• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.18 no.12
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• pp.1017-1024
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• 2006

In this work, pool boiling heat transfer coefficients (HTCs) of five hydrocarbon refrigerants of propylene, propane, isobutane, butane and dimethylether (DME) were measured at the liquid temperature of $7^{\circ}C$ on a 26 fpi low fin tube, Turbo-B, and Thermoexcel-E tubes. All data were taken from 80 to $10kW/m^2$ in the decreasing order of heat flux. The data of hydrocarbon refrigerants showed a typical trend that nucleate boiling HTCs obtained on enhanced tubes also increase with the vapor pressure. Fluids with lower reduced pressure such as DME, isobutane, and butane took more advantage of the heat transfer enhancement mechanism of enhanced tubes than those enhancement ratios of $2.3\sim9.4$ among the tubes tested due to its sub-channels and re-entrant cavities.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.8 no.3
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• pp.307-316
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• 1996

The objective of present study is to obtain information on the stenosis effects in the branch tubes for industrial piping system and atherogenesis processing in human arteries. Numerical solutions for flows of Newtonian and non-Newtonian fluids in the branch tubes are obtained by the finite volume method. Centerline velocity and pressure along the bifurcated tubes for water, blood and aqueous Separan AP-273 solution are computed and the numerical results of blood and the Separan solution are compared with those of water. Flow phenomena in the stenosed branch tubes are discussed extensively and predicted effectively. The effects of stenosis on the pressure loss coefficients are determined.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.14 no.8
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• pp.641-647
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• 2002

Objectives of this paper are to investigate the effect of roughness of micro-hatching tubes on the absorption performance and to develop on experimental correlation of Nusselt as a function of the roughness. Three different micro-scale hatched tubes and a bare tube were tested in the present experiment. $H_{2}O/LiBr$ solution is used as working fluid. It was found that absorption performance of micro-scale hatched tubes were improved upto 2 times with an error band of ${\pm}25%$ compared with the bare tube. An experimental correlation of Nusselt was developed as a function of the roughness.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.12 no.12
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• pp.1081-1089
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• 2000

Evaporating heat transfer coefficients of R-22 and R-134a were measured in smooth horizontal copper tubes with inner diameters of 1.77, 3.36 and 5.35mm, respectively. The experiments were conducted in a closed loop, which was driven by a magnetic gear pump. Experiments were performed for the following range of variables: mass velocity (200 to 400 kg/$m^2$.s), saturation temperature($0^{circ}C,; 5^{\circ}C$) and quality(0 to 1.0). Main results obtained are as follows: evaporating heat transfer coefficients in the small diameter tubes (ID<7mm) were observed to be strongly affected by various diameters and to differ from those in the large diameter tubers. The heat transfer coefficients of the small diameter tubes were higher than those of the large diameter tubs. And it was very difficult to apply some well-known previous predictions (Shah`s, Gungor-Winterton`s and Kandlikar`s correlation) to small diameter tubes.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.2 no.1
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• pp.74-83
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• 1990

The temperature distributions inside molds with heating or cooling tubes were calculated using special boundary element method. This special boundary element method was employed in order to reduce the error for small diameter tubes. Calculated temperature was compared with results using finite element method. It was found that the current method becomes more accurate as tubes' diameter gets smaller. Optimal arrangement of tubes for uniform temperature distribution along specific surface was found. CONMIN program was employed for the optimization.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.12 no.5
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• pp.494-501
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• 2000

Effectiveness of a 3-pass plate finned-tube heat exchanger is calculated using heat exchangelet method by changing the shape of fin and the arrangement of tubes. The alternative refrigerant R134a is taken in this study. Conduction between neighboring tubes along the fin is taken into account in addition to convection between the fin and the surrounding air. Governing equations are obtained by using energy balance in a small control volume containing a tube and fins. They are numerically solved following the tube. Effect of tube-to-tube conduction is investigated in single-phase and two-phase flows with various fin shapes and arrangements of tubes. Improvement of effectiveness by fin perforation is studied too. The results shows that perforating fins, increasing the number of tubes, and increasing the distance between neighboring tubes at the same fin area enhance the effectiveness.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.13 no.9
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• pp.827-835
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• 2001

In this study, condensation heat transfer characteristics were conducted with special heat transfer tubes of SH-C type. Experiments were carried out the saturated vapor temperature of 334K and the wall subcooling of 1.5-4.5K. The refrigerant was R-113 and the enhanced tubes used in the present study were SH-CDR, SH-CYR and SH-CHR. The experimental results showed that the condensation heat transfer coefficients of SH-C type tubes were about 23-66% higher than those of a low integral-fin tube. It was visualized that the condensed liquid on the outer surface of SH-C type tubes flowed continuously down unlike a low integral-fin tube and a plain tube, due to a 3-D extending fin on the outer surface of SH-C type tubes. As a result, the thermal resistance of the condensed liquid decreased and the heat transfer coefficient increased. Also, the enhancement ratio of SH-CDR tube was the highest, and it was about 9-11 times as compared to that of a plain tube.

• Explosives and Blasting
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• v.21 no.1
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• pp.49-58
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• 2003

현재 발파의 수행은 채석, 채굴 및 지하철과 도로, 건축, 토목공사 등 다양한 분야에 적용되고 있다. 이에 따라 발파특성의 연구와 관련하여 다양한 이론이 도출되고 있고 사용되지만 발파에 의한 공해인 진동 및 소음의 문제가 대두되면서 또한 이들의 취약점을 보완하기 위해 다양한 공법들이 시행되고 개선되어지고 있다. 본 연구에서는 기존의 발파공법과 Air tubes를 이용한 발파공법을 비교하므로 화약의 사용량을 보다 감소시키고, 기존 전색의 길이를 줄이므로 폭약의 투사면적을 증대시켜 진동 및 폭음을 감소시키고 상대적으로 양호한 파쇄입도를 얻고 상부의 대괴를 최소화하는데 목적이 있다. 따라서 발생하는 진동 및 발파폭음을 크게 감소시킬 수 있으므로 기존 노천발파 및 진동, 소음에 민감한 도심지 발파에서 그 이점을 활용하여 보다 안전적이고 경제적인 발파작업을 수행하는데 연구의 목적이 있다.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.24 no.7
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• pp.553-559
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• 2012

A plate-fin heat exchanger is a type of heat exchanger widely used in air conditioners, and tubes and fins are tightly assembled by the mechanical expansion process of tubes. The tube expansion process deforms the grooves inside the tube, and the groove shapes also affect the adhesion between tubes and fins. In this study, the adhesion and heat transfer performance affected by the tube expansion of the non-uniform groove shape tube with different heights are investigated by both analysis and experiments. From the analysis method, it was shown that the contact pressure of non-uniform groove tube is higher than that of the uniform groove tube, and the most appropriate high groove number of the non-uniform groove tube is designed for the maximum contact pressure. From the experimental results, the decreasing rate of the condensation heat transfer coefficient is smaller in the non-uniform groove tube with different heights, compared to the conventional uniform groove tube. Also, the air-side heat transfer coefficient of the non-uniform groove tube with different heights is higher than that of the uniform groove tubes.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.12 no.10
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• pp.932-940
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• 2000

An experimental study on evaporative heat transfer characteristics in micro-fin tubes before and after expansion process has been performed with R-22. Single-grooved micro-fin tubes with outer diameter of 9.52 mm were used as test sections, and it was uniformly heated by applying direct current to the test tubes. Experiments were conducted at mass flow rates of 20 and 30 kg/hr. For each mass flow rate condition, evaporation temperature was set at 5 and $15^{\circ}C$and heat flux was changed from 6 to 11 kW/$m^2$ The evaporative heat transfer coefficient of micro-fin tubes after expansion is decreased because of the crush of fins and enlargement of inner diameter compared to that before expansion. Convective boiling effect decreased remarkably at higher quality range in the micro-fin tube after expansion, and the difference of the heat transfer coefficient in micro-fin tubes before and after expansion was greater for higher quality region. The evaporative heat transfer coefficient of the micro-fin tube after expansion was 19.9% smaller on the average than that before expansion.