Vortex breakdown in non-isothermal gaseous swirling jets

Numerical simulations are performed to study vortex breakdown in low-Mach-number swirling jets at moderate Reynolds numbers with thermal effects. The jet-to-ambient density ratio Λ is varied from 1/5 to 5, with the temperature dependence of the gas density and viscosity as that of air. An effective Reynolds number, Re_eff, is defined based on the geometric mean of the jet and ambient fluid viscosities. As observed in constant density jets (Billant et al. 1998), two basic types of vortex breakdown are observed: the bubble and the cone configurations. A series of axisymmetric flow simulations at fixed Re_eff was used to determine the critical swirl number S for the onset of the bubble (S^*_B) and the cone (S^*_C), and results are compared with theoretical predictions. Values of S^*_B decrease monotonically as Λ increases due to an increased expansion of the flow. Values of S^*_C depend strongly on viscous effects. For small values of Λ, the low jet Reynolds number delays the transition to the cone while for large values of Λ, the large increase in ambient kinematic viscosity produces a corresponding trend that significantly increases S^*_C. Results from three-dimensional simulations provide details of the structure and behavior of these flows.