• Transactions of the Korean Society of Mechanical Engineers
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• v.16 no.12
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• pp.2226-2240
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• 1992

This paper studies the applicability of the method of caustics into anisotropic materials under mode I and mixed static loading conditions and introduces the procedure to obtain stress intensity factors(S.I.F.) in anisotropic materials by the method of caustics. The mapping equations for initial and caustic curves in anisotropic materials were introduced and their computer graphical images were compared to the experimental ones to check the validity of the mapping equations proposed in this paper. The agreement between them was found to satisfactory. Two kinds of method to determine S.I.F. in anisotropic materials by the method of caustics were proposed in this paper and applied into the orthotropic materials under various loading conditions. In the case of mode I loading condition, the S.I.F.'s obtained by this paper's methods were found to be quite similar to the results by other method, boundary element method(B.E.M) and in the case of mixed loading condtion, the S.I.F's by this paper and B.E.M. showed a little differences(2.2-24.4%) with respect to the slanted angle of crack.

• Proceedings of the Korean Society of Tribologists and Lubrication Engineers Conference
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• 1991.06a
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• pp.1-30
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• 1991

The lubrication state between contacting bodies with rolling and sliding under loaded condition is generally understood by the conception shown in Figure 1. When the lubricating oil film formation between facing bodies is good enough to separate these bodies by the hydrodynamic pressure, this state is called by the expression of "hydrodynamic lubrication". The thickness of oil film is so large that the lubricating oil between facing bodies behaves as fluid and metal-to-metal contact between surface roughness asperities on facing bodies does not occur. When the oil film thickness becomes thinner or when the surface roughness height becomes larger, top of surface roughness asperities on facing bodies reaches very near to each other and there the oil or absorbed oil molecules on the surface of facing bodies behave no more as fluid. Partly metal-to-metal contact of surface roughness asperities occurs. Such lubrication state is called by the expression "mixed-lubrication". When the oil film thickness becomes more thinner or surface roughness height becomes larger, metal-tometal contact or contact via absorbed oil molecules dominate at most of the part in contact zone. Such state is called by the expression "boundary lubrication". Schematic representation of these three regimes of lubrication is shown in Figure 1.rication is shown in Figure 1.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.22 no.1
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• pp.50-60
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• 1998

As a method to develop an enhanced heat transfer fluid, the fine particles of a phase-change material were mixed with a conventional heat transfer fluid. Paraffin, which can be obtained easily in domestic market, was used for the phase-change material and water was used as a carrier fluid. Fine liquid particles of paraffin were formed in water as an emulsion by using an emulsifier, and they were cooled rapidly to become solid particle, resulting in paraffin slurry. The average diameter of produced solid particles was inversely proportional to the amount of the added emulsifier, which was theoretically proved. The produced paraffin slurry was tested thermally in heat transfer test section having a constant-heat-flux boundary condition. The test section was made of a circular stainless-steel pipe, which was directly heated by the power supply having a maximum of 50 Volts-500 Amperes. DSC(Differential scanning calorimeter) tests showed that two kinds of phase change were involved in the melting of paraffin, and it was explained in two different ways. A five- region-melting model was developed by extending the conventional three-region-melting model, and was used to obtain the local bulk mean temperatures of paraffin slurry in the heating test section. The local heat transfer coefficient showed a maximum where the bulk mean temperature of the paraffin slurry reached at the melting temperature of paraffin.

• Journal of the Society of Naval Architects of Korea
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• v.47 no.3
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• pp.388-397
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• 2010

A two-dimensional time-domain, potential-theory-based fully nonlinear numerical wave tank (NWT) was developed by using boundary element method and the mixed Eulerian-Lagrangian (MEL) approach for free-surface node treatment. The NWT was applied to prediction of primary wave energy conversion efficiency of a bottom-mounted oscillating water column (OWC) wave power device. The nonlinear free-surface condition inside the chamber was specially devised to represent the pneumatic pressure due to airflow velocity and viscous energy loss at the chamber entrance due to wave column motion. The newly developed NWT technique was verified through comparison with given experimental results. The maximum energy extraction was estimated with various chamber-air duct volume ratios.

• Journal of the Korean Society of Safety
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• v.27 no.5
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• pp.9-15
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• 2012

The ignition characteristics in a confined axisymmetric coflow $CH_4$ jet were investigated numerically with the Fire Dynamics Simulator(FDS). The $CH_4$ fuel stream was diluted with main combustion product gases, such as $O_2$, $N_2$, CO, $CO_2$, and $H_2O$, and the mixed fuel stream was heated up to the sufficient temperature where a supplying fuel stream can be ignited. For the calculation of chemical reaction in the simulation, a 2-step global finite chemistry model was considered. Boundary condition for confined wall was optimized by investigating the effects of wall temperature on the ignition characteristics of fuel stream. In addition, the effects of composition of diluents in the fuel stream and fuel stream temperature on the ignition of fuel steam were investigated. The ignition characteristics of $CH_4$ stream with diluents were very sensitive to the wall temperature, composition of diluents in the fuel stream and fuel stream temperature.

• Transactions of Materials Processing
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• v.19 no.5
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• pp.277-282
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• 2010

Magnesium alloy has been known as lightweight material in automobile and electronic industry with aluminum alloy, titanium alloy and plastic material. Friction welding is useful to join various metals and nonferrous metals that are difficult to join by such as gas welding, resistance welding and electronic beam welding. In this study, friction joining was performed to investigate mechanical properties of Mg alloy with 20mm diameter solid bar. Also the optimal joining conditions for its application were determined on the basis of tensile test, and hardness survey. The joining parameters were chosen as heating pressure, heating time, upsetting pressure, and upsetting time. Heating and upsetting pressure were executed under the range of 10~40MPa and 20~80MPa, respectively. From the experimental results, optimal joining conditions were determined as follows; rotating speed=2000rpm, heating pressure=35MPa, upsetting pressure=70MPa, heating time=1sec, upsetting time=5sec. Also the hardness of jointed boundary showed as HV50 which was similar to that of base metal at the optimal condition, and it was supposed that zone of HAZ was 8mm. Finally two materials were strongly mixed at interface part to show a well-combined microstructure without particle growth or any defect.

• Transactions of Materials Processing
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• v.26 no.4
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• pp.216-221
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• 2017

Recently, interest in multifunctional energetic structural materials (MESMs) has grown due to their multifunctional potential, especially in military applications. However, there are few studies about extrinsic factors that govern the reactivity of MESMs. In this paper, a shock compaction process was performed on the nano Ni/Al-mixed powder to investigate the effect of particle size on the shock reaction condition. Additionally, heating the statically compacted specimen was also performed to compare the mechanical properties and microstructure between reacted and unreacted material. The results show that the agglomerated structure of nanopowders interrupts the reaction by reducing the elemental boundary. X-ray diffraction analysis shows that the NiAl and $Ni_3Al$ intermetallics are formed on the reacted specimen. The microhardness results show that the $Ni_3Al$ phase has a higher hardness than NiAl, but the portion of $Ni_3Al$ in the reacted specimen is minor. In conclusion, using Ni/Al composites as a reactive material should focus on energetic use.

• Journal of Energy Engineering
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• v.8 no.2
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• pp.224-232
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• 1999

Latent heat storage system using micro-encapsuled phase change material is effective method for floor heating of house and building. The temperature profile in capsule block and flow rate of hot water are important parameters for the development of heat storage system. In the present study, a mathematical model based on 3-D, non-steady state, Navier-Stokes equations, scalar conservation equations and turbulence model ($\kappa$-$\varepsilon$), is used to predict the temperature profiles in capsule and the velocity vectors in hot water pipe. The multi-block grids and fine grids embedding are used to join the circle in hot water pipe and square in capsule block. The phase change process of the capsule is quite complex not only because the size of phase change material is very small, but also because phase change material is mixed with the cement to form thermal storage block. In calculation, it's assumed that the phenomena of phase change is limited only the thermal properties of phase change material and the change of boundary is not happened in capsule. The purpose of this study is to calculate the temperature profiles in capsule block and velocity vectors in hot water pipe using the numerical calculation. Two kinds of thermal boundary condition were considered, the first (case 1) is the adiabatic condition for the both outside surfaces of the wall, the second (case 2) is the case in which one surface is natural convection with atmosphere and another surface is adaibatic. Calculation results are shown that the temperature profile in capsule block for case 1 is higher than that for case 2 due to less heat loss in adaibatic surface. Specially, in the domain of near Y=0, the difference of temperature is greater in case 1 than in case 2. The detailed experimental data of capsule block on the temperature profile and the thermal properties such as specific heat and coefficient of heat transfer with the various temperature are required to predict more exact phenomena of heat transfer.

• Journal of Powder Materials
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• v.17 no.4
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• pp.312-318
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• 2010

High speed steels (HSS) were used as cutting tools and wear parts, because of high strength, wear resistance, and hardness together with an appreciable toughness and fatigue resistance. Conventional manufacturing process for production of components with HSS was used by casting. The powder metallurgy techniques were currently developed due to second phase segregation of conventional process. The powder injection molding method (PIM) was received attention owing to shape without additional processes. The experimental specimens were manufactured with T42 HSS powders (59 vol%) and polymer (41 vol%). The metal powders were prealloyed water-atomised T42 HSS. The green parts were solvent debinded in normal n-Hexane at $60^{\circ}C$ for 24 hours and thermal debinded at $N_2-H_2$ mixed gas atmosphere for 14 hours. Specimens were sintered in $N_2$, $H_2$ gas atmosphere and vacuum condition between 1200 and $1320^{\circ}C$. In result, polymer degradation temperatures about optimum conditions were found at $250^{\circ}C$ and $480^{\circ}C$. After sintering at $N_2$ gas atmosphere, maximum hardness of 310Hv was observed at $1280^{\circ}C$. Fine and well dispersed carbide were observed at this condition. But relative density was under 90%. When sintering at $H_2$ gas atmosphere, relative density was observed to 94.5% at $1200^{\circ}C$. However, the low hardness was obtained due to decarbonization by hydrogen. In case of sintering at the vacuum of $10^{-5}$ torr at temperature of $1240^{\circ}C$, full density and 550Hv hardness were obtained without precipitation of MC and $M_6C$ in grain boundary.

• Journal of Environmental Science International
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• v.6 no.4
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• pp.323-333
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• 1997

An objective method for the generation of velocity streamfunction is presented for dealing with discretely sampled oceauc data. The method treats a Poisson equation (forced by vorticity) derived from Helmholtz theorem In which streamfunction is obtained by isolating the non-divergent part of the two-dimensional flow field. With a mixed boundary condition and vorticity field estimated from observed field, the method Is Implemented over the Texas-Louisiana show based on the current meter data of the Texas-Louisiana Shelf Circulation and Transport Processes Study (LATEX) measured at 31 moorings for 32 months (April 1992 - November 1994). The resulting streamfunction pattern is quote consistent with observations. The streamfunction field by this method presents an opportunity to initiauze and to verier computer models for local forecasts of enoronmental flow conditions for ell spill, nutrient and plankton transports as well as opportuuty to understand shelf-wlde low-frequency currents.