• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.8 no.1
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• pp.140-150
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• 1996

CFC-12, which has been used most widely in automobile air conditioners and household refrigerators is scheduled to be phased out soon because of its high ozone depletion potential. Now HFC-134a is suggested as an alternative refrigerant for CFC-12. In this Study, we intended to investigate how PAG oil influence evaporation heat transfer and flow pattern, using R-134a and PAG oil influences evaporation heat transfer and flow pattern, using R-134a and PAG oil in the horizontal miro-fin evaporation tube. Experiments were conducted under the flowing est conditions : mass velocity 86-250kg/$m^2$s, heat flux 5-30 ㎾/$m^2$, oil concentration 0-21 wt.% and saturation temperature 5$^{\circ}C$. Local evaporation heat transfer coefficients were found to be higher at the top, side and bottom of the tube in this order. Average heat transfer coefficients turned out to increase with oil concentration increment up to 3 wt.% oil concentration, whereas heat transfer coefficients gradually decreased over 3 wt.% oil concentration, because of oil-rich liquid film was formed on the heat transfer surface. Flow patterns were rapidly transitioned to annular regimes up to 3 wt.% oil concentration. In case of pure refrigerant, measured heat transfer coefficients in the experiments were similar to those of Kandlikar＇s correlation.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.11 no.5
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• pp.633-641
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• 1999

Experiments were carried out to investigate the condensation heat transfer characteristics for R22 and its alternatives, R407C (R32/125/134a, 23/25/52wt%) and R410A (R32/125, 50/50wt%). A concentric tube heat exchanger was made to conduct condensation heat transfer tests. Mass flux and saturation temperature of refrigerants at the test section inlet were varied to get the corresponding heat transfer coefficients. Serial and parallel input of secondary fluid (water) were applied to the test subsections. Compared with existing correlations of condensation heat transfer, experimental heat transfer coefficients obtained in this study were generally higher than the predicted values, and mean absolute deviations from several correlations were shown. Wall subcooling was introduced to get a new correlation for condensation heat transfer coefficients by modifying Shah's equation. The RMS deviation of the measured heat transfer coefficients from the new correlation in this study for R22 is 9.9% and that for R407C and R410A are 10.2% and 14.6%, respectively.

• Proceedings of the KSME Conference
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• 2004.11a
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• pp.1034-1039
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• 2004

The row-by-row heat transfer characteristics of fin-and-tube heat exchangers having wavy fins were experimentally investigated. Three samples having different rows (one, two and three) were tested. Results show that the heat transfer coefficient is strongly dependent on the tube row. The heat transfer coefficient of the first row is larger than those of second or third rows. However, the difference decreases as the Reynolds number increases. The heat transfer coefficients of the second and the third row are approximately the same, probably due to increased mixing of bulk flow by wavy channels. Although samples have different tube row, the heat transfer coefficients of same row are approximately the same.

• Transactions of the Korean Society of Mechanical Engineers
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• v.19 no.10
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• pp.2586-2594
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• 1995

Two phase flow phenomena are observed in many industrial facilities and make much importance of optimum design for nuclear power plant and various heat exchangers. This experimental study has been investigated the classification of the flow pattern, the local void distribution and convective heat transfer in swirl and non-swirl two phase flow under the isothermal and nonisothermal conditions. The convective heat transfer coefficients in the single phase water flow were measured and compared with the calculated results from the Sieder-Tate correlation. These coefficients were used for comparisons with the two-phase heat transfer coefficients in the flow orientations. The experimental results indicate, that the void probe signal and probability density function of void distribution can used into classify the flow patterns, no significant difference in voidage distribution was observed between isothermal and non-isothermal condition in non-swirl flow, the values of two phase heat transfer coefficients increase when superficial air velocities increase, and the enhancement of the values is observed to be most pronounced at the highest superficial water velocity in non-swirl flow. Also two phase heat transfer coefficients in swirl flow are increased when the twist ratios are decreased.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.4 no.4
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• pp.351-359
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• 1992

In order to improve the performance of heat exchanger, fluidized bed is often employed. The experiments are carried out in fluidized double pipe parallel flow heat exchanger in which finned tube is vertically immersed. And the heat transfer coefficients between the heated tube and fluidized bed of alumina beads(dp=0.41, 0.54, 0.65, 0.77mm) are calculated as a function of air fluidized velocity and pumping power. The effects of particle size, static bed height and pumping power on the heat transfer coefficients are investigated. And the heat transfer coefficients are compared with that of single phase forced convection heat exchanger. In particular, the heat transfer performance of each type heat exchanger is evaluated in relation to the pumping power.

• Journal of the Korean Society for Aviation and Aeronautics
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• v.14 no.2
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• pp.33-38
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• 2006

Heat transfer coefficients and static pressure distributions on a gas turbine vane endwall were experimentally investigated in a 5 bladed linear cascade. The Reynolds number based on an axial chord length and the cascade exit velocity was 500,000. Both heat transfer and pressure measurements on the vane endwall were made at the two different turbulence intensity levels of 6.8% and 10.8%. Detailed heat transfer coefficient distributions on the vane endwall region were measured using a hue detection based transient liquid crystals technique. Results show various regions of high and low heat transfer coefficients on the vane endwall surface due to several types of secondary flows and vortices. Heat transfer coefficient and endwall static pressure distributions showed similar trends for both turbulence intensity, however, the averaged heat transfer coefficients for higher turbulence intensity case was higher than the lower turbulence intensity case by 15%.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.25 no.12
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• pp.1676-1683
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• 2001

An experimental study on condensation heat transfer characteristics of helically coiled spiral tubes was performed. The refrigerant is R-113. A refrigerant loop was established to measure the condensation heat transfer coefficients. Experiments were carried out uniform heat flux of 15 kw/m$^2$, refrigerant quality of 0.1∼0.9, curvature ratio of 0.016, 0.025 and 0.045. The curvature of a coil was defined as the ratio of the inside diameter of the tube to the diameter of the bending circle. To compare the condensation heat transfer coefficients of coiled spiral tubes, the previous results on coiled plain tubes and straight plain tubes were used. The results shows that the condensation heat transfer coefficients of coiled spiral tubes largely increase, as increasing Re and quality, compared to those of coiled plain tubes and straight plain tubes. As increasing degree of subcooling, however, the condensation heat transfer coefficients on coiled spiral tubes decrease. It is found that the heat transfer enhancement is more better than coiled plain tubes and straight plain tubes, as increasing curvature ratio.

• Journal of Advanced Marine Engineering and Technology
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• v.20 no.3
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• pp.91-100
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• 1996

Experimental results for forced convection condensation of non-azeotropic refrigerant mixtures inside a horizontal smooth tube are presented. The mixtures of R-22+R-134a and pure refrigerants R-22 and R-134a are used as the test fluids and a double pipe heat exchanger of 7.5mm ID and 4800mm long inside tube is used. The range of parameters are 100-300kg/h of mass flow rate, 0-1.0 of quality, and 0, 33, 50, 67, and 100 weight percent of R-22 mass fraction in the mixtures. The heat flux, vapor pressure, vapor temperature and tube wall temperature were measured. Using the data, the local and average heat transfer coefficients for the condensation have been obtained. In the same given experimental conditions, the liquid heat transfer coefficients for NARMs were considerally lower than that of the pure refrigerant of R-22 and R-134a. Local heat transfer characteristics for NARMs were different from pure refrigerant R-22 and R-134a. In some regions, local heat transfer coefficients for NARMs were increased in the following order ; Bottom$\rightarrow$Top$\rightarrow$Side. The condensation heat transfer coefficients for NARMs increased with mass velocity, heat flux, and quality, but were considerably lower than that of pure refigerant R-22 and R-134a.

• Proceedings of the KSME Conference
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• 2001.11b
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• pp.330-336
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• 2001

Water heat transfer experiments were carried out in a uniformly heated annulus with a wide range of pressure conditions. The local heat transfer coefficients for saturated water flow boiling have been measured just before the occurrence of the critical heat flux (CHF) along the length of the heated section. The trends of the measured heat transfer coefficients were quite different from the conventional understanding for the heat transfer of saturated flow boiling. This discrepancy was explained from the nucleate boiling in the liquid film of annular flow under high heat flux conditions.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.24 no.2
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• pp.195-203
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• 2000

The local heat/mass transfer coefficients for arrays of impinging circular air jets on a plane surface are determined by means of the naphthalene sublimation method. Fluid from the spent jets is constrained to flow out of the system in one direction. Therefore, the spent fluid makes a crossflow in the confined space. The present study investigates effects of jet-orifice-plate to impingement-surface spacing and jet Reynolds number. The spanwise- and overall-averaged heat/mass transfer coefficients are obtained by numerical integrating the local heat transfer coefficients. The local maximum heat/mass transfer coefficients move further in the downstream direction due to the increase of crossflow velocity. At the mid-way between adjacent jets, the heat/mass transfer coefficients have a small peak owing to the collision of the adjacent wall jets and are affected strongly by the crossflow. The effect of the crossflow occurs strongly at the small orifice-to-impingement surface distance.