• Proceedings of the KSME Conference
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• 2002.08a
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• pp.291-294
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• 2002

This study describes the external spray characteristics of deflector nozzle such as the breakup procedures of liquid sheet, spray angle, discharge coefficient and bubble behaviors of spray and SMD at deflector nozzle. In order to visualize the spray behaviors shadow graphy technique were used. According to the increase of injection pressure, development of the spray passes through the dribbling, distoted jet, closed bubble due to the contraction by surface tension forces, the bubble opens into hollow tulip shape, and the curved surface straightened to form a conical sheet like as the simplex swirl atomizer. Spray cone angle was nearly 90 deg. Variations of SMD were examined in order to describe the dependency of SMD on the injection pressure and orifice diameter. The shape of deflector and oriffice diameter had an effect on the discharge coefficient.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.25 no.5
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• pp.246-254
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• 2013

Some characteristics of nitrogen-water slug flow were optically measured, in vertical acrylic tubes of 2, 5 and 8 mm diameter. Bubble velocity, bubble and unit cell lengths were measured, by analyzing the light intensity signals from two sets of dot laser-infrared sensor modules mounted along the transparent tubes. Optical images of the bubbles were also taken and analyzed, to measure bubble shapes and liquid film thickness. It was found that the measured bubble velocities were in good agreement with the empirical models in the literature, except for those measured under high superficial velocity condition in the 2 mm tube. Bubble length was found to be the longest in the 2 mm tube, being 4 to 5 times those of the other tubes. Liquid film was found to have developed early in the 2 mm tube, which made the blunt shape of the bubble head. Liquid film thickness in the 8 mm tube was measured at almost twice those of the other tubes.

• 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.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.28 no.5
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• pp.326-331
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• 2015

In this paper, we study the insulating properties of the liquid nitrogen(LN2) including the thermal bubbles. The shape of the thermal bubbles in accordance with the current change was observed in the 77 K and 65 K LN2. According to the temperature of liquid nitrogen, bubbles were generated differently. The round shape of the bubble is occurred in 77 K LN2. But the layer shape of bubble is occurred in 65 K LN2. When the bubbles present, the dielectric strength of liquid nitrogen is low. However, the breakdown patterns were different according to the electrode arrangement. AC breakdown voltage(BDV) was lower than the DC BDV due to the influence of bubbles. Therefore, the design of a high-voltage superconducting equipments should consider the bubbles.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.5 no.2
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• pp.85-93
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• 1993

The rise of a large gas bubble or slug in a Micro Two-Phase Closed Thermosyphon with a thin wire insert has been analiged by the potential flow theory. The effect of the interfacial surface tension is explicitly accounted by application of the Kelvin-Laplace equation and solved for the bubble shape. The solution is expressed in terms of the Stokes stream function which consists of an infinite series of Bessel functions. The conditions of the bubble movement in a Micro Two-Phase Closed Thermosyphon were theoretically ascertained.

• Journal of the Korean Society of Visualization
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• v.19 no.1
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• pp.28-35
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• 2021

In this study, an image processing method for the measurement of two-phase bubbly flow is developed. Shadowgraphy images obtained by high-speed camera are used for analysis. Some bubbles are generated as single unit and others are overlapped or clustered. Single bubbles can be easily analyzed using parameters such as bubble shape, centroid, and area. But overlapped bubbles are difficult to transform clustered bubbles into segmented bubbles. Several approaches were proposed for the bubble segmentation such as Hough transform, connection point method and watershed. These methods are not enough for bubble segmentation. In order to obtain the size distribution of bubbles, we present a method of splitting overlapping bubbles using watershed and approximating them to ellipse. There is only 5% error difference between manual and automatic analysis. Furthermore, the error can be reduced down to 1.2% when a correction factor is used. The ellipse fitting algorithm developed in this study can be used to measure bubble parameters accurately by reflecting the shape of the bubbles.

• Journal of the Korean Society of Manufacturing Technology Engineers
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• v.21 no.3
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• pp.473-478
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• 2012

In this paper, the air bubble formation mechanism in the rectangular and triangular line-and-space pattern during dispensing UV Nanoimprint Lithography (UV-NIL) at an atmospheric condition is studied. To investigate the air bubble formation, an analytic model based on geometric approach and a numerical model based on CFD(computational fluid dynamics) were used in the analysis. It was found in the numerical analysis that every time the flow front passed through a corner of the pattern, it proceeded with a newly formed shape, occurring due to interface reconfiguration, since the flow fronts were formed such that they minimized the surface energy. Moreover, the conditions for the air bubble formation were investigated by applying the analytic analysis based on geometric approach and the numerical analysis. Good overall agreement was found between the analytic and numerical analysis.

• Proceedings of the KSME Conference
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• 2004.11a
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• pp.1201-1206
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• 2004

Sonoluminescence (SL) characteristics such as pulse shape, radiance and spectrum radiance from submicron bubbles were investigated. In this study, a set of analytical solutions of the Navier-Stokes equations for the gas inside bubble and equations obtained from mass, momentum and energy equations for the liquid layer adjacent the bubble wall were used to estimate the gas temperature and pressure at the collapse point, which are crucial parameters to determine the SL characteristics. Heat transfer inside the gas bubble as well as at the liquid boundary layer, which was not considered in the most of previous studies on the sonoluminescence was taken it into account in the calculation of the temperature distribution inside the bubble. It was found that bremsstrahlung is a very possible mechanism of the light emission from either micron or submicron bubbles. It was also found that the peak temperature exceeding $10^{6}$ K in the submicron bubble driven at 1 MHz and 4 atm may be due to the rapid change of the bubble wall acceleration near the collapse point rather than shock formation.

• Journal of computational fluids engineering
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• v.13 no.4
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• pp.126-134
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• 2008

We developed a three dimensional Navier-Stokes code based on the level set method to simulate two phase flows with high density ratio. The Navier-Stokes equations with consideration of the surface tension effects are solved by using SIMPLE algorithm on a non-staggered grid. The present code is validated by simulating two test problems. First one is to simulate a rising bubble inside a cube. The thickness of the interface of the bubble is shown to affect the pressure distribution around the interface. As the thickness decreases, the pressure field around the interface becomes more oscillatory. As the bubble rises, a ring vortex is shown to form around the interface and the bubble eventually develops into an ellipsoidal shape. Merge of two bubbles inside a container is secondly tested to show the robustness of the present code for two phase flow simulation. Numerical results show stable and reliable behavior during the process of merging of two bubbles. The velocity and pressure fields around the interface of bubbles are shown oscillation free during the merging of two bubbles.

• Proceedings of the KSME Conference
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• 2003.11a
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• pp.85-89
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• 2003

In a closed square cavity filled with a liquid, a cooled the upper horizontal wall and a heated the lower horizontal wall, the flow isn't generated under the ground-based condition when Rayleigh number is lower than 1700. In such case the flow phenomena near an air bubble under a cooled horizontal wall were investigated. The temperature and the flow fields were studied by using the Thermo-sensitive Liquid-Crystal and the image processing. The qualitative analysis for the temperature and the flow fields were carried out by applying the image processing technique to the original data. Injecting bubble at the center point of upper cooled wall, the symmetry shape of two vortexes near an air bubble was observed. The bubble size increased, the size of velocity and the magnitude of velocity increased. In spite of elapsed time, a pair of two vortexes was the unique and steady-state flow in a square cavity and wasn't induce to the other flow in the surround region.