CONDENSATION OF STEAM ON THE SURFACE OF HARD COATED COPPER DISCS

At atmospheric pressure filmwise (FWC) and dropwise (DWC) condensation have been studied on the surface of copper discs which were coated by silicon-modified amorpheous hydrogenated carbon (a-C:H-Si) films of d ifferent thickness. On vertically oriented surfaces the DWC heat trans fer coefficients were found to be larger by a factor of about 10 than the FWC coefficients which follow as function of surface subcooling te mperature quite well Nusselt's theory. Varying the angle of surface in clination, the DWC coefficient decreased down to about 40% of the vert ical-surface values for 180 degrees (face down orientation). The mean value for all inclination angles between 30 degrees and 180 degrees wa s calculated to be 87.6% of the maximum value for the 90 degrees-orien tation. Partly coating of the copper surface indicates a strong heat t ransfer enhancement of DWC over FWC even for relative small coated par ts (e.g., 19%-coating yields an enhancement by a factor 2.3 for a cool ing water flow rate of 4 m(3)/h). The diamond like properties of the a -C:H-coatings promise long stand times and thus application also in re al technical condensation systems.