Dispersion of Evaporating Droplets in a Swirling Axisymmetric Jet

The effects of swirl and vaporization on droplet dispersion are investigated in the near region of a two-phase, swirling, heated jet. Simulations are based on the numerical solution of time-dependent, axisymmetric gas-phase equations.Droplets’ trajectories are tracked byusingaLagrangianapproach.Forall of the swirl cases investigated, the droplet dispersion exhibits a nonmonotonic behavior, with the maximum dispersion occurring near Stokes number (St) unity.As the swirl intensity is increased, the radial dispersion of both nonevaporatingandevaporating droplets is signiŽ cantly enhanced. We attribute this dispersion enhancement to two factors: 1) increased vortex pairing interactions caused by swirl and 2) transfer of gas-phase swirl momentum to droplets. The effect of vaporization on dispersion is characterized in terms of droplet lifetime compared to its response time and the characteristic  ow time. Forthe investigatedconditions, the dispersionof evaporatingdroplets is noticeablyreduced compared to that in the nonevaporating case. The dispersion function vs St behavior is also modiŽ ed due to vaporization. Droplets with diameters smaller than the optimum value are sufŽ ciently vaporized during their interaction with the vortex rings, and their dispersion approaches the low-St limit. On the other hand, droplets with diameters larger than the optimumare not vaporized signiŽ cantly. Consequently, their dispersion follows the large-St behavior.