• Nuclear Engineering and Technology
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• v.56 no.2
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• pp.624-635
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• 2024

In two-fluid simulations of flow boiling, the modeling of the mean bubble diameter is a key parameter in the closure relations governing the intefacial transfer of mass, momentum, and energy. Monodispersed approach proved to be insufficient to describe the significant variation in bubble size during flow boiling in a heated pipe. A population balance model (PBM) has been employed to address these shortcomings. During nucleate boiling, vapor bubbles of a certain size are formed on the heated wall, detach and migrate into the bulk flow. These bubbles then grow, shrink or disintegrate by evaporation, condensation, breakage and aggregation. In this study, a parametric analysis of the PBM aggregation and breakage models has been performed to investigate their effect on the radial distribution of the mean bubble diameter and vapor volume fraction. The simulation results are compared with the DEBORA experiments (Garnier et al., 2001). In addition, the influence of PBM parameters on the local distribution of individual bubble size groups was also studied. The results have shown that the modeling of aggregation process has the largest influence on the results and is mainly dictated by the collisions due to flow turbulence.

• Journal of Advanced Marine Engineering and Technology
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• v.39 no.9
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• pp.890-894
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• 2015

Recently, research on high-efficiency refrigeration cycles that apply an ejector to basic cycles has progressed actively. The role of the ejector and the performance of refrigeration cycles are subordinate to ejector locations. In this study, the performance of three refrigeration cycles with different ejector locations is compared and analyzed. The results showed an increased COP in all cycles due to the application of the ejector, with the highest increase of 44% compared to a basic refrigeration cycle. The ejector refrigeration cycle proposed in this study presents the highest COP, 3.47. Moreover, the decrease in condensation capacity in Bergander's cycle, Xing's cycle, and our proposed ejector refrigeration cycle went up to 21%. In refrigeration cycles applying the ejector, the pressure ratio of the ejector, the vapor fraction of discharge, and compression ratio are important factors for COP enhancement. For this reason, detailed and accurate control of these is significant.

• Journal of the Korean Ceramic Society
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• v.43 no.5 s.288
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• pp.299-303
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• 2006

In this paper, anode supported SOFC with columnar structured YSZ electrolyte was fabricated via Electron Beam Physical Vapor Deposition (EBPVD) method. Liquid condensation process was employed for the preparation of NiO-YSZ substrate and the high power electron beam deposition method was used for the deposition of YSZ electrolyte film. Double layered cathode with LSM-YSZ and LSM was printed on electrolyte via screen-printing method and fired at $1150^{\circ}C$ in air atmosphere for 3 h. The electrochemical performance and the long-term stability of $5{\times}5cm^2$ single cell were investigated with DC current-voltage characteristics and AC-impedance spectroscopy. According to the investigation, $5{\times}5cm^2$ sized unit cell showed the maximum power density of around $0.76W/cm^2$ at $800^{\circ}C$ and maintained the stable performance over 400 h.

• Proceedings of the KSME Conference
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• 2007.05b
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• pp.2113-2118
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• 2007

In this study, a mathematical model for a thermal analysis of a flat heat pipe with a grooved wick structure is presented. The effects of the liquid-vapor interfacial shear stress, the contact angle, and the amount of liquid charge have been included in the proposed model. In particular, the axial variations of the wall temperature and the evaporation/condensation rates are considered by solving the one-dimensional conduction and the augmented Young-Laplace equations, respectively. In order to verify the model, the results obtained from the model are compared to existing experimental data.

• Journal of computational fluids engineering
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• v.12 no.1
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• pp.9-15
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• 2007

A three dimensional numerical model to predict the flow and transport of mixtures and also the electrochemical reactions in polymer electrolyte membrane (PEM) fuel cells is developed. The numerical computation is base on vorticity- velocity method. Governing equations for the flow and transport of mixtures are coupled with the equations for electrochemical reactions and are solved simultaneously including production and condensation of vapor. Fuel cell performance predicted by this calculation is compared with the experimental results and resonable agreements are achieved.

• Proceedings of the Materials Research Society of Korea Conference
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• 2003.11a
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• pp.93-93
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• 2003

Tungsten and tungsten heavy alloys have widespread application as radiation shielding devices and heavy duty electrical contacts. High density and good room temperature mechanical properties have generated interest in evaluating tungsten and tungsten alloys as kinetic energy penetrators against armor. Nowdays ultra fine-grained tungsten powders are in great interest because higly dense structures can be obtained at low temperature, pressure and lower sintering time. Several physical md chemical methods are available for the synthesis of nanometric metal Powders: ball milling, laser abalation, vapor condensation, chemical precipitation, metallic wire explosion i.e. However production rates of the above mentioned methods are low and further efforts are needed to find out large-scale synthesis methods. From this point of view solid state combustion method ( known as SHS) represents undoubted interest.

• Proceedings of the Materials Research Society of Korea Conference
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• 2003.11a
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• pp.43-43
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• 2003

WC/Co계 초경 합금은 각종 공구 및 내마모 부품으로서 널리 사용되고 있다. 초경 합금의 특성은 WC 분말 크기에 크게 좌우되며 일반적으로 입자의 크기가 미세해 질수록 기계적 특성이 향상되는 것으로 알려지고 있다. 이를 위하여 지금까지 많은 연구가 진행되고 있으나, 현재 액상 공정을 이용하여 약 100nm급의 초경 분말이 개발되어지고 있으며, 그보다 미세한 분말의 제조는 아직 이루어지고 있지 않은 실정이다. 그러나 최근 기상 반응을 이용하여 수nm급의 분말 제조가 가능하게 되었으나, 주로 산화물계에 분말 제조에 국한되고 있다. 본 연구에서는 이러한 기상 반응법을 이용하여 초경 합금의 핵심 소재인 WC분말을 합성하고자 하였으며, 이를 위하여 장치의 설계 및 제작, 그리고 공정별 특성에 대하여 연구하였다.

• Journal of Powder Materials
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• v.10 no.1
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• pp.51-56
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• 2003

To investigate the effect of the parameters of the plasma arc discharge process on the particle formation and particle characteristics of the iron nano powder, the chamber pressure, input current and the hydrogen volume fraction in the powder synthesis atmosphere were changed. The particle size and phase structure of the synthesized iron powder were studied using the FE-SEM, FE-TEM and XRD. The synthesized iron powder particle had a core-shell structure composed of the crystalline $\alpha$-Fe in the core and the crystalline $Fe_3O_4$ in the shell. The powder generation rate and particle size mainly depended on the hydrogen volume fraction in the powder synthesis atmosphere. The particle size increased simultaneously with increasing the hydrogen volume fraction from 10% to 50%, and it ranged from about 45nm to 130 nm.

• The Transactions of the Korean Institute of Electrical Engineers C
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• v.55 no.9
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• pp.452-455
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• 2006

The technology of wire explosion have been used to product nanopowders. A new concept was proposed to produce metallic nanosized powders, which is wire explosion in liquid media. We have exploded the Ag or Cu wires of diameter of O.3mm, 40mm long, in the de-ionized water or acetone, respectively. Electrical energy of 1.1kJ was stored in 10uF capacitor and released to the wires through a triggered spark gap switch. The process was observed by high-speed camera. Those images showed that the powders were generated by vapor condensation in the shell formed by shock wave in the water. The particles were directly dispersed into the water with collapse of the shell. The sizes of Ag and Cu nanopowders were evaluated to 35nm and 17nm, respectively.

• Proceedings of the IEEK Conference
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• 2001.10a
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• pp.249-251
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• 2001

A novel technology for recycling valuable metals contained in waste dry-battery by vacuum metallurgy was devised by theoretical analysis. On the condition of the total chamber pressure of 1.013$\upsilon$10$^1$Pa, Hg, Cd and Zn are distilled in the temperature range of 773~973K, Pb is volatilized in the range of 1173~1273K while Mn, Cu, Fe and C are remained in residual. MnO$_2$and ZnO are reduced by carbon in waste dry-battery in 773~1273K. Pure metals including Zn, Cd, Hg and Pb can be recovered respectively from their mixed vapor by fractional condensation. Metal Cu and MnO$_2$can be obtained from the residual by hydrometallguical method. The technology can eliminate the pollution of Cd, Hg and Pb to environment.