Extension of a CLSVOF method for droplet-laden flows with a coalescence/breakup model

A multiple marker Coupled Level-Set/Volume-of-Fluid (CLSVOF) method for droplet-laden flows has been extended with a physical model to accommodate coalescence and breakup of the droplets. In the CLSVOF method each droplet has its own locally defined marker functions. This prevents the problem of numerical coalescence in conventional Level-Set and Volume-of-Fluid methods when two droplet interfaces are less than one grid cell apart from each other. In the present model coalescence is based on a computationally efficient film drainage model, which predicts if and when two colliding droplets will coalesce. If the contact time between two colliding droplets exceeds the predicted film drainage time, coalescence is numerically accomplished by merging the marker functions of the two separate droplets. Similarly, breakup of a droplet is accommodated by splitting the marker functions of the droplet. Simulations are shown for (near) head-on collision of two equal droplets at Weber numbers of 2.3, 9.33, 13.63, and 61.4, corresponding to four different collision regimes. The good agreement with the experimental data for each Weber number demonstrates the capability of the CLSVOF method to simulate coalescence and breakup with physical realism. A sensitivity study has shown a strong sensitivity of the collision dynamics on the prescribed film drainage time, while the influence of the spatial grid resolution appears to be small. This underlines the importance of accurate models for the film drainage time.