• Nuclear Engineering and Technology
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• v.44 no.6
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• pp.623-638
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• 2012

The volume of fluid (VOF) model of FLUENT and the lattice Boltzmann method (LBM) are used to simulate two-phase flows. Both methods are validated for static and dynamic bubble test cases and then compared to experimental results. The VOF method does not reduce the spurious currents of the static droplet test and does not satisfy the Laplace law for small droplets at the acceptable level, as compared with the LBM. For single bubble flows, simulations are executed for various Eotvos numbers, Morton numbers and Reynolds numbers, and the results of both methods agree well with the experiments in the case of low Eotvos numbers. For high Eotvos numbers, the VOF results deviated from the experiments. For multiple bubbles, the bubble flow characteristics are related by the wake of the leading bubble. The coaxial and oblique coalescence of the bubbles are simulated successfully and the subsequent results are presented. In conclusion, the LBM performs better than the VOF method.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2008.06a
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• pp.127-128
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• 2008

• International Journal of Naval Architecture and Ocean Engineering
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• v.13 no.1
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• pp.566-574
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• 2021

The process of cavitation involves generation, growth, coalescence, and collapse of small bubbles and is tremendously influenced by bubble-bubble interactions. To understand these interactions, a new cavitation model based on the transport equation is proposed herein. The modified Rayleigh-Plesset equation is analyzed to determine the bubble growth rate by assuming equal-sized spherical bubble clouds. The source term in the transport equation is then derived according to the bubble growth rate with the bubble-bubble interaction. The proposed model is validated by various test simulations, including microscopic bubble cloud evolution as well as macroscopical two- and three-dimensional cavitating flows. Compared with previous models, namely the Kunz and Zwart cavitation models, the newly proposed model does not require adjustable parameters and generally results in better predictions both microscopic and macroscopical cases. This model is more physical.

• Proceedings of the KSME Conference
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• 2008.11a
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• pp.1934-1938
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• 2008

Formation of air bubble is the one of common defects in UV nano imprint lithography. Location of dispensing and volume of droplets are among the most important parameters in the process. ]n this study, UV curable resin droplets with different volumes were dispensed at different locations and pressed to investigate air bubble formation. By varying volume of droplet and dispensing location, process conditions were found for minimum air bubble area.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.14 no.6
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• pp.1365-1373
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• 1994

A Monte-Carlo simulation model, developed to predict size distribution of air bubbles in turbulent shear flow, is applied to a laboratory-scale problem. Sensitivity to various numerical and physical parameters of the model is analyzed. Practical applicability of the model is explored through comparisons of results with experimental measurements. Bubble size increases with air-water discharge ratio and friction factor. Bubble size decreases with increasing mean flow velocity, but the total bubble surface area in the aeration region remains fairly constant. The effect on bubble size distribution of the longitudinal length increment in the simulation model is negligible. A larger radial length increment yields more small and large bubbles and fewer in between. Bubble size distribution is significantly affected by its initial distribution and the location of air injection. Collision efficiency is introduced to explain the discrepancy between collisions with and without coalescence.

• The KSFM Journal of Fluid Machinery
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• v.13 no.4
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• pp.31-36
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• 2010

Free energy based lattice Boltzmann method (LBM) has been used to simulate the rising bubble flows with large density ratio. LBM with compact discretization is able to reduce the spurious current of the static bubble test and be satisfied with the Laplace law. The terminal rise velocity and shape of the bubbles are dependent on Eotvos number, Morton number and Reynolds number. For single bubble flows, simulations are executed for various Eotvos number, Morton number and Reynolds number, and the results are agreed well with the experiments. For multiple bubbles, the bubble flow characteristics are related by the vortex pattern of the leading bubble. The coalescence of the bubbles are simulated successfully and the subsequent results are presented. The present method is validated for static, dynamic bubble test cases and compared to the numerical, experimental results.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.31 no.3 s.258
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• pp.248-255
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• 2007

Thermal bubble formation is a fundamental process in nucleate boiling heat transfer and many microelectromechanical thermal systems. One of the established facts is that heterogeneous nucleation is originated from vapors trapped inside cavities. Based on this, we performed an experimental study on the formation of thermal bubbles from microcavity fabricated by microfabrication technology on a copper plate. The cavity was filled with aluminum particles to enhance thermal bubble formation. We observed the thermal bubble behaviors, such as bubble incipience, diameter, frequency and coalescence during nucleate boiling. The experimental data showed that the superheat required to trigger the bubble formation was significantly reduced when the cavity was filled with microparticles. We found that the initial increase of superheat led to the increase of both the departure diameter and frequency while the further increase of superheat caused multiple bubbles to coalesce resulting in the decrease of departure frequency.

• Environmental Engineering Research
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• v.24 no.3
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• pp.382-388
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• 2019

Although nanobubbles attract significant attention, their characteristics and applications have not been thoroughly defined. There are diverse opinions about the definition of nanobubbles and controversy regarding methods that verify their characteristics. This study defines nanobubbles as having a size less than $1{\mu}m$. The generation of these sub-micron (nano) bubbles may be verified by induced coalescence or light scattering. The size of a sub-micron (nano) bubbles may be measured by optical, and confocal laser scanning microscopy. Also, the size may be estimated by the relationship of bubble size with the dissolved oxygen concentration. However, further research is required to accurately define the average bubble size. The zeta potential of sub-micron (nano) bubbles decreases as pH increases, and this trend is consistent for micron bubbles. When the bubble size is reduced to about 700-900 nm, they become stationary in water and lose buoyancy. This characteristic means that measuring the concentration of sub-micron (nano) bubbles by volume may be possible by irradiating them with ultrasonic waves, causing them to merge into micron bubbles. As mass transfer is a function of surface area and rising velocity, this strongly indicates that the application of sub-micron (nano) bubbles may significantly increase mass transfer rates in advanced oxidation and aeration processes.

• 한국신재생에너지학회:학술대회논문집
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• 2010.06a
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• pp.112.2-112.2
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• 2010

슬러리 기포탑 반응기는 열 및 물질 전달의 용이성, 낮은 운전비용 및 장치의 간단성의 장점을 가지고 있어서 Fischer-Tropsch 반응, bio-reaction 등에 많이 응용되고 있다. 그러나 기포탑 반응기 내의 물질 거동은 매우 복잡하기 때문에 많은 연구가 이루어지고 있음에도 불구하고 그 현상에 대한 명확한 이해는 어려운 상황이다. 특히 기포탑반응기내에 기체의 포집율(gas hold-up)을 증가시키는 것을 목적으로 하는 연구들이 활발히 진행되고 있다. 본 연구에서는 기체의 분사 방향에 따른 기체 포집율의 변화를 관찰하였다. 기체 분사는 0.6 mm의 pore가 66개로 구성된 perforated plate를 통해서 이루어졌고, 수직방향, 수평방향, 45도 그리고 수직/수평 조합의 네 가지 분사방향에 대해서 실험을 수행하였다. 반응기는 내경이 0.15 m이고 높이 2.0 m 아크릴 반응기를 이용하였다. 사용된 연속상은 수돗물을 사용하였고 분산상 기체로는 압축 공기를 이용하였다. 전체적인 기체 포집율은 수직방향의 분사방향에서 가장 높게 측정되었다. 그리고 수직/수평의 조합 분사방향의 경우, 기체 포집율이 가장 낮게 관찰되었다. 이것은 분사방향이 수직/수평으로 서로 엇갈릴 경우, 기포간의 충돌 가능성이 높아지고 bubble coalescence가 증가하였기 때문인 것으로 보인다. 실제로 homogeneous flow regime에서 heterogeneous flow regime으로 전환되는 기체선속도는 분사방향이 수직, 45도, 수평, 수직/수평 조합의 순서로 감소하였다. 즉 이 순서로 기체흐름의 와류가 증가하는 것을 알 수 있었다. 또한 Dynamic Gas Disengagement(DGD) 분석을 통하여 큰 기포가 발생하기 시작하는 기체 선속도의 변화를 관찰하였다. 이 경우, 예상되듯이 수직/수평 조합에서는 1.5 cm/sec 기체 선속도에서 큰 기포가 발생하기 시작한 반면 수직 방향 분사의 경우에는 2.5 cm/sec의 보다 높은 기체 선속도에서 관찰되기 시작하였다. 이러한 현상들을 종합하였을 때, 기체 분사방향을 수직으로 일정하게 했을 때, 기포간 출동을 최소화하고 와류발생을 최대한 지연시키며 전체 기체 포집율을 증가시킬 수 있음을 알 수 있다.

• Transactions of the Korean Society of Mechanical Engineers
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• v.16 no.4
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• pp.775-786
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• 1992

Studies are made of the turbulent separation bubble in a two-dimensional semi-infinite blunt plate aligned to a uniform free stream when the oncoming free stream contains a pulsating component. The discrete-vortex method is applied to simulate this flow situations because this approach is effective to represent the unsteady motions of turbulent shear layer and the effect of viscosity near the solid surface. The two key external paramenters in the free stream, i.e., the amplitude of pulsation, A, and the frequency parameter St[=fH/ $U_{1}$], are dealt with in the present numerical computations, A particular frequency gives a minimum reattachment which is related to the drag reduction and the most effective frequency is dependent on the most amplified shedding frequency. The turbulent flow structure is scrutinized. A comparison between the unperturbed flow and the perturbed at the particular frequency of the minimum reattachment length of the separation bubble suggests that the large-scale structure is associated with the shedding frequency and the flow instabilities.