Browsing by Subject "pool boiling"
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Item Effects of Carbon Nanotube Coating on Bubble Departure Diameter and Frequency in Pool Boiling on a Flat, Horizontal Heater(2011-08-08) Glenn, Stephen T.The effects of a carbon nanotube (CNT) coating on bubble departure diameter and frequency in pool boiling experiments was investigated and compared to those on a bare silicon wafer. The pool boiling experiments were performed at liquid subcooling of 10 degrees Celsius and 20 degrees Celsius using PF-5060 as the test fluid and at atmospheric pressure. High-speed digital image acquisition techniques were used to perform hydrodynamic measurements. Boiling curves obtained from the experiments showed that the CNT coating enhanced critical heat flux (CHF) by 63% at 10 degrees Celsius subcooling. The CHF condition was not measured for the CNT sample at 20 degrees Celsius subcooling. Boiling incipience superheat for the CNT-coated surface is shown to be much lower than predicted by Hsu's hypothesis. It is proposed that bubble nucleation occurs within irregularities at the surface of the CNT coating. The irregularities could provide larger cavities than are available between individual nanotubes of the CNT coating. Measurements from high-speed imaging showed that the average bubble departing from the CNT coating in the nucleate boiling regime (excluding the much larger bubbles observed near CHF) was about 75% smaller (0.26 mm versus 1.01 mm)and had a departure frequency that was about 70% higher (50.46 Hz versus 30.10 Hz). The reduction in departure diameter is explained as a change in the configuration of the contact line, although further study is required. The increase in frequency is a consequence of the smaller bubbles, which require less time to grow. It is suggested that nucleation site density for the CNT coating must drastically increase to compensate for the smaller departure diameters if the rate of vapor creation is similar to or greater than that of a bare silicon surface.Item Experimental and Numerical Investigation of Pool Boiling Heat Transfer on Engineered Nano-Finned Surfaces(2014-08-10) Yang, HongjooPool boiling experiments for nanocoatings (or nanostructured surfaces) show that despite the lower thermal conductivity values than carbon, silicon yielded higher values of CHF (critical heat flux). Subsequently numerical studies showed that the interfacial thermal resistance (Kapitza resistance or ?R_(k)?) between a nanofin and fluid molecules is the dominant component of the thermal impedance network. The values of R_(k) for silicon were predicted to be ~1000 times smaller than that of carbon in these numerical simulations. Since the total thermal impedance of silicon nanofins is lower than that of carbon they cause higher levels of enhancement of CHF. Surface adsorption of the liquid molecules on a nanofin results in the formation of dense ?compressed phase? which in turn induces thermal capacitance and diodic behavior. This is termed as the ?nanofin effect?: which implies that CHF is more sensitive to R_(k) than the thermal conductivity of the nanofin. Hence, the objective of this study was to verify the nanofin effect. Experimental and numerical investigation of transport phenomena during pool boiling were performed in this study for liquid subcooling of 0 ?, 5 ? and 10 ? on horizontal planar heater configuration. Surface temperature was measured using nanosensor (Thin Film thermocouple or ?TFT?) arrays. Heater surfaces (with or without nanofins of different heights) were composed of ceramic, oxide and metal surfaces. The nanofins were fabricated using Step and Flash Imprint Lithography (SFIL). Contact angle was measured both before and after the experiments. Nucleate pool boiling heat transfer was enhanced with increase in pillar height. Numerical predictions for R_(k) obtained from Molecular Dynamics (MD) simulations were found to be consistent with the level of heat flux enhancement observed in the experiments for the different nanofin configurations. Hence this study demonstrates that R_(k) is the more dominant parameter for heat transfer enhancement during pool boiling ? compared to the thermal conduction resistance (or material properties) of the nanofin itself. As an outcome of these investigations future topics of research are also proposed (such as, using temperature nano-sensors for the investigation of controlled fouling on pool boiling phenomena for heaters with micro/nano-structured surfaces).Item Pool boiling on nano-finned surfaces(2009-05-15) Sriraman, Sharan RamThe effect of nano-structured surfaces on pool boiling heat transfer is explored in this study. Experiments are conducted in a cubical test chamber containing fluoroinert coolant (PF5060, Manufacturer: 3M Co.) as the working fluid. Pool boiling experiments are conducted for saturation and subcooled conditions. Three different types of ordered nano-structured surfaces are fabricated using Step and flash imprint lithography on silicon substrates followed by Reactive Ion Etching (RIE) or Deep Reactive Ion Etching (DRIE). These nano-structures consist of a square array of cylindrical nanofins with a longitudinal pitch of 1 mm, transverse pitch of 0.9 mm and fixed (uniform) heights ranging from 15 nm ? 650 nm for each substrate. The contact angle of de-ionized water on the substrates is measured before and after the boiling experiments. The contact-angle is observed to increase with the height of the nano-fins. Contact angle variation is also observed before and after the pool boiling experiments. The pool boiling curves for the nano-structured silicon surfaces are compared with that of atomically smooth single-crystal silicon (bare) surfaces. Data processing is performed to estimate the heat flux through the projected area (plan area) for the nano-patterned zone as well as the heat flux through the total nano-patterned area, which includes the surface area of the fins. Maximum heat flux (MHF) is enhanced by ~120 % for the nanofin surfaces compared to bare (smooth) surfaces, under saturation condition. The pool boiling heat flux data for the three nano-structured surfaces progressively overlap with each other in the vicinity of the MHF condition. Based on the experimental data several micro/nano-scale transport mechanisms responsible for heat flux enhancements are identified, which include: ?microlayer? disruption or enhancement, enhancement of active nucleation site density, enlargement of cold spots and enhancement of contact angle which affects the vapor bubble departure frequency.