• 한국초전도학회:학술대회논문집
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• v.16
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• pp.27-27
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• 2006

• Proceedings of the KSME Conference
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• 2004.04a
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• pp.2094-2099
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• 2004

An experimental study on heat transfer characteristics near the critical pressure has been performed with an internally-heated vertical annular channel cooled by R-134a fluid. Two series of tests have been completed: (a) steady-state critical heat flux (CHF) and (b) heat transfer tests for pressure reduction transients through the critical pressure. In the present experimental range, the steady-state CHF decreases with the increase of the system pressure For a fixed inlet mass flux and subcooling, the CHF falls sharply at about 3.8 MPa and shows a trend toward converging to zero as the pressure approaches the critical point of 4.059 MPa. The CHF phenomenon near the critical pressure does not lead to an abrupt temperature rise of the heated wall because the CHF occurred at remarkably low power levels. In the pressure reduction transient experiments, as soon as the pressure passed through the critical pressure, the wall temperatures rise rapidly up to a very high value due to the occurrence of the departure from nucleate boiling. The wall temperature reaches a maximum at the saturation point of the outlet temperature, then tends to decrease gradually.

• Nuclear Engineering and Technology
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• v.36 no.5
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• pp.403-414
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• 2004

An experimental study of heat transfer characteristics near the critical pressure has been performed with an internally-heated vertical annular channel cooled by R-134a fluid. Two series of tests have been completed: (a) steady-state critical heat flux (CHF) tests, and (b) heat transfer tests for pressure reduction transients through the critical pressure. In the present experimental range, the steady-state CHF decreases with increase of the system pressure for fixed inlet mass flux and subcooling. The CHF falls sharply at about 3.8 MPa and shows a trend towards converging to zero as the pressure approaches the critical point of 4.059 MPa. The CHF phenomenon near the critical pressure does not lead to an abrupt temperature rise of the heated wall, because the CHF occurs at remarkably low power levels. In the pressure reduction transients, as soon as the pressure passes below the critical pressure from the supercritical pressure, the wall temperatures rise rapidly up to very high values due to the departure from nucleate boiling. The wall temperature reaches a maximum at the saturation point of the outlet temperature, and then tends to decrease gradually.

• Fire Science and Engineering
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• v.28 no.1
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• pp.26-30
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• 2014

A Compensation-type fire detector (CFD) is operated with two functions of a differential-temperature detector and as a fixed-temperature detector. The differential-temperature detector observes a rate of temperature increase, and the fixed-temperature detector measures a given fixed temperature. The differential-temperature detector does not observe the outbreak of fire in slowly increasing temperature conditions, whereas the fixed-temperature detector is not able to observe the outbreak of fire in conditions under predetermined temperature level. We developed a CFD to compensate for weaknesses of both detectors. To compensate for the disadvantages, a sensor of the sensor metal-insulator-transition critical-temperature sensor was used. Temperature coefficient of resistance is the sensitivity for sensor. At $55^{\circ}C$, temperature coefficient of resistance of metal-insulator-transition critical-temperature sensor was 14.15%. Temperature coefficient of resistance of thermistor was about 0.5%. This CFD was operated as two ways that fixed-temperature detector and differential-temperature detector in one sensor.

• 한국전산유체공학회:학술대회논문집
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• 2008.03b
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• pp.292-295
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• 2008

In a SCWR (SuperCritical pressure Water cooled Reactor), the coolant temperature initially at below the pseudo-critical temperature at the bottom of a reactor core increases as the coolant flows upward through the sub-channels of the fuel assemblies, and it finally becomes higher than the pseudo-critical temperature when it leaves the reactor core. At certain conditions, heat transfer deterioration occurs near the pseudo-critical temperature and it may cause a drastic rise of the fuel surface temperature resulting a fuel failure. Therefore, an accurate estimation of the heat transfer coefficient is very important for the thermal-hydraulic design of a reactor core. An experiment on heat transfer to the vertically upward flowing $CO_2$ at a supercritical pressure in a circular tube were performed at KAERI. The internal diameter of the test section is 6.32 mm, which corresponds to the hydraulic diameter of a sub-channel in the conceptional design proposed by KAERI. The test range of the mass flux is 285 to 1200 kg/m$^2$s and the maximum heat flux is 170 kW/m$^2$. The inlet pressure is maintained at 8.12 MPa, which is 1.1 times the critical pressure. A new correlation, which covers both the normal and deterioration heat transfer regimes was proposed and compared with the estimations by exiting correlations.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.34 no.2
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• pp.129-138
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• 2010

Technical ceramics, due to their unique physical properties, are excellent candidate materials for engineering applications involving extreme thermal and chemical environments. When ceramics are rapidly cooled, they receive thermal shock. The thermal shock parameter is defined as the critical temperature difference. The critical temperature difference for ceramic parts is influenced by its size, the convective heat transfer coefficient, etc. The thermal shock for a component is analyzed by using the transient thermal stress. If the transient thermal stress exceeds the modulus of rupture (MOR), cracking by thermal shock is initiated. The critical temperature difference for water is less than the critical temperature difference for air. The three-way catalyst substrate used in this study has an adequate performance against thermal shock because its radial and axial temperature differences existed below the critical temperature differences.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.11 no.2
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• pp.154-161
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• 1998

The effects of melting temperature and holding time on the critical current density($J_c$) of $YBa_2Cu_3O_x$ based superconducting bulk fabricated by MPMG process were investigated. The amount of the formed $Y_2BaCuO_5$ phases increased with the melting temperature. However, the value of critical current density was highest at 1120 $^{\circ}C$. With this proper melting temperature, the effect of holding time on the critical characteristics was also investigated. In the case of Ag addition, the volume of the formed $Y_2BaCuO_5$ phase when the amount of Ag addition was 10 wt% and 20 wt% was observed to be highest at 20 minute and 40 minute respectively. But in the specimen without Ag, volume of $Y_2BaCuO_5$ phase increased as the holding time increased. The proper melting temperature and the holding time obtained were 1120 $^{\circ}C$ and 20 minute. The long holding time was not effective for the $J_c$ improvement as well as the formation of $Y_2BaCuO_5$.

• Fire Science and Engineering
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• v.33 no.4
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• pp.70-76
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• 2019

This paper reports development of a low-power standalone heat detector using a Critical-Temperature Switch. The Critical-Temperature Switch, which is a thermally sensitive and passive component whose resistance decreases significantly at 70 ℃ due to a metal-insulator transition, provides reliable temperature measurements. This digital-like behavior of the Critical-Temperature Switch can detect fires without a microcontroller, meaning that it can minimize the power consumption of the standalone heat detector. The experimental results showed that the standalone heat detector using the Critical-Temperature Switch complied with the Notification of the National Emergency Management Agency. Compared to conventional standalone heat detectors, only 70% of the power was consumed monitoring the fires.

• Bulletin of the Korean Chemical Society
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• v.24 no.10
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• pp.1449-1454
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• 2003

A new equation has been derived on the basis of ${\delta}G^o$ = -RT lnK, linear behavior of the enthalpy of micellization with temperature, and the Gibbs-Helmholtz relation. It describes correctly the dependence of critical micelle concentration $(X_{CMC})$ on temperature and has yielded excellent fitting results for various surfactant systems. The new equation results in the linear behavior of the entropy of micellization with temperature and accounts for the compensation phenomena observed for the micellization in aqueous solutions, along with the linear dependence of the enthalpy of micellization on temperature. These results imply that the new equation of $X_{CMC}(T)$ accounts for the temperature dependence of CMC correctly.

• Journal of the Korean Society of Fisheries and Ocean Technology
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• v.35 no.4
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• pp.421-427
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• 1999

The critical fracture energy and failure mechanisms of GF/PP composites are investigated in the temperatures range of the ambient temperature to $-50^{\circ}C$ The critical fracture energy increase as fiber volume fraction ratio increased The critical fracture energy shows a maximum at ambient temperature and it tends to decrease as temperature goes up. Major failure mechanisms can be classfied such as fiber matrix debonding, fiber pull-out and/or delamination and matrix deformation.