• Nuclear Engineering and Technology
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• v.55 no.3
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• pp.1105-1117
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• 2023

In this study, the pressure drop behavior of single- and two-phase flows of air and water through the porous beds filled with uniform and non-uniform sized spherical particles was examined. The pressure drop data in the single-phase flow experiments for the uniform particle beds agreed well with the original Ergun correlation. The results from the two-phase flow experiments were analyzed using numerical results based on three types of previous models. In the experiments for the uniform particle beds, the data on the two-phase pressure drop clearly showed the effect of the flow regime transition with a variation in the gas flow rate under stagnant liquid condition. The numerical analyses indicated that the predictability of the previous models for the experimental data relied mainly on the sub-models of the flow regime transitions and interfacial drag. In the experiments for the non-uniform particle beds, the two-phase pressure loss could be predicted well with numerical calculations based on the effective particle diameter. However, the previous models failed to accurately predict the counter-current flooding limit observed in the experiments. Finally, we propose a relation of falling liquid velocity into the particle bed by gravity to appropriately simulate the CCFL phenomenon.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.41 no.3
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• pp.171-181
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• 2017

Two-phase cross flow exists in many shell-and-tube heat exchangers such as condensers, evaporators, and nuclear steam generators. The drag force acting on a tube bundle subjected to air/water flow is evaluated experimentally. The cylinders subjected to two-phase flow are arranged in a normal square array. The ratio of pitch to diameter is 1.35, and the diameter of the cylinder is 18 mm. The drag force along the flow direction on the tube bundles is measured to calculate the drag coefficient and the two-phase damping ratio. The two-phase damping ratios, given by the analytical model for a homogeneous two-phase flow, are compared with experimental results. The correlation factor between the frictional pressure drop and the hydraulic drag coefficient is determined from the experimental results. The factor is used to calculate the drag force analytically. It is found that with an increase in the mass flux, the drag force, and the drag coefficients are close to the results given by the homogeneous model. The result shows that the damping ratio can be calculated using the homogeneous model for bubbly flow of sufficiently large mass flux.

• Nuclear Engineering and Technology
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• v.54 no.2
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• pp.546-553
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• 2022

Narrow rectangular channels are employed in nuclear research reactors that use plate-type nuclear fuels, high heat-flux compact heat exchangers, and high-performance micro-electronics cooling systems. Two-phase flow in narrow rectangular channels is important, and it needs to be better understood because it is considerably different than that in round tubes. In this study, mechanistic models were developed for the flow regime transition criteria for various flow regimes in co-current air-water two-phase flow for vertical downward flow inside a narrow rectangular channel. The newly developed criteria were compared to a flow regime map of downward air-water two-phase flow inside a narrow rectangular channel with a 2.35-mm gap width under ambient temperature and pressure conditions. Overall, the proposed model showed good agreement with the experimental data.

• Nuclear Engineering and Technology
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• v.37 no.6
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• pp.525-536
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• 2005

The interfacial area transport equation dynamically models the changes in interfacial structures along the flow field by mechanistically modeling the creation and destruction of dispersed phase. Hence, when employed in the numerical thermal-hydraulic system analysis codes, it eliminates artificial bifurcations stemming from the use of the static flow regime transition criteria. Accounting for the substantial differences in the transport mechanism for various sizes of bubbles, the transport equation is formulated for two characteristic groups of bubbles. The group 1 equation describes the transport of small-dispersed bubbles, whereas the group 2 equation describes the transport of large cap, slug or chum-turbulent bubbles. To evaluate the feasibility and reliability of interfacial area transport equation available at present, it is benchmarked by an extensive database established in various two-phase flow configurations spanning from bubbly to chum-turbulent flow regimes. The geometrical effect in interfacial area transport is examined by the data acquired in vertical fir-water two-phase flow through round pipes of various sizes and a confined flow duct, and by those acquired In vertical co-current downward air-water two-phase flow through round pipes of two different sizes.

• Journal of the Korean Applied Science and Technology
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• v.27 no.4
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• pp.415-420
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• 2010

This study is focused on the channel design of bipolar plate in the electrode of hydrogen gas generator. The characteristics of hydrogen gas generation was studied in view of efficiency of hydrogen gas generation rate and a tendency of gas flow through the riv design of electrode. Since the flow rate and flow pattern of generated gas in the two phase flow system are the most crucial in determining the efficiency of hydrogen gas generator, we adopted the commercial analytical program of COMSOL MultiphysicsTM to calculate the theoretical flow rate of hydrogen gas from the outlet of gas generator and flow pattern of two phase fluid in the electrode. In this study, liquid electrolyte flows into the bipolar plate and decomposed into gas phase, two phase flow simulation is applied to measure the efficiency of hydrogen gas generation.

• Proceedings of the KSME Conference
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• 2000.04b
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• pp.71-76
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• 2000

This experimental study was conducted to figure out the drag reduction and convective heat transfer in vertical downward two-phase flow with polymer additives. The drag reduction effect were analyzed by using the difference of the pressure drop between the flow with polymer additives and without it. Experimental results show that the pressure drop with polymer additives is less than the pressure drop without polymer in vertical downward two-phase flow. And the convective heat transfer has decreased with increasing the polymer concentration in vertical downward two-phase flow.

• 한국전산유체공학회:학술대회논문집
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• 2003.08a
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• pp.43-48
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• 2003

When the Eulerian-Lagrangian method is used to analyze the particle laden two-phase flow, a large number of particles should be used to obtain statistically meaningful solutions. Then it takes too much time to track the particles and to average the particle properties in the numerical analysis of two-phase flow. The purpose of this paper is to reduce the computation time by means of a set of particle gird separate to the flow grid. Particle motion equation here is the simplified B-B-O equation, which is integrated to get the particle trajectories. Particle turbulent dispersion, wall collision, and wall roughness effects are considered but the two-way coupling effects between gas and particles are neglected. Particle laden 2-D channel flow is solved and it is shown that the computational efficiency is indeed improved by using the current method

• Transactions of the Korean Society of Automotive Engineers
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• v.4 no.3
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• pp.176-186
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• 1996

The phenomena of drag reduction using small quantities of a liner macromolecules has attracted the attention of many experimental investigations. On the other hand drag reduction in two phase flow can be applied to the transport of crude oil, phase change system such as chemical reactor, pool and boiling flow, and to flow with cavitation which occurs pump impellers. But the research on dragreduction in two phase flow is not sufficient. The purpose of the present work is to evaluate the drag reduction by measuring pressure drop, void fraction, mean liquid velocity and turbulent intensity whether polymer additives a horizontal single and two phase system or not. Flow pattern of air-water two phase flow was classified by electrical conductivity probe signal. Velocities and turbulent intensities of signal were measured simultaneously with a Hot-film anemometer.

• Korean Journal of Materials Research
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• v.24 no.8
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• pp.429-433
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• 2014

As the fabrication technology used in FPDs(flat-panel displays) advances, the size of these panels is increasing and the pattern size is decreasing to the um range. Accordingly, a cleaning process during the FPD fabrication process is becoming more important to prevent yield reductions. The purpose of this study is to develop a FPD cleaning system and a cleaning process using a two-phase flow. The FPD cleaning system consists of two parts, one being a cleaning part which includes a two-phase flow nozzle, and the other being a drying part which includes an air-knife and a halogen lamp. To evaluate the particle removal efficiency by means of two-phase flow cleaning, silica particles $1.5{\mu}m$ in size were contaminated onto a six-inch silicon wafer and a four-inch glass wafer. We conducted cleaning processes under various conditions, i.e., DI water and nitrogen gas at different pressures, using a two-phase-flow nozzle with a gap distance between the nozzle and the substrate. The drying efficiency was also tested using the air-knife with a change in the gap distance between the air-knife and the substrate to remove the DI water which remained on the substrate after the two-phase-flow cleaning process. We obtained high efficiency in terms of particle removal as well as good drying efficiency through the optimized conditions of the two-phase-flow cleaning and air-knife processes.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.37 no.1
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• pp.9-18
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• 2013

Two-phase vapor-liquid flows exist in many shell and tube heat exchangers such as condensers, evaporators, and nuclear steam generators. To understand the fluid dynamic forces acting on a structure subjected to a two-phase flow, it is essential to obtain detailed information about the characteristics of a two-phase flow. The characteristics of a two-phase flow and the flow parameters were introduced, and then, an experiment was performed to evaluate the pressure loss in the tube bundles and the fluid-dynamic force acting on the cylinder owing to the pressure distribution. A two-phase flow was pre-mixed at the entrance of the test section, and the experiments were undertaken using a normal triangular array of cylinders subjected to a two-phase cross-flow. The pressure loss along the flow direction in the tube bundles was measured to calculate the two-phase friction multiplier, and the multiplier was compared with the analytical value. Furthermore, the circular distributions of the pressure on the cylinders were measured. Based on the distribution and the fundamental theory of two-phase flow, the effects of the void fraction and mass flux per unit area on the pressure coefficient and the drag coefficient were evaluated. The drag coefficient was calculated by integrating the measured pressure on the tube by a numerical method. It was found that for low mass fluxes, the measured two-phase friction multipliers agree well with the analytical results, and good agreement for the effect of the void fraction on the drag coefficients, as calculated by the measured pressure distributions, is shown qualitatively, as compared to the existing experimental results.