Numerical Simulation Of A Single Particle Freeze Process Of Spray-Freez Drying

Droplets freezing process by spray freezing generally determines their complex microstructures due to the formation of ice crystals within droplets.This thesis studid the process of a single droplet freezing durying spray freezing from different angles. First of all numerical simulation was undertaken to study the affects of droplet size, gas velocity and environmental temperature on the freezing process for a single droplet, and the particle residence times in the flow and the frozen state when it "disappeared". Through the experiment of a single droplet impact on low temperature, the impacting process was undersood more in-depth.The numerical simulation results show that the total solidification time for larger droplet is longer than that for the smaller one. Freezing rate is enhanced as gas velocity increases and environmental temperature decreases. Using variance analysis method it is found that droplet size, gas velocity and environmental temperature have significant affects on droplet total solidification time. During cooling process, the ratio of droplet mass loss due to convection decreases to lesser extent with the increases of droplet size and gas velocity. Lower environmental temperature, which is an influential parameter, causes lower droplet mass loss ratio.In spray freezing process a question must be considered is that atomized droplets collide with the wall caused by impacting. To accurately understand the wall boundary condition of spray-freezing flow field, a single droplet impact on low temperature solid surface was investigated. When contacting with the cold plate under different Weber numbers, the droplet spreads, retracts, rebounds (sometimes) and freezes on the surface. Using a high-speed camera we analyzed the effects of droplet’s size, impact velocity, and surface temperature on droplet spreading diameter and complete solidification time.Through flow analysis it is found that small size particles were more easily affected by airflow in the chamber and driven by the reback gas within a long time, however the large ones more easily "disappeared" form flow filed. And found the large particles needed a long time to be completely frozen by state analys, leading to these particles did not reach frozen state when "disappeared". This can be a reference for designing the chamber in engineering application.