• Nuclear Engineering and Technology
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• v.56 no.2
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• pp.624-635
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• 2024

In two-fluid simulations of flow boiling, the modeling of the mean bubble diameter is a key parameter in the closure relations governing the intefacial transfer of mass, momentum, and energy. Monodispersed approach proved to be insufficient to describe the significant variation in bubble size during flow boiling in a heated pipe. A population balance model (PBM) has been employed to address these shortcomings. During nucleate boiling, vapor bubbles of a certain size are formed on the heated wall, detach and migrate into the bulk flow. These bubbles then grow, shrink or disintegrate by evaporation, condensation, breakage and aggregation. In this study, a parametric analysis of the PBM aggregation and breakage models has been performed to investigate their effect on the radial distribution of the mean bubble diameter and vapor volume fraction. The simulation results are compared with the DEBORA experiments (Garnier et al., 2001). In addition, the influence of PBM parameters on the local distribution of individual bubble size groups was also studied. The results have shown that the modeling of aggregation process has the largest influence on the results and is mainly dictated by the collisions due to flow turbulence.

• Journal of Korean Society of Water and Wastewater
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• v.25 no.4
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• pp.471-478
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• 2011

A series of laboratory experiments carried out to investigate the particle separation efficiency and flotation characteristics using $CO_2$ bubbles. The primary objective of this study was to find out the feasibility of $CO_2$ bubbles as an applicable unit of flotation process in tap-water and wastewater treatment plant. The fundamental measurements were conducted to characterize the $CO_2$ bubble from the physical viewpoint in water including bubble size distribution and rising velocity under various operational conditions. In addition, the removal efficiency of solid was experimented using the lab scale plant applied $CO_2$ bubbles, namely the dissolved carbon dioxide flotation (DCF) process. The DCF process using carbon dioxide bubble, which is an advantage as the decrease and the reuse of Green-House gas, can be a promising technology as an water treatment process. On the other hand, the further research to decrease the bubble size distribution of $CO_2$ is required to enhance the particle separation efficiency.

• Journal of the Korean Society of Manufacturing Process Engineers
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• v.17 no.1
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• pp.82-88
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• 2018

An experimental study was performed on the orifice nozzle system that generates micro-bubbles by air self-suction using a venturi nozzle. This study experimentally investigates the amount of air sucked into the venturi nozzle and the number of micro-bubbles generated by the orifice nozzle system in Cases 1 and 2. The experimental conditions were varied by changing the diameter of the orifice nozzle (d=2~7 mm) and the number of holes of the perforated plate nozzle (n = 2-12). In Case 1, the air self-suction was more than 2 LPM at $d{\leq}4mm$. When d = 4 mm, the total number of bubbles was 29,777, and it was confirmed that micro-bubbles occupied approximately 65% of the total number of bubbles. In Case 2, the air self-suction was maintained constant at approximately 2.5 LPM regardless of the number (n) of holes. The total amount of bubbles increased when n increased but remained constant at approximately 44,000 when $n{\geq}7EA$. It was also confirmed that more than 80% of all bubbles were micro-bubbles when $n{\geq}10EA$. Thus, the number of micro-bubbles increased by approximately 15% compared to the experimental result of Case 1, which was optimized with d = 4 mm.

• The Journal of the Acoustical Society of Korea
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• v.27 no.3E
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• pp.69-76
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• 2008

Measurements of bubble size and distribution in the surface layer of the sea, wind speed, and variation of ocean environments were made continually over a four-day period in an experiment conducted in the South Sea of Korea during 17-20 September 2007. Theoretical background of bubble population model indicates that bubble population is a function of the depth, range and wind speed and bubble effects on sound speed shows that sound speed varies with frequency. Observational evidence exhibited that the middle size bubble population fit the model very well, however, smaller ones can not follow the model probably due to their short lifetime. Meanwhile, there is also a hysteresis effect of void fraction. Observational evidence also indicates that strong changes in sound speed are produced by the presence of swarms of micro bubbles especially from 7 kHz to 50 kHz, and calculation results are consistent with the measured data in the high frequency band, but inconsistent in the low frequency band. Based on the measurements of the sound speed and high frequency transmission configuration in the bubble layer, we present an estimation of underwater acoustic channel capacity in the bubble layer.

• Nuclear Engineering and Technology
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• v.51 no.6
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• pp.1575-1588
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• 2019

Fracture near the U-10Mo/cladding material interface impacts fuel service life. In this work, a mesoscale stress model is developed with the fuel foil considered as a porous medium having gas bubbles and bearing bubble pressure and surface tension. The models for the evolution of bubble volume fraction, size and internal pressure are also obtained. For a U-10Mo/Al monolithic fuel plate under location-dependent irradiation, the finite element simulation of the thermo-mechanical coupling behavior is implemented to obtain the bubble distribution and evolution behavior together with their effects on the mesoscale stresses. The numerical simulation results indicate that higher macroscale tensile stresses appear close to the locations with the maximum increments of fuel foil thickness, which is intensively related to irradiation creep deformations. The maximum mesoscale tensile stress is more than 2 times of the macroscale one on the irradiation time of 98 days, which results from the contributions of considerable volume fraction and internal pressure of bubbles. This study lays a foundation for the fracture mechanism analysis and development of a fracture criterion for U-10Mo monolithic fuels.

• Journal of Environmental Science International
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• v.26 no.6
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• pp.711-717
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• 2017

Dielectric Barrier Discharge (DBD) plasma is a new technique for use in environmental pollutant degradation, which is characterized by the production of hydroxyl radicals as the primary degradation species. Due to the short lifetime of the chemically active species generated during the plasma reaction, the dissolution of the plasma gas has a significant effect on the reaction performance. The plasma reaction performance can be enhanced by combining the basic plasma reactor with a homogenizer system in which the bubbles are destroyed and turned into micro-bubbles. For this purpose, the improvement of the dissolution of plasma gas was evaluated by measuring the RNO (N-dimethyl-4-nitrosoaniline, an indicator of the generation of OH radicals). Experiments were conducted to evaluate the effects of the diameter, rotation speed, and height of the homogenizer, pore size, and number of the diffuser and the applied voltage on the plasma reaction. The results showed that the RNO removal efficiency of the plasma reactor combined with a homogenizer is two times higher than that of the conventional one. The optimum rotor size and rotation speed of the homogenizer were 15.1 mm, and 19,700 rpm, respectively. Except for the lowest pore size distribution of $10-16{\mu}m$, the pore size of the diffuser showed little effect on RNO removal.

• International Journal of Fluid Machinery and Systems
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• v.8 no.2
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• pp.73-84
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• 2015

Aerating hydroturbines have recently been proposed as an effective way to mitigate the problem of low dissolved oxygen in the discharge of hydroelectric power plants. The design of such a hydroturbine requires a precise understanding of the dependence of the generated bubble size distribution upon the operating conditions (viz. liquid velocity, air ventilation rate, hydrofoil configuration, etc.) and the consequent rise in dissolved oxygen in the downstream water. The purpose of the current research is to investigate the effect of location of air injection on the resulting bubble size distribution, thus leading to a quantitative analysis of aeration statistics and capabilities for two turbine blade hydrofoil designs. The two blade designs differed in their location of air injection. Extensive sets of experiments were conducted by varying the liquid velocity, aeration rate and the hydrofoil angle of attack, to characterize the resulting bubble size distribution. Using a shadow imaging technique to capture the bubble images in the wake and an in-house developed image analysis algorithm, it was found that the hydrofoil with leading edge ventilation produced smaller size bubbles as compared to the hydrofoil being ventilated at the trailing edge.

• Korean Chemical Engineering Research
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• v.43 no.2
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• pp.272-277
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• 2005

Characteristics of bubbling behavior and bubble properties were investigated in a gas-liquid countercurrent bubble column of in diameter 0.152 m and 3.5 m in height, respectively. Effects of gas and liquid velocities and bubble distribution mode(even, wall-side, central or asymmetric distribution) on the bubble properties such as chord length, frequency, rising velocity and holdup in the reactor were measured and examined by means of dual resistivity probe method. The bubble size, frequency and holdup increased with increasing gas($U_G$) or liquid velocity($U_L$). The rising velocity of bubbles increased with increasing $U_G$, whereas decreased with increasing $U_L$. The uniformity of bubble size distribution and bubble holdup decreased when the distribution mode of bubbles at the gas distributor was changed from even to wall-side, central or asymmetric. The central distribution of bubbles was better than asymmetric mode but worse than wall-side distribution, in considering the bubble holdup and uniformity of distribution.

• Membrane and Water Treatment
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• v.10 no.4
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• pp.307-311
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• 2019

The ejector type microbubble generator, which is the method to supply air to water by using cavitation in the nozzle, does not require any air supplier so it is an effective and economical. Also, the distribution of the size of bubbles is diverse. Especially, the size of bubbles is smaller than the bubbles from a conventional air diffuser and bigger than the bubbles from a pressurized dissolution type microbubble generator so it could be applied to the aeration tank for wastewater treatment. However, the performance of the ejector type microbubble generator was affected by hydraulic pressure and MLSS(Mixed Liquor Suspended Solid) concentration so many factors should be considered to apply the generator to aeration tank. Therefore, this study was performed to verify effects of hydraulic pressure and MLSS concentration on oxygen transfer of the ejector type microbubble generator. In the tests, the quantity of sucked air in the nozzle, dissolved oxygen(DO) concentration, oxygen uptake rate(OUR), oxygen transfer coefficient were measured and calculated by using experimental results. In case of the MLSS, the experiments were performed in the condition of MLSS concentration of 0, 2,000, 4,000, 8,000 mg/L. The hydraulic pressure was considered up to $2.0mH_2O$. In the results of experiments, oxygen transfer coefficient was decreased with the increase of MLSS concentration and hydraulic pressure due to the increased viscosity and density of wastewater and decreased air flow rate. Also, by using statistical analysis, when the ejector type microbubble generator was used to supply air to wasterwater, the model equation of DO concentration was suggested to predict DO concentration in wastewater.

• Journal of the Korean Society of Visualization
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• v.17 no.3
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• pp.24-31
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• 2019

Recently, an air-lift bio-reactor operated by micro bubbles has been utilized to product hydrogen fuel. To enhance the performance, characteristics of hydrodynamics inside the bio-reactor were analyzed using a numerical simulation for two-phase flow. An Eulerian model was employed for both of liquid and gas phases. The standard k-ε model was used for turbulence induced by micro bubbles. A Population Balance Model was employed to consider size distribution of bubbles. A hollow cylinder was introduced at the center of the reactor to reduce a dead area which disturbs circulation of CO bubbles. An appropriate diameter of the draft tube and hollow cylinder were optimized for better performance of the bio-reactor. The optimum model could be obtained when the cross-sectional area ratio of the hollow cylinder to the reactor, and the width ratio of the riser to the downcomer approached 0.4 and 3.5, respectively. Consequently, it is expected that the optimum model could enhance the performance of the bio-reactor with the homogeneous distribution and higher density of CO, and more effective mixing.