喷雾冷冻法单个液滴冻结过程模拟

Numerical simulation of a single droplet freezing process

喷雾冷冻液滴的冻结过程决定着干燥产品的微结构。本文以单个雾化液滴为研究对象,利用数值模拟的方法研究了液滴大小、气体流速和环境温度3个参数对其冻结过程的影响。结果发现,液滴越大冻结时所需的形核时间和完全固化时间越长,而且冻结过程随着气体流速的增大和环境温度的降低而缩短。通过方差分析发现,液滴大小较气体流速和环境温度对液滴完全固化时间的影响有较显著差异。液滴冷冻过程中,其质量损失率随着液滴大小的增大而略有减小,随着气体流速的增加及环境温度的降低而减小,其中环境温度对液滴质量损失率的影响最大。

Droplets freezing process by spray freezing leads to their complex microstructures due to the formation of ice crystals within droplets. Numerical simulation was conducted to study the effect of droplet size, gas velocity and environmental temperature on the freezing process for a single droplet. Uniform temperature model was adopted in the simulation of droplet freezing process. The results show that the total solidification time for larger droplet is about 3 orders 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 fotmd that droplet size, gas velocity and environmental temperature have significant impacts on droplet total solidification time （p=0.0063〉0.05） . 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, which is about 2%. Lower environmental temperature, which is an influential parameter, results in lower droplet mass loss ratio.