• Transactions of the Korean Society of Mechanical Engineers B
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• v.25 no.11
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• pp.1659-1666
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• 2001

The objective of this study was to investigate the characteristics of air flow and heat transfer caused by trapezoid rods array in impinging air jet system. Trapezoid rods have been set up on front of flat plate to act as a turbulence promoter. Local Nusselt numbers were determined as a function of three parameters : (a) the space from re(Is to heating surface(C=1, 2, 4mm), (b) the pitch between each rods(P=30, 40, 50mm), (c) the distance from nozzle exit to flat plate(H/B=2, 6, 10). The measurements were compared with those of the experiment without trapezoid rods. As a result, when rods are installed in front of the impinging palate, the acceleration of the flow and the eddies due to the rods seem to contribute to the heat transfer enhancement. Heat transfer performance was best under the condition of C=1mm and as the pitch is 30mm. The maximum rate of heat transfer augmentation is about 1.9 times greater compared to that without trapezoid rods.

• Proceedings of the SAREK Conference
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• 2008.06a
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• pp.430-435
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• 2008

Total lengths of domestic pipe network for district heating system are above about 2,500Km. A lots of pumping power for heat transportation through long pipe are required by the flow friction of pipe surface. Until now there have been considered about various methods to reduce the flow friction for district heating system such as using surfactants and turbulence promoters by swirl flow and baffles etc. At this study, swirl flow generator was tested about the possibility to increase the heat transfer ratio at the heat exchanger in the case which the suppling water temperature increased from $50^{\circ}C$ until $120^{\circ}C$. Experimental results showed that the heat transfer ratio increased and also pressure increase ratio increased simultaneously in the case which swirl flow generator installed. The amount of the increasing ratio for heat transfer and pressure were reached until 4.33% and 11% at the case of $120^{\circ}C$ suppling temperature which domestic district heating system were using.

• Transactions of the Korean Society of Mechanical Engineers
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• v.10 no.1
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• pp.96-101
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• 1986

The average heat transfer coefficient of 2-D impinging jet has been augmented as much as 60% on the wall with large-scale wavy roughness. The mechanism of this heat transfer augmentation is studied with emphasis on two primary flow structures in the impinging flow region by using either the surface floating method or the smoke-wire technique. They are the stream-wise vortex-like structure, which is characteristic to the impining jet, and the spanwise vortiecs associated with the flow separation around the roughness. The combined effect of these structures can effectively augment the heat transfer particularly in the downstream region where the teat transfer usually deteriorates consicerably.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.17 no.9
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• pp.855-862
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• 2005

An experimental investigation has been carried out for aluminum foam heat sink inserted into the annulus to examine the feasibility as a heat sink for high performance forced water cooling in the annulus. The local wall temperature distribution, inlet and outlet pressures and temperatures, and heat transfer coefficients were measured for heat flux of 13.6, 18.9, 25.1, 31.4 $kw/m^2$ and Reynolds number ranged from 120 to 2000. Experimental results show that the friction factor is higher than clear annulus without aluminum foam, while the significant augmentation in Nu is obtained. This technique can be used for the compactness of the heat exchanger.

• Journal of Aerospace System Engineering
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• v.2 no.4
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• pp.7-12
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• 2008

Averaged heat transfer coefficients were measured in a turbine blade internal cooling passage model with three blockage walls. Each blockage wall was equipped with 9 staggered holes or slots in order to create different shaper of repeated jet impingement. The effect of jet shape on the averaged heat transfer coefficient was studied by the copper-thermocouple method and three Reynolds number of 10,000, 20,000, and 30,000 were tested. Results showed that the repeated stagger jets could increase the averaged heat transfer coefficient by at least 9 times compared to the smooth channel cases. Due to the large pressure drop induced by the repeated jet impingement, the thermal performance was less than 1 for all cases and decreased as the Reynolds number increased. Among the tested cases, the widest slot showed the best thermal performance. The measurement results showed that the thermal performance of the heat transfer augmentation by repeated stagger jets could be improved by altering the jet shape, and other shape of impingement jet will be studied in near future.

• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.1
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• pp.203-211
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• 1994

An experiment was carried out to evaluate the heat transfer and pressure drop performances of the smooth tube and two augmented tubes using R-113 under horizontal condensation condition. The augmented tubes are a spirally-twisted tube and an internally-finned tube. The test tube is 13.88 mm in diameter and 3.2 m long. Five different inlet pressure of 0.13, 0.16, 0.18, 0.21 and 0.23 MPa were employed and the mass flux was varied from 80 to 265 $kg/m^{2}s.$ The results showed that the overall heat transfer coefficient for the spirally-twisted tube and internally-finned tube were enhanced by 30-85% and 130-180%, respectively, over that for the smooth tube. The increase in total pressure drop for the spirally-twisted tube and internally-finned tube were reached up to 250-350% and 1100-1600%, respectively, over that for the smooth tube. Correlations were proposed for predicting the condensation heat transfer coefficient for the smooth tube and two augmented tubes.

• Solar Energy
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• v.13 no.1
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• pp.11-21
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• 1993

The purpose of this study was to investigate the enhancement of heat transfer without additional external power in the case of rectangular impingement air jet vertically on the flat heating surface. The technique used in the present study was placement of square rod bundles as a turbulence promoter in front of the heat transfer surface. The results obtained through this study were summerized as follws. High heat transfer enhancement was achieved by inserting rods in front of the heating flat plate. According to visulaization, it was examined because of flow acceleration and separation and disturbance of boundary layer. The smaller clerance between rod and heating plate was, the larger heat transfer effect became at each H/B. Arverage Nusselt number reached maximum at H/B=10 and the local augmentation rate of heat transfer became maximum at H/B=2. The maximum average heat transfer enhancement rate increase about 43% for the case of X/B=2 and C=1mm, compared to a flat plate without rods. The correlating equation of average Nusselt number and Reynolds number was obatined. As follws : ${\overline{Nu}}_0=1.249Re^{0.465}(C/A)^{-0.033}(H/B)^{0.013}$.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.2 no.3
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• pp.226-233
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• 1990

Fixed bed is known to be an effective heat transfer augmentation device which could be applied to heat exchangers. In this study, pressure drop in vertical cylindrical and annular fixed beds with air flowing through was experimentally investigated. Based on the experimental data and some analytical considerations, the demensionless correlation of pressure drop has been determined and shown in empirical forms. In particular, the experimental equation for the pressure drop was derived using the particle diameter and the bed diameter as variables, which would be more practical and useful in the design of heat transfer devices, instead of void fraction which had been used previously as the major variables by others. The present empirical equation obtained for the cylindrical fixed bed were found to be applicable also to an annular fixed bed when the concept of effective diameter was introduced.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.29 no.6 s.237
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• pp.737-746
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• 2005

The present study investigates heat/mass transfer and flow characteristics in a ribbed rotating passage with turning region. The duct has an aspect ratio (W/H) of 0.5 and a hydraulic diameter ($D_h$) of 26.67 mm. Rib turbulators are attached in the cross arrangement on the leading and trailing surfaces of the passage. The ribs have a rectangular cross section of $2\;mm\;(e){\times}\;mm\;(w)$ and an attack angle of $70^{\circ}$. The pitch-to-rib height ratio (p/e) is 7.5, and the rib height-to-hydraulic diameter ratio ($e/D_h$) is 0.075. The rotation number ranges from 0.0 to 0.20 while the Reynolds number is constant at 10,000. To verify the heat/mass transfer augmentation, internal flow structures are calculated for the same conditions using a commercial code FLUENT 6.1. The heat transfer data of the smooth duct for various Ro numbers agree well with not only the McAdams correlation but also the previous studies. The cross-rib turbulators significantly enhance heat/mass transfer in the passage by disturbing the main flow near the surfaces and generating one asymmetric cell of secondary flow skewing along the ribs. Because the secondary flow is induced in the first-pass and turning region, heat/mass transfer discrepancy is observed in the second-pass even for the stationary case. When the passage rotates, heat/mass transfer and flow phenomena change. Especially, the effect of rotation is more dominant than the effect of the ribs at the higher rotation number in the upstream of the second-pass.

• Journal of Energy Engineering
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• v.15 no.1 s.45
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• pp.60-66
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• 2006

An experimental investigation on the flow and convective heat transfer characteristics has been carried out far aluminum foam heat sink inserted into the annulus to examine the feasibility as a heat sink. Two aluminum foams or different permeability were selected to provide the friction factor and heat transfer correlations as function of Darcy, Reynolds and Prandtl number. Experimental results show that the friction factor is higher than clear annulus without aluminum foam, while $6\sim10$ times augmentation in Nusselt number is obtained. This technique can be used for the compactness of the heat exchanger.