Identification of the extinction mechanism of lean limit hydrogen flames based on Lewis number effect

Abstract

The conventional flame stretch analysis showed some technical limitations in describing the observed extinction mechanism without considering the heat transfer characteristics related to flame flow patterns. To improve the flame stretch analysis, this study investigated the extinction mechanism of lean limit hydrogen flames with the stabilized flame method. Multidimensional lean limit hydrogen flames stabilized in a tube of inner diameter of 25 mm were simulated numerically. Unlike the methane flames, the positive effect of a stretched hydrogen flame (Le≪1) on the flame tip dominated the negative stretch effect by increasing the preferential diffusion. Hence the primary extinction of hydrogen flames occurred at the trailing edge by heat loss mechanism. The simulation confirmed that the radiation heat loss rate did not attain 5% of the conduction heat loss rate at the trailing edge. When the flame skirt length was shortened enough by the primary extinction, an enhanced recirculation flow was generated. The combined effect of conduction and recirculation flow associated with the Lewis number effect was the key to understand the extinction mechanism of lean limit hydrogen flames. Finally, generation of recirculating flow causing the stretch extinction plays a dominant role in final flame extinction.