• Nuclear Engineering and Technology
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• 제47권1호
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• pp.126-134
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• 2015

A simple Couette flow velocity profile with an appropriate correlation for the free terminal rise velocity of a single bubble in a quiescent liquid can produce reliable results for the trajectories of small spherical air bubbles in a low-viscosity liquid (water) provided the liquid remains under uniform shear flow. Comparison of the model adopted in this paper with published results has been accomplished. Based on this study it has also been found that the lift coefficient in water is higher than its typical value in a high-viscosity liquid and therefore a modified correlation for the lift coefficient in a uniform shear flow of water within the regime of the $E\ddot{o}tv\ddot{o}s$ number $0.305{\leq}Eo{\leq}1.22$ is also presented.

• 한국전산유체공학회지
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• 제13권4호
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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.

• Membrane and Water Treatment
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• 제10권4호
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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.

• 한국유체기계학회 논문집
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• 제8권5호
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• pp.7-12
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• 2005

Experimental study was conducted to increase the oxygen transfer efficiency of air diffusers in clean water. By measuring the bubble size from the bubbly two-phase flow visualization with several air diffusers, the size of air bubbles near the top surface of aeration tank seems to be independent on the diffuser types. Considering design parameters for the better break-up of larger bubbles around the air diffusers, advanced conceptual air diffusers using nozzle-type throat showing the higher oxygen transfer efficiencies were made.

• 한국가시화정보학회:학술대회논문집
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• 한국가시화정보학회 2003년도 추계학술대회 논문집
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• pp.45-46
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• 2003

The x-ray micro-imaging technique was employed to measure the size and velocity of micro-bubbles moving in an opaque tube simultaneously. Phase contrast images were obtained at interfaces of micro-bubbles between water and air due to different refractive index. Micro-bubbles of $20\~120{\mu}m$ diameter moving upward in an opaque tube $(\phi=2.7mm)$ were tested. For two different working fluids of tap water and DI water, the measured velocity of micro-bubbles is roughly proportional to the square of bubble size.

• 한국환경과학회지
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• 제19권5호
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• pp.647-653
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• 2010

The objectives of this research are to evaluate and compare the oxygen transfer coefficients($K_{La}$) in both a general bubbles reactor and a micro-nano bubbles reactor for effective operation in sewage treatment plants, and to understand the effect on microbial kinetic parameters of biomass growth for optimal biological treatment in sewage treatment plants when the micro-nano bubbles reactor is applied. Oxygen transfer coefficients($K_{La}$) of tap water and effluent of primary clarifier were determined. The oxygen transfer coefficients of the tap water for the general bubbles reactor and micro-nano bubbles reactor were found to be 0.28 $hr^{-1}$ and 2.50 $hr^{-1}$, respectively. The oxygen transfer coefficients of the effluent of the primary clarifier for the general bubbles reactor and micro-nano bubbles reactor were found be to 0.15 $hr^{-1}$ and 0.91 $hr^{-1}$, respectively. In order to figure out kinetic parameters of biomass growth for the general bubbles reactor and micro-nano bubbles reactor, oxygen uptake rates(OURs) in the saturated effluent of the primary clarifier were measured with the general bubbles reactor and micro-nano bubbles reactor. The OURs of in the saturated effluent of the primary clarifier with the general bubbles reactor and micro-nano bubbles reactor were 0.0294 mg $O_2/L{\cdot}hr$ and 0.0465 mg $O_2/L{\cdot}hr$, respectively. The higher micro-nano bubbles reactor's oxygen transfer coefficient increases the OURs. In addition, the maximum readily biodegradable substrate utilization rates($K_{ms}$) for the general bubbles reactor and micro-nano bubbles reactor were 3.41 mg COD utilized/mg active VSS day and 7.07 mg COD utilized/mg active VSS day, respectively. The maximum specific biomass growth rates for heterotrophic biomass(${\mu}_{max}$) were calculated by both values of yield for heterotrophic biomass($Y_H$) and the maximum readily biodegradable substrate utilization rates($K_{ms}$). The values of ${\mu}_{max}$ for the general bubbles reactor and micro-nano bubbles reactor were 1.62 $day^{-1}$ and 3.36 $day^{-1}$, respectively. The reported results show that the micro-nano bubbles reactor increased air-liquid contact area. This method could remove dissolved organic matters and nutrients efficiently and effectively.

• 한국기계가공학회지
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• 제14권1호
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• pp.98-104
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• 2015

This experiment was a study of a Vortex nozzle designed to produce micro-bubbles due To investigate air self-suction and the generation of micro-bubble by the Vortex nozzle, the dimensions of air intake region, the nozzle shape, and the nozzle exit diameter ($d_n=5,7,9.2,12.3mm$)werevaried. The air self-suction rate was ~1,000 to 2,000 cc/min at the orifice nozzle (7 mm), and ~100 and ~22 cc/min at the sector nozzles (9.2 and 12.3 mm, respectively). The most bubbles were detected in the orifice nozzle, but bubbles less than $50{\mu}m$ were found in the 12.3-mm sector nozzle. The dissolved oxygen in the tank water was much greater in Case 2 than in Case 1, at both the orifice and sector nozzles. Moreover, the reduction rate of dissolved oxygen was found to be less at the sector nozzles, than at the orifice nozzle.

• 한국지반공학회지:지반
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• 제13권2호
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• pp.39-54
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• 1997

연약지반 개량공법으로 국내에서 생산된 플라스틱 보드 드레인(PBD)이 많이 사용되고 있으나 이들에 대한 물리적 성질 및 배수성능의 검증이 미약한 실정이다. 또한 PBD의 웰 레지스턴스(well resistance)에 크게 영향을 미치는 공기(기포)의 영향에 대한 인식이 부족하다고 판단된다. 따라서 본 연구에서는 대형 실험장치를 이용하여 먼저 국산 PBD를 중심으로 물리적 성질 및 배수성능 시험을 실시하여 제품의 질적 수준을 규명하고 대표적인 국외제품과 비교했다. 아울러 PBD재의 꺾임 부분에 누적된 공기(기포)가 통수능력에 지장을 초래하는 것을 재확인하였으며 현장에서 적용할 수 있는 진공펌프를 이용하여 필기(기포)를 제거시키므로써 통수능력의 회복이 가능하다는 것을 나타냈다.

• 한국소음진동공학회논문집
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• 제16권2호
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• pp.190-197
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• 2006

It is well known that the acoustic characteristics of the sea are significantly affected by bubbles which have their own inherent characteristics at the undersea. In this study, the shape and acoustic characteristics of air bubbles generated by an underwater nozzle are calculated numerically, and are measured with a high speed camera and a hydrophone at various air flow rates in the experimental apparatus. As a result of analysis, the shape calculated numerically well matched with measured values at low flow rates, but in case of relatively higher flow rates. the use of correction coefficient is needed for more accurate estimation of the bubble shape. And also the rising velocity of a single bubble is constant regardless of both the bubble size and the flow rate. and the acoustic signal generated when the bubble is produced by an underwater nozzle has the same characteristic of natural frequency of the bubble pulsation, and is agreed with Minnaert's equation if the correction coefficient is considered in accordance with the flow rate.

• 대한기계학회논문집B
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• 제28권6호
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• pp.659-664
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• 2004

It is important to measure precisely the size and velocity of micro-bubbles used in various field. The synchrotron X-ray micro-imaging technique was employed to measure the size and velocity of micro-bubbles moving in an opaque tube simultaneously. Phase contrast images were obtained at interfaces of micro-bubbles between water and air due to their different refractive indices. The X-ray micro-imaging technique was found to measure an optical fiber with an accuracy of 0.2%. Micro-bubbles of 20∼60$\mu\textrm{m}$ diameter moving upward in an opaque tube (${\Phi}$＝2.7mm) were tested to measure bubble size and up-rising velocity. For DI water, the measured velocity of micro-bubbles is nearly proportional to the square of bubble size, agreed well with the theoretical result. In addition, the synchrotron X-ray micro-imaging technique can measure accurately the size and velocity of several overlapped micro-bubbles.