SUBCOOLED FLOW BOILING HEAT TRANSFER ENHANCEMENT IN MICROCHANNELS/TUBES WITH MODIFICATIONS OF SURFACE CHARACTERISTICS WITH MICRO/NANO STRTUCTRES AND FILMS
Mechatronics Engineering, MSc. Thesis Defense, 2015
Prof. Dr. Ali Koşar (Thesis Advisor), Assoc. Prof. Gözde Özaydın Ince, Assoc. Prof. Tansel Karabacak
Date & Time: 30th, 2015 – 15:30 PM
Place: FENS L067
Keywords: Heat Transfer Enhancement, Subcooled Flow Boiling, Enhanced Surfaces, Microchannels/tubes
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. The results also 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) alloy 2024 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 different grit sizes and immersion in boiling DI water were implemented to prepare micro and nano structured (hierarchical) Al plates of 150 × 150 mm. 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 roughened with grit sizes of 36 and 60 showed deterioration in HTC compared to the untreated plate. The reason for heat transfer enhancement was due to the increased heating surface area and more active nucleation sites. Visualization was performed for visualizing bubble departures and the size of emerging bubbles on micro and nano structured plates.