• Transactions of the Korean Society of Mechanical Engineers B
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• v.34 no.1
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• pp.9-18
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• 2010

There are two kinds of models in two-phase pressured drop; homogeneous flow model and separated flow model. Many previous researchers have developed correlations for two-phase pressure drop in a microchannel. Most correlations were modified Lockhart and Martinelli's correlation, which was based on the separated flow model. In this study, experiments for adiabatic liquid water and nitrogen gas flow in rectangular microchannels were conducted to investigate two-phase pressure drop in the rectangular microchannels. Two-phase frictional pressure drop in the rectangular microchannels is highly related with flow regime. Homogeneous model with six two-phase viscosity models: $Owen^{(21)}$'s, $MacAdams^{(22)}$'s, Cicchitti et ${al.}^{(23)}$'s, Dukler et ${al.}^{(24)}$'s, Beattie and ${Whalley}^{(25)}$'s, Lin et ${al.}^{(26)}$'s models and six separated flow models: Lockhart and $Martinelli^{(27)}$'s, ${Chisholm}^{(31)}$'s, Zhang et ${al.,}^{(15)}$'s, Lee and ${Lee}^{(5)}$'s, Moriyama and ${Inue}^{(4)}$'s, Qu and $Mudawar^{(8)}$'s models were assessed with our experimental data. The best two-phase viscosity model is Beattie and Whalley's model. The best separated flow model is Qu and Mudawar's correlation. Flow regime dependency in both homogeneous and separated flow models was observed. Therefore, new flow pattern based correlations for both homogeneous and separated flow models were individually proposed.

• Transactions of the Korean Society of Mechanical Engineers
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• v.17 no.5
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• pp.1313-1320
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• 1993

Flow patterns inside the riser section and the effects of the heater inlet-and exit-restrictions, liquid charging level and the heater inlet subcooling on the flow characteristics inside an open two-phase natural circulation loop were studied experimentally. Three basic circulation modes were observed ; periodic circulation (A)(flow oscillations with incubation(no boiling) period), continuous circulations(stable operation mode with no flow oscillations), and periodic circulation (B) (flow oscillations with continuous boiling). The circulation rate increases and then decreases with the increase of the heating rate and the maximum circulation rate appears with the continuous circulation mode. The decrease of the inlet-restriction or the increase of the exitrestriction destabilizes the system. When the liquid charging level or the inlet subcooling decreases, the continuous circulation mode starts at the lower heating rate and the system is stabilized.

• Transactions of the Korean Society of Mechanical Engineers
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• v.14 no.5
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• pp.1290-1299
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• 1990

Characteristics of flow patterns in horizontal gas-liquid two-phase flow for two different sizes of pipe were investigated based upon a statistical analysis of differential pressure fluctuations at an orifice. The probability density function and the power spectral density function of the traces indicate peculiar shapes depending upon the two-phase flow regime. Mixed and separated flows also could be identified by the autocorrelation function. The transition region from separated flow to mixed flow also could be identified by these statistical properties. The experimental data determined by this method were compared with the flow pattern maps suggested by other investigators. The result indicates that the statistical characteristics of differential pressure fluctuations at orifices may be a useful tool for identifying flow patterns of horizontal gas-liquid two-phase flow.

• Transactions of the Korean Society of Mechanical Engineers
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• v.11 no.5
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• pp.810-818
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• 1987

A new method is proposed to identify two-phase flow regimes in horizontal gas-liquid flow, based upon a statistical analysis of instantaneous pressure drop curves at an orifice. The probability density functions of the curves indicate distinct patterns depending upon the two-phase flow regime. The transition region also could be identified by the distribution shape of the probability density function. The statistical properties of the pressure drop are analyzed for various flow regimes and transitions. Finally, the data of flow patterns determined by the proposed method are compared with the flow pattern maps suggested by other investigators.

• Proceedings of the KSME Conference
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• 2000.11b
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• pp.136-141
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• 2000

A study of counter-current two-phase flow in narrow rectangular channels has been performed. Two-phase flow regimes were experimentally studied in 760 mm long and 100 mm wide test sections with 2.0 and 3.0mm gaps. The resulting data have been compared to previous transition models. For the transition from bubbly to slug flow the superficial velocity of gas increased as the gap width increased. The comparison of experimental data to the transition model developed by Taitel and Barnea showed relatively good agreement for the bubbly-to-slug transition in the case of 2mm gap width. For the criteria of Mishima and Ishii to be applicable to the slug-to-churn transition the distribution parameter should be well defined for narrow channels. Even though the gap width of narrow channels increased the superficial gas velocity did not change for the transition form chum to annular flow regime. For the chum-to-annular transition the model of Taitel and Barnea showed discrepancies with experimental data, especially in the channel with larger gap.

• Journal of Advanced Marine Engineering and Technology
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• v.22 no.4
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• pp.505-514
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• 1998

Flow pattern of air-water two phase flow depends on the conditions of pressure void fraction and channel geometry. We classify the flow pattern by measuring the output signal of the conductivity probe. under the classified flow pattern we mount a visualization equipment on the test section and take pictures. We vary the concentration of pure solvent and polymer to measure local void fraction. We know that the maximum point position of local void fraction distribution move from the center of the pipe to the wall of the pipe as JSL increase when JSA is constant in two phase flow. But we find that the maximum point position of local void friction move from the wal of the pipe to the center of the pipe when polymer concentration increase.

• Journal of the Korean Institute of Gas
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• v.2 no.4
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• pp.53-59
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• 1998

Flow pattern of air-water two phase flow depends on the conditions of pressure drop, void fraction, and channel geometry. Drag reduction in the two phase flow can be applied to the transport of crude oil, phase change systems such as chemical reactor, pool and boiling flow, and to present cavitation which occurs in pump impellers. But the research on drag reduction in two phase flow is not intensively investigated. Therefore, experimental investigations have been carried out to analyze the drag reduction produced and void fraction by Co-polymer(A611p) addition in the two phase flow system. We find that the maximum point position of local void friction moves from the wall of the pipe to the center of the pipe when polymer concentration increases. Also we find that the polymer solution changes the characteristics of the two phase flow. And then we predict that it is closely related with the drag reduction.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.10 no.4
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• pp.398-410
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• 1998

Two-phase flow phenomena are observed in many industrial facilities and make much importance of optimum design for nuclear power plant and the liquid transportation system. The particular flow pattern depends on the conditions of pressure, flow velocity, and channel geometry. However, the research on drag reduction in two-phase flow is not intensively investigated. Therefore, experimental investigations have been carried out to analyze the drag reduction and void fraction by polymer addition in the two-phase flow system. We find that the polymer solution changes the characteristic of two-phase flow. The peak position of local void friction moves from tile wall of the pipe to the center of the pipe when polymer concentration increase. And then we predict that it is closely related with the frau reduction.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.27 no.2
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• pp.75-80
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• 2015

The effect of inlet mixer geometries on the two-phase flow patterns in square micro-channel with $600{\times}600{\mu}m$ was investigated experimentally in this paper. The 4 different mixer configurations based on the Y, Impacting, and two T types (gas and liquid inlets were switched) were used. The test fluids were nitrogen and water. The liquid and gas superficial velocities were 0.01~10 m/s and 0.1~100 m/s, respectively. Several distinctive flow patterns, namely, annular, slug-annular, slug, slug-bubbly, bubbly, and churn flow could be seen. The flow pattern maps for each mixer were suggested, and it can be concluded that two-phase flow patterns are not very sensitive to the mixer geometries. But the mixing behaviors of gas and liquid for each mixer were different for slug and bubbly flow. Thus, the characteristics of slug and bubble for each case were not same.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.33 no.12
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• pp.939-946
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• 2009

Wettability is a critical parameter in micro-scale two-phase system. Several previous results indicate that wettability has influential affect on two-phase flow pattern in a microchannel. However, previous studies conducted using circular microtube, which was made by conventional fabrication techniques. Although most applications for micro thermal hydraulic system has used a rectangular microchannel, data for the rectangular microchannel is totally lack. In this study, a hydrophilic rectangular microchannel was fabricated using a photosensitive glass. And a hydrophobic rectangular microchannel was prepared using silanization of glass surfaces with OTS (octa-dethyl-trichloro-siliane). Experiments of two-phase flow in the hydrophilic and the hydrophobic rectangular microchannels were conducted using water and nitrogen gas. Visualization of twophase flow pattern was carried out using a high-speed camera and a long distance microscope. Visualization results show that the wettability was important for two-phase flow pattern in rectangular microchannel. In addition, two-phase frictional pressure drop was highly related with flow patterns. Finally, Two-phase frictional pressure drop was analyzed with flow patterns.