results in an increase of the flame stretch rate at the flame base. This, in turn leads to higher flame temperatures and higher burning velocities, making the survival of the flame below the flammability limits possible. Along with the anomalous blow-off behavior, normal blow-off occurring at increased velocity was observed for mixtures with high hydrogen content at the lowest tested equivalence ratios. The observation of the flame shape evolution showed that, when the normal blow-off limit is approached, a flame narrows slightly above the flame base, forming a neck. The flame fronts merge at the neck location and flame breaks there, leading to complete flame extinction or leaving a very small flamelet near the flame holder. It is suggested that the flame local extinction in that case occurs as the result of the excessive flame stretch at the flame neck which leads to the flame fronts merging and to the incomplete reaction. (C) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.