Ageing of fouling layers

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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.