• 한국가시화정보학회지
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• 제21권1호
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• pp.47-56
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• 2023

Bubble condensation, which involves the interaction of bubbles within the subcooled liquid flow, plays an important role in the effective control of thermal devices. In this study, numerical simulations are performed using a VOF (Volume of Fluid) model to investigate the effect of tube diameter on bubble condensation. As the tube diameter decreases, condensation bubbles persist for a long time and disappear at a higher position. It is observed that for small tube diameters, the heat transfer coefficients of condensation bubbles, which is a quantitative parameter of condensation rate, are smaller than those for large tube diameters. When the tube diameter is small, the subcooled liquid around the condensing bubble is locally participated in the condensation of the bubble to fill the reduced volume of the bubble due to the generation of a backflow in the narrow space between the bubble and the wall, so that the heat transfer coefficient decreases.

• Journal of Mechanical Science and Technology
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• 제16권11호
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• pp.1511-1521
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• 2002

High-speed moist air or steam flow has long been of important subject in engineering and industrial applications. Of many complicated gas dynamics problems involved in moist air flows, the most challenging task is to understand the nonequilibrium condensation phenomenon when the moist air rapidly expands through a flow device. Many theoretical and experimental studies using supersonic wind tunnels have devoted to the understanding of the nonequilibrium condensation flow physics so far. However, the nonequilibrium condensation can be also generated in the subsonic flows induced by the unsteady expansion waves in shock tube. The major flow physics of the nonequilibrium condensation in this application may be different from those obtained in the supersonic wind tunnels. In the current study, the nonequilibrium condensation phenomenon caused by the unsteady expansion waves in a shock tube is analyzed by using the two-dimensional, unsteady, Navier-Stokes equations, which are fully coupled with a droplet growth equation. The third-order TVD MUSCL scheme is applied to solve the governing equation systems. The computational results are compared with the previous experimental data. The time-dependent behavior of nonequilibrium condensation of moist air in shock tube is investigated in details. The results show that the major characteristics of the nonequilibrium condensation phenomenon in shock tube are very different from those in the supersonic wind tunnels.

• 대한기계학회논문집B
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• 제20권5호
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• pp.1725-1734
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• 1996

Experimental condensation and evaporation heat transfer coefficients were measured in a horizontal smooth tube and a horizontal micro-finned tube with HFC-134a. The test sections are straight, horizontal tubes with have a 9.52mm outside diameter and about 5000mm long. The micro-finned tube had 60 fins with a height of 0.12mm and a spiral angle of 25.deg.. The condensation test section was a double-pipe type with counter flow configuration. The evaporation test section employed an electic heating method. Enhancement factors which is defined as a ratio of the heat transfer coefficient for micro-finned tube to that for smooth tube, varied from 1.3 to 1.6(mass flux:110~190kg/m$^{2}$s) for condensation and 1.2 to 1.5 (mass flux:70~160kg/m$^{2}$s) for evaporation. The experimental data of condensation and evaporation heat transfer coefficients were compared to several empirical correlations. Based on these comparisons, modified correlations of the condensation and evaporation heat transfer coefficient for both smooth and micro-finned tubes were proposed.

• 대한설비공학회:학술대회논문집
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• 대한설비공학회 2006년도 하계학술발표대회 논문집
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• pp.657-662
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• 2006

The present paper dealt with an experimental study of condensation heat transfer coefficients with refrigerant R-22, R-407C and R-410A, and was focused on pressure gradient and heat transfer coefficient in horizontal tube-in-tube heat exchangers using inner diameter of 4 mm, 3 mm and 2 mm in a 16.91 mm tube and length of 3,000 mm. Experiments were performed at inlet saturation temperature of 35 to $45^{\circ}C$ and mass flux ranges from 200 to $600 kg/m^2s$. The pressure gradient with inner tube diameter of 4.0 mm is higher 2.5 times than that of 8.0 mm. In tube-in-tube HEX, the pressure gradient of R-410A were lower than those of R-22 and R-407C. The condensation heat transfer coefficients increased with mass flux increase, but they decreased with saturation temperature increased. Condensation heat transfer coefficients of R-410A were a little higher than those of R-22 and R-407C. The condensation heat transfer coefficients of tube-in-tube HEX were about 40% higher than those of double tube HEX.

• Journal of Mechanical Science and Technology
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• 제17권12호
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• pp.2066-2077
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• 2003

The time-dependent behavior of unsteady condensation of moist air through the Ludwieg tube is investigated by using a computational fluid dynamics (CFD) work. The two-dimensional, compressible, Navier-Stokes equations, fully coupled with the condensate droplet growth equations, are numerically solved by a third-order MUSCL type TVD finite-difference scheme, with a second-order fractional time step. Baldwin-Lomax turbulence model is employed to close the governing equations. The predicted results are compared with the previous experiments using the Ludwieg tube with a diaphragm downstream. The present computations represent the experimental flows well. The time-dependent unsteady condensation characteristics are discussed based upon the present predicted results. The results obtained clearly show that for an initial relative humidity below 30% there is no periodic oscillation of the condensation shock wave, but for an initial relative humidity over 40% the periodic excursions of the condensation shock occurs in the Ludwieg tube, and the frequency increases with the initial relative humidity. It is also found that total pressure loss due to unsteady condensation in the Ludwieg tube should not be ignored even for a very low initial relative humidity and it results from the periodic excursions of the condensation shock wave.

• 설비공학논문집
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• 제7권2호
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• pp.298-309
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• 1995

Low-fin tubes are widely used to enhance condensation heat transfer. In this study, condensation heat transfer experiment was conducted on the low-fin tube using R-11. Three different fin densities-787 fpm (fins per meter), 1102 fpm. 1378 fpm-were tested. The results show that low-fin tube enhances the condensation heat transfer considerablely. The enhancement increases as the fin density increases. It was also found that the fin shape and height have a significant effect on the condensation heat transfer coefficient. Slender or high fins showed a higher condensing heat transfer coefficient compared with fat, low fins. For the tube with 1378 fpm, however, excessive fin height decreased the condensing heat transfer coefficient. The reason may be attributed to the increasing condensate retention angle as the fin density increases. The experimental data are compared with existing prediction models. Results show that Webb's surface tension model predicted the data best (within ${\pm}20%$), which confirms that surface tension plays the major role in low-fin tube condensation.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 추계학술대회논문집B
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• pp.450-455
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• 2001

The time-dependent behavior of nonequilibrium condensation of moist air through the Ludwieg tube is investigated with a computational fluid dynamics(CFD) method. The two-dimensional, compressible, Navier-Stokes equations, fully coupled with the condensate droplet growth equations, are numerically solved by a third-order MUSCL type TVD finite-difference scheme, with a second-order fractional time step. Baldwin-Lomax turbulence model is employed to close the governing equations. The computational results are compared with the previous experiments using the Ludwieg tube with a downstream diaphragm. The results clearly show that for an initial relative humidity below 30% there is no periodic oscillation of the condensation shock wave, but for an initial relative humidity over 40% the periodic excursions of the condensation shock occurs in the Ludwieg tube, and the frequency increases with the initial relative humidity. It is also found that total pressure loss due to nonequilibrium condensation in the Ludwieg tube should not be ignored even for a very low initial relative humidity, and the periodic excursions of the condensation shock wave are responsible for the total pressure loss.

• 한국산학기술학회논문지
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• 제19권4호
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• pp.517-524
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• 2018

본 연구의 목표는 수평 관외 초소수성 튜브에서 비응축가스(NCG)의 영향을 관찰하고, 이를 일반 알루미늄 튜브의 응축 현상과 비교하는 것이다. 초소수성 튜브 제작을 위해 Self-Assembled Monolayer(SAM) 코팅으로 알루미늄 튜브의 외부를 표면개질 했다. 응축 성능을 나타내기 위해 총합 열전달계수를 사용하였고, 이 값으로 응축 성능을 비교 검토하였다. 본 연구의 주요 변수는 비응축가스 질량 분율로, 0.08에서 0.45의 범위에서 실험을 진행하였다. 응축 실험을 통해 비응축가스 질량 분율이 낮아질수록 응축 성능이 SAM 튜브와 일반 튜브에서 모두 향상되는 것을 확인했다. SAM 튜브에서 적응축 열전달 성능은 일반 튜브 대비하여 평균 약 1.9배에서 2.5배 정도 큰 것을 관찰하였다. SAM 튜브에서 비응축가스 질량 분율이 낮아지면서 응축 성능 상승폭이 감소하게 되는데, 이는 Flooded 응축 현상이 발현되었기 때문이다. 응축이 더 활발하게 진행되면 SAM 튜브에서 막응축이 일어나는 것을 관찰하였고, 이 때 성능은 일반 알루미늄 튜브보다도 저하된 성능을 보였다. SAM 알루미늄 튜브에서 Flooded 응축과 막응축이 일어나는 원인으로 표면에서의 Pinning 효과를 이용하여 설명하였다. 결론적으로, SAM 튜브를 실제 응축기에 적용해 표면개질로 인한 응축 성능 개선 효과를 얻기 위해서는 적응축 또는 Flooded 응축이 일어나는 조건으로 응축기 내 환경 조성을 해야 한다.

• 대한기계학회논문집
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• 제18권1호
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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.

• 설비공학논문집
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• 제12권6호
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• pp.533-540
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• 2000

Condensation heat transfer characteristics have been investigated experimentally when a water vapor is condensed on the outside of a horizontal copper tube in a condenser. This problem is of particular interest in the design of a LiBr-water absorption system. Hydrophilic surface modification was performed to increase the wettability on the copper tube. The optimum hydrophilic treatment condition using acethylene and nitrogen as reaction gas is also studied in detail. The results obtained indicate that the optimum reaction gas ratio of acethylene to nitrogen for hydrophilic surface modification was found to be 7 : 3 for the best condensation heat transfer. In the wide ranges of coolant inlet temperatures, and coolant mass flow rates, both the condensation heat transfer rate and the condensation heat transfer coefficient of a hydrophilic copper tube are increased substantially, compared with those of a conventional copper tube used in a condenser. It is also found that the condensation heat transfer enhancement by the hydrophilic surface modification still emains even after a hundred cycles of wet/dry processes.