Response of hydrogen-air opposing-jet diffusion flame to different types of perturbations

Studies on the structure of a strained flame in a steady or unsteady state are important to the understanding of turbulent-flame behavior. A numerical investigation was performed to explore the similarities and differences in unsteady flame structures that were established through various modes of perturbation in an opposed-jet diffusion flame. A diluted hydrogen-nitrogen mixture was used as the fuel. Vortices were driven toward the flame surface with different velocities from the air side, the fuel side, or both sides. Changes in the structure of the flame during its interaction with the incoming vortex/vortices were investigated using a time-dependent CFDC code that incorporates 13 species and 74 reactions. Calculations have identified two types of unsteady flames: traveling and stationary. It was found that when a vortex was issued from either the air or the fuel side, the flame not only became strained but also moved with the vortex head, creating a traveling unsteady flame. On the other hand, if two vortices were issued, one from each side of the flame, then the flame was subjected to an unsteady strain rate however, it was locked in a spatial location, leading to a stationary unsteady flame. The structures of theses two types of unsteady flames near the extinction limits were studied. Stationary unsteady flames were also established by flue tuating the fuel and air jets simultaneously and sinusoidally, and their structures were compared with those obtained during the vortex-flame interactions. It was found that in comparison with stationary unsteady flames, traveling unsteady flames can be subjected to higher strain rates without causing extinction.