• 한국연소학회:학술대회논문집
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• 2014.11a
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• pp.81-84
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• 2014

A Experimental study on flame extinction behavior was investigated using He curtain flow with counter triple co-flow burner. Buoyancy force was suppressed up to a microgravity level of $10^{-2}-10^{-3}g$ by using He curtain flow. The stability maps were provided with a functional dependency of diluent mole fraction and global strain rate to clarify the differences in flame extinction behavior. The flame extinction curves had C-shapes at various global strain rates. The oscillation and extinction modes were different each other in terms of the global strain rate, and the flames extinction modes could be classified into five modes such as (I) and (II): an extinction through the shrinkage of the outmost edge flame forward the flame center after self-excitation and without self-excitation, respectively, (III): an extinction through rapid advancement of a flame hole while the outmost edge flame is stationary, (IV): self-excitation occurs in the outermost edge flame and the center edge flame and then a donut shaped flame is formed and/or the flame is entirely extinguished, (V): shrinkage of the outermost edge flame without self-excitation followed by shrinkage or survival of the center flame. These oscillation and extinction modes could be identified well to the behavior of edge flame. The result also showed that the edge flame was influenced significantly by the conductive heat losses to the flame center or ambient He curtain flow.

• Transactions of the Korean hydrogen and new energy society
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• v.23 no.1
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• pp.8-18
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• 2012

This paper presents a three-dimensional, transient cold-start polymer electrolyte fuel cell (PEFC) model to numerically evaluate the effects of membrane electrode assembly (MEA) design and cell location in a PEFC stack on PEFC cold start behaviors. The cold-start simulations show that the end cell experiences significant heat loss to the sub-freezing ambient and thus finally cold-start failure due to considerable ice filling in the cathode catalyst layer. On the other hand, the middle cells in the stack successfully start from $-30^{\circ}C$ sub-freezing temperature due to rapid cell temperature rise owing to the efficient use of waste heat generated during the cold-start. In addition, the simulation results clearly indicate that the cathode catalyst layer (CL) composition and thickness have an substantial influence on PEFC cold-start behaviors while membrane thickness has limited effect mainly due to inefficient water absorption and transport capability at subzero temperatures.

• Journal of the Korean Society of Propulsion Engineers
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• v.13 no.4
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• pp.45-54
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• 2009

An exergy analysis is carried out for the regenerative steam-injection gas turbine systems which has a potential of enhanced thermal efficiency and specific power. Using the analysis model in the view of the second law of thermodynamics, the effects of pressure ratio, steam injection ratio, ambient temperature and turbine inlet temperature are investigated on the performance of the system such as exergetic efficiency, heat recovery ratio of heat exchangers, exergy destruction, loss ratios, and on the optimal conditions for maximum exergy efficiency. The results of computation show that the regenerative steam-injection gas turbine system can make a notable enhancement of exergy efficiency and reduce irreversibilities of the system.

• Journal of Bio-Environment Control
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• v.29 no.1
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• pp.9-19
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• 2020

Convective heat transfer is the main component of greenhouse energy loss because the energy loss by this mechanism is greater than those of the other two components (radiative and conductive). Previous studies have examined the convective heat transfer coefficients under natural conditions, but they are not applicable to symmetric thermal screens with zero porosity, and such screens are largely produced and used in Korea. However, the properties of these materials have not been reported in the literature, which causes selectivity issues for users. Therefore, in this study, three screens having similar color and zero porosity were selected, and a mathematical procedure based on radiation balance equations was developed to determine their convective heat transfer coefficients. To conduct the experiment, a hollow wooden structure was built and the thermal screen was tacked over this frame; the theoretical model was applied underneath and over the screen. Input parameters included three components: 1) solar and thermal fluxes; 2) temperature of the screen, black cloth, and ambient air; and 3) wind velocity. The convective heat transfer coefficients were determined as functions of the air-screen temperature difference under open-air environmental conditions. It was observed from the outcomes that the heat transfer coefficients decreased with the increase of the air-screen temperature difference provided that the wind velocity was nearly zero.

• Journal of Biosystems Engineering
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• v.43 no.1
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• pp.21-29
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• 2018

Purpose: The objective of this study is to evaluate the performance of the conical solar concentrator (CSC) system, whose design is focused on increasing its collecting efficiency by determining the optimal conical angle through a theoretical study. Methods: The design and thermal performance analysis of a solar concentrator system based on a $45^{\circ}$ conical concentrator were conducted utilizing different mass flow rates. For an accurate comparison of these flow rates, three equivalent systems were tested under the same operating conditions, such as the incident direct solar radiation, and ambient and inlet temperatures. In order to minimize heat loss, the optimal double tube absorber length was selected by considering the law of reflection. A series of experiments utilizing water as operating fluid and two-axis solar tracking systems were performed under a clear or cloudless sky. Results: The analysis results of the CSC system according to varying mass flow rates showed that the collecting efficiency tended to increase as the flow rate increased. However, the collecting efficiency decreased as the flow rate increased beyond the optimal value. In order to optimize the collecting efficiency, the conical angle, which is a design factor of CSC, was selected to be $45^{\circ}$ because its use theoretically yielded a low heat loss. The collecting efficiency was observed to be lowest at 0.03 kg/s and highest at 0.06 kg/s. All efficiencies were reduced over time because of variations in ambient and inlet temperatures throughout the day. The maximum efficiency calculated at an optimum flow rate of 0.06 kg/s was 85%, which is higher than those of the other flow rates. Conclusions: It was reasonable to set the conical angle and mass flow rate to achieve the maximum CSC system efficiency in this study at $45^{\circ}$ and 0.06 kg/s, respectively.

• Fashion & Textile Research Journal
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• v.8 no.3
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• pp.349-356
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• 2006

The purpose of this study was to compare the thermophysiological responses and subjective sensations of clothing materials with different water transfer property investigated in exercising and resting subjects at an ambient temperature of $20^{\circ}C$ and a relative humidity of 40%. Two kinds of clothing ensemble were tested: 100% cotton with highly water-absorbent but slowly dry properties(C) and 100% polyester with quickly water-absorbent and dry properties by four capillary channels(QADP). Seven apparently healthy male participants each undertook two series of experiments comprised 10-min of rest, 20-min of exercise with 70% of $VO_{2max}$ on a treadmill and 20-min of recovery. Mean skin temperature was significantly lower in QADP than in C during exercise and recovery. Clothing microclimate temperature was significantly lower in QADP during exercise and clothing surface temperature was also lower in QADP especially during recovery. Also, clothing surface humidity was significantly higher in QADP after the later half of exercise. The concentration of blood lactic acid tended to decrease to a lower level at recovery 3 minutes when wearing QADP rather than C clothing ensemble. Metabolic energy was marginally significantly less during the second half of exercise in QADP. Body mass loss tended to be greater in C than in QADP. The participants had better scores in thermal sensation, comfortable sensation and wetness in QADP during exercise and recovery. These results show that functional materials with quickly water-absorbent and dry properties can alleviate heat strain and induce more comfortable clothing microclimates and subjective sensations in the exercise-induced hyperthermia.

• Horticultural Science & Technology
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• v.32 no.2
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• pp.184-192
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• 2014

Various types of unit packaging methods were applied for 'Fuji' apples during short-term cold storage and export simulation. Gas tightness of the package was controlled stepwise in the successive two-year experiments using different perforation treatments (none, punch hole, or pinhole) and sealing methods (tie v s. heat seal). Risk of tight packaging and effectiveness of macroperforation on weight loss and quality maintenance were analyzed as related to changes in gas concentration inside the packages. Immediately after harvest, each 5 apple units were packaged in $40{\mu}m$ polypropylene (PP) film bags, stored 4 weeks at $0^{\circ}C$, and then put on the shelf for one week at ambient temperature in the preliminary experiment, In the main experiment, export process was imposed after storage simulating 2 week refrigerated container shipment at $0^{\circ}C$ plus one week local marketing at ambient temperature. Non-perforated film packaging with relatively high gas tightness induced flesh browning caused by carbon dioxide accumulation regardless of the sealing methods. Among perforated film packaging, in contrast, atmospheric modification was partly established only in the pinhole treatment and flesh browning symptom was not observed in all the treatments. Even the punch hole perforated film packaging without gas tightness effectively reduced the weight loss, whereas had slight benefits for quality maintenance. Reduced perforation using pinhole treatment seemed to improve sensory texture, while effects on physicochemical quality were insignificant. Overall results suggest the need of more minute perforation treatments on the packaging film to ensure modified atmosphere effects on quality maintenance.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.23 no.1
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• pp.80-86
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• 2011

Vertical open display cabinets are widely used in shopping mall, supermarkets, retail stores. Maintaining the temperature of foods in the display cabinet is vitally important to retailers to ensure optimal food quality and safety. The purpose of this study is to reduce the infiltration of air and heat loss from ambient space to display cabinet. The three-dimensional Computational Fluid Dynamics(CFD) simulation is used for the analysis of air flow patterns and temperature distribution in refrigerated display cabinets. Under several operating conditions which vary both the inner and outer jet velocities in the range from 0.3 to 1.1 m/s, simulations were carried out. This paper presents a performance of display cabinets with single jet and double jet. The energy consumption due to thermal entrainment ratio is plotted with varying Re. It was found that the double jet system is better than single jet system in terms of temperature distribution and energy saving.

• Journal of Biosystems Engineering
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• v.31 no.6 s.119
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• pp.529-534
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• 2006

The greenhouse heating system with far-infrared heater was built to analyze various thermal characteristics, such as greenhouse air temperature, soil temperature, energy flow, energy consumption in far-infrared heater, and other factors, which could be used in comparison with other greenhouse heating system in this study. The results showed that the inside air temperature of the far-infrared greenhouse heating system was $5^{\circ}C$ higher than that of hot air heating system. Heat loss of daytime was found to be larger than that of night time as much as 44.8% for the heating system with far-infrared heater. In the heating system with far-Infrared heater, when the lowest ambient temperature was -8 $\sim$ -7$^{\circ}C$, the air temperature of greenhouse was 12 $\sim$ 15$^{\circ}C$, thus the far-infrared heating system was shown to be feasible for heating system. Energy consumption of far-infrared heating system was shown to be less than that of hot air heating system.

• Journal of the Korean Society for Nondestructive Testing
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• v.27 no.2
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• pp.142-147
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• 2007

In order to measure corrosion level of steel reinforcement rebar which is inside reinforced concrete structure, infrared thermographic technique was employed. Experimental test parameters were four different ambient temperatures and various levels of corrosion states (0, 1, 3, 5, 7 and 10% of weight loss). After analysis of temperature distributions of concrete surface, the amount of heat flux from the concrete surface is directly proportional to the corrosion level which is inside of concrete.