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 mu m. Initiated chemical vapor deposition (iCVD) was employed for coating inner walls of the microtubes with different coating thicknesses of similar to 50 and similar to 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/m(2) 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 micro tubes. 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 mu m with the coating thickness of 160 nm, at G = 8000 kg/m(2) 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

Enhancemet of flow boiling heat transfer in pHEMA/pPFDA coated microtubes with longitudinal variations in wettability

Flow boiling heat transfer was investigated in stainless steel hypodermic microtubes, whose surfaces were enhanced by gradient crosslinked polyhydroxyethylmethacrylate (pHEMA)/polyperfluorodecylacrylate (pPFDA) coatings thereby offering variations in wettability along the surface as well as high porosity. The initiated chemical vapor deposition (iCVD) method was implemented for coating the inner walls of the microtubes with an inner diameter of 502 mu m, and deionized water was used as the working fluid. Experimental results were obtained from the coated microtubes, where one end corresponded to the pHEMA (hydrophilic) coated part and the other end was the most hydrophobic location with the pPFDA (hydrophobic) coating so that wettability varied along the length of the microtube. The results of both the hydrophobic and hydrophilic inlet cases were compared to their plain surface counterparts at the mass flux of 9500 kg/m(2)s. The experimental results showed a remarkable increase in boiling heat transfer with the coatings. The highest heat transfer coefficients were attained for the pHEMA coated (hydrophobic inlet and hydrophilic outlet) outlet case with a maximum heat transfer enhancement ratio of similar to 64%. The reason for the enhanced heat transfer with the coated microtubes can be attributed to the increased nucleation site density and bubble release as well as enhanced convection and bubble motion near the surface due to the variation in wettability along the length. The results proved that gradient pHEMA/pPFDA coatings can be utilized as a viable surface enhancement method in microscale cooling applications

Fabrication and characterization of temperature and pH resistant thin film nanocomposite membranes embedded with halloysite nanotubes for dye rejection

In this study, nanofiltration (NF) membranes with high pH and temperature resistance were fabricated by introducing halloysite nanotubes (HNTs) into the organic phase during the interfacial polymerization step. HNTs were dispersed in cyclohexane with the concentration 0% (TFNO), 0.02% (TFN0.02), 0.04% (TFN0.04) and 0.06% (TFN0.06) as w/v (%). Fabricated membranes were characterized by FT-IR spectroscopy, SEM and AFM analyses, optical profilometry, contact angle and zeta potential measurements. Filtration performance tests were conducted with 2000 ppm MgSO4 and NaCl solutions and 100 ppm of synthetic dye solutions (Setazol Red Reactive and Reactive Orange dyes), respectively. For pH -resistance tests, synthetic dyes were filtrated in acidic, neutral and base conditions (pH = 4-7-11) to measure changes in flux and rejection. The effect of the temperature of the feed stream on membranes were determined by the filtration of pure water and dye solutions at 15 degrees C, 25 degrees C and 40 degrees C. Among different membranes fabricated with varying HNTs content, TFN0.04 membrane showed increased water flux without considerable salt and dye rejection loss