• 한국자동차공학회논문집
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• 제14권6호
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• pp.1-8
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• 2006

One of the most important subjects to develop a LPDi engine is to suppress the bubble generated inside the liquid LPG direct injector. For the purpose of this, the analogy visualization injector to visualize the generation and behaviors of bubble is manufactured, and the bubbling phenomenon and behaviors of bubble are visualized and investigated according to the change of the temperature around an injector wall, fuel pressure and a needle configuration. As results, it was found that the bubble inside the injector is generated around an injector hole and after rising by buoyancy it disappears around the top of a nozzle. The number of bubbles generated is little changed regardless of the lapse of time but it remarkably increases as the temperature around the injector increases. Also, it was known that as the sac volume in LPDi injector decreases the generation of bubble is more active and the rising velocity of bubble generated is increased.

• 설비공학논문집
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• 제27권10호
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• pp.520-526
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• 2015

The characteristics of gas-liquid Taylor (Slug) flow in a square micro-channel of $600{\sim}600{{\mu}m}$ were investigated experimentally in this paper. The test fluids were nitrogen and water. The liquid and gas superficial velocities were 0.01~3 m/s and 0.1~3 m/s, respectively. Bubble and liquid slug length, bubble velocity, and frequency were measured by analyzing optical images using a high speed camera. Bubble length decreased with higher liquid flow rate, which increased dramatically with higher gas flow rate. However, slug length did not vary with changes in inlet liquid conditions. Additionally, bubble velocities and frequencies increased with higher liquid and gas flow rates. It was found that measured bubble lengths were in good agreement with the empirical models in the existing literature, but slug lengths were not.

• 한국가시화정보학회지
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• 제21권3호
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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.

• 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.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2008년도 춘계학술대회논문집
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• pp.185-188
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• 2008

In this study, we experimentally investigated that the optimization of pulsating jet to reduce the separated flow region behind the vertical fence. The vertical fence was submerged in the turbulent boundary layer in the circulating water channel and we applied phase averaged PIV method to measure the instantaneous velocity fields around the fence. One cycle of pulsating jet is divided into 20 phases and grabbed total 200 instantaneous velocity fields at each phase. The experiments were performed by varying the frequency, maximum jet velocity and the shape of pulsating jet wave. Pulsating jet was precisely made by piston-type pump controlled by the computer. The obtained results were compared with normal fence flow. From this study, we found there is the specific frequency which is effective in reducing the reattachment region.

• 수산해양기술연구
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• 제30권2호
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• pp.109-115
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• 1994

This paper presents a new concept to reduce turbulent frictional drag by injecting micro-bubble into near the buffer layer of turbulent boundary layer on flat plate. The concentrations of micro bubble distribution in the boundary was calculater by eddy viscosity equations in the governing equations. When near region of the buffer layer of turbulent boundary layer is filled with micro-bulle of air and viscous of the region is kept low, the velocity profile in the near region should be changed substantially. Then the Reynolds stress in the region becomes less, which guide to lower velocity gradient there. It results in reduction of velocity gradient at the viscous sublayer, which gives the reduction of shear stress at the wall.

• 대한환경공학회지
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• 제34권11호
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• pp.772-779
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• 2012

본 연구에서는 원뿔형으로 개발된 미세기포형 산기관의 성능향상을 위하여 운용적 측면에서 실험과 전산모사적 방법을 이용하였다. 산기관 잠김깊이의 변화에 대해 성능실험이 용이한 실증 규모의 타워형 생물반응기에 산기관을 장착하여, 잠김깊이가 표준산소전달계수($K_{L}a_{20}$) 및 표준산소전달효율(SOTE) 등 산소전달 성능에 미치는 영향을 실험적으로 연구하였다. 또한, 2상 유동에 관한 유체역학적 전산모사를 이용하여 산기관 수와 잠김깊이에 대한 유동현상을 파악함으로써 산소전달 성능변화에 대한 원인을 규명하였다. 산소전달 성능실험결과, 산기관의 잠김깊이를 6 m에서 12 m로 증가시킴에 따라 표준산소 전달계수는 7% 증가하였으나 표준산소전달효율은 39~72% (5.6 %/m)로 대폭 상승하였다. 유동 해석결과, 산기관의 수가 증가함에 따라 공기 체적분율 및 공기와 시험수의 유속 모두 증가하였으며, 공기와 시험수의 유속 경향은 유사하게 나타났다. 잠김깊이가 증가함에 따라서 공기 체적분율, 공기 및 시험수의 유속은 조금씩 감소하는 경향을 나타냈다. 선회 병류유동은 기포류의 확산과 상승속도를 결정하는 주요 인자로서 선회강도가 과도하게 큰 경우에는 기포 체류시간과 체류량이 감소되므로 산소전달 성능을 저하를 예측할 수 있었다.

• 대한기계학회논문집
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• 제19권6호
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• pp.1518-1528
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• 1995

The deformation and departure processes of a bubble attached to the wall are studied experimentally and numerically to understand the phenomena of the nucleate boiling heat transfer enhancement under DC electric field. An air-bubble is injected in a dielectric liquid with different electric fields generated by changing three types of electrode system (Type 1,2 and 3) in the bubble generator. Experimental variables are the electric field strength and the distance and the shape of the electrodes under DC electric field. From experimental results, it is observed that the bubble under Dc electric field is elongated in the same direction as the electric field and the contact angle increases. For the parallel plate electrode which generates a uniform electric field, bubble departure volume doesn't seem to decrease within our experimental range. However, when a needle is raised a few millimeters from the lower electrode to make a nonuniform electric field around the needle, bubble departure volume decreases continuously with the increase of an applied voltage. The reduction effect of bubble departure volume is the most effective under a strong nonuniform electric field generated with Type 3. As the nonuniformity of the electric field due to the shape of a electrode increases, the terminal velocity and the acceleration of a bubble increase largely. For the comparison with visualization results, the deformation of a bubble attached to the electrode is carried out by a numerical method. Numerical results show good agreement qualitatively with experimental results.

• Korean Chemical Engineering Research
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• 제49권5호
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• pp.582-587
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• 2011

점성액체 기포탑에서 기포, wake 및 연속액상들의 체류량 특성을 고찰하였다. 기포탑의 직경(0.051, 0.076, 0.102 and 0.152 m ID), 기체 유속(0.02~0.16 m/s) 그리고 연속액상의 점도(0.001~0.050 $Pa{\cdot}s$)가 기포, wake 및 연속액상의 체류량에 미치는 영향을 검토하였다. 기포, wake 그리고 연속액상들은 이중 전기 저항탐침방법에 의하여 성공적으로 구별할 수 있었다. 압축된 여과공기와 물 또는 CMC 수용액을 각각 기체와 연속액상으로 사용하였다. 기포탑에서 기포와 wake 상들을 연속적으로 검침하기위해 자료수집장치(DT 2805 Lab Card)와 컴퓨터를 사용하였다. 탐침 써키트로부터 수집된 아날로그 자료는 디지털 자료로 변환되었으며, 이들 자료를 이용하여 기포탑에서 상승하는 단일기포 뿐만이 아니라 다중기포들의 후면에서 wake 상을 검침할 수 있었다. 기포와 wake 상의 체류량은 각각 기포탑의 직경과 연속액상의 점도가 증가함에 따라 감소하였으나 연속액상의 체류량은 증가하였다. 그러나, 기포와 wake의 체류량은 각각 기체 유속이 증가함에 따라 증가한 반면 연속액상의 체류량은 감소하였다. wake 상 체류량에 대한 기포 체류량의 비율은 기포탑의 직경 또는 기체의 유속이 증가함에 따라 감소한 반면 연속액상의 점도가 증가함에 점성액체 기포탑에서 기포, wake 그리고 연속액상의 체류량은 본 연구의 실험범위에서 다음과 같은 실험변수의 상관식으로 나타낼 수 있었다. ${\varepsilon}_B=0.043D^{-0.18}U_G^{0.56}{\mu}_L^{-0.13}$, ${\varepsilon}_W=0.003D^{-0.85}U_G^{0.46}{\mu}_L^{-0.10}$, ${\varepsilon}_C=1.179D^{0.09}U_G^{-0.13}{\mu}_L^{0.04}$.

• 한국원자력학회:학술대회논문집
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• 한국원자력학회 1999년도 추계학술발표회요약집
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• pp.123.2-123
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• 1999