• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.23 no.2
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• pp.147-152
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• 2011

The cryogenic cooling load from the top plate of cryostat to liquid helium surface, including wall conduction, thermal radiation and current leads, is investigated in a closed cryostat system for superconducting magnet. In general methods of load estimation, individual load is calculated separately, however they are actually coupled each other because of natural convection of helium vapor. Using relevant heat transfer analysis, we calculate cryogenic load with taking into account the effect of natural convection. Cryogenic load is under-estimated approximately 1% when the natural convection is ignored. The difference between actual cooling load and cooling load by individual calculation increases with supplying current.

• Geomechanics and Engineering
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• v.21 no.3
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• pp.237-245
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• 2020

The process of evaporation from free water surface was simulated in a large scale environmental chamber under various controlled atmospheric conditions and also was modelled by a new mass transfer model. Six evaporation tests were conducted with increasing wind speed and air temperature in the environmental chamber, and hence the effect of atmosphere parameters on the evaporation process and the corresponding response of water were investigated. Furthermore, based on the experiment results, seven general types of mass transfer models were evaluated firstly, and then a new model consisted of wind speed function and air relative humidity function was proposed and validated. The results show that the free water evaporation is mainly affected by the atmospheric parameters and the evaporation rate increases with the increasing air temperature and wind speed. Both the air and soil temperatures are affected by the energy transformation during water evaporation. The new model can satisfactorily describe the evaporation process from free water surface under different atmospheric conditions.

• Journal of the Korea Safety Management & Science
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• v.20 no.4
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• pp.15-19
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• 2018

The purpose of this study is to enhance heat insulation effect and to decrease fire hazard by attaching aluminum foil to expanded polystyrene, which is mainly used for insulating materials, to have fire retardant. The result of the test confirmed that the insulating materials, expanded polystyrene of $10kg/m^3$ and $14kg/m^3$ of density attached aluminum foil on both sides, showed 12%, 14% of improved heat transfer coefficient respectively compared to existing expanded polystyrene of the same density. Besides, they met all the standards for the testing of heat release and gas hazard. On the other hand, the one made of general expanded polystyrene could not meet the standards of the heat release test and the gas hazard test.

• Nuclear Engineering and Technology
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• v.32 no.1
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• pp.34-45
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• 2000

This study is concerned with the experimental test and numerical analysis of the heat transfer and solidification of the molten metal pool with overlying coolant with boiling. In the test, the metal pool is heated from the bottom surface and coolant is injected onto the molten metal pool. Experiments were performed by changing the test section bottom surface temperature of the metal layer and the coolant injection rate. The two-phase boiling coolant experimental results are compared against the dry test data without coolant or solidification of the molten metal pool, and against the crust formation experiment with subcooled coolant. Also, a numerical analysis is performed to check on the measured data. The numerical program is developed using the enthalpy method, the finite volume method and the SIMPLER algorithm. The experimental results of the heat transfer show general agreement with the calculated values. The present empirical test and numerical results of the heat transfer on the molten metal pool are apparently higher than those without coolant boiling. This is probably because this experiment was performed in concurrence of solidification in the molten metal pool and the rapid boiling of the coolant. The other experiments were performed without coolant boiling and the correlation was developed for the pure molten metal without phase change.

• Journal of the Korean Society for Precision Engineering
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• v.21 no.7
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• pp.185-194
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• 2004

In order to analyze the motion errors of the aerostatic stage, it is necessary to consider the influence of the moment variation occurred inside the pads. In this paper, a motion error analysis method utilizing the transfer functions on the reaction force and moment is proposed, and general characteristics of the transfer functions are discussed. Calculated motion errors by the proposed method show good agreement with the ones calculated by Multi fad Method, which is considered the entire table as an analysis object. Also, by the introduction of the transfer function of motion errors, which represent the relationship between the spatial frequency components of the rail form error and motion errors, motional characteristics of the porous aerostatic stage can be generalized. In detail, the influence of the spatial frequencies is analyzed qualitatively, and the patterns of the insensitive frequencies which almost do not affect the linear motion error or angular motion error according to the rail length ratio and the number of the pad are verified. The relationship between the moment variation occurred inside the pads and the motion errors is also verified together.

• Journal of Mechanical Science and Technology
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• v.15 no.12
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• pp.1882-1891
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• 2001

The present paper investigated the effect of ultrasonic vibrations on the melting process of a phase-change material (PCM). Furthermore, the present study considered constant heat flux boundary conditions unlike many of the previous researches adopted constant wall temperature conditions. Therefore, in the present study, modified dimensionless parameters such as Ste* and Ra* were used. Also, general relationships between melting with ultrasonic vibrations and melting without ultrasonic vibrations were established during the melting of PCM. Experimental observations show that the effect of ultrasonic vibrations on heat transfer is very important throughout the melting process. The results of the present study reveal that ultrasonic vibrations accompany the effects like agitation, acoustic streaming, cavitation, and oscillating fluid motion. Such effects are a prime mechanism in the overall melting process when ultrasonic vibrations are applied. They enhance the melting process as much as 2.5 tildes, compared with the result of natural melting. Also, energy can be saved by applying ultrasonic vibrations to the natural melting. In addition, various time-wise dimensionless numbers provide conclusive evidence of the important role of ultrasonic vibrations on the melting phenomena.

• Fire Science and Engineering
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• v.33 no.4
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• pp.152-162
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• 2019

This paper proposes improvement measures of a school safety education program by analyzing the factors affecting the cognition of school safety on the school safety transfer intention and their mediating effects. In summary, the satisfaction with the contents was represented as a variable with the greatest influence on the transfer intention followed in order by satisfaction with the instructor and satisfaction with the environment. On the other hand, satisfaction with the teaching method was not significant. In addition, the general cognition on school safety education showed that it has mediating effects on the relationship between the satisfaction with the school administrator safety education and the school safety transfer intention.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.26 no.8
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• pp.1172-1182
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• 2002

In the present study a general criterion for local thermal equilibrium is presented in terms of parameters of engineering importance which include the Darcy number, the effective Prandtl number of fluid, and the Reynolds number. For this, an order of magnitude analysis is performed for the case when the effect of convection heat transfer is dominant in a porous structure. The criterion proposed in this study is more general than the previous criterion suggested by Carbonell and Whitaker, because the latter is applicable only when conduction is the dominant heat transfer mode in a porous medium while the former can be applied even when convection heat transfer prevails. In order to check the validity of the proposed criterion for local thermal equilibrium, the forced convection phenomena in a porous medium with a microchanneled structure subject to an impinging jet are studied using a similarity transformation. The proposed criterion is also validated with the existing experimental and numerical results for convection heat transfer in various porous materials that include some of the parameters used in the criterion such as a microchannel heat sink with a parallel flow, a packed bed, a cellular ceramic, and a sintered metal. It is shown that the criterion presented in this work well-predicts the validity of the assumption of local thermal equilibrium in a porous medium.

• Proceedings of the Membrane Society of Korea Conference
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• 1994.04a
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• pp.51-52
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• 1994

High permeability, selectivity and stability are the basic properties also required for membrane gas separations. The $CO_2$ separation by liquid membranes has been developed as a new technique to improve the permeability and selectivity of polymeric membranes. Sirkar et al.(1) have atlempted the hollow-fiber contained liquid membrane technique under four different operational modes, and permeation models have been proposed for all modes. Compared to a conventional liquid membrane, the diffusional resistance decreased by the work of Teramoto et al.(2), who referred to a moving liquid membrane. Recently, Shelekhin and Beckman (3) considered the possibility of combining absorption and membrane separation processes in one integrated system called a membrane absorber. Their analysis could be predicted effectively the performance of flat sheet membrane, however, there are restrictions for considering a flow effect. The gas absorption rate is determined by both an interfacial area and a mass transfer coefficient. It can be easily understood that although the mass transfer coefficients in hollow fiber modules are smaller than in conventional contactors, the substantial increase of the interfacial area can result in a more efficient absorber (4). In order to predict a performance in the general system of hollow-fiber membrane absorber, a gas-liquid mass transfor should be investigated inevitably. The influence of liquid velocity on both a mass transfer and a performance will be described, and then compared with experimental results. A present study is attempted to provide the fundamentals for understanding aspects of promising a hollow-fiber membrane absorber.

• Magazine of the Korea Concrete Institute
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• v.6 no.1
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• pp.89-100
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• 1994

The current test procedure for transfer length, which determine transfer length by measuring concrete strain, has an actual bond stress state in the prestressed pretensioned member : however, it is difficult to determine the bond properties of maximum bond stress and bond stiffness with this method. It is also difficult for design engineer to understand and select a correct safety criterion from the widely distributed results of such a ransfer test alone. An alternative testing procedure is provided here to determine the bond properties without measuring the concrete strain. In this test the bond stress is measured directly by creating a similar boundary condition within the transfer length in a real beam during the transfer of prestressing force. The prestressing force was released step by step by step from the unloading side. The release of force induces a swelling of the strand at the unloading side of concrete block, bonding force in the block, and a bond slip of the strand toward the other side of the block. Two center-hole load cells are used to record the end loads until the point of general bond slip(maximum bond stress). It is suggested that this test procedure be performed with the ordinary transfer test when determining the transfer length in a prestressed, pretensioned concrete beam.