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Improved condensation on hierachically structured surfaces

Subject Area Chemical and Thermal Process Engineering
Experimental Condensed Matter Physics
Term from 2020 to 2024
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 441180250
 
Final Report Year 2025

Final Report Abstract

In this project, microstructured ridge/trench systems made of silicon, which were expanded into hierarchical structures by nanostructuring, were examined for their efficiency in condensation. By choosing small web/trench widths, the condensate mass on the surface could be reduced and thus its thermal resistance kept low. Two experiments were set up to characterize the condensate behaviour: on the one hand, the condensate dripping from the samples was measured over long periods of time in a climatic chamber using a precision balance and the condensation on the surface was simultaneously examined macroscopically and microscopically using two camera systems. On the other hand, a test rig was developed to determine the thermal transition coefficient very precisely (AG HJB). The expectation of the application that condensate droplets would grow in and out of the trench in order to run down the bars almost hysteresis-free had to be abandoned. Two phenomena occurred with silicon samples: On the one hand, large droplets formed less frequently on the surface due to continued coalescence of small droplets and these eventually slid off gravitationally. On the other hand, droplets were sucked off the bar surfaces into the trenches and transported away. This required an initialization phase in which the trenches were first filled. A high-speed camera was used to observe both the suction and the transport in the trenches. The driving force is the gradient of the Laplace pressure between the small droplets on the bar surface, which are sucked into the trench by coalescence, and larger droplets in the trench or at the edge of the sample. From the microscopic images, the droplet distribution and thus the sum frequency distribution for each image could be calculated using the Hough circle transformation algorithm. This was used to determine the temporal condensate mass on the surface, which showed a periodic behavior with decreasing oscillation amplitude. This behavior was interrupted by a statistically occurring decrease in the amount of condensate, which was triggered by sliding processes of large droplets. The oscillation itself was interpreted as a drainage process of condensate droplets from the ridges into the trenches. These types of trench structures of different dimensions were provided in the HJB working group as master stamps for hot embossing in order to imprint corresponding structures in the surfaces of special plastics and to be able to examine them later in condensation experiments. The experiments of the HJB working group aimed to investigate the condensation behavior and heat transfer properties of stainless steel (SS), graphite composite (GC) and superhydrophobic graphite composite (SHGC). To produce the SHGC surface, a hot stamping process was developed using microstructured silicon structures as a matrix. In addition, a hierarchical structure of micro- and nanostructures was created using a metal oxide nanostructure (MONSTR) (contact angle >170°, contact angle hysteresis <5°). Condensation heat transfer experiments were performed at relative humidities (RH) of 60%, 80% and 100%. SS showed better results at low RH with a heat flux density of 541 W/m² at 60% RH. However, with increasing RH, GC and SHGC clearly outperformed SS. At 100% RH, GC achieved the highest heat flux density of 1,320 W/m², which is 36.2% higher than SS. SHGC reached 1210 W/m², but remained below the performance of GC. This is attributed to the Wenzel pinning effect and thermal resistances due to the MONSTR and lauric acid coating. In addition, a hierarchical structure of micro- and nanostructures was created using a metal oxide nanostructure (MONSTR) (contact angle >170°, contact angle hysteresis <5°). Condensation heat transfer experiments were performed at relative humidities (RH) of 60%, 80% and 100%. SS showed better results at low RH with a heat flux density of 541 W/m² at 60% RH. However, with increasing RH, GC and SHGC clearly outperformed SS. At 100% RH, GC achieved the highest heat flux density of 1,320 W/m², which is 36.2% higher than SS. SHGC reached 1210 W/m², but remained below the performance of GC. This is attributed to the Wenzel pinning effect and thermal resistances due to the MONSTR and lauric acid coating.

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