Flame stabilization in recirculation zones of jets with swirl

Abstract

An experimental study has been made of recirculation zones set up in the central region of a large diameter swirl burner with air introduced into the burner through tangential slots. Measurements of temperature, velocity, and flow direction were made in flames in which fuel gas was injected into the burner. Flames and recirculation zones extended to distances of more than two diameters inside the burner mouth. High intensity combustion was completed at distances of 1 1/2 diameters downstream of the burner exit. Recirculation zones occupy 75% of the cross-sectional area at the burner mouth, with reverse flow velocities into the burner of the same order of magnitude as mean exit velocities. The shape and size of recirculation zones is slightly reduced under combustion conditions as compared to isothermal conditions. Temperature measurements show high temperatures (1300°C), with small variation throughout the flame and with rapid temperature decay for x/D between 1.5 and 2. Maximum reverse mass flow rates are 80% of input mass-flow rates under cold conditions and 70% for combustion conditions. Kinetic energy of turbulence, as measured by a hot-wire anemometer in cold isothermal conditions, shows maximum values at 160% at the burner exit. Temperature decay closely follows the decay of kinetic energy of turbulence. Results of the experiments show that, under strong swirl conditions, aerodynamic forces are so dominant that little change occurs to flow fields as a result of chemical reaction in the flame. Quantitative information is provided for calculation of heat transfer in recirculation zones required in the theory of flame stabilization in reversal zones.