Interaction of a vortex with a flat flame formed between opposing jets of hydrogen and air

Abstract

Studies on individual vortex-flame interactions constitute important elements for the understanding of the turbulent-flame structure. Vortices having sufficiently high normal velocity can pass through the flame by extinguishing it locally. In several circumstances they deform the flame surface significantly before attaining extinction conditions. The development of curvature on the flame surface, especially in hydrogen flames, could lead to different quenching patterns. An experimental/numerical investigation is performed to explore possible quenching patterns in opposing-jet diffusion flames. A diluted hydrogen-nitrogen mixture is used as the fuel. Vortices are driven toward the flame surface with different velocities from the air side. The changes in the structure of the flame during its interaction with the incoming vortex are recorded by measuring instantaneous OH-concentration field using the laser-induced fluorescence (LIF) technique. A time-dependent CFDC code that incorporates 13 species and 74 reactions is used for the simulation of these vortex-flame interactions. Both the experiments and calculations have identified two types of quenching patterns: namely, point and annular. It is found that when an air-side vortex is forced toward the flame at a relatively high speed, then the flame at the stagnation line quenches, resulting in a well-known point-quenching pattern. On the other hand, when the vortex is forced at a moderate speed, the flame surface deforms significantly, and quenching develops in an annular ring away from the stagnation line, resulting in an unusual annular-quenching pattern. Detailed analyses performed just before the development of annular quenching and 1 ms later suggest that this unusual annular quenching did not result from the strain rate. Based on the understanding gained from previous investigations on curvature effects in coaxial hydrogen jet flames and the findings made in the present study, it is argued that such quenching develops as a result of the combined effect of preferential diffusion and flame curvature.