• Carbon letters
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• v.25
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• pp.25-32
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• 2018

In this study, a thermal-gradient chemical vapor infiltration (TG-CVI) process was numerically studied in order to enhance the deposition uniformity within the preform. The computational fluid dynamics technique was used to solve the governing equations for heat transfer and gas flow during the TG-CVI process for two- and three-dimensional (2-D and 3-D) models. The temperature profiles in the 2-D and 3-D models showed good agreement with each other and with the experimental results. The densification process was investigated in a 2-D axisymmetric model. Computation results showed the distribution of the SiC deposition rate within the preform. The results also showed that using two-zone heater gave better deposition uniformity.

• Fire Science and Engineering
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• v.33 no.4
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• pp.35-40
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• 2019

In this study, the mass flow rate of the heat release rate equation, which is the major factor of the oxygen consumption method, was analyzed for the fundamental investigation of the cone-calorimeter (5 m length and 0.3 m diameter). The shapes of a completely empty inside, 3 mm pore diameter mesh and pore diameter 10 mm honeycomb with 0.76 porosity were constructed using the cone-calorimeter. To calculate the mass flow rate, four bi-directional probes and thermocouples were installed in a uniform position in the vertical direction of flow. The velocity gradient and flow perturbation were measured from the increase in Reynolds number. As the flow capacity increased, the speed gradient increased in all three shapes relative to the turbulence intensity. In addition, the deviation of extended uncertainty to the mass flow was completely low in the order of empty space, mesh (d_p = 3 mm) and honeycomb (d_p = 10 mm and 𝜖 = 0.76) at the 95% confidence level. The results can be used in designs to improve the flow stability of the cone calorimeter.

• Journal of the Korean Society for Precision Engineering
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• v.19 no.8
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• pp.92-99
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• 2002

This work investigated the optimal condition for an uniform deposition growth rate in the vertical cylindric CVD chamber. Heat transfer, surface chemical reaction and mass diffusion in the flow field of CVD chamber h,id been computed using Fluent v5.3 code. A SIMPLE based finite Volume Method (FVM) was adopted to solve the fully elliptic equations for momentum, temperature and concentration of a chemical species. The numerical analysis results show good agreements with the measurements obtained by N. Yoshikawa. The results obtained by the numerical analysis showed that the film growth rate in the center of a susceptor is increasing, as the inner flow approaches to the forced convection. To the contrast, as it approaches to the natural convection, that in the outside of a susceptor is increasing. As the Reynolds number increases, the uniformity may not hold due to the larger temperature gradient at a susceptor surface. Therefore, when the temperature gradient on the surface of a susceptor is zero, the film growth rate becomes uniform on most surface.

• Proceedings of the KSME Conference
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• 2004.04a
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• pp.1999-2004
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• 2004

Experiments were conducted to investigate the flow characteristics of water through rectangular PDMS microchannels with a hydraulic diameter ranging from 66.67 to 200 ${\mu}m$. In the experiments, the flow rate and pressure drop across the microchannels were measured at steady states. The experimental results were compared with the predictions from the conventional laminar flow theory. A significant difference between the experimental data and the theoretical predictions was found. Experimental results indicate that the pressure gradient and flow friction in microchannels are higher than those from the conventional laminar flow theory. This may be attributed to the fact that there exists effect of surface roughness of the microchannels. In this study, a surface roughness model is implemented to interpret the experimental data. A good agreement between the experimental data and the numerical predictions with a surface roughness model were found.

• Journal of Ship and Ocean Technology
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• v.9 no.4
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• pp.1-10
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• 2005

The effect of buoyancy orientation on turbulent channel flow has been investigated using DNS (direct numerical simulation). Grashof number is kept at 9.6 $\times 10^{5}$ while changing the orientation of the buoyancy vector to be parallel or perpendicular to the channel walls. Four study cases can be distinguished during this research namely; streamwise, wall-normal unstable stratification, wall-normal stable stratification and spanwise oriented buoyancy. The driving mean pressure gradient used in all cases is adjusted to keep mass flow rate constant while friction Reynolds number is around 150. At this Grashof number, the skin friction shows decrement in the unstable and stable stratification and increment in the other two cases. Analyses of the changes of flow structure for the four cases are presented highlighting on the mean quantities and second order statistics.

• Economic and Environmental Geology
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• v.39 no.4 s.179
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• pp.485-494
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• 2006

This paper summarizes the history of geothermal research in Korea since 1920s and also describes the present status of research on heat flow, origin of thermal waters and geothermal exploitation and utilization. Geothermal research in Korea has been mainly related with hot spring investigation until 1970s. 1t was not until 1980s before heat flow study became continuous by research institute and academia and first nation-scale geothermal gradient map and heat flow map were published in 1996. Also in 1990s, geochemical isotope analysis of Korean hot spring waters and measurements of heat production rate of some granite bodies were made. Attempts to develop and utilize the deep geothermal water has been tried from early 1990s but field scale exploitations for geothermal water was activated in 2000s. Considering recent increase of demands on both deep and shallow geothermal energy utilization, outlook on future goethermal research and development is encouraging.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.33 no.10
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• pp.820-826
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• 2009

The anisotropic phonon conductions with varying Joule heating rate of the silicon film in Silicon-on-Insulator devices are examined using the electron-phonon interaction model. It is found that the phonon heat transfer rate at each boundary of Si-layer has a strong dependence on the heating power rate. And the phonon flow decreases when the temperature gradient has a sharp change within extremely short length scales such as phonon mean free path. Thus the heat generated in the hot spot region is removed primarily by heat conduction through Si-layer at the higher Joule heating level and the phonon nonlocality is mainly attributed to lower group velocity phonons as remarkably dissimilar to the case of electrons in laser heated plasmas. To validate these observations the modified phonon nonlocal model considering complete phonon dispersion relations is introduced as a correct form of the conventional theory. We also reveal that the relation between the phonon heat deposition time from the hot spot region and the relaxation time in Si-layer can be used to estimate the intrinsic thermal resistance in the parallel heat flow direction as Joule heating level varies.

• The Bulletin of The Korean Astronomical Society
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• v.45 no.1
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• pp.41.2-42
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• 2020

Understanding how the filamentary structure plays a role in the formation of the prestellar cores and stars is a key issue to challenge. We have observed two prestellar cores in surrounding filamentary environments in 13CO, C180 (3-2) and HCO+ (4-3) molecular lines with the Heterodyne Array Receiver Program (HARP) of the James Clerk Maxwell Telescope (JCMT), in order to search for the evidence related to the possible flow motions along the filament and/or the radial accretion (or infalling motions) of gas material toward the dense cores from their surrounding filamentary cloud. In L1544, the velocity gradient of 1.6 km s-1 pc-1 toward the core was measured in a small branch of filament lying on a radial direction of main filament while no velocity gradient along the main axis of filament in both 13CO and C18O lines. In L694-2, we found the velocity gradient of 0.6 km s-1 pc-1 along the filament in only 13CO lines. The projected accretion rate of ~6 M◉ Myr-1 was estimated in both cases. The infall (or radially contracting) velocity of gas material was measured ~0.16 km s-1 in both 13CO and HCO+ lines and in both L1544 and L694-2, which leads to estimate a mass infall rate of ~20 M◉ Myr-1. Our analysis suggests that our targets are at a stage where the gravitational contraction dominates the mass accretion through the surrounding filamentary cloud. This is consistent with the fact that our targets are highly evolved prestellar cores on a verge of star formation. More detailed results will be presented at the meeting.

• Journal of the Korean Geosynthetics Society
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• v.9 no.2
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• pp.11-20
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• 2010

This paper studies the discharge capacity of vertical drain boards that is controlling hydraulic gradient among many factors in the specification. The KS K 0940(2008), a testing method based on the conventional Delft type method for measuring the discharge capacity of a vertical drain, was specified in Korea Standard recently. In this test method, the variation in hydraulic gradient can result in large differences in the discharge capacity for the same vertical drain board.

• Proceedings of the KSME Conference
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• 2000.04b
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• pp.631-636
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• 2000

The experimental study concerns the characteristics of a transitional flow in a concentric annulus with a diameter ration of 0.52, whose outer cylinder is stationary and inner one rotating. The pressure drops and skin-friction coefficients have been measured for the fully developed flow of water and that of glycerine-water solution (44%) at a inner cylinder rotational speed of $0{\sim}600$ rpm, respectively. The transitional flow have been examined by the measurement of pressure drops and the visualization of flow field, to reveal the relation of the Reynolds and Rossby numbers with the skin-friction coefficients and to understand the flow instability mechanism. The present results show that the skin-friction coefficients have the significant relation with the Rossby numbers, only for laminar regime. The occurrence of transition has been checked by the gradient changes of pressure drops and skin-friction coefficients with respect to the Reynolds numbers. The increasing rate of skin-friction coefficient due to the rotation is uniform for laminar flow regime, whereas it is suddenly reduced for transitional flow regime and, then, is gradually declined for turbulent flow regime. Consequently, the critical (axial-flow) Reynolds number decreases as the rotational speed increases. Thus, the rotation of inner cylinder promotes the early occurrence of transition due to the excitation of taylor vortices.