• Proceedings of the Korean Powder Metallurgy Institute Conference
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• 2001.11a
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• pp.46-46
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• 2001

The microstructure of the 1020 mild steel target in the region ahead of craters, made by colliding against Cu and W-Cu shaped-charge jets. has been investigated in the present work. The region ahead of the crater impacted by the Cu shaped-charge jet reveals grain refinement implying the formation of sub-grains, while that of W-Cu one leads to martensitic transformation indicating that the region was heated up to an austenitic region which was followed by rapid cooling. The pressure of W-Cu shaped-charge jet impacting against the target when calculated is higher than that of Cu one. The microhardness of the region ahead of the crater impacted by the W-Cu shaped-charge jet is also higher than that of the Cu one. The microstructure of W-Cu slug that remains inside of the crater depicts the occurrence of the remarkable elongation of W particles during the liner collapse. The microstructural evolution of the region ahead of the crater is discussed on the basis of the pressure dependency of the ferrite/austenite transformation in the steel.

• Transactions of the Korean Society of Mechanical Engineers
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• v.11 no.5
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• pp.856-866
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• 1987

In the present paper a statistical method using probability density function has been applied to investigate experimentally the flow patterns and fluctuations of time-averaged local void fraction in air-water two-phase mixtures which flow vertically upwards in concentric annuli. This study was carried out using three vertical concentric annuli. The annular test section consists of a lucite outer tube whose inside diameter is 38mm and a stainless steel inner rod. The rod diameter is either 12mm, 16mm or 20mm. The two-phase flow patterns observed in the experiment were bubbly, slug, annular and each transition patterns. It was first demonstrated that the variance, coefficients of skewness and kurtosis calculated from probability density function on time-averaged local void fraction can be used to identify the flow patterns in the annular passage, and the fluctuation of time-averaged local void fraction varies with the radial position in annular gap and the flow pattern.

• Nuclear Engineering and Technology
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• v.29 no.4
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• pp.336-347
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• 1997

During postulated severe accidents in Light water Reactors, molten corium which was ejected from the reactor vessel bottom, may erode the concrete basemat of the containment and there by threaten the containment integrity. This study experimentally examines the molten core-concrete interaction (MCC) using 20kg of thermite melt (Fe + $Al_2$O$_3$) and the concrete, used in Yonggwang Nuclear Power Plant Units 3 and 4 (YGN 3 & 4) in Korea. The measured data are the downward heat fluxes, concrete erosion rate, gases and particle generation rates during MCCI. Transient results ore compared with those of TURCIT experiment conducted by SNL in USA. The peak downward heat flux to the concrete was measured to be about 2.1㎿/$m^2$. The initial concrete erosion rate was 175cm per hour, decreasing to 30cm per hour. It was shown from the post-test that the erosion was progressed downward up to 18mm in the concrete slug.

• Wind and Structures
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• v.24 no.1
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• pp.79-93
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• 2017

Wind-induced fluctuating internal pressures in a building with a dominant opening can be described by a second-order non-linear differential equation. However, the accuracy and efficiency of the governing equation in predicting internal pressure fluctuations depend upon two ill-defined parameters: inertial coefficient $C_I$ and loss coefficient $C_L$, since $C_I$ determines the un-damped oscillation frequency of an air slug at the opening, while $C_L$ controls the decay ratio of the fluctuating internal pressure. This study particularly focused on the value of loss coefficient and its influence factors including: opening configuration and location, internal volumes, as well as wind speed and approaching flow turbulence. A simplified formula was presented to predict loss coefficient, therefore an approximate relationship between the standard deviation of internal and external pressures can be estimated using Vickery's approach. The study shows that the loss coefficient governs the peak response of the internal pressure spectrum which, in turn, will directly influence the standard deviation of the fluctuating internal pressure. The approaching flow characteristic and opening location have a remarkable effect on the parameter $C_L$.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.23 no.5
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• pp.637-645
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• 1999

Horizontal two-phase flow pressure drop within rectangular channels with small gap heights have been examined experimentally. The gap heights range from 0.4mm to 4mm corresponding to aspect ratios(the channel height divided by the width) from 0.02 to 0.2. Water and air were used as the test fluids with the superficial velocity ranges being 0.03-2.39m/s and 0.05-18.7m/s, respectively. The experimental results In rectangular channels were compared with the Lockhart-Martinelli correlation, which are widely used for conventional round tube. The Lockhart-Martinelli correlation turned out to be Inappropriate to represent the present experimental data. In this respect, considering the aspect ratio and gap-height effects, an empirical correlation on two-phase flow pressure drop was proposed. The proposed correlation successfully covers the bubbly, plug, slug and annular flow regimes.

• Nuclear Engineering and Technology
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• v.48 no.5
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• pp.1096-1108
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• 2016

Metal fuel is being developed for the prototype generation-IV sodium-cooled fast reactor (PGSFR) to be built by 2028. U-Zr fuel is a driver for the initial core of the PGSFR, and U-transuranics (TRU)-Zr fuel will gradually replace U-Zr fuel through its qualification in the PGSFR. Based on the vast worldwide experiences of U-Zr fuel, work on U-Zr fuel is focused on fuel design, fabrication of fuel components, and fuel verification tests. U-TRU-Zr fuel uses TRU recovered through pyroelectrochemical processing of spent PWR (pressurized water reactor) fuels, which contains highly radioactive minor actinides and chemically active lanthanide or rare earth elements as carryover impurities. An advanced fuel slug casting system, which can prevent vaporization of volatile elements through a control of the atmospheric pressure of the casting chamber and also deal with chemically active lanthanide elements using protective coatings in the casting crucible, was developed. Fuel cladding of the ferritic-martensitic steel FC92, which has higher mechanical strength at a high temperature than conventional HT9 cladding, was developed and fabricated, and is being irradiated in the fast reactor.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.20 no.1
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• pp.384-398
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• 1996

An improved multi-channel Impedance Void Meter (IVM) is developed to measure an area-averaged void fraction. It consists of a main sensor, a reference sensor and a signal processor. The sensor was designed to be flush-mounted to the inner wall of the test section to avoid the flow disturbances. Guard electrodes are used to obtain evenly distributed electrical field in a measuring volume. A reference sensor is also installed to eliminate the drift in void signal caused by the changes in electrical properties of working fluid. The signal processor with three channels is specially designed so as to minimize the inherent error due to the phase difference between channels. As an example of applications, the mean and fluctuating components of void fraction are measured for bubbly and slug flow regime, and it is shown that IVM has good dynamic resolution which is required to investigate the structural developments of bubbly flow and the propagation of void waves in a flow channel.

• Nuclear Engineering and Technology
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• v.39 no.4
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• pp.327-336
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• 2007

The influence of density differences on the mixing of the primary loop inventory and the Emergency Core Cooling (ECC) water in the downcomer of a Pressurised Water Reactor (PWR) was analyzed at the ROssendorf COolant Mixing (ROCOM) test facility. ROCOM is a 1:5 scaled model of a German PWR, and has been designed for coolant mixing studies. It is equipped with advanced instrumentation, which delivers high-resolution information for temperature or boron concentration fields. This paper presents a ROCOM experiment in which water with higher density was injected into a cold leg of the reactor model. Wire-mesh sensors measuring the tracer concentration were installed in the cold leg and upper and lower part of the downcomer. The experiment was run with 5% of the design flow rate in one loop and 10% density difference between the ECC and loop water especially for the validation of the Computational Fluid Dynamics (CFD) software ANSYS CFX. A mesh with two million control volumes was used for the calculations. The effects of turbulence on the mean flow were modelled with a Reynolds stress turbulence model. The results of the experiment and of the numerical calculations show that mixing is dominated by buoyancy effects: At higher mass flow rates (close to nominal conditions) the injected slug propagates in the circumferential direction around the core barrel. Buoyancy effects reduce this circumferential propagation. Therefore, density effects play an important role during natural convection with ECC injection in PWRs. ANSYS CFX was able to predict the observed flow patterns and mixing phenomena quite well.

• Journal of Soil and Groundwater Environment
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• v.25 no.2_spc
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• pp.55-69
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• 2020

Permeability-analyzing methods commonly involve small-scale drilling, such as pumping or slug test, but it is difficult to identify overall distribution of permeability of the entire target sites due to high cost and time requirement. Spectral induced polarization (SIP) method is known to be capable of providing distributions of both the porosity and the pore size, the two major parameters determining permeability of the porous medium. The relationship between SIP variables and permeability has been studied to identify the hydrological characteristics of target sites. Kozeny-Carman formula has been improved through many experiments to better predict fluid permeability with electrical properties. In this work, the permeability prediction techniques based on SIP data were presented in accordance with the hydrogeological and electrical characteristics of a porous medium. Following the summary of the techniques, various models and related laboratory experiments were analyzed and examined. In addition, the field applicability of the prediction model was evaluated by field case analysis.

• Journal of the Microelectronics and Packaging Society
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• v.12 no.4 s.37
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• pp.331-338
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• 2005

The rapid advances in high power light sources and arrays as encountered in incandescent lamps have induced dramatic increases in die heat flux and power consumption at all levels of high power LED packaging. The lifetime of such devices and device arrays is determined by their temperature and thermal transients controlled by the powering and cooling, because they are usually operated under rough environmental conditions. The reliability of packaged electronics strongly depends on the die attach quality, because any void or a small delamination may cause instant temperature increase in the die, leading sooner or later to failure in the operation. Die attach materials have a key role in the thermal management of high power LED packages by providing the low thermal resistance between the heat generating LED chips and the heat dissipating heat slug. In this paper, thermal transient characteristics of die attach in high power LED package have been studied based on the thermal transient analysis using the evaluation of the structure function of the heat flow path. With high power LED packages fabricated by die attach materials such as Ag paste, solder paste and Au/Sn eutectic bonding, we have demonstrated characteristics such as cross-section analysis, shear test and visual inspection after shear test of die attach and how to detect die attach failures and to measure thermal resistance values of die attach in high power LED package. From the structure function oi the thermal transient characteristics, we could know the result that die attach quality of Au/Sn eutectic bonding presented the thermal resistance of about 3.5K/W. It was much better than those of Ag paste and solder paste presented the thermal resistance of about 11.5${\~}$14.2K/W and 4.4${\~}$4.6K/W, respectively.