• Journal of Advanced Marine Engineering and Technology
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• v.36 no.7
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• pp.869-875
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• 2012

In the past decades, extensive studies on convection heat transfer on internal flow have been conducted by using high specific surface area, by increasing heat transfer coefficient and swirl flow, and by improving the transport properties. In this study, we applied a tangential slot swirl generator to improve heat transfer in a horizontal circular copper tube. The Al-Mg particles (approximately $100{\mu}m$ to $130{\mu}m$) were employed for this experimental work. The copper tube was heated uniformly by winding a heating coil with a resistance of 9 ohm per meter for heat transfer. Using Al-Mg particles, experiments were performed in the Reynolds number range of 5,000 to 13,130, with and without swirl. Experimental data transfers or comparisons between Nusselt numbers with and without swirl along the test tube and Reynolds numbers are presented. The Nusselt number is improved by increasing Reynolds numbers or swirl intensities along the test tube.

• Journal of the Korean Society of Visualization
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• v.10 no.3
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• pp.16-20
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• 2012

In the past decades, extensive studies on convection heat transfer on internal flow have been conducted by using high specific surface area, by increasing heat transfer coefficient and swirl flow, and by improving the transport properties. In this study, we applied a tangential slot swirl generator to improve heat transfer in a horizontal circular copper tube. The Al-Mg particles (approximately $100{\mu}m$ to $130{\mu}m$) were employed for this experimental work. The copper tube was heated uniformly by winding a heating coil with a resistance of 9ohm per meter for heat transfer. Using Al-Mg particles, experiments were performed in the Reynolds number range of 5,000 to 13,130, with and without swirl. Experimental data transfers or comparisons between Nusselt numbers with and without swirl along the test tube and Reynolds numbers are presented. The Nusselt number is improved by increasing Reynolds numbers or swirl intensities along the test tube.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.30 no.5 s.248
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• pp.389-396
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• 2006

For the successful design of heat exchangers, it is very important to understand local heat transfer phenomena on the circular tube of heat exchangers. In the present study, experiments are performed for single circular tube and tube banks with and without heat transfer promoters. The naphthalene sublimation technique is employed to measure the local mass transfer coefficients, and the measured local mass transfer data are converted to the local heat transfer data using heat and mass transfer analogy. The distribution pattern of local Nusselt numbers on single circular tube with heat transfer promoters is similar to that without the heat transfer promoter, but average Nusselt numbers are greatly increased. In case of tube banks without the heat transfer promoter, the Nusselt numbers are much lower in the first row than those of other rows, but the local heat transfer coefficients on all rows are equalized when the heat transfer promoter is installed.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.32 no.1
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• pp.16-24
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• 2022

A computational study of combined thermal and solutal convection (double diffusive convection) in a sealed crystal growth reactor is presented, based on a two-dimensional numerical analysis of the nonlinear and strongly coupled partial differential equations and their associated boundary conditions. The average Nusselt numbers for the source regions are greater than those at the crystal regions for 9.73 × 10³ ≤ Gr_t ≤ 6.22 × 10⁵. The average Nusselt numbers for the source regions varies linearly and increases directly with the thermal Grashof number form 9.73 × 10³ ≤ Gr_t ≤ 6.22 × 10⁵ for aspect ratio, Ar (transport length-to-width) = 1 and 2. Additionally, the average Nusselt numbers for the crystal regions at Ar = 1 are much greater than those at Ar = 2. Also, the occurrence of one unicellular flow structure is caused by both the thermal and solutal convection, which is inherent during the physical vapor transport of Hg₂Br₂. When the aspect ratio of the enclosure increases, the fluid movement is hindered and results in the decrease of thermal buoyancy force.

• Journal of Mechanical Science and Technology
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• v.17 no.4
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• pp.581-589
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• 2003

Film absorption involves simultaneous heat and mass transfer in the gas-liquid system. While the non-absorbable gas does not participate directly In the absorption process. its pretence does affect the overall heat and mass transfer. An experimental study was performed to investigate the heat and mass transfer characteristics of LiBr-H$_2$O solution flow ing over 6-row horizontal tubes with the water vapor absorption in the pretence of non-absorbable gases. The volumetric concentration of non-absorbable gas, air, was varied from 0.17 to 10.0%. The combined effects of the solution flow rate and its concentration on the heat and mass transfer coefficients were also examined. The presence of 2％ volumetric concentration of air resulted in a 25% reduction in the Nusselt number and 41% reduction in the Sherwood number Optimum film Reynolds number was found to exist at which the heat and mass transfer reach their maximum value independent of air contents. Reduced Nusselt and Sherwood numbers. defined as the ratio of Nusselt and Sherwood numbers at given non-absorbable gas content to that with pure water vapor, were correlated to account for the reduction in the heat and mass transfer due to non-absorbable gases in a falling film absorption process.

• Journal of Advanced Marine Engineering and Technology
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• v.37 no.7
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• pp.677-684
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• 2013

A lot study of convection heat transfer on internal flow has been extensively conducted in the past decades using of high specific surface area, increasing heat transfer coefficient, swirling flow and improving the transport properties. This study concerned with the application of a tangential slot swirl generator for improving heat transfer in a horizontal circular copper tube. The Al particles(about $100{\sim}130{\mu}m$) was employed for this experimental work. 3D PIV(particle image velocimetry) technique has employed to measure velocity profiles of Al particles with and without swirl flow. The copper tube is heated uniformly by winding of a heating coil for heat transfer work, having a resistance of 9 ohm per meter. Experiments are performed in the Reynolds number range of 6,800~12,100 with swirl and without swirl using Al particles. Experimental data for comparison of Nusselt number is presented that of with swirl and without swirl along the test tube for the Reynolds numbers. The Nusselt number is improved with increasing of Reynolds numbers or swirl intensities along the test tube. The Nusselt number with swirl flow is about 60.0% to 119.0% higher than that obtained by the Dittus-Boelter equation.

• Solar Energy
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• v.15 no.3
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• pp.91-103
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• 1995

Non-gray radiation with convection in the entrance region of a smooth tube is numerically investigated. The fluid is a mixture of carbon dioxide, water vapor, and nitrogen to simulate combustion products of propane. The flow is assumed to be laminar and hydrodynamically and thermally developing. The P-1 approximation is used to simplify the radiative transfer equation and the exponential wide band model is adapted to model the spectral absorption coefficients of non-gray gas mixture. The bulk mean temperature and Nusselt number variation along the tube axis are shown for several inlet and wall temperature pairs to show the effect of temperature on the heat transfer characteristics. Nusselt numbers for simultaneously developing flow are compared to those for thermally developing flow. In addition, the effect of the mole fraction of the non-gray gases on convective and radiative Nusselt numbers is investigated.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.7 no.1
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• pp.112-119
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• 1995

Measurements of the local heat transfer coeffcients on a spherically concave surface with a round impinging jet are presented. The liquid crystal transient method was used for these measurements. This method, which is a variation on the transient method, suddenly exposes a preheated wall to an impinging jet while video recording the response of liquid crystals for the measurement of the surface temperature. The Reynolds numbers used were 1,000, 23,000 and 50,000 and the nozzle-to-jet distance was L/d=2, 4, 6, 8, 10. Presented results are compared to previous measurements for flat plate. In the experiment, the local heat transfer Nusselt numbers on a concave surface are higher than those on a flat plate. Maximum Nusselt number at all region occured at L/d=6 and second maximum in the Nusselt number occured at R/d=2 for both Re=50,000 and Re=23,000 in case of L/d=2 and for only Re=50,000 in case of L/d=4. All other cases exhibit monotonically decreasing value of the Nusselt number along the curved surface.

• Solar Energy
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• v.7 no.2
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• pp.86-97
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• 1987

Natural convection in the annulus between a horizontal conducting tube and a cylinder with spacers has been studied by 2-dimensional numerical method with finite difference techniques. The effects of Rayleigh number, conductivities of conducting tube and spacer, and position of spacers were studied analytically. In case of vertical spacers, the maximum local Nusselt number appears at ${\theta}{\approx}50^{\circ}$ in a conducting tube and ${\theta}{\approx}30^{\circ}$ in an outer cylinder, The local Nusselt numbers show positive values on the lower spacer, but negative values on the surface of the upper spacer. In case of horizontal spacers, the flow over the spacer is more active than that of under the spacer as the Rayleigh number increases. The maximum local Nusselt appeares at ${\theta}=180^{\circ}$ in a conducting tube and at ${\theta}=0^{\circ}$ in an outer cylinder. The local Nusselt numbers show positive values on the upward surface, but negative values on the downward surface of spacer. As the dimensionless conductivity increases, the mean Nusselt number remarkably increases at $K_w/K_f<48$ and show almost even at $K_w/K_f{\ge}48$. The mean Nusselt number of a conducting tube with vertical spacers is 5.12 percent less and with horizontal spacers is 11.33 percent less than that of a conducting tube without spacer at $Ra=10^4$, Pr = 0.7 and $K_w/K_f=48$.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.20 no.10
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• pp.3304-3312
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• 1996

An experimental study on the convective heat transfer characteristics was performed for a two-dimensional wall attaching offset jet(WAOJ). Thermochromic liquid crystal was used to measure the plate wall temperature. The Nusselt number was measured for Reynolds numbers from 6, 500 to 39, 000, and the offset ratios from 0.5 to 15. The maximum Nusselt number point coincides with the time-averaged reattachment point and Nusselt number decreases monotonically after the jet reattaches on the wall. In the recirculation region Nusselt number minimize near the upstream corner and then increases as X/D decreases to vanishes. This suggests the existence of secondary vortices, causing an additional mixing of the flow in the corner. The correlations between the local Nusselt number and Reynolds number, Re, offset ratio, H/D, and streamwise distance, X/D are presented.