Assessment of Numerical Methods for Investigating Single Droplet Falling Velocity and Evaporation*

Droplet evaporation is a fundamental yet complex process involving mass and heat transfer at the interface. The evaporation process depends on numerous environmental conditions, such as temperature, humidity, droplet composition, and for droplets in motion, the relative motion and drag forces also have significant impact. Experiments are prohibitively expensive and limited by the droplets size. Experimental data is scarce specifically for free falling droplets, since they are more difficult to capture as compared to droplets that are acoustically levitating or suspended. Computational Fluid Dynamics (CFD) is a viable alternative to predict such behaviour.

Here, we present experimental data conduced in-house for both acoustically levitating and free-falling droplets which serve for both verification and validation purposes. Two numerical approaches are explored here: Eulerian-Eulerian (E-E) (such as Volume of Fluid, Mixture and Eulerian) and Eulerian-Lagrangian (E-L) (Discrete Phase Model “DPM”). The results obtained are validated and compared. E-L methods are found to be more practical from the computational resources point of view, and therefore can be easily extended to multiple dispersed droplets. E-E models have the advantage of the ability to drastically refine the mesh more than DPM, hence acquiring more flow details around and inside the droplet.