Spray drying is an industrial scale drying method where a bulk liquid suspension is transformed into a solid powder. The bulk liquid is first atomized into a set of droplets and ligaments. These droplets are heated up in a drying chamber, where eventually they dry and form a powder. This technique is used in the dairy industry for the creation of dried milk powders. During the atomization phases, droplets are likely to collide, which can lead to the creation of larger or smaller droplets. This can be problematic for the quality of the product, as the droplet size largely influences the rate of evaporation. Even though this drying method is often used, the physics on which it is based are not completely understood. Therefore, it can be very beneficial to investigate the behavior during the droplet collision in the atomization part of this process. This could lead to a better understanding of this part of the process and more control over the final product. In this work, collisions for milk-like solutions are numerically reproduced and validated experimentally. For the numerical part, Direct Numerical Simulations (DNS) are used to solve the fields (i.e., velocity and pressure) and the Local Front Reconstruction Method (LFRM) is used to track the interfaces between the liquid and gas phase. The reason for choosing the LFRM is its combined ability of accurately tracking phase interfaces, similar to interface tracking methods, with its ability to easily handle topological changes, similar to interface capturing methods. The experiments are performed using two droplet jets which are placed so the droplets collide. These collision trajectories are captured with a high-speed camera and used to validate the numerical results. The goal is to show the abilities of the LFRM in simulating the droplet collisions observed during the atomization stage of the spray drying process. The results obtained in this analysis show a good agreement between the simulations and experiments. The LFRM is shown to be able to accurately track the droplet interfaces for most collision regimes and able to resolve complex topological changes.
Numerical simulations of binary droplet collisions using the Local Front Reconstruction Method
Timmermans, B. A. G. (Auteur). nov. 2021
Scriptie/Masterproef: Master