• Coupled systems mechanics
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• v.9 no.5
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• pp.411-432
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• 2020

This research is devoted to the study of plane wave propagation in homogeneous transversely isotropic (HTI) magneto-thermoelastic rotating medium with combined effect of Hall current and two temperature due to multi-dual-phase lag heat transfer. It is analysed that, for 2-D assumed model, three types of coupled longitudinal waves (quasi-longitudinal, quasi-transverse and quasi-thermal) are present. The wave characteristics like phase velocity, specific loss, attenuation coefficients, energy ratios, penetration depths and amplitude ratios of transmitted and reflected waves are computed numerically and illustrated graphically and compared for different theories of thermoelasticity. Some particular cases are also derived from this research.

• Journal of the Korean Society of Industry Convergence
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• v.21 no.5
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• pp.191-195
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• 2018

By using an ANSYS product suite (CFX, Ansys Multiphysics), which is a powerful tool for multiphysics analysis of complicated physical phenomena, we performed a structural stress analysis based on fluid flow and heat transfer phenomena within a quick connect/disconnect (QC/DC) bellows system. Considering the extremely low temperatures in the QC/DC environment, an approach to the problem based on complex multi-physics phenomena, where different phenomena interact with each other, is crucial. Therefore, we use a numerical analysis technique where fluid-thermal-structural interactions are combined. In conclusion, when low temperature fluids flow inside bellows, the expected service life is conspicuously reduced due to the thermal stress caused by heat transfer. Therefore, in future research, a structure with considerably reduced thermal stress by robust design optimization will be derived.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.27 no.5
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• pp.543-552
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• 2003

The effects of a directionally emitting and reflecting dielectric surface on the wall heat flux and medium temperature distribution are studied. The system is an infinite square duct enclosing an absorbing and emitting medium. The emissivity and reflectivity of opaque and gray wall vary with direction. Combined effect of conductive and radiative heat transfer is analyzed using finite difference and the direct discrete-ordinates method. The parameters under study are conduction to radiation parameter, optical depth, refractive index ratio. The results with directional and diffuse properties deviate each other when the conduction to radiation parameter is less than around 0.01. The wall heat flux differs fur optical thickness less than around 0.1. However, the medium temperature profiles differ for optical thickness greater than around 1. Deviations from diffuse property calculations are larger for hot wall with directional property than cold wall with directional property. As n increases from 1.5, the trend changes are observed fur refractive index ratio about n=6.10

• Journal of Korean Tunnelling and Underground Space Association
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• v.12 no.3
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• pp.223-237
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• 2010

The initial heating rate is well known as one of the most influencing factors on the occurrence of spalling and the loss of strength in concrete after fire initiation. In this study, a series of fire tests were carried out at different initial heating rates to find out its effects on the deterioration of tunnel structural members. Heat transfer analyses combined with an element elimination model were also carried out to verify its applicability in the same conditions as the fire tests. Moreover, the convection heat transfer coefficients compatible with fire test results were derived from parametric studies. In this course, their time-dependent variations were also analyzed at different initial heating rates. Finally, a numerical formula to estimate the heat transfer coefficients at the various initial heating rates was proposed by the interpolation of the results of numerical analyses.

• Journal of the Korean Solar Energy Society
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• v.38 no.2
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• pp.33-43
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• 2018

PV/Thermal module is the combined system, which consist of a photovoltaic module and solar thermal collector that can obtain electrical power and thermal energy simultaneously. Thus the power generation can be increase by decreasing the temperature of photovoltaic module and thermal energy retrieved from module also can be used for heating system. In this study, Heat transfer performance of air type PV/Thermal module was confirmed with various bottom obstacles that can be installed easily to real photovoltaic module by CFD (computational fluid dynamics) analysis. Eight type obstacles were investigated according to the shape and arrangement. As a result, nusselt number represent heat transfer performance was increased about 86% compare with the basic type PV/Tthermal module that has no obstacle and triangle type obstacle had higher value than other types. But pressure drop was also increased with increment of heat transfer enhancement. Thus the performance factor considering both heat transfer and pressure drop was confirmed and V-fin type obstacle arranged in a row for Reynolds number below 9,600 and protrusion type obstacle arranged in zigzag for Reynolds number above 14,400 were shown higher performance factor than other types. From these results, V-fin type obstacle arranged in row and protrusion type obstacle arranged in zigzag were considered as a proper type for applying to real PV/thermal module according to operating condition. But the heat transfer performance can be changed by the geometric conditions of obstacle such as height, width, length and arrangement. Thus, it could also confirmed that the optimal condition and arrangement of this obstacle need to be found in further study.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.30 no.1
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• pp.47-53
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• 2017

A boundary layer integral combined with a 1-D isentropic core flow model has been successfully used to determine heat transfer rate on the surface of a supersonic nozzle. However its accuracy is affected by the core flow condition which is used as a boundary condition for the integral calculation. Because flow behavior near a nozzle throat deviates from 1-D isentropic condition due to 2-D flow turning and interaction between core flow and boundary layer, accuracy of heat transfer calculation decreases at a nozzle throat. Therefore, CFD is adopted to deduce improved core flow condition and increase accuracy of boundary layer integral at nozzle throat in this research. Euler model and SST $k-{\omega}$ model is solved by CFD code and used as a boundary condition for boundary layer integral. Developed code is tested in the supersonic nozzle from the previous research and improvement in accuracy is observed, especially at nozzle throat and diverging section of the nozzle. Error between experimental result and calculation result reduced by 16% when a calculation is made based on the SST $k-{\omega}$ model. Method developed in this research is expected to be used in thermal design of the rocket nozzle.

• Transactions of the Korean Society of Mechanical Engineers
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• v.11 no.2
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• pp.331-344
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• 1987

A numerical analysis has been conducted on the interaction of the thermal radiation and natural convection in a rectangular enclosure filled with a gray fluid. P-1 approximation is adopted for the radiative transfer and its application limit is examined. Considered are the Stark number effect, the optical thickness effect and the wall emissivity effect on the flow and heat transfer characteristics. As the Stark number increase or the optical thickness decreases, the boundary layer thickness and the flow velocity increase. Transition to turbulence is retarded with the increase of the radiation effect. When the optical thickness is one, the radiation effect is negligible for the Stark numbers larger than 10.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.10 no.7
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• pp.1465-1472
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• 2009

The effect of heat loss rate on NOx formation of $CH_4/air$premixed flame were examined numerically in a perfectly stirred reactor. The following conclusions were drawn. Under the adiabatic wall condition, an increase in the residence time causes a remarkable increases in NOx emission. Under the heat loss conditions, however, NOx decreases significantly as the heat transfer coefficient and residence time increase. As the heat loss rate increases, Thermal NO mechanism and Re-burning NO mechanism play an important role in the NOx reduction, but Prompt NO mechanism and $N_2O$-intermediate NO mechanism lead to the increase in NOx production. Although the NOx formation is actually related to complex NOx mechanism with the changes in the heat transfer coefficient and residence time, it was found that NOx concentration can be represented by independent Thermal NO mechanism. From these results, new NOx correlation combined with the heat loss rate and residence time was suggested for predicting the NOx concentration in a practical $CH_4/air$premixed combustor.

• Solar Energy
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• v.15 no.3
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• pp.3-14
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• 1995

The Direct Contact heat Exchanger(DCHX) has been widely studied in the chemical industry for many years due to its inherent simplicity as a counter-current divice for heat and mass transfer. In many solar systems, the DCHX unit can be combined with the thermal storage unit, or alternatively, it can be used separately from the storage unit, much like an external(to storage) closed heat exchanger system. In the present work, the spray column type of direct contact heat exchangers are studied extensively to harness the solar energy for hot water and spaced heating. Some of the major considerations that are involved in the design of heat exchangers in this study are that : working fluid is a hydrocaabon(such as Texaterm) or water which is either lighter or heavier than storage medium. The experimental data have revealed some interesting characteristics concerning the application of DCHXs for solar energy utilization. These experiments are carried out in the line of solar heating system, major results are as follows : 1) the flow and aspect of working fluid drop for maxium heat transfer 2) efficiency and volumetric heat transfer coefficient of D.C.H.X with a heavier working fluid are higher than those of D.C.H.X with a lighter working fluid.

• Transactions of the Korean Society of Mechanical Engineers
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• v.16 no.11
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• pp.2196-2202
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• 1992

The optimum size combination of heat exchangers in a Joule-Thomson(J-T) circuit for small cryogenic systems has been sought analytically, when the circuit is combined with a two-stage Gifford-McMahon(GM) cooler. Full thermodynamic cycle analysis was carried out to predict the performance of the combined refrigeration system. Relevant convective heat transfer coefficients, the computerized properties of helium, and the refrigeration capacity curve of a typical GM cooler have been used in the analysis. The result showed that, by changing the configuration(heat exchanger area ratio) of the system, the performance of the commonly-used GM/J-T refrigerators could be optimized. For the maximum refrigeration performance, the optimum mass flow rate of the refrigerant and the relative size between the heat exchangers have been obtained, when the cooling load was 0.1W at 3.995K with the total heat exchanger area being given.