Ageing of fouling layers

污垢，即在换热和/或传质表面形成不想要的绝缘层，是一种在空间和时间上变化的现象。这会导致沉积物随时间发生材料变化(老化)，从而影响其热性能和机械性能。有了这些过程的知识，可以更详细地描述热传递和清洁行为。这项研究建议建立在一个假设的基础上，即可以通过依赖于温度的动力学方法来定量地描述随时间推移的老化过程。这可以分别从导热系数、强度以及结构和组成来计算。为此，食品加工过程中产生的矿物质和蛋白质沉淀物在层流通道中以不同的工艺参数产生，如散装和壁面温度、流速和成分。首先，将老化速率与沉积和去除速率解耦，并确定污垢横截面积内离散亚层的导热系数。为了实现后者，设计了一种将热垢阻力和膜层厚度相结合的实验方法。此外，还量化了内聚力和附着力、剪切强度、耐磨性和抗渗性，以表征其力学性能。为了能够以简化和分离的方式研究老化机理和动力学，前面提到的材料性能也是在均匀的模型层上确定的。除了将现有的热老化模型转移到两个新的材料系统之外，还将根据结构和力学性能的信息对沉积层内的结合力进行新的定义。这可以被整合到一个经典的污垢模型中。然后，可以根据先前阐明的热、机械和结构参数的相互作用来预测老化过程的进展和由此产生的材料变化。

Fouling, i.e. the unwanted formation of insulating layers on heat and/or mass transfer surfaces, is a phenomenon varying in space and time. This leads to a time-dependent material change (ageing) of the deposits, which influences their thermal and mechanical properties. With the knowledge of such processes the heat transfer and the cleaning behavior can be described more detailed. This research proposal builds on the hypothesis that a thorough understanding of the ageing processes over time can be described quantitatively by means of a temperature dependent kinetic approach. This can be figured out for thermal conductivity, strength and the structure and composition re-spectively. For this purpose, mineral and protein containing deposits from food processing are produced in a laminar flow channel at varying process parameters such as bulk and wall temperature, flow velocity and composition. First, the ageing rate is decoupled from the deposition and removal rate and the thermal conductivity in discrete sublayers within the cross-sectional area of the fouling layer is determined. In order to achieve the latter, an experimental procedure was designed to combine thermal fouling resistance and layer thickness. Furthermore, cohesion and adhesion, shear strength, wear and penetration resistance are quantified to characterize the mechanical properties. In order to be able to examine ageing mechanisms and kinetics in a simplified and sepa-rated manner, the material properties mentioned before are also determined on homogeneous model layers.In addition to the transfer of existing models of thermal ageing to two new material systems, a new definition of binding forces within deposits will be undertaken based on the information on structure and mechanical properties. This can be integrated into a classical fouling model. The progression of the ageing process and the resulting material changes can then be predicted on the basis of the previously elucidated interaction of thermal, mechanical and structural parameters.