Numerical Simulation Of A Single Droplet Impact On The Cold Surfaces

The freeze deposition of atomized droplets on the equipment walls appeared in spray freeze-drying (SFD).The problem caused not only the decline of product collection rate but also the secondary pollution to the product, it may also affect the heat and mass transfer rates of the equipment. The research of single droplet impact on the cold plate will help to solve this problem.Based on the previous experiments, this thesis used a single distilled water droplet and a pullulan solution droplet as the research objects, simulated the impacting and solidification process of the single droplet(3.2mm in diameter, at 20 ℃) impact the cold plate(below 0 ℃).The thesis analyzed the effect of impact height (H=100 mm,250 mm, 500 mm), the temperature of the cold plate(T=253 K,268 K) and the dip angle of the cold plate(0 °,30°,45 °nd 60 °) in the spreading and solidification process. Based on previous studies, the thesis analyzed the process of the droplet spread and freeze on the cold plate from the theoretical perspective. The mathematical model to describe the droplet spreading and solidification process was established and the two-dimensional and three- dimensional physical model of the droplet spreading was set up. Combined the level set method and the model of solidification, this study used the heat conduction equation based on temperature instead of the original enthalpy equation and added latent heat of fusion in heat capacity source terms. The heat transfer problem with phase change was solved by using COMSOL Multiphysics 4.2(?) software which coupled the laminar two-phase flow, level set (tpf) and the heal transfer in fluids (ht) models.The results showed that the impact height and temperature of the horizontal cold plate play an important role to the droplet spreading process. When the droplet impacted the inclined plate, the dip angle of the cold plate affected the droplet freezing and deposition. With the increase of impact velocity and dip angle, the influence of temperature on droplet impact dynamics behavior became smaller gradually, reducing the chances of the droplet deposition. Material properties, such as viscosity could also have major effect the droplet deposition. For high viscosity materials, temperature was not a determinant, viscosity affected the spreading rate and diameter of the droplet and the trend was more obvious. In this paper, the simulation and experimental results are in good agreement, the results showed temperature change and the simulation conditions of the droplet during the spreading and freeze process which conducive to explain the droplets deposition.