Influence of density ratio on the secondary atomization of liquid droplets under highly unstable conditions

• A lower density ratio results in a higher deformation rate.
• The density ratio has a non-monotonic effect on droplet deformation.
• A larger recirculation region may lead to a higher drag coefficient.
• The droplet dynamics is notably affected by the density ratio even if it exceeds 32.
• A greater density ratio results in more intensive fragmentation.

In this paper, the influence of density ratio on the atomization of liquid droplets under highly unstable conditions is numerically investigated with a coupled volume-of-fluid and level-set method. By employing the adaptive mesh refinement technique, the computational cost is significantly reduced and good agreements with experimental measurements on both the morphology and trajectory of droplets are obtained. Specifically, at We = 13, the present model correctly predicts bag breakup, which is consistent with earlier studies. Based on the present model, the effects of density ratio on the deformation and fragmentation of liquid droplets are investigated from the aspects of the time-resolved evolutions of gaseous Weber number, drag coefficient, liquid surface structure, as well as the morphology of liquid droplets, with special attention drawn to gain more in-depth insight into the dynamics of fragmentation under the highly unstable conditions (initial gaseous Weber number Weg,i = 225, initial gaseous Reynolds number Reg,i = 10062.5). The results indicate that the drag coefficient is determined by the recirculation region and therefore largely affected by the density ratio. A lower density ratio may lead to a higher deformation rate, while a higher density ratio results in more intensive fragmentation. Furthermore, the density ratio has a significant effect on the deformation and fragmentation in the investigated conditions when the density ratio exceeds 32 at a small Ohnesorge number.