• Journal of Advanced Marine Engineering and Technology
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• v.40 no.7
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• pp.605-612
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• 2016

A numerical analysis was performed on the effect of the design parameters of a bubble jet type microactuator on its liquid flow characteristics. The numerical models included the ink flow from the reservoir, bubble formation and growth, ejection through the nozzle, and dynamics of the refilling process. Because the bubble behavior is a very important parameter for the overall actuator performance, the bubble growth and collapse phenomena in an open pool were simulated in the present study. The drop ejection and refill process were numerically predicted for various geometries of the nozzle, chamber, and restrictor of the bubble jet microactuator. The numerical results from varying the design parameters can help with predicting the performance and optimizing the design of a microactuator.

• Journal of the Korean Geotechnical Society
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• v.19 no.2
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• pp.15-25
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• 2003

To evaluate the effect of trapped air bubbles in horizontal drains on discharge capacity, unsaturated discharge capacity tests are carried out for four types of drains selected according to the size of section as well as the shape of core. Unsaturated discharge capacities with the elapse of time, the increase of confining pressures, and hydraulic gradients are examined and are compared with saturated discharge capacities. It is found that the unsaturated discharge capacities at a hydraulic gradient of 0.01 decreased by 17%~80% due to the remained air bubbles in the drains compared with the saturated discharge capacities. It is caused by the fact that the horizontal direction of water flow is not consistent with the direction of movement of floating air bubbles in case of horizontal drains. Especially, far the drain with filament shaped core, discharge capacities decreased significantly due to the difficulty in removing air bubbles.

• Journal of the Korean Society of Visualization
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• v.17 no.1
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• pp.60-68
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• 2019

Flow visualizations have been carried out to analyze the characteristics of bubby flows exhausted from a venturi-type bubble generator with an air vent. For various design parameters and operating conditions of the bubble generator, the images of bubbly flows was recorded using a high-speed camera and a microscope. Then the amount and size distribution of bubble was evaluated by an image processing technique. The results show that for increasing the amount of bubble, it is more effective to reduce the venturi throat than to enlarge the air vent diameter. If the water flow rate increases, the bubble generation rate increases but reaches a status of saturation, whose condition depends on Reynolds number at a given air vent diameter. The bubble size increases as the diameter of venturi throat decreases and Reynolds number increases. However, the air vent diameter is not a significant factor on bubble size.

• Journal of the Korean Society of Visualization
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• v.21 no.3
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• pp.75-84
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• 2023

In this study, we conducted analysis of bubble dynamics and flow of liquid phase change material(PCM) using shadowgraphy and particle image velocimetry(PIV). Characteristics of internal flow varied depending on locations of injection when solid PCM was liquefied from heated vertical wall. When bubbles rose immediately, they exhibited elliptical shape and zigzag trajectory. In contrast, when bubbles rose after merging at the bottom of solid PCM, with equivalent diameter for the inter-wall distance of 0.64 or greater, they showed a jellyfish shape and strong rocking behavior. It was observed by the PIV that the small ellipse bubbles made most strong flow inside the liquid PCM. Furthermore, the flow velocity was highest in the case of front injection, as the directions of temperature gradients and bubble-driven flow were aligned. The results underscore the significant influence of injection location on various characteristics, including bubble size, shape, rising path of bubbles, and internal flow.

• Journal of Korea Foundry Society
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• v.8 no.3
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• pp.322-328
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• 1988

3.9%C-2.0%Si-Fe 조성의 압분체(壓粉體)를 $1350^{\circ}$, Ar가스 분위기에서 50분간 용해한 용탕에 시판(市販) misch metal과 misch metal hydride인 MmH와 $MmH_2$를 각각 여러비율로 첨가(添加)한후 서냉(徐冷)및 급냉(急冷)한 시편의 흑연형상(黑鉛形狀)의 변화로부터 misch metal hydride의 구상화능(球狀化能)과 흑연구상화(黑鉛球狀化) 기구(機構) 로서의 기포설(氣泡說)의 타당성을 고찰한 결과 다음과 같은 결론을 얻었다. 1) Misch metal은 0.5% 이상, 그리고 misch metal hydride는 0.25% 이상 첨가한 때 구상흑연(球狀黑鉛)과 CV 흑연(黑鉛)이 나타났으며, misch metal hydride를 첨가한 때가 misch metal에 비하여 흑연립수(黑鉛粒數)가 더 많았다. 이와같이 misch metal hydride의 첨가량이 misch metal에 비하여 적었음에도 흑연립수(黑鉛粒數)가 더 많은 것을 보면 Mm으로부터 방출되는 다수(多數)의 수소기포(水素氣泡)가 구상흑연(球狀黑鉛)의 정출장소로 되는 것으로 해석되기 때문에 기포설(氣泡說)이 타당하다고 생각된다. 2) Misch metal 과 misch metal hydride의 첨가량(添加量)이 1.0%로 증가되면 두경우 모두 흑연립수(黑鉛粒數)가 감소하였다. 이는 용탕중에 개재된 수소기포(水素氣泡)의 실수율차이에 의한 잔류 수소기포수의 증가율둔화와 첨가량(添加量)의 증가에 따른 잔류 misch metal량의 상대적(相對的) 증가로 인한 다량의 잔류 misch metal이 응고과정에서 수소를 재흡수 용해하여 흑연입수를 결정하는 용탕중의 유효수소기포수(有效水素氣泡數)를 감소시켰기 때문으로 생각된다.

• Journal of the Korean Institute of Gas
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• v.26 no.6
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• pp.9-15
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• 2022

In order to verify the durability of the high-pressure hydrogen tank in the operating pressure range, a hydraulic rupture test should be performed. However, if the bubbles generated by the initial injection process of water are attached to the inner wall of the tank and remain, a sudden pressure change of the bubbles during the rupture of the pressurized tank may cause shock and noise. Therefore, in this study, the flow velocity required to remove the bubbles remaining on the inner wall of the tank was predicted through simplified formulas, and the shape of the injection nozzle to maintain the flow velocity was determined based on the shape of the hydrogen tank for the hydrogen bus. In addition, a numerical model was developed to predict the change in flow velocity according to the inlet pressure, and an experiment was performed through a model tank to prove the validity of the prediction result. As a result of the experiment, the flow velocity near the tank wall was similar to the predicted value of the analysis model, and when the inlet pressure was 1.5 to 5.5 bar, the minimum size of the removable bubble was predicted to be about 2.2 to 4.6 mm.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.37 no.9
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• pp.837-845
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• 2013

An airlift pump can be used to pump liquids and sediments within itself, which cannot easily be pumped up by a conventional method, by using the airlift effect. This characteristic of the airlift pump can be exploited in a DCFC (Direct Carbon Fuel Cell) so that molten fuel with high temperature may be carried or transported. The basic characteristics of airlift are investigated. A simple system is constructed, where the reservoir is filled with water, a tube is inserted, and air is supplied from the bottom of the tube. Then, water is lifted and its flow rate is measured. Bubble patterns in the tube are observed in a range of air flow rates with the parameters of the tube diameter and submergence ratio, leading to four distinct regimes. The pumping performance is predicted, and the correlation between the supplied gas flow rate and the induced flow rate of water is found.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.39 no.6
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• pp.519-526
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• 2015

To measure the physical parameters of the simple microlayer model for the prediction of the heat flux and heat transfer rate due to the evaporation of the microlayer during nucleate boiling, the microlayer geometry was experimentally examined. The parameters, including initial thickness, moving velocity and microlayer radius, were measured by total reflection and interferometry techniques using a laser. Single-bubble nucleate boiling experiments were conducted using saturated water on a horizontal surface under atmospheric pressure. The geometric characteristics of the microlayer underneath the bubbles periodically nucleating at a nucleation site at an average heat flux of $200kW/m^2$ were analyzed. The experimental results in the present study show that the maximum initial thickness of the microlayer and the horizontal moving velocity are $5.4{\mu}m$ and 0.12 m/s, respectively.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2004.05a
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• pp.81-81
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• 2004

등통로각압축(ECAP, Equal Channel Angular Pressing)공정은 다결정의 재료 덩어리를 두 채널(channel)이 일정하게 교차하는 형태의 금형에 통과시켜 단면적과 단면 형상의 큰 변화 없이 압출하는 성형법으로 다른 공정에 비해 상대적으로 낮은 압력으로 재료에 소성변형을 발생시켜 입자를 미세화 시킬 수 있으며, 기존의 분말야금에 의한 방법에 비해 상용재료를 포함한 광범위한 금속 및 합금에 적용이 용이한 점과 재료 내부에 기포가 거의 잔류하지 않는 점등의 장점을 가지고 있다.(중략)

• Transactions of the Korean Society of Mechanical Engineers B
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• v.40 no.5
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• pp.275-281
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• 2016

In this study, we investigate the pool boiling enhancement caused by perforated plates on top of a smooth surface. We conduct tests using R-123 at atmospheric pressure. It was shown that perforated plates significantly enhanced the pool boiling of the smooth surface. The reason may be attributed to the increased bubble contact area between the plates. The results showed that the enhancement ratio was dependent on the heat flux. At high heat flux, the enhancement ratio increased as the porosity increased. However, at low heat flux, the enhancement ratio decreased as the porosity increased. For the present investigation, the optimum configuration had a pore diameter of 2.0 mm, pore pitch of $2.5mm{\times}5.0mm$ or $5.0mm{\times}5.0mm$, and a gap width of 0.5 mm, which yielded heat-transfer coefficients that are close to those of GEWA-T. The optimum porosity for R-123 was significantly larger than that of water or ethanol. The reason for this may be the large liquid-to-vapor density ratio along with the small latent heat of vaporization of R-123. The perforated plates yielded smaller boiling hysteresis compared with that of the smooth surface.