Structure and propagation of triple flames in partially premixed hydrogen-air mixtures

The characteristics of triple flames in a hydrogen-air mixing layer are studied using direct numerical simulation with detailed chemistry. Triple flames are initiated by imposing a temperature ignition source in the center of a scalar mixing layer of nonuniform thickness, thereby forming a pair of freely propagating triple flames. Two different fuel streams are studied: pure hydrogen and hydrogen diluted with nitrogen. During the ignition stage, the initial ignition runaway is followed by a secondary peak as the ignition kernel transitions to a triple flame, consistent with previous observations. For both diluted and undiluted cases, the triple flame structure exhibits more similarity with a diffusion flame than with a premixed flame, such that the triple point, defined as the location of maximum heat release, is always in the proximity of the stoichiometric mixture fraction line. Similar to a previous study of methanol-air triple flames, the enhancement in the stabilization speed is attributed mainly to flow divergence, and its value is proportional to the square root of the density ratio across the flame. In the undiluted case, however, the asymmetric flame structure results in distinct locations where the stabilization speed and the displacement speed are maximum. The effect of unsteady strain rate is also studied by imposing a pair of vortices on the propagating triple flames. The negative strain rate results in the collapse of the premixed flame branches onto the diffusion flame, forming an edge flame structure. Excessive compressive strain and curvature at the triple flame tip leads to a negative displacement speed. A mixture fraction/temperature parameterization is shown to be useful in representing the structure of a triple flame subjected to unsteady strain rate.