Numerical simulations of coaxial swirling jets and flames

Three-dimensional numerical simulations are used to study vortex breakdown in a coaxial jet configuration with a central non-swirling jet surrounded by a swirling annular jet. This flow, which is commonly used in non-premixed combustion applications, is investigated for a series of jet Reynolds numbers, ranging from laminar (Re=200) to transitional (Re=1000) conditions. In the non-reacting isothermal flow, for increasing values of the swirl number S, vortex breakdown occurs, and a bubble-type recirculation zone forms near the jet inlet. Critical values for the onset of breakdown (S^*) are determined, and the transition is explained through vorticity dynamics. The same flow is then investigated for a gaseous non-premixed flame, with the central jet containing methane, surrounded by a swirling annular jet of air. The chemical reaction, which is assumed to occur with an infinite rate (Burke-Schumann limit), increases the viscosity in the gas mixture, leading to a delay in the transition to bubble breakdown. The resulting flow structure and dynamics are compared with the isothermal case and analyzed by quantifying effects of thermal expansion and baroclinic torque.