No abstract

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 μm, 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/m2s. 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 ∼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.

-In this study, heat transfer performance of micro and nanostructured plates embedded into a rectangular microchannel was experimentally investigated for their potential use in cooling applications. A simple and environmentally friendly technique was proposed to provide the micro and nanostructured plates having superhydrophilic property. Aluminium alloy (Al Alloy) 4041 substrates of 1.5 × 1.5 cm were roughened by sandblasting method using medium size aluminum oxide abrasive to provide microfeatures; followed by hot water treatment (HWT) process in which nanograss structures were developed on the microstructured substrates. The micro and nanostructured plates were placed near the exit of an aluminum (Al) microchannel with a length, width, and depth of 14 cm, 1.5 cm, and 500 μm, respectively. Four cartridge heaters connected to a DC power supply were utilized to apply heat to the test section. De-ionized (DI) water was used as the working fluid and passed through the microchannel using a micro gear pump at three mass fluxes of 100, 300, and 500 kg/m 2 s. The test section dimensions and experimental parameters were selected such that fully developed flow conditions were obtained. The results from the microchannel with untreated Al alloy surfaces were considered as the baseline data. Experimental results of the micro and nanostructured plates revealed that there is no significant change in nucleate boiling heat transfer compared to the control sample. The reason is related to wettability and roughness of the micro and nanostructured plates.

In this study, two-dimensional (2D) numerical simulations of liquid slip flows in parallel-plate microchannels have been performed to obtain heat transfer characteristics and entropy generation rate under asymmetric heating conditions. Heat transfer analysis has been conducted along with second-law analysis through utilizing temperature-dependent thermophysical properties. The results indicate that temperature-dependent thermophysical properties have a positive effect on convective heat transfer and entropy generation. Nusselt numbers of the upper and lower plates and global entropy generation rates are significantly affected by slip parameter and heat flux ratio. It is shown that Nusselt number of the lower plate may have very large but finite values at a specific heat flux ratio. This finding resembles to analytical solutions, where singularities leading to an infinite Nusselt number exist.