Near-field flow stability of buoyant methane/air inverse diffusion flames

Experiment and simulation were performed to investigate buoyant methane/air inverse diffusion flames, with emphasis on the near-field flow dynamics under non-reacting and reacting conditions. In the non-reacting flow, the initial shear flow and the buoyancy effect induce opposite-direction vortices, which interact with each other and cause flow instability similar to the mechanism forming the von Karman vortex street. The instability is greatly intensified at around unity Richardson number, when the two vortices are comparably strong. In the reacting flows, the density gradient is reversed due to chemical heat release and so is the buoyancy-induced vortex that has the same direction with the vortex of the initial shear flow. As a result, the buoyancy-induced vorticity generation would facilitate the growth of the initial shear layer, thus the near-field flow remains stable. However, the growing shear flow would eventually lead to the development of the Kelvin–Helmholtz instability in the far field.