• Nuclear Engineering and Technology
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• v.38 no.1
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• pp.69-80
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• 2006

Fluid-elastic instability in an air-water two-phase cross-flow has been experimentally investigated using two different arrays of straight tube bundles: normal square (NS) array and rotated square (RS) array tube bundles with the same pitch-to-diameter ratio of 1.633. Experiments have been performed over wide ranges of mass flux and void fraction. The quantitative tube vibration displacement was measured using a pair of strain gages and the detailed orbit of the tube motion was analyzed from high-speed video recordings. The present study provides the flow pattern, detailed tube vibration response, damping ratio, hydrodynamic mass, and the fluid-elastic instability for each tube bundle. Tube vibration characteristics of the RS array tube bundle in the two-phase flow condition were quite different from those of the NS array tube bundle with respect to the vortex shedding induced vibration and the shape of the oval orbit of the tube motion at the fluid-elastic instability as well as the fluid-elastic instability constant.

• Nuclear Engineering and Technology
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• v.45 no.5
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• pp.613-624
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• 2013

Multi-dimensional two-phase phenomena occur in many industrial applications, particularly in a nuclear reactor during steady operation or a transient period. Appropriate modeling of complicated behavior induced by a multi-dimensional flow is important for the reactor safety analysis results. SPACE, a safety analysis code for thermal hydraulic systems which is currently being developed, was designed to have the capacity of multi-dimensional two-phase thermo-dynamic phenomena induced in the various phases of a nuclear system. To validate the performance of SPACE, a two-dimensional two-phase flow test was performed with slab geometry of the test section having a scale of $1.43m{\times}1.43m{\times}0.11m$. The test section has three inlet and three outlet nozzles on the bottom and top gap walls, respectively, and two outlet nozzles installed directly on the surface of the slab. Various kinds of two-dimensional air/water flows were simulated by selecting combinations of the inlet and outlet nozzles. In this study, two-dimensional two-phase void fraction profiles were quantified by measuring the local gap impedance at 225 points. The flow conditions cover various flow regimes by controlling the flow rate at the inlet boundary. For each selected inlet and outlet nozzle combination, the water flow rate ranged from 2 to 20 kg/s, and the air flow rate ranged from 2.0 to 20 g/s, which corresponds to 0.4 to 4 m/s and 0.2 to 2.3 m/s of the superficial liquid and gas velocities based on the inlet port area, respectively.

• 한국전산유체공학회:학술대회논문집
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• 2010.05a
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• pp.365-367
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• 2010

In this paper, we presented the exact Riemann solver for the air-water two-phase shock tube problems where the strength of the propagated sock wave is moderately weak. The shock tube has a diaphragm in the middle which separates water medium in the left and air medium in the right. By rupturing the diaphragm, various waves such as rarefaction wave, shock wave and contact discontinuity are propagated into water and air. Both fluids are treated as compressible, with the linearized equations of state. We used the isentropic relations for the air and water assuming a weak shock wave. We solved the shock tube problem considering a high pressure in the water and a low pressure in the air. The numerical results cleary showed a left-traveling rarefaction wave in the water, a right-traveling shock wave in the air, and the right-traveling material interface.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.19 no.6
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• pp.441-446
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• 2007

An experimental study on the countercurrent two-phase flow in narrow rectangular channels has been peformed. Countercurrent flow limitation (CCFL) was investigated using air and water in 760mm long, 100mm wide, vertical test sections with 1 and 3mm channel gaps. Tests were systematically performed with downward liquid superficial velocities and upward gas velocities covering 0 to 0.125 and 0 to 3.5m/s ranges, respectively. As the gap width of rectangular channel increased the CCFL water superficial velocity decreased for the given air superficial velocity. Slight increase of the air superficial velocity resulted in the abrupt decrease of water velocity when $j_g=2{\sim}4m/s$. The critical superficial velocity of air, at which the downward flow of water was no longer allowed, also decreased with the increase of gap width. The experimental results were compared with the previous correlations, which were mainly for round tubes, and the qualitative trends were found to be partially acceptable. However the quantitative discrepancies were hardly neglected. New correlation of CCFL was developed and showed good agreement with the experimental data.

• Nuclear Engineering and Technology
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• v.37 no.6
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• pp.595-604
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• 2005

A new instrument, an average BDFT (Birectional Flow Tube), was proposed to measure the flow rate in single and two phase flows. Its working principle is similar to that of the Pilot tube, wherein the dynamic pressure is measured. In an average BDFT, the pressure measured at the front of the flow tube is equal to the total pressure, while that measured at the rear tube is slightly less than the static pressure of the flow field due to the suction effect downstream. The proposed instrument was tested in air/water vertical and horizontal test sections with an inner diameter of 0.08m. The tests were performed primarily in single phase water and air flow conditions to obtain the amplification factor(k) of the flow tube in the vertical and horizontal test sections. Tests were also performed in air/water vertical two phase flow conditions in which the flow regimes were bubbly, slug, and churn turbulent flows. In order to calculate the phasic mass flow rates from the measured differential pressure, the Chexal drift-flux correlation and a momentum exchange factor between the two phases were introduced. The test results show that the proposed instrument with a combination of the measured void fraction, Chexal drift-flux correlation, and Bosio & Malnes' momentum exchange model could predict the phasic mass flow rates within a $15\%$ error. A new momentum exchange model was also proposed from the present data and its implementation provides a $5\%$ improvement to the measured mass flow rate when compared to that with the Bosio & Malnes' model.

• Proceedings of the KSME Conference
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• 2007.05b
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• pp.2108-2112
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• 2007

The air and water flow distribution are experimentally studied for a round header-ten microchannel tube configuration. Three different inlet orientations (parallel, side, normal) were investigated. Tests were conducted with downward flow configuration for the mass flux from 70 to 130 kg/$m^2s$, quality from 0.2 to 0.6, non-dimensional protrusion depth (h/D) from 0.0 to 0.5. It is shown that, for almost all the test conditions, normal inlet yielded the best flow distribution, followed by side and parallel inlet. Possible reasoning is provided using flow visualization results.

• Journal of the Korean Society of Visualization
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• v.11 no.1
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• pp.34-40
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• 2013

Gas/liquid two-phase ejector is a device without moving parts, in which liquid is used to drive gas of a low-pressure source. In this paper, the hydrodynamic characteristics of a vertical down type two-phase ejector were studied using an air-water loop system. Entrained air flow rates were measured with inlet and outlet pressures of the ejector with varying water flow rate. Homogeneous bubbly flows in the discharge pipe were confirmed by the high speed flow visualization method. Quantitative measurements of void fraction were made using a newly developed fiber optic probe system.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.30 no.5 s.248
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• pp.489-495
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• 2006

Proton exchange membrane (PEM) should be sufficiently hydrated with a careful consideration of heat and water management. Water management has been a critical operation issue for better understanding the operation and optimizing the performance of a PEM fuel cell. The flooding on cathode side resulting from excess water can limit the fuel cell performance. In this study, the visual cell was designed and fabricated fur the visualization of liquid water droplet dynamics related to cathode flooding in flow channels. The experiment was carried out to observe the formation, growth and removal of water droplets using CCD imaging system. Effects of operating conditions such as cell temperature, air flow rate and air relative humidity on cathode flooding characteristics were mainly investigated. Based on this study, we can get the basic insight into flooding phenomena and its two-phase flow nature. It is expected that data obtained can be effectively used fur the setup and validation of two-phase PEM fuel cell models considering cathode flooding.

• Proceedings of the Korean Nuclear Society Conference
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• 2002.10a
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• pp.142-142
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• 2002

• Proceedings of the Korea Water Resources Association Conference
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• 2016.05a
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• pp.386-390
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• 2016

Water aeration is an effective water treatment process, which involves the injection of air or air-water mixture into water treatment reservoir commonly through pipes. The main purpose of water aeration is to maintain healthy levels of dissolved oxygen (DO), which is the most important water quality factor. The pipes' operating conditions are important for controlling the efficiency and effectiveness of aeration process. Many studies have been conducted on two-phase flows in pipes, however, there are a few studies to deal with small s ale in millimeter. The main objective of this study is to perform 2-dimensional two-phase simulations inside various straight pipes using the computational fluid dynamic (CFD) OpenFOAM (Open source Field Operation And Manipulation) tools to examine the influence of flow patterns on bubble size, which is closely related to DO concentration in a water body. The both flow regimes, laminar and turbulence, have been considered in this study. For turbulence, Reynolds-averaged Navier-Stokes (RANS) has been applied. The coalescence and breakage of bubbles caused by random collisions and turbulent eddies, respectively, are considered in this research. Sauter mean bubble diameter and water velocity are compared against experimental data. The simulation results are in good agreement with the experimental measurements.