Mixing and flame structure study of inverse swirl diffusion flames

Abstract

Swirling flames appear in many practical applications to enhance flame stabilization. However, relatively few studies have investigated the inverse swirl diffusion flames (ISDFs) in detail. In ISDFs, a central swirling air jet is surrounded by an annular methane fuel jet. In the first part of this work, the stability map of ISDFs is studied. This map shows two distinctive stability regions: a swirling-jet-like region at low swirl and a compact swirling flame region at high swirl intensity. Regardless of the bulk air jet velocity (U), these two regions are separated by an optimum swirl intensity -leading to the highest flame stability. Then across these two regions, the development of the fuel–air mixing and the flame-flow interaction were investigated via acetone-PLIF and simultaneous PIV/OH-PLIF measurements. At the same U, increasing the swirl intensity leads to different mixing patterns. An annular-rich jet exists in the swirling-jet-like region versus a compact short mixing field in the compact flame stability region. Moreover, the development of the fuel–air mixing field downstream of the burner exit impacts the flame-flow interaction. The fast and early fuel–air mixing enhances the mutual interaction between the lower edge of the flame and the wrinkled flame, and the spots of the high heat released from the downstream regions - hence a highly stable flame is obtained. The lower flame edge is seen to track the instantaneous lower stagnation point of the swirling flow, while the increased heat release spots/flame wrinkling track the shear layer vortices. Beyond the optimum swirl intensity, increasing the swirl intensity retarding the fuel–air mixing and weakens the mixture ignitability, straining the flame in the vicinity of the flame edge or the vortices of the shear layer. These results suggest that suitable modifications of the mixing field close to the burner nozzle of ISDFs may increase flame stability.