Macro-structured surfaces

For the cleaning of widespread surfaces a big part of the cleaning effect is often realised by the liquid falling film. For this on the one hand mechanical effects like the wall shear stress play a role but on the other hand most of the material removal is done by diffusion. The effect of the diffusion again is usually limited by the viscous sublayer. Because of the laminar flow characteristics the soil concentration there is very high what slows down the diffusion. Analogies to this problem can be found on the field of heat transfer. When this parameter is examined for a film flow, the formation of a thermal boundary layer can be detected. This layer is also dominated by laminar flow patterns what leads to a low temperature gradient. This impedes the heat transfer. In this field it could be shown that a significant improvement can be achieved by using macro-structured surfaces, which lead to stronger turbulences and a higher mixing within the film flow. Thus the thermal boundary layer is reduced und the temperature gradient is increased. The best results were determined with dimple-structures similar to the ones on a golf ball. In this project the transferability of those results shall be examined for the field of mass transfer which is relevant for cleaning. For this purpose stainless steel samples with promising macro-structures are produced. These samples are soiled reproducibly with a polysaccharide layer by using dip coating. In comparison this is also done with even samples without macro-structures. In this way it can be determined if the macro-structures lead to a higher soiling propensity. Subsequently the probes are used for cleaning tests, where inclination angle and temperature are held constant and the wetting rate is varied. The cleaning progress is analysed locally and temporally so that inferences can be drawn about the suitability of the method and about which macro-structures are most promising for an industrial application.

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