Improving heat transfer by surfaces structuring

Micro- and Nanostructured surfaces

Nano- and micorstructured surfaces will be implemented to enhance the efficiency of evaporators and condensers. These structures can significantly increase the heat transfer coefficient through the facilitation of early bubble boiling and drop wise condensation. Different techniques, such as sol-gel-coatings and colloidal lithography, for surface structuring and functionalization will be combined to obtain the desired effects. Sol-gel coatings can lead to strong hydrophobicic structures compared to metal surfaces. Such coatings can be combined with hydrophilic material such as aluminium dioxide nanoparticles to generate hydrophilic spots on an else hydrophobic surface. In colloidal lithography polystyrene nano- or microparticles are spray coated to serve as a mask on the substrate. An inorganic film is then deposited using a PVD technique.

The polystyrene mask is afterwards removed, which leaves a hole structure at the substrate. By varying the size and coverage of the polystyrene beads, various different surface structures can be generated.

Structure
Figure 1: Sketch of colloidal lithography. Polystyrene beads are spray coated on the substrate. A PVD film is deposited, and the polystyrene is afterwards removed.

In order to test the heat transfer improvement of the structured surfaces, a test setup simulating pool boiling has been developed. The evaporation in heat pumps occurs as flow boiling as there is forced convection present. However, pool boiling, which is boiling in the absence of forced convection, shows comparable effects to flow boiling, and thus results from this test setup can be used as indication of the heat transfer improvements in heat pumps. A large scale heat transfer setup simulating flow boiling is being built and will be used to test the most promising structured surfaces.

Testcell
Figure 2: Sketch and picture of the pool boiling setup.