Subcooled flow boiling heat transfer enhancement in microchannels/tubes with modifications of surface characteristics with micro/nano structures and films
Nedaei, Masoumeh (2015) Subcooled flow boiling heat transfer enhancement in microchannels/tubes with modifications of surface characteristics with micro/nano structures and films. [Thesis]
In this study, subcooled flow boiling was experimentally investigated in micro tubes/channels, whose surfaces were enhanced by various methods. In all the experiments, deionized (DI) water was used as the working fluid. In the first study, initiated chemical vapor deposition (iCVD) method was employed to coat the inner walls of stainless steel hypodermic microtubes having inner diameter of 502 μm with polyhydroxyethylmethacrylate (pHEMA)/polyperfluorodecylacrylate (pPFDA) coatings. This coating altered wettability along the surface of the microchannels and also offered high porosity. To investigate the effect of wettability, the experiments were accomplished for both hydrophilic and hydrophobic ends of the microtubes such that one end corresponded to the most hydrophilic (pHEMA) location, while the other end corresponded to the most hydrophobic (pPFDA) location. The results were compared to the results with the bare surface microtube. The experimental results revealed a remarkable increase in subcooled boiling heat transfer with the coatings, while 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 showed that gradient pHEMA/pPFDA coatings obtained by iCVD method can be utilized as a viable surface enhancement method in microscale cooling applications. In the second study, a horizontal microchannel made of Aluminum (Al) with a length of 14 cm, width of 1.5 cm, and depth of 500 μm was utilized to investigate subcooled boiling heat transfer. Environmentally friendly and simple methods of sanding with sandpapers in different grit sizes and immersion in boiling DI water were used to prepare the micro and nano structured (hierarchical) Al alloy 2024 plates (150 × 150 mm) which were integrated near the exit of the microchannel. The experiments were carried out at different mass fluxes of 70, 100, and 125 kg/m²s. The results revealed that plates sanded with grit sizes of 400 and 1000 enhanced heat transfer coefficients while plates sanded with grit sizes of 36 and 60 deteriorated heat transfer coefficients compared to the untreated plate. The reason for heat transfer enhancement was related to an increase in both the heating surface area and active nucleation sites. A high speed camera was used for visualizing bubble departures in boiling experiments.
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