FVV1455: CFD Simulation of Droplet Separators

For industrial applications, the two-fluid model is preferred due to its efficient modelling of small-scale interfaces. Whereas, a thin film model, based on a long wave approximation, is used for the unresolved interfaces to obtain the film features by solving the 2D Navier-Stokes equations for wall films. Within the project, the target is to develop an experimentally validated 3D-CFD model to investigate the separation efficiency of droplet separators for fuel cell systems. A hybrid model is developed, which couples the two-fluid model with a thin film model via mass transfer terms for droplet deposition, droplet entrainment and film separation. A two-way coupling between droplets and the thin film is established using mass and momentum source terms, derived analytical and from available experiments. The droplet separator is an essential component of an automotive fuel cell system that segregates a significant amount of liquid fractions from the air-water mixture. The flow dynamics inside a droplet separator consist of a dispersed gas and liquid with a wall adhered thin liquid film. The modelling is divided into the following stages due to the complex fluidic phenomenon inside a generic droplet separator: Droplet deposition model, Film separation model, Film transition model, and Population balance model. The numerical simulations are validated and fine-tuned with experiments carried out at HZDR.

针对工业应用场景，双流体模型（two-fluid model）因可高效模拟小尺度界面而被优先采用。对于未解析界面，则采用基于长波近似的薄膜模型（thin film model），通过求解壁面液膜的二维纳维-斯托克斯（2D Navier-Stokes）方程以获取液膜特征。
本项目的目标是开发经实验验证的三维计算流体动力学（3D-CFD）模型，用以研究燃料电池系统用液滴分离器的分离效率。
本项目开发了一种混合模型，通过传质项将双流体模型与薄膜模型耦合，覆盖液滴沉积、液滴夹带及液膜分离过程。基于解析推导与现有实验结果得到的质量与动量源项，建立了液滴与薄膜之间的双向耦合关系。
液滴分离器是车用燃料电池系统的关键部件，可从气水混合流中分离出大量液相组分。液滴分离器内部的流动包含弥散态气液两相以及附着于壁面的薄液膜。
由于通用型液滴分离器内部存在复杂的流体力学现象，建模工作分为以下四个阶段：液滴沉积模型、液膜分离模型、液膜过渡模型以及群体平衡模型（population balance model）。
数值模拟结果通过在HZDR开展的实验完成了验证与参数微调。