Analysis and modelling the coating of solid particles

在该研究项目的框架内，计划分析基本原理，并推导液滴与较大固体颗粒及其涂层碰撞的模型，用于欧拉/拉格朗日计算。液滴和较大的固体颗粒之间的碰撞的基本过程将使用成像技术进行实验分析。为此，将产生液滴链，固体颗粒以限定的速度和频率射入液滴链中。由于液滴与较大颗粒的碰撞，可以观察到弹跳、沉积或分解。碰撞事件将使用高速摄像机和LED阵列和激光照明进行可视化。将通过图像处理对结果进行分析。对于所有相关的影响参数，首先需要确定碰撞的结果，并根据重要的无量纲数（例如，Ohnesorge数和撞击力的数值）对其进行总结。所考虑的影响参数是尺寸比（液滴/颗粒）、液滴液体性质（粘度和表面张力）、颗粒性质（温度和表面粗糙度）、液滴在颗粒表面上的冲击速度和冲击位置（即中心和横向冲击）。如前所述，液滴比颗粒小，并且液体是湿润的。所有这些影响都需要考虑，以使用相关的无量纲数来推导碰撞结果之间的物理边界。对于飞溅或部分沉积的状态，液滴碎片的尺寸分布也将被分析和建模。下一步是实验确定颗粒上最终液体膜的尺寸，包括膜厚度和膜的横向尺寸。为此目的，液滴液体将掺杂有荧光染料，以便允许与颗粒的图像更好地区分。为了开发与碰撞条件和相关无量纲数相关的涂层模型，应开发基于物理的相关性。此外，为了分析碰撞结果，将基于能量平衡进行理论研究。基于薄膜理论研究了颗粒表面的液膜形成过程，推导出了适用于技术涂层工艺拉格朗日计算框架的涂层模型，该模型考虑了广泛的参数范围，并首次考虑了横向冲击。

In the frame of the research project it is planned to analyse the fundamentals and to derive models for the collision of droplets with larger solid particles and their coating for Euler/Lagrange calculations. The elementary process of collisions between droplets and larger solid particles will be analysed experimentally using imaging techniques. For that purpose a droplet chain will be produced into which solid particles are shot with defined velocity and frequency. As a result of the collision of droplets with larger particles one may observe bouncing, deposition or disintegration. The collision event will be visualised using high-speed cameras and illumination by LED-arrays and a laser. The results will be analysed by image processing. For all the relevant parameters of influence it is first required to determine the outcome of the collision and summarise them with respect to the important non-dimensional numbers, e.g. Ohnesorge-number and impact Reynolds-number. The considered parameters of influence are the size ratio (droplet/particle), droplet liquid properties (viscosity and surface tension), particle properties (temperature and surface roughness), impact velocity and impact location of the droplet on the particle surface (i.e. central and lateral impacts). As mentioned before, the droplets are smaller than the particles and the liquid is wetting. All these effects need to be considered for deriving physically-based boundaries between the collision outcomes using the relevant non-dimensional numbers. For the regime splashing or partial deposition the size distribution of the droplet fragments also will be analysed and modelled.The next step is the experimental determination of the dimensions of the final liquid film on the particles, including film thickness and lateral dimension of the film. For that purpose the droplet liquid will be doped with fluorescing dye in order to allow a better distinction from the image of the particle. For the development of the coating model with respect to the impact conditions and the relevant non-dimensional numbers, physically based correlations shall be developed.In addition, for analyzing the collision outcomes, theoretical studies will be conducted based on an energy balance. The liquid film formation on the particle will be studied based on film theory.The developed coating model, which considers a broad range of parameters and also for the first time lateral impact, will be derived for being used in the frame of Lagrangian calculations of technical coating processes.