TaeJoo Kim

Enhanced heat transfer is dependent on thickness of graphene films: the heat dissipation during boiling

Boiling heat transfer (BHT) is a particularly efficient heat transport method because of the latent heat associated with the process. However, the efficiency of BHT decreases significantly with increasing wall temperature when the critical heat flux (CHF) is reached. Graphene has received much recent research attention for applications in thermal engineering due to its large thermal conductivity. In this study, graphene films of various thicknesses were deposited on a heated surface, and enhancements of BHT and CHF were investigated via pool-boiling experiments. In contrast to the well-known surface effects, including improved wettability and liquid spreading due to micron- and nanometer-scale structures, nanometer-scale folded edges of graphene films provided a clue of BHT improvement and only the thermal conductivity of the graphene layer could explain the dependence of the CHF on the thickness. The large thermal conductivity of the graphene films inhibited the formation of hot spots, thereby increasing the CHF. Finally, the provided empirical model could be suitable for prediction of CHF.

Research on water discharge characteristics of PEM fuel cells by using neutron imaging technology at the NRF, HANARO

An investigation into the water discharge characteristics of proton exchange membrane (PEM) fuel cells is carried out by using a feasibility test apparatus and the Neutron Radiography Facility (NRF) at HANARO. The feasibility test apparatus was composed of a distilled water supply line, a compressed air supply line, heating systems, and single PEM fuel cells, which were a 1-parallel serpentine type with a 100 cm(2) active area. Three kinds of methods were used: compressed air supply-only; heating-only; and a combination of the methods of a compressed air supply and heating, respectively. The resultant water discharge characteristics are different according to the applied methods. The compressed air supply only is suitable for removing the water at a flow field and a heating only is suitable for water at the MEA. Therefore, in order to remove all the water at PEM fuel cells, the combination method is needed at the moment.

Fabrication and characterization of the source grating for visibility improvement of neutron phase imaging with gratings

The fabrication of gratings including metal deposition processes for highly neutron absorbing lines is a critical issue to achieve a good visibility of the grating-based phase imaging system. The source grating for a neutron Talbot-Lau interferometer is an array of Gadolinium (Gd) structures that are generally made by sputtering, photo-lithography, and chemical wet etching. However, it is very challenging to fabricate a Gd structure with sufficient neutron attenuation of approximately more than 20 μm using a conventional metal deposition method because of the slow Gd deposition rate, film stress, high material cost, and so on. In this article, we fabricated the source gratings for neutron Talbot-Lau interferometers by filling the silicon structure with Gadox particles. The new fabrication method allowed us a very stable and efficient way to achieve a much higher Gadox filled structure than a Gd film structure, and is even more suitable for thermal polychromatic neutrons, which are more difficult to stop than cold neutrons. The newly fabricated source gratings were tested at the polychromatic thermal neutron grating interferometer system of HANARO at the Korea Atomic Energy Research Institute, and the visibilities and images from the neutron phase imaging system with the new source gratings were compared with those fabricated by a Gd deposition method.

EXPERIMENTAL APPROACHES FOR WATER DISCHARGE CHARACTERISTICS IN PEMFC USING NEUTRON IMAGING TECHNIQUE AT CONRAD, HMI

In this investigation, we prepared a 1 and 3-parallel serpentine single PEMFC, which has an active area of 100 cm² and a flow channel cross section of 1x1mm. Distribution and transport of water in a non-operating PEMFC were observed by varying flow types and the flow rates (250, 400, and 850 cc/min). This investigation was performed at the neutron imaging facility at the COld Neutron RAdiography facility CONRAD), HMI, Germany of which the collimation ratio and neutron fluence rate are 250, 1 10 6 n/s/cm², respectively. The neutron image was continuously recorded by a scintillator and lens- CCD coupled detector system every 10 seconds. It has been observed that although the distilled water was supplied into the cathode channel only, the neutron image showed a water movement from the cathode to the anode channel. The water at the cathode channel was completely discharged as soon as the pressurized air was supplied. But the water at the anode channel was not easily removed by the pressurized air except for the 3-parallel serpentine type with 850 cc/min of air flow rate. Moreover, the water at the MEA wasn’t removed for any of the cases.

Modulation of gut microbiota and delayed immunosenescence as a result of syringaresinol consumption in middle-aged mice

Age-associated immunological dysfunction (immunosenescence) is closely linked to perturbation of the gut microbiota. Here, we investigated whether syringaresinol (SYR), a polyphenolic lignan, modulates immune aging and the gut microbiota associated with this effect in middle-aged mice. Compared with age-matched control mice, SYR treatment delayed immunosenescence by enhancing the numbers of total CD3+ T cells and naïve T cells. SYR treatment induced the expression of Bim as well as activation of FOXO3 in Foxp3+ regulatory T cells (Tregs). Furthermore, SYR treatment significantly enhanced the Firmicutes/Bacteroidetes ratio compared with that in age-matched controls by increasing beneficial bacteria, Lactobacillus and Bifidobacterium, while reducing the opportunistic pathogenic genus, Akkermansia. In addition, SYR treatment reduced the serum level of lipopolysaccharide-binding protein, an inflammatory marker, and enhanced humoral immunity against influenza vaccination to the level of young control mice. Taken together, these findings suggest that SYR may rejuvenate the immune system through modulation of gut integrity and microbiota diversity as well as composition in middle-aged mice, which may delay the immunosenescence associated with aging.

Experimental Study on the Heat and Mass Transfer of Teflon-Coated Tubes for the Latent Heat Recovery

Abstract The heat and mass transfer on stainless steel and Teflon-coated tube surfaces in a steam-air mixture flow were experimentally studied to obtain design data for a heat exchanger to be used in the latent heat recovery from flue gases. The test section consisted of three horizontal tubes with a pitch of 67 mm. The diameter of the tubes was 25.4 mm, and the tube wall thickness was 1.2 mm. The steam-air mixture flowed vertically from top to bottom. The Teflon coating degraded the overall heat transfer coefficient by 19%, as compared to the bare stainless steel tube. The degradation of the overall heat transfer coefficient of the Teflon-coated tube comes from the additional heat transfer resistance due to Teflon coating. Its magnitude on heat transfer resistance is comparable to the in-tube heat transfer resistance. Nusselt and Sherwood numbers of Teflon-coated surfaces have a different trend compared to bare stainless steel surface in the higher Reynolds number of our test. The higher contact angle of the condensate on the Teflon-coated surface, about 25°, is the reason for the kind of trend observed.

Interactive Ion-Mediated Sap Flow Regulation in Olive and Laurel Stems: Physicochemical Characteristics of Water Transport via the Pit Structure

Sap water is distributed and utilized through xylem conduits, which are vascular networks of inert pipes important for plant survival. Interestingly, plants can actively regulate water transport using ion-mediated responses and adapt to environmental changes. However, ionic effects on active water transport in vascular plants remain unclear. In this report, the interactive ionic effects on sap transport were systematically investigated for the first time by visualizing the uptake process of ionic solutions of different ion compositions (K+/Ca2+) using synchrotron X-ray and neutron imaging techniques. Ionic solutions with lower K+/Ca2+ ratios induced an increased sap flow rate in stems of Olea europaea L. and Laurus nobilis L. The different ascent rates of ionic solutions depending on K+/Ca2+ ratios at a fixed total concentration increases our understanding of ion-responsiveness in plants from a physicochemical standpoint. Based on these results, effective structural changes in the pit membrane were observed using varying ionic ratios of K+/Ca2+. The formation of electrostatically induced hydrodynamic layers and the ion-responsiveness of hydrogel structures based on Hofmeister series increase our understanding of the mechanism of ion-mediated sap flow control in plants.

Measurement of Ballooning Gap Size of Irradiated Fuels Using Neutron Radiography Transfer Method and HV Image Filter

A transfer method of neutron radiography was developed to measure the size of the end plug and a gap of an intact K102L-2, the irradiated fuel of a ballooned K174L-3, a ballooned and ruptured K98L-3. A typical irradiation time of 25 min. was determined to obtain a film density of between 2 and 3 of SR X-ray film with neutrons of 1.5 × 10¹¹ nㆍ㎝?². To validate and calibrate the results, a RISO fuel standard sample, Cd plate and ASTM-BPI/SI were used. An activated latent image formed in the 100 ㎛ Dy foil was subsequently transferred in a dark room for more than 8 hours to the SR film which is a maximum of three half-lives. Due to the L/D ratio an unsharpness of 9.82-14 ㎛ and a magnification of 1.0003 were given. After digitizing an image of SR film, the ballooning gap of the plug was discernible by an H/V filter of image processing. The gap size of the ballooned element, K174L-3, is equal to or greater than 1.2 ㎜. The development of a transfer method played a pivotal role in developing high burn-up of Wolsung and PWR nuclear fuel type.

The visualization of water distribution at the PEMFC using the neutron imaging technique: feasibility test

Neutron imaging technique was performed to visualize the water distribution and movement at flow channels on operating proton exchange membrane fuel cell (PEMFC). According to operation conditions (inlet humidity ratio, temperature, and flow field geometry), the phenomena of water content and movement are investigated and their effects for the fuel cell efficiency are discussed

Continuous cropping of endangered therapeutic plants via electron beam soil-treatment and neutron tomography

Various medicinal plants are threatened with extinction owing to their over-exploitation and the prevalence of soil borne pathogens. In this study, soils infected with root-rot pathogens, which prevent continuous-cropping, were treated with an electron beam. The level of soil-borne fungus was reduced to ≤0.01% by soil electron beam treatment without appreciable effects on the levels of antagonistic microorganism or on the physicochemical properties of the soil. The survival rate of 4-year-old plant was higher in electron beam-treated soil (81.0%) than in fumigated (62.5%), virgin (78%), or untreated-replanting soil (0%). Additionally, under various soils conditions, neutron tomography permitted the monitoring of plant health and the detection of root pathological changes over a period of 4–6 years by quantitatively measuring root water content in situ. These methods allow continual cropping on the same soil without pesticide treatment. This is a major step toward the environmentally friendly production of endangered therapeutic herbs.