• Journal of the Korean Society of Safety
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• v.15 no.1
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• pp.36-42
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• 2000

An experimental study was carried out to investigate the effects of inclined angles of channel on thermal stability of electronic components. In this study, it is focused on the natural convection heat transfer from an inclined parallel channel with discrete protruding heat sources. The material used for the inclined parallel channel was epoxy-resin, while air as the cooling fluid. Heat transfer phenomena for inclined angles of $\psi$=$15^{\circ}$, $30^{\circ}$, $45^{\circ}$, $60^{\circ}$ and for the range of $9.52{\times}10^5/ were analyzed. The thermal fields in the channel were visualized by Mach-Zehnder interferometer. Also, local temperatures were measured by thermocouples along the channel wall and heat sources surface. As a result, for the range of $4.29{\times} 10^5/, a useful correlation of mean Nusselt number was proposed as a function of modified channel Rayleigh number.

• Transactions of the Korean Society of Mechanical Engineers
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• v.9 no.6
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• pp.750-756
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• 1985

Experimental investigation was carried out to study the natural convection of water and silicon oil due to end temperature differences in a horizontally insulated rectangular enclosure of aspect ratio 0.1 with a special attention on the core configuration in the laminar boundary-layer flow regime. Rayleigh number ranges covered herein are Ra=4.40 * 10$^{6}$ -9.64 * 10$^{7}$ for water and Ra=1.69*10$^{5}$ -3.80*10$^{6}$ for silicon oil, respectively. In the case of water, for Ra.geq.2.21 * 10$^{7}$ there appeared distinct horizontal thermal layers adjacent to the horizontal boundaries in the core and the temperature distribution outside the horizontal thermal layers, i.e., in the mid-core region, is vertically stratified. The core flow pattern was shown to be nonparallel with a weak back flow in the mid-core for Ra.geq.3.63 *10$^{7}$ . In the case of silicon oil, distinct horizontal thermal layers appeared along the core horizontal boundaries for Ra.geq.1.27 * 10$^{6}$ with a stratified temperature distribution in the mid-core, but the core flow pattern in this case was shown to be parallel. In addition, secondary flow appeared near the hot wall for Ra.geq.3.80 * 10$^{6}$ . Nusselt number, Nu, was found to be proportional to R $a^{0.3}$ for water and R $a^{0.28}$ for silicon oil in the boundary-layer flow regime. There also in an indication from the comparison with other results that Nu is independent of aspect ratio for water in the boundary-layer flow regime in low aspect ratio enclosures.res.

• Solar Energy
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• v.19 no.3
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• pp.29-41
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• 1999

Three-dimensional natural convective heat transfer in a vertical channel with a protruding single module was investigated experimentally. The particular interest was in the removal of thermal energy from the module by convective heat transfer. Hence radiative and conductive heat losses were estimated by using thermocouples and heat flux sensor respectively. The flow fields in the channel were visualized by means of a smoke-method. Also, local temperatures were measured by thermocouples inside the channel, along the vertical wall and module surface. It is found that convective heat transfer was promoted at the lower comer of the module and was decreased at the upper comer due to a recirculation zone. A general correlation of the critical channel ratios was found as a function of Rayleigh number. For the range of $8.28{\times}10^3<Ra^*_c<3.48{\times}10^6$, a useful correlation for the mean Nusselt number was proposed as a function of modified channel Rayleigh number.

• Journal of computational fluids engineering
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• v.16 no.3
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• pp.29-36
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• 2011

The natural convection in a horizontal enclosure heated from the bottom wall, cooled at the top wall, and having a square adiabatic body at its centered area was studied. Three different Prandtl numbers (0.01, 0.7 and 7) were considered for an effect of the Prandtl number on natural convection. A two-dimensional solution for unsteady natural convection was obtained, using Chebyshev spectral methodology for different Rayleigh numbers varying over the range of $10^4$ to $10^6$. It had been experimentally and numerically reported [1,2] that the heat transfer mode becomes oscillatory when Pr is out of a specific Pr band beyond the critical Ra. In this study, we reproduced this phenomenon numerically. The variation of time- and surface-averaged Nusselt numbers on the hot and cold walls for different Rayleigh numbers and Prandtl numbers was presented to show the overall heat transfer characteristics in the system. And also, the isotherms and streamline distributions were presented in detail to compare the physics related to their thermal behavior.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.40 no.11
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• pp.717-726
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• 2016

This study evaluated the effect of aspect ratio of an enclosure with a heated inner circular cylinder on three-dimensional natural convection. The immersed boundary method was used to model the inner circular cylinder based on the finite volume method. The Rayleigh number was varied between $10^5$ and $10^6$, and the Prandtl number was maintained at 0.7. The aspect ratio of the three-dimensional enclosure was changed in steps of 1 within a range of 1-4 by increasing the width of the enclosure. In this study, the flow and thermal fields in the enclosure reached the steady state, and showed a mirror-symmetric pattern with respect to the center plane (x=0). In addition, the surface-averaged Nusselt number of the inner circular cylinder increased, while the total surface-averaged Nusselt number of the enclosure walls decreased with increase in the aspect ratio of the enclosure.

• Transactions of the Korean Society of Mechanical Engineers
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• v.16 no.11
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• pp.2181-2188
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• 1992

Two-dimensional natural convection within a circular trapezoidal cavity with parallel cylindrical top and bottom walls at different temperatures and two adiabatic side walls has been solved by finite-difference methods. This study has been conducted to evaluate the effects of aspect ratio and inclination in the natural convection for various Rayleigh numbers. The minimum average Nusselt number occurs at the point of transition from a conductive heat transfer to a convective heat transfer. Numerical results are compared to experimental results with qualitatively good agreement.

• Proceedings of the KSME Conference
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• 2001.06e
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• pp.530-535
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• 2001

Variations of temperature field in a Hele-Shaw convection cell (HSC) were measured using a holographic interferometry with varying Rayleigh number. Experimental results show a steady flow pattern at low Rayleigh numbers and a time-dependent periodic flow at high Rayleigh numbers. Especially, the period of oscillation at $Ra = 6.35{\times}10^6$ was 62 seconds. Two different measurement methods of holographic interferometry, double-exposure method and real-time method, were employed to measure the temperature field variations of HSC convective flow. In the double-exposure method, unwanted waves can be eliminated and reconstruction images are clear, but transient flow structure cannot be observed clearly. On the other hand, transient flow can be observed and reconstructed well using the real-time method. However, the fringe patterns reconstructed by the real-time method contain more noise, compared with the double-exposure method. The two holographic interferometer techniques employed complementary in this study were proved to be useful for analyzing the temperature field variations of unsteady thermal fluid flows.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.25 no.11
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• pp.1624-1631
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• 2001

Variations of temperature field in a Hele-Shaw convection cell (HSC) were measured using a holographic interferometry with varying Rayleigh number. Experimental results show a steady flow pattern at low Rayleigh numbers and a time-dependent periodic flow at high Rayleigh numbers. Especially, the period of oscillation at Ra = 6.35 $\times$ 10$^{6}$ was 62 seconds. Two different measurement methods of holographic interferometry, double-exposure method and real-time method, were employed to measure the temperature field variations of HSC convective flow. In the double-exposure method, unwanted waves can be eliminated and reconstruction images are clear, but transient flow structure cannot be observed clearly. On the other hand, transient flow can be observed and reconstructed well using the real-time method. However, the fringe patterns reconstructed by the real-time method contain more noise, compared with the double-exposure method. The two holographic interferometer techniques employed complementary in this study were proved to be useful fur analyzing the temperature field variations of unsteady thermal fluid flows.

• Journal of the Korean Solar Energy Society
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• v.21 no.3
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• pp.33-41
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• 2001

This paper showed the a numerical analysis of the thermal and fluid flow in solar concentration absorber with tilt angle, and the purpose of this study is to obtain the optimum tilt angle of the absorber. The boundary conditions of a numerical model were assumed as flows : (1) The heat source is located at the center of absorber (3) The bottom wall is opened and adiabatic. (3) The top, right and left walls are cooled wall. The parameters for the numerical analysis are tilt angles and Rayleigh numbers i.e., tilt angle $\theta=0^{\circ},\;15^{\circ},\;30^{\circ},\;45^{\circ},\;60^{\circ},\;75^{\circ},\;90^{\circ}$ and 101 $\leq$ Ra $\leq$ 103. The velocity vectors and isotherms were dense at wall side and the heat source. The mean Nusselt number had a maximum value at $\theta=0^{\circ}$ and showed a low value as the tilt angles were increased. Finally, the decrease rate of mean Nusselt number was appeared small with tilt angle when Rayleigh numbers were increased.

• Journal of Energy Engineering
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• v.20 no.1
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• pp.44-53
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• 2011

The laminar natural convection of air in 2-D rectangular enclosure in which two opposing isothermal walls were kept at different temperatures is investigated numerically for Rayleigh number up to $10^6$. Computations were performed for the width-to-height ratios of 1, 2, and 4, and for the inclination angle range of $0^{\circ}{\leq}{\theta}{\leq}90^{\circ}$. For each aspect ratio, the influence of the inclination angle on the flow patterns and heat transfer rates were examined for $10^3{\leq}Ra{\leq}10^6$. It is found that the growth of secondary flow in the corners led to the decrease in overall heat transfer for small aspect ratio case, and the transition from a three-cell structure to a unicell flow pattern in large aspect ratio led to a step-like change in heat transfer. A new correlation of mean Nusselt number is presented for the vertical case of ${\theta}=90^{\circ}$.