Subcooled flow boiling heat transfer enhancement using polyperfluorodecylacrylate (pPFDA) coated microtubes with different coating thicknesses

In this study, enhanced subcooled boiling heat transfer was achieved at high mass fluxes by applying a new surface enhancement method. In this method, polyperfluorodecylacrylate (pPFDA) was applied on the inner walls of the 4 cm long stainless steel hypodermic microtubes with inner diameters of 889 and 600 μm. Initiated chemical vapor deposition (iCVD) was employed for coating inner walls of the microtubes with different coating thicknesses of ∼50 and ∼160 nm. iCVD could serve for a surface deposition method for closed geometries like microtubes and offered a uniform coating. The experiments were performed at high mass fluxes of 6000, 7000, and 8000 kg/m2 s with de-ionized (DI) water (as the coolant). The Joule heating method was used for applying heat to the test section, which was located at the end (the last 2 cm) of the microtube. Temperature measurements were done at the very end of the microtubes. The experimental results indicated that pPFDA coated microtubes could significantly enhance flow boiling heat transfer. The largest heat transfer enhancement was achieved as ∼61% pertinent to the coated microtube of an inner diameter of 889 μm with the coating thickness of 160 nm, at G = 8000 kg/m2 s, relative to its bare surface counterpart (at the same heat flux). The coatings were proven to be reliable and reproducible by analyzing the coated microtubes after performing boiling experiments with the Raman spectroscopy method.