Dropwise condensation heat transfer on ion implanted aluminum surfaces

Stable dropwise condensation (DWC) of saturated steam has been achieved on an aluminum alloy Al 6951 disc with an average surface finish of about 0.15 μm by means of ion beam implantation technology with an ion dose of 1016 N+ cm−2 and an implantation energy of 20 keV. Measurements of the condensation heat transfer coefficient at steam pressures of 1200 and 1400 mbar were carried out as a function of surface subcooling on vertical plates of the same material which is commonly used for heat transfer equipment. Probably due to alloy inhomogeneities, only on about 50% of the plate surface DWC could be achieved, resulting in a maximum enhancement factor of 2.0 for DWC in comparison with theoretical values calculated by a corrected form of the Nusselt film theory. The heat transfer coefficient increases with increasing steam pressure and decreases with increasing surface subcooling. Furthermore, it was shown that condensation heat transfer cannot be enhanced if the ion implantation does not induce DWC. For the investigations, two different condensers have been used, one for the stability tests on discs and one for the heat transfer measurements on plates.