• The Magazine of the Society of Air-Conditioning and Refrigerating Engineers of Korea
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• v.17 no.5
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• pp.583-589
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• 1988

Condensation of water vapor into an absorbent liquid of LiBr-water solution falling over a bank of water cooled horizontal tubes was investigated theoretically. The governing conservation equation for a re-defined physical transport phenomena were solved numerically using a finite difference method. Raw parameters were used in this study, since reliable experimental data is required prior to a dimensionless parametric study. The average values of wall heat transfer coefficient and interfacial absorption rate were defined to see the system performance. Other parameters include tube diameter, streamwise coordinate (and number of tubes in row), mass flow rate, and the wall temperature. The effects of these quantities on the absorption processes and suggestions for a rational system design have been presented.

• Progress in Superconductivity
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• v.9 no.1
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• pp.5-10
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• 2007

We prepared four different $MgB_2$ films on $Al_2O_3$ by hybrid physical chemical vapor deposition method with thicknesses ranging from $0.65\;{\mu}m$ to $1.2\;{\mu}m$. X-ray diffraction patterns confirm that all the $MgB_2$ films are c-axis oriented perpendicular to $Al_2O_3$ substrates. The superconducting onset temperature of $MgB_2$ films were between 39.39K and 40.72K. The residual resistivity ratio of the $MgB_2$ films was in the range between 3.13 and 37.3. We measured the angle dependence of critical current density ($J_c$) and resistivity, and determined the upper critical field ($H_{c2}$) from the temperature dependence of the resistivity curves. The anisotropy ratios defined as the ratio of the $H_{c2}$ parallel to the ab-plane to that perpendicular to the ab-plane were in the range of 2.13 to 4.5 and were increased as the temperature was decreased. Some samples showed increase of $J_c$ and decrease of resistivity when a magnetic field in applied parallel to the c-axis. We interpret this angle dependence in terms of enhanced flux pinning due to columnar growth of $MgB_2$ along the c-axis.

• Korean Chemical Engineering Research
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• v.52 no.1
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• pp.75-80
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• 2014

The convective flow structures in the vapor phase on earth are shown to be single unicellular, indicating the solutally dominant convection is important. These findings reflect that the total molar fluxes show asymmetrical patterns in a viewpoint of interfacial distributions. With decreasing the gravitational level form $1g_0$ down to $1.0{\times}10^{-4}g_0$, the total molar fluxes decay first order exponentially. It is also found that the total molar fluxes decay first order exponentially with increasing the partial pressure of component B, PB (Torr) form 5 Torr up to 400 Torr. Under microgravity environments less than $1g_0$, a diffusive-convection mode is dominant and, results in much uniformity in front of the crystal regions in comparisons with a normal gravity acceleration of $1g_0$.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.14 no.4
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• pp.160-168
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• 2004

We investigate the effects of solutal convection on the crystal growth rate in a horizontal configuration for diffusive-convection conditions and purely diffusion conditions achievable in a low gravity environment for a nonlinear thermal gradient. It is concluded that the solutally-driven convection due to the disparity in the molecular weights of the component A $(Hg_2Br_2)$ and B (CO) is stronger than thermally-driven convection for both the nonlinear and the linear thermal profiles, corresponding to $Gr_t= 8.5{\times}10^3,\; Gr_s = 1.05{\times}10^5$. For both solutal and thermal convection processes, the growth rates for the linear thermal profile (conducting walls) are greater than for the nonlinear case. With the temperature humps, there are found to be observed in undersaturation for diffusive-convection processes ranging from $D_{AB}$ = 0.087 to 0.87. For the vertical configurations, the diffusion mode is so much dominated that the growth rate and interfacial distribution is nearly regardless of the gravitational accelerations. Also, the diffusion mode is predominant over the convection for the gravity levels less than 0.1 $g_0$ for the horizontally oriented configuration.

• International Journal of Air-Conditioning and Refrigeration
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• v.16 no.1
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• pp.22-29
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• 2008

A sequence of visually experimental observations was conducted to investigate the flow boiling and two-phase flow in a coiled tube. Different boiling modes and bubble dynamical evolutions were identified for better recognizing the phenomena and understanding the two-phase flow evolution and heat transfer mechanisms. The dissolved gases and remained vapor would serve as foreign nucleation sites, and together with the effect of buoyancy, centrifugal force and liquid flow, these also induce very different flow boiling nucleation, boiling modes, bubble dynamical behavior, and further the boiling heat transfer performance. Bubbly flow, plug flow, slug flow, stratified/wavy flow and annular flow were observed during the boiling process in the coiled tube. Particularly the effects of flow reconstructing and thermal non-equilibrium release in the bends were noted and discussed with the physical understanding. Coupled with the effects of the buoyancy, centrifugal force and inertia or momentum ratio of the two fluids, the flow reconstructing and thermal non-equilibrium release effects have critical importance for flow pattern in the bends and flow evolution in next straight sections.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.25 no.1
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• pp.1-5
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• 2015

For the special usage, the effort of developing AlN single crystals has been very hot in the world. The AlN-base UV LEDs are used on the field of sterilization, purification, curing and analyzing, which can advance human's living and medical processes etc.. AlN single crystals were grown by the PVT (Physical vapor transport) method. On the growing process, carbon crucibles with three different types such as normal size, taller than normal and wider than normal were used for comparison. The processing temperature was in the range of $1900{\sim}2100^{\circ}C$ and ambient pressure was 200~1 Torr. When the taller crucible was used, the sublimation mass was greater than normal dimension one but the best condition of growth changes widely. However the wider one gave much sublimation mass and growing condition was more stable than normal dimension. On limited growing furnace system, the changes of crucible dimension of PVT method provide the change of best condition for growth rate, as-grown crystal quality and growth condition stability.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.31 no.2
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• pp.63-68
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• 2021

This study was focused to investigate the effect of NbC-coated crucible on the quality of the SiC crystals. Then, the different properties between SiC crystals grown in a conventional graphite crucible and NbC-coated crucible were systematically compared. SiC crystals were grown using the Physical Vapor Transport (PVT) method at a temperature of 2300℃ and a pressure of 5 Torr in Ar atmosphere. After grinding and polishing, the polytype of the grown SiC crystal was analyzed using Raman spectroscopy, and crystallinity was confirmed by HR-XRD. Furthermore, the defect density and the concentration of impurities were analyzed by an optical microscope and a SIMS, respectively.

• Fashion & Textile Research Journal
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• v.3 no.1
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• pp.15-19
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• 2001

The purpose of this study was to determine the thermal characteristics of men's suits ensembles and their fabrics. For the study, 100% wool fabrics were woven with various fabric structure, fabric density and yam count and With the use of these, 12 men's suits were made with the same design. Physical characteristics that affect thermal transport properties, including drapery, cover factor; bulk density, keeping warmth ratio, vapor permeability, air permeability and porosity of the fabrics were measured. In addition, thermal resistance of men's suit ensembles, including Y-shirts, inner wear and socks was measured on the thermal manikin in the environmental chamber. The result of the study was as follows: 1. In terms of fabric structure, keeping warmth ratio of plain woven fabrics was higher than those of twill and satin woven fabrics and also, vapor and air permeability and porosity of plain woven fabrics were higher than those of twill and satin woven fabrics. 2. The result showed that thermal resistance of 12 ensembles were in the range of 0.77clo~0.97clo. 3. There was little correlation between woven condition such as, including structure, fabric density and yam count and thermal resistance of ensembles.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.34 no.2
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• pp.48-54
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• 2024

During the Hg₂Br₂ physical vapor transport process with self-diffusion, it is concluded that for 10^-3g₀≤ g ≤ 1g₀ the thermal buoyancy driven convection is dominant in the vapor phase; at the gravitational level of g = 10^-4g₀, the transition region from the convection to diffusion occurs; for 10^-6g₀ ≤ g ≤ 10^-5g₀, the diffusion mode is predominant. The total molar flux of Hg₂Br₂ decays exponentially with the decreasing of one tenth of gravitational magnitude. For 10℃ ≤ ΔT ≤ 50℃, the total molar flux increases linearly and directly with the temperature difference between the source and crystal regions.

• Journal of the Korean Physical Society
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• v.81
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• pp.942-947
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• 2022

We report the electrical transport properties of a dual-gate graphene device placed on a CaF₂ substrate. A hexagonal boron nitride top-gate dielectric was introduced to confirm the electrical characteristics of the CaF₂/graphene transistor because it is difficult to inject sufficient carriers through the CaF₂ substrate owing to its thickness of 500 ㎛, and the typical ambipolar behavior of graphene with a slight n-doping effect was clearly observed. In addition, we used a polymethyl methacrylate (PMMA) film as a top-gate dielectric for large-scale graphene devices grown via chemical vapor deposition, which was transferred onto a CaF₂ substrate. We controlled the high gate leakage current caused by the breakdown of the polymer due to non-uniformity by applying the film-transfer process rather than the direct coating method on the graphene device. Furthermore, the transport properties of large-area graphene in contact with CaF₂ are discussed with respect to the effect of top-contacted PMMA.