• Transactions of the Korean Society of Mechanical Engineers B
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• v.20 no.10
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• pp.3344-3354
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• 1996

An experimental and numerical study on the three-dimensional natural convection-radiation conjugate heat transfer in the enclosure with heat generating chip has been performed. A 3-dimensional simulation model is developed by considering heat transfer phenomena by conduction-convection and radiation. Radiative transfer was analyzed with the discrete ordinates method. Experiments are conducted in order to validate the numerical model. Comparisons with the experimental data show that good agreement is obtained when the radiation effect is considered. The effects of the thermal conductivity of the substrate and power level on heat transfer are investigated. It is shown that radiation is the dominant heat transfer mode and the conductivity of the substrate has important effects on the heat transfer in the enclosure.

• Nuclear Engineering and Technology
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• v.52 no.1
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• pp.27-36
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• 2020

Natural convection and thermal stratification phenomena are found in large water pools that are being used as heat sinks for decay heat removal from the reactor core using passive heat removal systems. In this study, the two-phase (water and air) natural convection and thermal stratification phenomena with conjugate heat transfer in the rectangular enclosure were investigated numerically using ANSYS Fluent 17.2 code. The transient numerical simulations of these phenomena in the full-scale computational domain of the experimental facility were performed. Generation of water vapour bubbles around the heater rod and evaporation phenomena were included in this numerical investigation. The results of numerical simulations are in good agreement with experimental measurements. This shows that the natural convection is formed in region above the heater rod and the water is thermally stratified in the region below the heater rod. The heat from higher region and from the heater rod is transferred to the lower region via conduction. The thermal stratification disappears and the water becomes well mixed, only after the water temperature reaches the saturation temperature and boiling starts. The developed modelling approach and obtained results provide guidelines for numerical investigations of thermal-hydraulic processes in the water pools for passive residual heat removal systems or spent nuclear fuel pools considering the concreate walls of the pool and main room above the pool.

• Transactions of the Korean Society of Mechanical Engineers
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• v.16 no.5
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• pp.952-964
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• 1992

The interaction of natural convection and radiation heat transfer in a two dimensional square enclosure containing absorbing, emitting and linear anisotropically scattering gray medium is numerically analyzed. P-1 and P-3 approximation is introduced to calculate radiation heat transfer. The effects of scattering albedo, wall emissivity, scattering anisotropy, and optical thickness on the characteristics of the flow and temperature field and heat transfer are investigated. Temperature and velocity profiles depend a great deal on the scattering albedo, and the importance of this effect increases with decrease in albelo. Planck number is another important parameter in radiation heat transfer. The increase in scattering albedo increases convection heat transfer and decreases radiation heat transfer at hot wall. However, the increase in scattering albedo decreases both convection and radiation heat transfer at cold wall. The increase in optical thickness decreases radiation heat transfer. The scattering anisotropy has important effects on the radiation heat transfer only. The highly forward scattering leads to an increase of radiation heat transfer whereas the highly backward scattering leads to an decrease of radiation heat transfer. The effect of scattering anisotropy decreases when reducing the wall emissivity.

• Transactions of the Korean Society of Mechanical Engineers
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• v.13 no.1
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• pp.153-160
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• 1989

A numerical and experimental study has been performed on natural convection heat transfer from a horizontal annulus with spacers. The mode of heat transfer in the annulus is changed from conduction to convection at Ra = 10$^{3}$. By increasing wall conductivity, mean Nusselt number is apparently increased at $K_{w}$/K$_{f}$ .leg. 48, but at /K$_{w}$/K$_{f}$ > 48, slightly increased for no spacers, and decreased for vertical spacers and horizontal spacers. The mean Nusselt number can be represented in an exponential function of Grashof number at all conditions. The characterics of natural convection heat transfer show similiarity for no spacers and vertical spacers but show difference for horizontal spacers. The presence of the horizontal spacers increased the convective heat transfer by an average 6 percent over that for the no forced cooling to outer cylinder. The maximum local Nusselt number appears at .theta. = 150.deg. in a conducting tube and .theta. = 30.deg. in an outer cylinder for vertical spacers, and appears at .theta. = 180.deg. in a conducting tube and .theta. = 0.deg. in an outer cylinder for horizontal spacers.spacers.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.14 no.8
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• pp.676-682
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• 2002

This paper has been made to investigate the thermal performance characteristics for the several types of heat sinks such as extruded heat sink, aluminum foam heat sink, layered heat sink. The various types heat sinks are prepared and tested for natural convection as well as forced convection. The experimental results for natural convection are compared to those for three types of heat sink so that the appropriate heat sink can be designed or chosen according to the heating conditions. The overall heat transfer performances for layered heat sink, extruded heat sink and aluminum foam heat sink are almost comparable to those under natural convection and forced convection. The forced convection of layered heat sink become 1.2 times as high as those of extruded heat sink, and the forced convection of extruded heat sink become 1.2 times as high as those of aluminum foam heat sink. This study shows that bar height, bar distance and number of bar for layered heat sink are important parameters, which have a serious influence on thermal performance for layered heat sinks.

• Proceedings of the SAREK Conference
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• 2009.06a
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• pp.1172-1176
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• 2009

In the present study, natural convection over a heat sink with a horizontal circular base and rectangular fins was numerically analyzed. To calculate natural convection heat transfer, the assumptions of ideal gas and laminar flow were made for air. Flow patterns around the heat sink were chimney-like. The resultant temperature distribution on the circular base appeared almost uniform. Parametric studies were performed to compare the effects of fin length, fin height, the ideal number of fins, and heat flux on the average temperature of a heat sink and the average heat transfer coefficient from the heat sink array. Correlation to predict the average Nusselt number was presented.

• Journal of Advanced Marine Engineering and Technology
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• v.22 no.6
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• pp.844-853
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• 1998

Natural and forced convection experiments were carried out in order to investigate the effects of channel spacing gap between protrusions and number of rows of protrusion, In natural convection the optimum channel spacing was found to be approximately 20mm regardless of the protrusion gaps. For optimum channel spacing the heat transfer coefficients were converged to an asymptotic value after the fourth row. The heat transfer coefficient for each row approaches to constant values for protrusion gaps larger than 10 mm. An experimental correlation has been suggested by using a modified Rayleigh number based on the dimensionless characteristic length(G/L). In forced convec-tion the heat transfer coefficients were not merged to an asymptote until the fifty row and increases as the channel spacing at the constant Reynolds number decreases.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.31 no.9
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• pp.807-813
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• 2007

Recent research on nanofluids under forced convection experiment shows that there is little relationship between convective heat transfer and thermal conductivity increase of nanofluids. This kind of new findings are totally different from the traditional theory of nanofluids, which says that the higher thermal conductivity is a prerequisite for convective heat transfer enhancement. To elucidate this controversial issue in a very comprehensible manner, simple natural convection experiment has been carried out for the water- and oil-based nanofluids. ($water-Al_2O_3$, transformer $oil-Al_2O_3$) Present research shows that there exists strong dependence between natural convection performance and thermal conductivity increase of nanofluids.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.33 no.3
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• pp.170-177
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• 2009

In the present study, a finite element analysis of conjugate heat transfer problem inside a cavity with a heat-generating conducting body, where constant heat flux is generated, is conducted. A conduction heat transfer problem inside the solid body is automatically coupled with natural convection inside the cavity by using a finite element formulation. A finite element formulation based on SIMPLE type algorithm is adopted for the solution of the incompressible Navier-Stokes equations coupled with energy equation. The proposed algorithm is verified by solving the benchmark problem of conjugate heat transfer inside a cavity having a centered body. Then a conjugate natural heat transfer problem inside a cavity having a heat-generating conducting body with constant heat flux is solved and the effect of the Rayleigh number on the heat transfer characteristics inside a cavity is investigated.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.4 no.4
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• pp.323-331
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• 1992

A two-dimensional Benard-convection system with a phase-change material inside has been analysed. The main purpose of the present study is to clarify the basic reason of the hysteresis found by the previous investigators. The interface between the solid and the liquid is assumed to be planar. The analysis was performed with heat transfer rates under the steady state on the interface. It was found that the hysteresis occurs due to the abrupt increase in the heat transfer rate at the onset of natural convection in the classical Benard-convection system. The spectral method was applied to obtain the steady solution of the natural convection for the specific material and to confirm the hysteresis phenomenon.