Unsteady spray behavior in a heated jet shear layer: Droplet-vortex interactions

Processes of droplet vortex interactions have been studied numerically in a dynamic evaporating spray. The spray is formed between a droplet-laden heated nitrogen jet and a coflowing air stream. The jet velocity and temperature have been considered in a range, where large-scale vortex structures develop due to convective Kelvin-Helmholtz instability of the jet shear layer. Numerical simulations of the heated jet without droplets show the presence of organized vortex structures and their pairing interaction. The fundamental frequency of these structures scale with the jet diameter and velocity, yielding a Strouhal number of 0.36. Results concerning droplet dispersion indicate that 1) the dispersion of intermediate-sized droplets is enhanced due to their interaction with vortex rings during the vortex-pairing process, 2) a second Stokes number based on a dtoplet transit time can be used to characterize the dispersion of larger droplets, and 3) the evaporation during droplet-vortex interaction modifies dispersion significantly. Results on the dynamics of two-way coupled system indicate that for a non-evaporating spray at a mass loading of unity, the dynamics of shear layer and vortex rings are strongly influenced by the dispersed phase. The locations of shear-layer rollup, vortex formation and pairing are shifted downstream, and their respective frequencies are reduced compared to those for the one-way coupled system. In addition, it is demonstrated that the shear-layer stability can be modulated by changing the droplet injection characteristics. For an evaporating spray, the effect of two-way coupling is more complex compared to that for a non-evaporating spray. In general, the shear-layer dynamics becomes much less organized compared to the one-way coupled system.