Effect of Droplet Collisions on Evaporation in Spray-drying

<p>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 are very sensitive to the droplet size, and the droplet size is heavily influenced by collisions that can coalesce or split droplets. 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. The results show a linear relationship between the Weber number and the evaporation rate at low droplet number densities. However, 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.</p>

<p>In the dataset, "NC" is the "no collision" model which does not take into account the collision of droplets and lets them pass through each other. This model provides a benchmark case. The "Evap" folder is the model with "collisions", the extra modules included in this folder enables droplet collision dynamics in the DEM software and the results refer to a case droplets collide with each other.</p>

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喷雾干燥系统中过量的蒸汽含量会引发多种问题，例如产品出现非预期的团聚或结块。喷雾干燥机的大部分干燥过程发生在进料仍为液态的初始阶段。干燥动力学对液滴尺寸极为敏感，而液滴尺寸又极易受到碰撞的影响——碰撞可使液滴发生聚并或分裂。液滴碰撞动力学被集成至欧拉-拉格朗日（Eulerian-Lagrangian）框架中：该框架以拉格朗日参考系追踪液滴，并将背景气体建模为连续介质。该碰撞动力学模型可基于从文献实验数据构建的工况图，预测碰撞的结果。碰撞液滴间的相对速度与韦伯数（Weber number），会决定液滴是发生聚并，还是分裂为卫星液滴。该建模框架包含液滴相与气相之间完全耦合的相间传热过程。随后，通过解析模型与数值模型对简化的伪一维（pseudo-1D）喷雾系统进行分析。研究结果表明：在低液滴数密度条件下，韦伯数与蒸发速率呈线性关系。然而，研究还发现，当液滴数密度较高时，蒸发速率与韦伯数的关系变为非线性；而在极高液滴数密度下，即便韦伯数较高，蒸发速率也会出现下降。

本数据集内，“NC”代表“无碰撞模型（no collision model）”，该模型不考虑液滴碰撞，允许液滴相互穿越，可作为基准对照案例。“Evap”文件夹对应的为带碰撞的模型，该文件夹内的额外模块可在DEM软件中启用液滴碰撞动力学功能，其结果对应液滴间发生碰撞的工况。