Abstract: Phase change through boiling is used in a variety of heat-transfer and chemical reaction applications. The state of the art in nucleate boiling has focused on increasing the density of bubble nucleation using porous structures and microchannels with characteristic sizes of tens of micrometers. Traditionally, it is thought that nanoscale surfaces will not improve boiling heat transfer, since the bubble nucleation process is not expected to be enhanced by such small cavities. In the experiments reported here, we observed unexpected enhancements in boiling performance for a nanostructured copper (Cu) surface formed by the deposition of Cu nanorods on a Cu substrate. Moreover, we observed striking differences in the dynamics of bubble nucleation and release from the Cu nanorods, including smaller bubble diameters, higher bubble release frequencies, and an approximately 30-fold increase in the density of active bubble nucleation sites. It appears that the ability of the Cu surface with nanorods to generate stable nucleation of bubbles at low superheated temperatures results from a synergistic coupling effect between the nanoscale gas cavities (or nanobubbles) formed within the nanorod interstices and micrometer-scale defects (voids) that form on the film surface during nanorod deposition. For such a coupled system, the interconnected nanoscale gas cavities stabilize (or feed) bubble nucleation at the microscale defect sites. This is distinct from conventional-scale boiling surfaces, since for the nanostructured surface the bubble nucleation stability is provided by features with orders-of-magnitude smaller scales than the cavity-mouth openings.

Abstract: The ability of reentrant cavities to suppress flow boiling oscillations and instabilities in microchannels was experimentally studied. Suppression mechanisms were proposed and discussed with respect to various instability modes previously identified in microchannels. It was found that structured surfaces formed inside channel walls can assist mitigating the rapid bubble growth instability, which dominates many systems utilizing flow boiling in microchannels. This, in turn, delayed the parallel channel instability and the critical heat flux (CHF) condition. Experiments were conducted using three types of 200 X253 μm 2 parallel microchannel devices: with reentrant cavity surface, with interconnected reentrant cavity surface, and with plain surface. The onset of nucleate boiling, CHF condition, and local temperature measurements were obtained and compared in order to study and identify flow boiling instability.

Abstract: Nucleate pool boiling of ZrO2 based aqueous nanofluid has been studied. Though enhancement in nucleate boiling heat transfer has been observed at low volume fraction of solid dispersion, the rate of heat transfer falls with the increase in solid concentration and eventually becomes inferior even to pure water. While surfactants increase the rate of heat transfer, addition of surfactant to the nanofluid shows a drastic deterioration in nucleate boiling heat transfer. Further, the boiling of nanofluid renders the heating surface smoother. Repeated runs of experiments with the same surface give a continuous decrease in the rate of boiling heat transfer.

Abstract: World energy crisis has triggered more attention to energy saving and energy conversion systems. Enhanced surfaces for boiling are among the applications of great interest since they can improve th ...

Abstract: Recent studies relating to saturated flow boiling in minichannels and microchannels are reviewed, with a focus on the functional dependence of the heat transfer characteristics on thermodynamic vapor quality. While there is general agreement in the literature that the heat transfer coefficient increases with increases in heat flux, conflicting trends have been reported on the influence of vapor quality. A compilation and analysis of the results from recent investigations in the literature on flow boiling in small channels is presented. Further, correlations for flow boiling proposed in these studies are quantitatively assessed by comparing them against 10 independent data sets from the published literature covering a range of hydraulic diameters from 0.16 to 2.01 mm. Recommended topics for future research in this field are also identified. [Supplemental materials are available for this article. Go to the publisher's online edition of Nanoscale and Microscale Thermophysical Engineering for the following fr...

Abstract: The present work investigates and compares boiling heat transfer and two-phase flow of water in a single shallow uniform-cross-section microchannel and a diverging one with a diverging angle of 0.183°. Both types of microchannel are with a depth of about 20 µm and a mean hydraulic diameter of about 33.3 µm. Experiments are conducted to study the effect of channel geometry on boiling heat transfer in the microchannel. It is found that the slug bubbles tend to grow exponentially in the present shallow microchannels, uniform-cross section or diverging. The results reveal that the diverging microchannel presents better performance in boiling heat transfer than that of the uniform-cross-section one, primarily due to more stable two-phase flow in the diverging microchannel. Empirical correlations based on convective boiling are developed, respectively, for both types of microchannel. For the same mass flow rate, the diverging microchannel presents a higher single-phase flow pressure drop, while the two-phase flow in both types of channel shows approximately the same pressure drop for boiling at the same heat flux.

Abstract: The white potato (Solanum tuberosum L.) is a valuable source of potassium in the human diet. While most consumers benefit from high levels of potassium in potato tubers, individuals with compromised kidney function must minimize their potassium intake. This study was undertaken to determine the effects of leaching and boiling on levels of potassium and other minerals in potato tubers. Leaching alone did not significantly reduce levels of potassium or other minerals in tubers. Boiling tuber cubes and shredded tubers decreased potassium levels by 50% and 75%, respectively. Reductions in mineral amounts following boiling were observed for phosphorus, magnesium, sulfur, zinc, manganese, and iron. There was no difference between the leaching and boiling treatment and the boiling treatment. In addition, mineral levels in tubers of 6 North American potato cultivars are reported. Significant differences in mineral levels were detected among cultivars, but they were too small to be nutritionally important. Individuals wishing to maximize the mineral nutrition benefits of consuming potatoes should boil them whole or bake, roast, or microwave them. Those who must reduce potassium uptake should boil small pieces before consuming them.

Abstract: This paper explores experimentally stabilizing flow boiling water in ten parallel microchannel heat sinks with a diverging cross-section design. Each diverging microchannel has a mean hydraulic diameter of 120 µm and a diverging angle of 0.5° while the channel depth is uniform at 76 µm. Flow visualization shows that heat flux and mass flux significantly affect the stability of flow boiling in the parallel microchannels. The extent of pressure drop oscillations may be regarded as an index for the onset of flow boiling instability. The stability boundary is plotted on the plane of the subcooling number (Nsub) against the phase change number (Npch) and compared with microchannels with a uniform cross-section design in the literature. The present study confirms that, in terms of stability performance, the flow boiling in the parallel microchannel heat sinks with a diverging cross-section design is superior to a uniform cross-section design.

Abstract: Despite shortcomings, boiling is the most common means of treating water at home and the benchmark against which emerging point-of-use water treatment approaches are measured. In a 5-month study, we assessed the microbiological effectiveness and cost of the practice among 218 self-reported boilers relying on unprotected water supplies. Boiling was associated with a 99% reduction in geometric mean fecal coliforms (FCs; P < 0.001). Despite high levels of fecal contamination in source water, 59.6% of stored drinking water samples from self-reported boilers met the World Health Organization standard for safe drinking water (0 FC/100mL), and 5.7% were between 1 and 10 FC/100 mL. Nevertheless, 40.4% of stored drinking water samples were positive for FCs, with 25.1% exceeding 100 FC/100 mL. The estimated monthly fuel cost for boiling was INR 43.8 (US$0.88) for households using liquid petroleum gas and INR 34.7 (US$0.69) for households using wood.