• Coupled systems mechanics
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• v.5 no.4
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• pp.341-353
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• 2016

Manufactures of multi-crystalline silicon ingots by means of the directional solidification system (DSS) is important to the solar photovoltaic (PV) cell industry. The quality of the ingots, including the grain size and morphology, is highly related to the shape of the crystal-melt interface during the crystal growth process. We performed numerical simulations to analyze the thermo-fluid field and the shape of the crystal-melt interface both for steady conditions and transient processes. The steady simulations are first validated and then applied to improve the hot zone design in the furnace. The numerical results reveal that, an additional guiding plate weakens the strength of vortex and improves the desired profile of the crystal-melt interface. Based on the steady solutions at an early stage, detailed transient processes of crystal growth can be simulated. Accuracy of the results is supported by comparing the evolutions of crystal heights with the experimental measurements. The excellent agreements demonstrate the applicability of the present numerical methods in simulating a practical and complex system of directional solidification system.

• Journal of Advanced Marine Engineering and Technology
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• v.33 no.5
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• pp.637-645
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• 2009

The main purpose of this study lies on the improvement of micro dilution tunnel based on the typical porous tube type chamber. The characteristics of flow and temperature fields for steady state has been obtained by numerical analysis using FLUENT. Three different geometrical variations of the porous tube; a) increase of thickness at center, b) step increase of thickness at center and downstream, c) tapered increase of thickness, have been proposed. Accordingly results are obtained and compared in terms of penetration velocity and velocity ratio to therrmophoretic velocity for improvement against particulate deposition inside the tube. The penetration velocity and velocity ratio distributions in the upstream portion and portion of impinging of dilution air are apparently shown to be improved for the case of the step and tapered change of porous tube. The tapered change of tube thickness addition are shown to be the most effective among three geometrical changes. In addition, the considerable improvement against deposition are shown that its thickness should be at least 2mm.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.7 no.4
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• pp.654-666
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• 1995

R502 has been extensively used as a working fluid in transport refrigerating vehicles and low temperature refrigerating machines but is to be phased out by the end of 1995 due to ozone layer depletion problem. In this study, both theoretical cycle analysis and experiments were carried out to examine the best substitutes for R502. Theoretical results indicate that the alternatives available in the market today may replace R502 without significant changes in the system without suction line heat exchanger(SLHX). When the system contains a SLHX, however, COPs of the alternatives increase up to approximately 15~20% than those without the SLHX. But simultaneously, the discharge temperatures of the compressor also increrease significantly with the SLHX. Actual test results obtained from the experiment with a transport vehicle's refrigerator indicate the similar trend as those of the theoretical results. Especially, HFCs and their mixtures show lower discharge temperature than those of R502, which is one of the desirable features. Further research is needed to study the effect of the SLHX on the performance of the real machine as well as on the oil return for reliability of the system.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.28 no.7
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• pp.747-754
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• 2004

Thermo-fluid characteristics in coke oven, sintering machine and blast furnace in iron-making facility are key processes related to the quality and productivity of the pig iron. Solid material in the processes usually forms a bed in a gas flow. For simulation of the processes by mathematical model, the solid beds are idealized to be a continuum and a reacting solid flow in the gas flow. Governing equations in the form of partial differential equations for the solid material can be constructed based on this assumption. Iron ore sintering bed is simulated and limited amount of parametric study have been performed. The results have a good agreement with the experimental results or physical phenomena, which shows the validity and applicability of the model.

• Plant Journal
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• v.12 no.1
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• pp.29-36
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• 2016

It is becoming increasingly important to make use of alternative energy source. because It is not able to rely on only fossil fuel for the recent increasing demand of energy consumption. With this situation, lots of studies for utilizing low grade energy such as industrial waste heat, solar energy, and geothermal energy have been conducted. The aim of this study is to predict the operation characteristics of working fluid by using performance analysis program (ThermoFlex) through the system analysis which is not mixing district return water but using ORC(Organic Rankine Cycle, hereinafter ORC) as a downstream cycle when accumulating district heating (hereinafter DH). In this study, We conducted the performance analysis for the case which has the district heating water temperature($120^{\circ}C$) and Flow rate of $163m^3/h$ (including District Heating return water flow), and examined several working fluid which is proper to this temperature. The case using R245fa (which is the best-case) showed 269.2kW power output, 6.37% efficiency. Additionally, Cut down on fuel was expected because of the boiler inlet temperature increase by being Formed $57.3{\sim}85^{\circ}C$ in a temperature of district heating return water, depending on a pressure change of a condenser in ORC system.

• International Journal of Aeronautical and Space Sciences
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• v.17 no.4
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• pp.518-525
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• 2016

This paper presents the characteristics of non-fundamental multi-mode combustion instability and the effects of high-harmonic components on the Rayleigh criterion. Phenomenological observations of multi-harmonic-mode dynamic pressure waves regarding the intensity of harmonic components and the source of wave distortion have been explained by introducing examples of second- and third-order harmonics at various amplitudes. The amplitude and order of the harmonic components distorted the wave shapes, including the peak and the amplitude, of the dynamic pressure and heat release, and consequently the temporal Rayleigh index and its integrals. A cause-and-effect analysis was used to identify the root causes of the phase delay and the amplification of the Rayleigh index. From this analysis, the skewness of the dynamic pressure turned out to be a major source in determining whether multi-mode instability is driving or damping, as well as in optimizing the combustor design, such as the mixing length and the combustor length, to avoid unstable regions. The results can be used to minimize errors in predicting combustion instability in cases of high multi-mode combustion instability. In the future, the amount of research and the number of applications will increase because new fuels, such as fast-burning syngases, are prone to generating multi-mode instabilities.

• Journal of the Korean Magnetics Society
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• v.13 no.3
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• pp.127-132
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• 2003

Natural convection of a magnetic fluid is different from that of Newtonian fluids because magnetic-body force exists in addition to gravity and buoyancy. In this paper, natural convection of a magnetic fluids (W-40) in a cubic cavity was examined by experimental method. One side wall was kept at a constant temperature (25 $^{\circ}C$), and the opposite side wall was also held at a constant but lower temperature (20 $^{\circ}C$). The magnetic fields of various magnitude were applied up and down by permanent magnets. We measured temperatures at 5 points which are the most suitable places in cavity by the analysis record. The thermo-sensitive liquid crystal film (R20C5A) was utilized in order to visualize wall-temperature distributions. Several kinds of experiments were carried out in order to clarify the influence of direction and intensity of magnetic fields on the natural convection. It was found that the natural convection of a magnetic fluids could be controlled by the direction and intensity of the magnetic fields.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.43 no.8
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• pp.677-683
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• 2015

Hybrid rocket combustion displays low frequency instability(LFI, 10~30Hz) at a certain condition. Vortex shedding in the post-chamber is suspected to cause the occurrence of LFI. This study focused on the visualization of flow image using light emissions from high temperature combustion gas. Results shows that combustion pressure oscillates at a frequency of about 18 Hz, which is in phase with oscillations of light emission. Since LFI is not a property of thermo-acoustic instability, this result suggested there exists a physical coupling of pressure fluctuations with light emissions proportional to chemical reaction. Also POD analysis shows that dominant symmetric spatial modes in the stable combustion shift suddenly into asymmetric spatial pattern with the appearance of LFI. Especially, the appearance of mode 3 is a typical change of flow dynamics in unstable combustion representing a rotational fluid motions associated with vortex shedding.

• Journal of the Korean Applied Science and Technology
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• v.31 no.2
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• pp.231-237
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• 2014

The supercritical fluids (SCFs) have been widely used for material synthesis and processing due to their remarkable properties including low viscosity, high diffusivity and low surface tension. Carbon dioxide is one of the suitable solvents in SCFs processes in terms of its advantages such as easy processibility (with low critical temperature and pressure), inexpensive, nonflammable, nontoxic, and readily available. However, it has generally low solubility for high molecular weight polymers with the exception of fluoropolymers and siloxane polymers. Therefore, hydrocarbon solvents and hydrochlorofluorocarbons have been used for various SCFs process by its high solubility for high molecular weight polymers. In this report, a PMMA/clay nanocomposites were fabricated by using supercritical fluid process. The $Na^+$-MMT(montmorillonites)was modified by a fluorinated surfactant which is able to enhance compatibility with the chlorodifluoromethane(HCFC-22) and thus, improve dispersability of the clay in the polymer matrix. The PMMA/fluorinated surfactant modified clay nanocomposite shows enhanced mechanical and thermal properties which characterized by X-raydiffraction(XRD), Thermo gravimetric analysis(TGA), Dynamic mechanical analysis (DMA) and Transmission electron microscopy (TEM).

• Journal of the Korean Society of Propulsion Engineers
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• v.9 no.1
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• pp.35-45
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• 2005

Numerical study is carried out to investigate the effects of chemistry and thermal radiation on the rocket plume flow field at various altitudes. Navier-Stokes equations for compressible flows were solved by a fully-implicit TVD code based on the finite volume method. An infinitely fast chemistry module for hydrocarbon mixture with detailed thermo-chemical properties and a thermal radiation module for optically thick media were incorporated with the fluid dynamics code. The plume flow fields of a kerosene-fueled rocket flying at Mach number zero at sea-level, 1.16 at altitude of 5.06 km and 2.90 at 17.34 km were numerically analyzed. Results showed the plume structures at different altitude conditions with the effects of chemistry and radiation. It is understood that the excess temperature by the chemical reactions in the exhaust gas may not be ignored in the view point of propulsion performance and thermal protection of the rocket base, especially at higher altitude conditions.