Nucleate Boiling Comparison between Teflon-Coated Plain Copper and Cu-HTCMC in Water

Abstract

The hydrophobic, Teflon-coated surfaces on plain copper and Cu-HTCMC (High temperature Thermally Conductive Microporous Coating) compared on pool boiling heat transfer of water. The HTCMC was created by sintering of copper powders with the average particle size of 67 µm and about 300 µm coating thickness that showed a good boiling heat transfer and the CHF enhancement from the previous study at saturation of water [1]. The Teflon-coated surfaces were created by coating of Amorphous Fluoroplastic (AF) 2400 resin on both plain copper and Cu-HTCMC. The static angles of both surfaces showed hydrophobic as about 120-130°. The departure bubble sizes created by merged bubbles of both surfaces are comparable as about 7 mm at 5 kW/m2 and the sizes are increased as heat flux increases. However, unlike to the plain surface, the smaller bubbles on Cu-HTCMC are not observed at the heat flux of 5 kW/m2 because the number of nucleation sites created in the porous structure are huge smaller bubbles are merged as soon as they grow from pores. As heat flux reaches the surfaces are covered by vapor film and reached the critical heat flux (CHF) at much lower heat fluxes compared to hydrophilic surfaces but the CHF values of Teflon-coated Cu-HTCMC is 640 kW/m2 and the value is more than tenfold higher than that of Teflon-coated plain copper.