Abstract

Excess vapor content in the spray-drying system leads to various issues such as undesired agglomeration or caking of products. The majority of the drying in spray-dryers occurs during the initial phase, where the feed is still liquid. Drying kinetics is very sensitive to the droplet size, and the droplet size is heavily influenced by collisions that can coalesce or split droplets. An analytical model is developed to examine the role of droplet collisions on drying kinetics, and the evaporation rate during the first phase of drying. In addition, droplet collision dynamics are implemented into an Eulerian-Lagrangian framework, where droplets are tracked in the Lagrangian frame, and the background gas is modeled as a continuum. The collision dynamics is capable of predicting the outcome of collisions based on a regime map constructed from the experimental data from the literature. The relative velocity and the Weber number between colliding droplets affect whether droplets coalesce or split into satellite droplets. The modeling framework includes fully coupled interphase heat transfer between the droplet and gas phases. The analytical and numerical models are then used to analyze a simplified pseudo-1D spray system. Both models show a linear relationship between the Weber number and the evaporation rate at low droplet number densities. Further, comparing both methods’ results reveals droplet number density as another critical factor influencing the evaporation rate. It is shown that at high droplet number densities, the relationship between the evaporation rate and the Weber number becomes non-linear, and at extremely high droplet number densities, the evaporation rate decreases even at high Weber numbers.

摘要

喷雾干燥系统中过多的蒸气含量会导致各种问题，例如不希望的附聚或产品结块。喷雾干燥机中的大部分干燥发生在初始阶段，此时进料仍然是液体。干燥动力学对液滴尺寸非常敏感，并且液滴尺寸会受到碰撞的影响，碰撞可能会合并或分裂液滴。建立了一个分析模型，以检查液滴碰撞对干燥动力学的影响，以及干燥第一阶段的蒸发速率。此外，将液滴碰撞动力学实现到欧拉-拉格朗日框架中，在拉格朗日框架中跟踪液滴，并将背景气体建模为连续体。碰撞动力学能够基于从文献中的实验数据构建的状态图来预测碰撞的结果。碰撞液滴之间的相对速度和韦伯数影响液滴是否会聚结或分裂为卫星液滴。建模框架包括液滴和气相之间的完全耦合的相间传热。然后将分析模型和数值模型用于分析简化的伪1D喷涂系统。在低液滴数密度下，两个模型都显示了韦伯数与蒸发速率之间的线性关系。此外，比较这两种方法的结果表明，液滴数密度是影响蒸发速率的另一个关键因素。结果表明，在高液滴数密度下，