• Journal of Energy Engineering
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• v.25 no.1
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• pp.69-75
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• 2016

Bubble size is a key parameter for an accurate prediction of bubble behaviours in the multi-dimensional two-phase flow. In the current STAR CCM+ CFD code, a mechanistic bubble size model $S{\gamma}$ is available for the prediction of bubble size in the flow channel. As another model, Yun model is developed based on DEBORA that is subcooled boiling data in high pressure. In this study, numerical simulation for the gas-liquid two-phase flow was conducted to validate and confirm the performance of $S{\gamma}$ model and Yun model, using the commercial CFD code STAR CCM+ ver. 10.02. For this, local bubble models was evaluated against the air-water data from DEDALE experiments (1995) and Hibiki et al. (2001) in the vertical pipe. All numerical results of $S{\gamma}$ model predicted reasonably the two-phase flow parameters and Yun model is needed to be improved for the prediction of air-water flow under low pressure condition.

• Journal of Korean Society of Environmental Engineers
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• v.35 no.10
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• pp.710-716
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• 2013

New and renewable energy sources have drawn attention because of climate change. Many studies have been carried out in waste-to-energy field. Fast pyrolysis of waste lignocelluosic biomass is one of the waste-to-energy technologies. Bubbling fluidized bed (BFB) reactor is widely used for fast pyrolysis of the biomass. In BFB pyrolyzer, bubble behavior influences on the chemical reaction. Accordingly, in the present study, hydrodynamic characteristics and fast pyrolysis reaction of waste lignocellulosic biomass occurring in a BFB pyrolyzer are scrutinized. The computational fluid dynamics (CFD) simulation of the fast pyrolysis reactor is carried out by using Eulerian-Granular approach. And two-stage semi-global kinetics is applied for modeling the fast pyrolysis reaction of waste lignocellulosic biomass. To summarize, generation and ascendant motion of bubbles in the bed affect particle behavior. Thus biomass particles are well mixed with hot sand and consequent rapid heat transfer occurs from sand to biomass particles. As a result, primary reaction is observed throughout the bed. And reaction rate of tar formation is the highest. Consequently, tar accounts for 66wt.% of the product gas. However, secondary reaction occurs mostly in the freeboard. Therefore, it is considered that bubble behavior and particle motions hardly influences on the secondary reaction.

• Proceedings of the Korean Nuclear Society Conference
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• 1997.10a
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• pp.487-494
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• 1997

비등이 발생하는 경사유로에서 이상유동 구조 파악을 위해 국부적 이상유동 변수의 측정이 이루어졌다. 국부 기포율, 국부 기포빈도, 국부기포속도 그리고 국부 기포크기와 같은 기상 관련 이상유동변수는 이중 전기전도도 탐침으로, 액상속도 분포는 Pilot tuba로 측정하였다. 유로의 경사도가 이상유동 구조에 미치는 영향을 파악하기 위해 실험은 수직, 수직으로부터 30도 경사도 및 60도 경사도에서 유량과 열유속을 변화시켜가며 수행하였으며 유동변수의 측정은 경사진 경우에 측면 방향으로의 대칭성을 고려하여 유로 반 단면내 총 91개 지점에서, 수직인 경우에는 13개 지점에서 이루어졌다. 유동조건은 1.4m/s 이하의 평균 액상 겉보기 속도에 제한되었고 유로내 압력은 대기압이며 유동양식은 미포화 비등에 국한되었다. 측정된 이상유동 변수의 분포를 이용하여 경사유로에 적용할 수 있는 distribution parameter와 drift velocity 같은 dirft flux parameter에 대한 상관식이 개발되었다.

• Journal of the Korean Society of Propulsion Engineers
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• v.15 no.4
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• pp.65-72
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• 2011

Numerical simulation has been performed to investigate the bubbly flow in the liquid with vertical temperature gradient. The objective of this study is to establish an accurate numerical prediction program of gas-liquid two-phase flows in a vertical temperature gradient condition, whose mathematical model is formulated by the Eulerian-Lagrangian model. The present numerical results reveal the temperature mixing mechanism and the fluid dynamical characteristics induced by the bubbly flow in the liquid with stratified temperature. The effects of bubble radius, void fraction, and gas flow rate on bubble-driven convective flow are considered, too.

• Proceedings of the Korean Nuclear Society Conference
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• 1998.05a
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• pp.457-462
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• 1998

유동장에서의 기포거동 정보의 중요성 때문에 이를 정확히 측정하기 위한 실험방법이 여러 가지로 발전해 왔지만 아직까지도 기포분포에 대한 정확한 정보 추출에는 도달하지 못하고 있다. 본 연구에서는 원래 의공학분야에서 새로운 tomography 기술로 연구되고 있는 EIT(Electrical Impedance Tomography) 기술을 2상유동에서의 기포분포 측정방법 개발에 적용하기 위한 기초연구와 기포분포 가시화를 위한 전산실험을 수행하였다. 기포분포 가시화를 위해서는 EIT inverse problem solver로 많이 사용되는 iNR(improved Newton-Raphson) 계열의 EIT 염상복원 프로그램을 본 연구진이 유전알고리즘(Genetic Algorithm)과 fuzzy-based mesh grouping 방법을 추가하여 개선한 영상복원프로그램을 사용하였다. 전산실험 결과 본 영상복원프로그램으로는 12$\times$12의 분해능으로 모사되는 기포분포를 저항률 오차한도 $\pm$1%의 신뢰도로 PC상에서 복원이 가능함을 확인하였다.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.34 no.5
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• pp.471-479
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• 2010

The adiabatic two-phase flow in single rectangular microchannels was studied for different aspect ratios. The working fluids were liquid water and nitrogen gas. The hydraulic diameters of the rectangular microchannels were 490, 322, and $143\;{\mu}m$, and the widths of the microchannels were around $500\;{\mu}m$. The two-phase flow pattern was visualized using a high-speed camera and a long-distance microscope. This study was focused on bubble flow regimes. From the visualized images, the bubble velocity, bubble length, number of bubbles, and void fraction were evaluated. Further, the pressure drop in a single bubble was evaluated by using a unit cell model. The bubble velocity is proportional to the superficial velocity. Further, the relationship between the void fraction and the volumetric quality is linear. The pressure drop in a single elongated bubble is strongly related to the aspect ratio. Finally, the new correlation about the pressure drop of a single elongated bubble in the rectangular microchannel was proposed.

• Proceedings of the Korean Nuclear Society Conference
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• 1998.05a
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• pp.469-474
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• 1998

이상 유동 지배 방정식에 두 상의 경계면 두께 개념을 도입하여 액체 내부에서의 기포 거동에 대한 동적 안정성을 이론적으로 해석하였다. 현재까지 기포의 안정성은 실험과 거시적인 물리적 이론에 의하여만 정적으로 연구되어 왔으나, 상간의 압력 불연속성을 표면장력 모델링을 이용하여 해석적으로 해결 함으로서 기포 안정성에 대한 정량적인 분석을 가능하게 하였다. 그 결과 기포의 안정성은 기존의 무차윈 Weber수 이외에 Reynolds수 그리고 유체의 점성력과 Capillary파와 관련된 무차원 수에 의해서도 영향을 받는 것으로 밝혀졌다.

• Journal of the Korean Society for Nondestructive Testing
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• v.22 no.5
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• pp.545-556
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• 2002

Bubble behaviors in two-phase flows have been analyzed by tomography methods such as an algebraic reconstruction technique (ART) and a multiplicative algebraic reconstruction technique (MART). Initially, a bubbly flow and an annular flow have been investigated by cross-sectional view using computer synthesized phantoms. Two tomography methods have been compared to obtain more accurate results of the two-phase flows. Then, reconstruction of three-dimensional density distributions of phantoms with two and three bubbles have been accomplished by the MART method which provided the better results for the two-dimensional reconstructions accurately to analyze the bubble behaviors in the two-phase flow.

• Journal of Advanced Marine Engineering and Technology
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• v.18 no.5
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• pp.24-28
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• 1994

In the present paper, the behavior of bubbles in a fluidized bed has been investigated experimentally. The bubble size, distribution of bubble, bubble rising velocity and pressure fluctuation in the fluidized bed are obtained at different air velocity. The results are discussed and compared study the effect of air velocity on the behavior of a bubbles in fluidized bed.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.39 no.8
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• pp.633-643
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• 2015

In this study, we develop a highly efficient conical-air diffuser that generates fine bubble. By inserting a sufficient number of aerotropic microorganisms with dissolved oxygen from an air diffuser and minimizing the air-channel blockages within the air diffuser, we expect to improve the efficiency and durability of the decomposition process for organic waste. To upgrade the conventional air diffuser, we perform experiments and numerical analysis to develop a conical-type that generates fine bubble, and which is free from nozzle blockage. We complement the air-diffuser design by numerically analyzing the internal air-flow pattern within the diffuser. Then, by applying the diffuser to a mockup bioreactor, we experimentally and numerically study the bubble behavior observed in the diffuser and the 2-phase fluid flow in the bioreactor. The results obtained include statistics of the cord length and increased velocity, and we investigate the mechanisms of the fluid-flow characteristics including bubble clouds. Throughout the study, we systemize the design procedures for the design of efficient air diffusers, and we visualize the fluid-flow patterns caused by bubble generation within the mockup bioreactor. These results will provide a meaningful basis for further study as well as the detection of oxygen transfer and fluid-flow characteristics in real-scale bio-reactors using sets of air diffusers.