• 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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• 제21권2호
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• pp.316-328
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• 1997

There were appreciable progresses on the study of shock wave / boundary layer interaction control in the transonic flow without nonequilibrium condensation. But in general, the actual flows associated with those of the airfoil of high speed flight body, the cascade of steam turbine and so on accompany the nonequilibrium condensation, and under a certain circumstance condensation shock wave occurs. Condensation shock wave / boundary layer interaction control is quite different from that of case without condensation, because the droplets generated by the result of nonequilibrium condensation may clog the holes of the porous wall for passive control and the flow interaction mechanism between the droplets and the porous system is concerned in the flow with nonequilibrium condensation. In these connections, it is necessary to study the condensation shock wave / boundary layer interaction control by passive cavity in the flow accompanying nonequilibrium condensation with condensation shock wave. In the present study, experiments were made on a roof mounted half circular arc in an indraft type supersonic wind tunnel to evaluate the effects of the porosity, the porous wall area and the depth of cavity on the pressure distribution around condensation shock wave. It was found that the porosity of 12% which was larger than the case of without nonequilibrium condensation produced the largest reduction of pressure fluctuations in the vicinity of condensation shock wave. The results also showed that wider porous area, deeper cavity for the same porosity of 12% are more favourable "passive" effect than the cases of its opposite. opposite.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2008년도 제31회 추계학술대회논문집
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• pp.341-344
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• 2008

Numerical simulations have been carried out for a supersonic two-dimensional flow over open, rectangular cavities (length-to-depth ratios are L/D = 1.0) in order to investigate the effect of non-equilibrium condensation of moist air on supersonic flows around the cavity for the flow Mach number 1.83 at the cavity entrance. In the present computational investigation, a condensing flow was produced by an expansion of moist air in a Laval nozzle. The results obtained showed that in the case with non-equilibrium condensation for L/D = 1.0, amplitudes of oscillation in the cavity became smaller than those without the non-equilibrium condensation. Furthermore, the occurrence of the non-equilibrium condensation reduced the peaks of power spectrum density and the frequency of the flow field oscillation increased in comparison with the case of $S_0$ = 0.

• 한국유체기계학회 논문집
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• 제17권3호
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• pp.33-40
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• 2014

In this study, a new model for calculating the liquid film thickness and condensation heat transfer coefficient in a vertical condenser tube is proposed by considering the effects of gravity, liquid viscosity, and vapor flow in the core region of the flow. In order to introduce the radial velocity profile in the liquid film, the liquid film flow was regarded to be in Couette flow dragged by the interfacial velocity at the liquid-vapor interface. For the calculation of the interfacial velocity, an empirical power-law velocity profile had been introduced. The resulting liquid film thickness and heat transfer coefficient obtained from the proposed model were compared with the experimental data from other experimental study and the results obtained from the other condensation models. In conclusion, the proposed model physically explained the liquid film thinning effect by the vapor shear flow and predicted the condensation heat transfer coefficient from experiments reasonably well.

• 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.

• 대한기계학회논문집B
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• 제26권7호
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• pp.951-958
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• 2002

Rapid expansion of a moist air or a stream through a supersonic nozzle often leads to non-equilibrium condensation shock wave, causing a considerable energy loss in flow field. Depending on amount of latent heat released due to non-equilibrium condensation, the flow is highly unstable or a periodical oscillation accompanying the condensation shock wave in the nozzle. The unsteadiness of the condensation shock wave is always associated with several kinds of instabilities as well as noise and vibration of flow devices. In the current study, a passive control technique using a porous wall with a plenum cavity underneath is applied for the purpose of alleviation of the condensation shock oscillations in a transonic nozzle. A droplet growth equation is coupled with two-dimensional Navier-Stokes equation system. Computations are carried out using a third-order MUSCL type TVD finite-difference scheme with a second-order fractional time step. An experiment using an indraft wind tunnel is made to validate the present computational results. The results show that the oscillations of the condensation shock wave are completely suppressed by the current passive control method.

• 설비공학논문집
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• 제14권2호
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• pp.175-183
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• 2002

Flow Condensation heat transfer coefficients (HTCs) of Rl2, R22, R32, Rl23, Rl25, R134a, R142b were measured experimentally on a horizontal plain tube. The experi- mental apparatus was composed of three main parts; a refrigerant loop, a water loop and a water-glycol loop. The test section in a refrigerant loop was made of a copper tube of 8.8 mm inner diameter and 1000 mm length respectively. The refrigerant was cooled by passing cold water through an annulus surrounding the test section. All tests were performed at a filed refrigerant saturation temperature of 4$0^{\circ}C$ with mass fluxes of 100, 200, 300 kg/$m^2$s. The experimental result showed that flow condensation HTCs increase as the quality, mass flux, and latent heat of condensation increase. At the same mass flux, the HTCs of R32 and R142b were higher than those of R22 by 35~45% and 7~14% respectively while HTCs of R134a and Rl23 were similar to those of R22. On the other hand, HTCs of Rl25 and Rl2 were lower than those of R22 by 28 ~30% and 15 ~25% respectively Finally, a new correlation for flow condensation HTCs was developed by modifying Dobson and Chato's correlation with the latent heat of condensation considered. The correlaton showed an average deviation of 13.1% for all pure fluids data indicating an excellent agreement.

• Journal of Mechanical Science and Technology
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• 제20권8호
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• pp.1275-1283
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• 2006

Condensation heat transfer coefficients in a 7.92 mm inside diameter copper smooth tube were obtained experimentally for R22, R134a, and R410A. Working conditions were in the range of $30-40^{\circ}C$ condensation temperature, $95-410 kg/m^2s$ mass flux, and 0.15-0.85 vapor quality. The experimental data were compared with the eight existing correlations for an annular flow regime. Based on the heat-momentum analogy, a condensation heat transfer coefficients correlation for the annular flow regime was developed. The Breber et al. flow regime map was used to discern flow pattern and the Muller-Steinhagen & Heck pressure drop correlation was used for the term of the proposed correlation. The proposed correlation provided the best predicted performance compared to the eight existing correlations and its root mean square deviation was less than 8.7%.

• 대한기계학회논문집
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• 제12권3호
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• pp.582-589
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• 1988

본 연구에서는 작동유체로서 습공기를 사용하였을때 수반되는 응축에 의한 전 압손실을 실험과 수치해석을 통하여 정체점에 있어서의 습공기의 온도, 과포화도 및 노즐 팽창율의 효과와 액적 질량비 (g)과의 관계로서 구명하였다.

• 대한기계학회논문집B
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• 제20권12호
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• pp.3980-3990
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• 1996

When a moist air is rapidly expanded in a supersonic nozzle, nonequilibrium condensation occurs at a supersaturation state. Condensation shock wave appears in the nozzle flow if the releasing latent heat due to condensation goes beyond a critical value. It has been known that self-excited oscillations of the condensation shock wave generate in an air or a steam nozzle flow with a large humidity. In the present study, the passive control technique using porous wall with a cavity underneath was applied to the condensation shock wave. The effects of the passive control on the steady and self-excited condensation shock waves were experimentally investigated by Schlieren visualization and static pressure measurements. The result shows that the present passive control is a useful technique to suppress the self-excited oscillations of condensation shock wave.