• Journal of The Korean Astronomical Society
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• v.37 no.5
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• pp.571-574
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• 2004

Clusters of galaxies are filled with X-ray emitted hot gas with the temperature of T ${\~}$2-10 keV. Recent X-ray observations have been revealing unexpectedly that many cluster cores have complicated, peculiar X-ray structures, which imply dynamical motion of the hot gas. Moreover, X-ray spectra indicate that radiative cooling of the cool gas is suppressed by unknown heating mechanisms (the 'cooling flow problem'). Here we propose a novel mechanism reproducing both the inhomogeneous structures and dynamics of the hot gas in the cluster cores, based on state-of-the-art hydrodynamic simulations. We showed that acoustic-gravity waves, which are naturally expected during the process of hierarchical structure formation of the universe, surge in the X-ray hot gas, causing a serous impact on the core. This reminds us of tsunamis on the ocean surging into an distant island. We found that the waves create fully-developed, stable turbulence, which reproduces the complicated structures in the core. Moreover, if the wave amplitude is large enough, they can suppress the cooling of the core. The turbulence could be detected in near-future space X-ray missions such as ASTRO-E2.

• Journal of Energy Engineering
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• v.28 no.4
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• pp.103-110
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• 2019

The domestic innovative power reactor named iPOWER will employ the passive molten corium cooling system(PMCCS) to cool down and stabilize the core melt in the severe accident. The final design concept of the PMCCS is yet to be determined, but the in-vessel retention through external reactor vessel cooling has been also considered as a viable strategy to cope with the severe accident. In this study, the two-phase natural circulation flow established between the reactor vessel and the insulation was simulated using a thermal-hydraulic system code, MARS-KS. The flow path of cooling water was modeled with one-dimensional nodes, and the boundary condition of the heat load from the molten core was defined to estimate the naturally-driven flow rate. The evolution of major thermal-hydraulic parameters were also evaluated, including the temperature and the level of cooling water, the void fraction around the lower head of the reactor vessel, and the heat transfer mode on its external surface.

• Asian-Australasian Journal of Animal Sciences
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• v.21 no.4
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• pp.555-560
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• 2008

The effects of utilizing evaporative cooling system equipped with tunnel ventilation on postpartum ovarian activities, energy balance and milk production of early lactating dairy cows under hot and humid climates were studied from parturition to 22 wk postpartum. Thirty-four crossbred Holstein-Friesian (93.75% HF$\times$.25% Bos indicus) primiparous cows were randomly assigned to one of two groups. Cooled cows (n = 17; treatment) were housed in the tunnel ventilated barn equipped with evaporative cooling system and uncooled (n = 17; control) were housed in the naturally ventilated barn without supplemental cooling system. Cooled cows had greater (p<0.05) dry matter intake and milk production than uncooled cows. Days to the energy balance (EB) nadir did not differ between groups. However, days to equilibrium EB for uncooled cows was longer (p<0.05) than for cooled cows. There was no significant difference in postpartum anovular condition between cooled and uncooled cows. The interval from parturition to first postpartum ovulation did not differ between groups ($31.4{\pm}4.3$ and $26.1{\pm}3.6$ day, respectively). These results suggest that the evaporative cooling and tunnel ventilation has the potential to decrease the severity of heat stress and improve both milk production and metabolic efficiency during early lactation without affecting reproductive function in dairy cows under hot and humid climates.

• Nuclear Engineering and Technology
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• v.53 no.10
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• pp.3196-3206
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• 2021

For the Korean design of the PCCS (passive containment cooling system) in an innovative PWR, the overall thermal resistance around a condenser tube is dominated by the heat transfer coefficient of steam condensation on the exterior surface. It has been reported, however, that the calculated heat transfer coefficients by thermal-hydraulic system codes were much lower than measured data in separate effect tests. In this study, a new empirical model of steam condensation in the presence of a noncondensable gas was implemented into the MARS-KS 1.4 code to replace the conventional Colburn-Hougen model. The selected correlation had been developed from condensation test data obtained at the JERICHO (JNU Experimental Rig for Investigation of Condensation Heat transfer On tube) facility, and considered the effect of the Grashof number for naturally circulating gas mixture and the curvature of the condenser tube. The modified MARS-KS code was applied to simulate the transient response of the containment equipped with the PCCS to the large-break loss-of-coolant accident. The heat removal performances of the PCCS and corresponding evolution of the containment pressure were compared to those calculated via the original model. Various thermal-hydraulic parameters associated with the natural circulation operation through the heat transport circuit were also investigated.

• Journal of The Korean Society of Agricultural Engineers
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• v.54 no.3
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• pp.113-124
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• 2012

Reliable estimation of energy load inside the greenhouse and the selection of cooling and heating facilities are very important preceding factors to save energy as well as initial and maintenance costs of operating a greenhouse. Recently, building energy simulation (BES) technique to simulate a model similar to the actual conditions through a variety of dynamic simulation methods, and predict and analyze the flow of energy is being actively introduced and developed. As a fundamental research to apply the BES technique which is mainly used for analysis of general buildings, to greenhouse, this research designed four types of naturally-ventilated greenhouses using one of commercial programs, TRNSYS, and then compared and analyzed their energy load properties, by applying meteorological data collected from six regions in Korea. When comparing the greenhouse load of each region depending on latitude and topographical characteristics through simulation, Chuncheon had nearly 9~49 % higher heating load per year than other regions, but its annual cooling load was the reverse to it. Except for Jeju, 1-2W type greenhouses in five regions showed about 17 % higher heating load than a widespan type greenhouse, and 1-2W type greenhouses in Chuncheon, Suwon, Cheongju, Daegu, Cheonju and Jeju had 23 %, 20 %, 17 %, 16 %, 18 % and 20 % higher cooling load respectively than a wide span-type one. Glasshouse and vinyl greenhouse showed 8~11 % and 10~12 % differences respectively in heating load, while 2~10 % and 7~10 % differences in cooling load respectively.

• Journal of Bio-Environment Control
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• v.14 no.1
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• pp.29-37
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• 2005

In order to provide fundamental data on utilization of dehumidifier in greenhouses, a condensing type dehumidifier using ground water as a coolant was developed and tested dehumidification performance. The developed dehumidifier was applied to greenhouse with fog cooling system and effect of dehumidification on improvement of evaporative cooling efficiency was analyzed. Results of the dehumidifier performance test showed that dehumidification using ground water as a coolant was sufficiently possible in fog cooling greenhouse. When the set point temperature of greenhouse cooling was $32^{\circ}C$ and as temperatures of ground water rose from $15^{\circ}C\;to\;18^{\circ}C,\;21^{\circ}C\;and\;24^{\circ}C$, dehumidification rates decreased by $17.7\%,\;35.4\%\;and\;52.8\%$, respectively. As flow rates of ground water reduced to $75\%\;and\;50\%$, dehumidification rates decreased by $12.1\%\;and\;30.5\%$, respectively. Cooling efficiency of greenhouse equipped with fog system was distinctly improved by artificial dehumidification. When the ventilation rate was 0.7 air exchanges per minute, dehumidification rates of the fog cooling greenhouse caused by natural ventilation were 53.9%-74.4% and they rose up to 75.4%-95.9% by operating the dehumidifier. In case of using the ground water of $18^{\circ}C$ and flow rate of design condition, it was analyzed that whole fog spraying water can be dehumidified even if the ventilation rate is 0.36 exchanges per minute. As a utilization of dehumidifier, it is possible to improve cooling efficiency of fog system in naturally ventilated greenhouses.

• Korea-Australia Rheology Journal
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• v.13 no.1
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• pp.1-12
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• 2001

This paper describes the application of our recently developed two-phase model for flow-induced crystallization (FIC) to the simulation of fiber spinning and film blowing. 1-D and 2-D simulations of fiber spinning include the combined effects of (FIC), viscoelasticity, filament cooling, air drag, inertia, surface tension and gravity and the process dynamics are modeled from the spinneret to the take-up roll device (below the freeze point). 1-D model fits and predictions are in very good quantitative agreement with high- and low-speed spinline data for both nylon and PET systems. Necking and the associated extensional softening are also predicted. Consistent with experimental observations, the 2-D model also predicts a skin-core structure at low and intermediate spin speeds, with the stress, chain extension and crystallinity being highest at the surface. Film blowing is simulated using a "quasi-cylindrical" approximation for the momentum equations, and simulations include the combined effects of flow-induced crystallization, viscoelasticity, and bubble cooling. The effects of inflation pressure, melt extrusion temperature and take-up ratio on the bubble shape are predicted to be in agreement with experimental observations, and the location of the frost line is predicted naturally as a consequence of flow-induced crystallization. An important feature of our FIC model is the ability to predict stresses at the freeze point in fiber spinning and the frost line in film blowing, both of which are related to the physical and mechanical properties of the final product.l product.

• Journal of Bio-Environment Control
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• v.9 no.1
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• pp.1-10
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• 2000

This study was performed to find an efficient method to overcome extremely high temperature in greenhouses during summer season. The actual utilization of greenhouses during hot summer season showed that about 21.6％ of the investigated greenhouse farms were in fallow state, and most of greenhouse farms were cultivated under the very inferior environment. Thermal environment of greenhouses according to the evaporative cooling method and several assistant cooling methods such as ventilation, shading screen, roof sprinkling were examined. As the each assistant cooling method was used, about 74.8％, 25.9％, and 58.2％ of temperatures measured at intervals of ten minutes between ten and seventeen o'clock were above 35$^{\circ}C$. When shading screen and evaporative cooling system were operated, most greenhouse air temperatures were maintained below 35$^{\circ}C$, and showed a drop of 3.8~4.2$^{\circ}C$ as compared with naturally ventilated greenhouse.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.26 no.9
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• pp.401-408
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• 2014

This paper proposes an optimum operation method for open type ground heat exchangers. A series of TRTs and artificial heating/cooling operations were carried out while monitoring temperature in the hole of SCW. The ground temperature naturally increases with depth, but a switch between the cooling/heating mode results in a change in the distribution of ground temperature. The effect of the mode change was evaluated by performing LMTD and COMSOL multiphysics analysis for a reduced model with the depth of 150 m. As a result, in the cooling mode, the upstream operation is more efficient than the downstream operation and reduces EWT by $2.26^{\circ}C$. On the other hand, in the heating mode, the downstream operation is advantageous over the upstream operation and increases EWT by $3.19^{\circ}C$. The merit of the optimum operation will be enhanced for the typical dimension of SCW with a depth of 400~500 m. In the future, an open type ground heat exchanger system adopting the optimum operation with variation in the ground temperature will be used in practice.

• Proceedings of the Korean Society For Composite Materials Conference
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• 2000.04a
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• pp.43-48
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• 2000

Composite materials have been increasingly used in automotive and aircraft industries, naturally leading to active researches on the materials. The carbon-epoxy composite is selected to study its thermal characteristics. During multiple thermal cycles composed of repeated cooling and heating variations of elastic constants are investigated to understand thermal effects on the carbon-epoxy composite. In this investigation longitudinal resonance method and flexural resonance method was used to characterize. The values of $E_1$ show small amount of increases depending on number of cycles of the thermal fatigue processes whereas values of $G_13$ do not indicate noticeable changes. Also, in cases of $E_2$ and $G_23$ their values decrease to a certain extend in initial stages after applications of thermal fatigue processes. However, the number of cycles of the applied thermal fatigue processes does not seem to affect their values.