• Title/Summary/Keyword: Channel -flow

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Flow Structures Around a Freely-falling, Rectangular Cylinder (자유 낙하하는 사각 실린더 주위의 유동 구조)

  • Jeon, Chung-Ho;Lee, Chang-Yeol;Yoon, Hyun-Sik
    • Journal of Ocean Engineering and Technology
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    • v.24 no.5
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    • pp.8-15
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    • 2010
  • The flow around a two-dimensional, rectangular cylinder that is freely falling in a channel was simulated using the immersed boundary method with direct forcing to determine the interactions between the fluid and the structure. The results of the present study were in good agreement with previous experimental results. Regardless of the H/L ratio (where H and L are the height and width of the rectangular cylinder, respectively), the flow structures had essentially the same pattern as the two symmetrical circulations that form about the horizontal center of the cylinder, with those centers located at each lateral position near the wake. When the cylinder approaches very close to the bottom, a jet-like flow appeared between the bottom of the rectangular cylinder and the channel. When the jet-like flow goes through the channel, surrounding fluids are sucked into this jet, forming the secondary vortices.

Air-water Countercurrent Flow Limitation in Narrow Rectangular Channels (협소 사각유로에서 공기-물 대향류 유동한계)

  • Kim, Byong-Joo
    • Korean Journal of Air-Conditioning and Refrigeration Engineering
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    • v.19 no.6
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    • pp.441-446
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    • 2007
  • An experimental study on the countercurrent two-phase flow in narrow rectangular channels has been peformed. Countercurrent flow limitation (CCFL) was investigated using air and water in 760mm long, 100mm wide, vertical test sections with 1 and 3mm channel gaps. Tests were systematically performed with downward liquid superficial velocities and upward gas velocities covering 0 to 0.125 and 0 to 3.5m/s ranges, respectively. As the gap width of rectangular channel increased the CCFL water superficial velocity decreased for the given air superficial velocity. Slight increase of the air superficial velocity resulted in the abrupt decrease of water velocity when $j_g=2{\sim}4m/s$. The critical superficial velocity of air, at which the downward flow of water was no longer allowed, also decreased with the increase of gap width. The experimental results were compared with the previous correlations, which were mainly for round tubes, and the qualitative trends were found to be partially acceptable. However the quantitative discrepancies were hardly neglected. New correlation of CCFL was developed and showed good agreement with the experimental data.

Effects of stenotic severity on the flow structure in a circular channel under a pulsatile flow

  • Kim, Kyung-Won;Cheema, Taqi-Ahmad;Park, Cheol-Woo
    • Journal of Advanced Marine Engineering and Technology
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    • v.38 no.2
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    • pp.140-146
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    • 2014
  • Stenosis is the drastic reduction in the cross-sectional area of blood vessel caused by accumulations of cholesterol. It affects the blood flow property and structure from the fluid dynamic point of view. To understand the flow phenomenon more clearly, a particle image velocimetry method is used and the fluid dynamic characteristics in a circular channel containing stenosis structure is investigated experimentally in this study. Different stenotic-structured models made of acrylic material are subjected to a pulsatile flow generated by an in-house designed pulsatile pump. The inner diameter of the tube inlet is 20 mm and the length of reduced area for stenosis ranges between 35mm and 40mm. It is circulated continuously through a circular channel by the pump system. Pressure is measured at four different sections during systolic and diastolic phase changes. The phase-averaged velocity field distribution shows a recirculation regime after the stenotic structure. The effects of the stenotic obstructions are found to be more severe when the aspect ratio is varied.

Numerical Simulation of Hydro-Acoustic Flow in Piezo Inkjet Print Head (피에조 잉크젯 헤드의 음향파 거동의 수치 해석)

  • Lee, You-Seop;Wee, Sang-Kwon;Oh, Se-Young;Chung, Jae-Woo
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.31 no.1 s.256
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    • pp.51-61
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    • 2007
  • This paper presents numerical and theoretical studies of acoustic wave interactions in slightly compressible liquids within piezoelectrically driven inkjet print heads. The interconnected flow channels may cause jet crosstalk, resulting in poor printing quality. It should be reduced by modifying the channel structure with the acoustic wave interactions considered. Compressible gas flow driven by the sudden movement of a top wall in the channel is calculated using Flow3D and is validated with the narrow gap theory. Limited compressibility model of the Flow3D is employed to calculate pressure waves of slightly compressible ink flow. It is found that reducing restrictor width can damp out the jet crosstalk by inhibiting the pressure wave propagation. The degree of crosstalk has been quantified using the maximum values of cross-correlations between neighboring channels and a critical channel dimension for acceptable crosstalk has been proposed. This finding is verified by drop visualization experiments using silicon-micromachined piezo inkjet print heads that are fabricated by our group.

Optimization of Angled Ribs for Heat Transfer Enhancement in a Square Channel with Bleed Flow (유출유동을 가진 정사각유로 내 열전달 향상을 위한 경사진 요철 최적설계)

  • Lee, Hyun;Kim, Kyung-Min;Lee, Dong-Hyun;Cho, Hyung-Hee
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.32 no.4
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    • pp.300-306
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    • 2008
  • In the present study, the second order response surface method (RSM) is carried out to get optimum thermal design for enhancing heat transfer in a square channel with bleed flow. The RSM is used as an optimization technique. To calculate the heat transfer, RNG k-epsilon model and enhanced wall function are used. To design optimum rib turbulators, two design variables such as attack angle of rib $({\alpha})$ and rib pitch-to-rib height ratio (p/e) are optimized. In these analyses, the channel inlet Reynolds number was fixed at 10,000 in both non-bleeding and bleeding cases. The response surfaces of two design variables are constructed in cases with and without bleed flow. As a result, the optimum (or highest) heat transfer values are almost the same in ranges of two cases with and without bleed flow. However, the friction losses in the case with bleed flow are lower than those without bleed flow.

Numerical analysis of the gas flow-rate uniformity in the anode flow channel of indirect internal reforming molten carbonate fuel cell (MCFC) under different pressure drop and temperature conditions (간접 내부 개질형 용융탄산염 연료전지 anode 채널에서의 압력 강하 및 온도 조건 변경에 따른 유량 균일도에 관한 수치 해석적 연구)

  • Cho, Jun-Hyun;Ha, Tae-Hun;Kim, Han-Sang;Min, Kyoung-Doug;Park, Jong-Hoon;Chang, In-Gab;Lee, Tae-Won
    • 한국신재생에너지학회:학술대회논문집
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    • 2009.11a
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    • pp.127-130
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    • 2009
  • The uniform gas distribution between anode channels of the indirect internal reforming type molten carbonate fuel cell (MCFC) is crucial design parameter because of the electric performance and the durability problems. A three-dimensional computational fluid dynamics (CFD) analysis is performed to investigate flow characteristics in the anode channels and manifold under different pressure drop and channel temperature conditions. The combined meshes consists of hexadral meshes in the channels and polyhedral meshes in the manifold are adopted and chemical reactions inside the MCFC system are not included because of computational difficulties associated with the size and geometric complexity of the system. Results indicate that the uniformity in flow-rate is in the range of $\pm$ 0.048 % between the anode channels when the pressure drop of anode channel is about 150 Pa. A gas flow-rate uniformity decreases as the pressure drop of anode channels decreases and as the temperature difference between indirect internal reforming (IIR) channels and anode channels increases.

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Characteristics of Sediment and Flow with Channel Patterns in Alluvial Rivers (충적하천(沖積河川)의 수로양상(水路樣相)에 따른 유사(流砂) 및 흐름특성(特性))

  • Lee, Jong Seok;Lee, Dae Cheol;Pai, Dong Man;Cha, Young Kee
    • KSCE Journal of Civil and Environmental Engineering Research
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    • v.14 no.5
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    • pp.1177-1189
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    • 1994
  • This paper aims to develop the numerical model for prediction of the channel migration by analyzing of sediment and flow characteristics with patterns of channel in alluvial rivers. Flow in rivers constitutes to be the meandering or the braided form and rarely straight channel through morphologically stable patterns with mutual actions between the flowing water and bed materials. In order to develop the model for simulation of the channel migration, the channels are divided into two types with positive or negative sign by the direction of curvature radius of the centerline channel ($r_c$). That is, the single bend-channel consists of only one curvature of positive or negative sign and the multi-bend channel consists of two more curvatures of positive or negative sign, respectively. The model analyzes the sediment and flow characteristics under the influence of superelevation, spiral motion, irregularity in bed topography and depth-averaged velocity of channels. For reliability of this model, the single bend-channel and the multi bend channel are compared with experiment data in other models and the measured field data in the Keum-River, respectively. As a result, the both com parisians turn out to be excellent.

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Numerical Investigation of the Moving Wall Effects in Turbulent Channel Flows (난류채널유동에서 움직이는 벽면에 대한 수치연구)

  • Hwang, Jun Hyuk;Lee, Jae Hwa
    • Journal of the Korean Society of Visualization
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    • v.15 no.3
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    • pp.27-33
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    • 2017
  • Direct numerical simulations of turbulent channel flows with moving wall conditions on the top wall are performed to examine the effects of the moving wall on the turbulent characteristics. The moving wall velocity only applied to the top wall with the opposite direction to the main flow is systematically varied to reveal the sustained-mechanism for turbulence. The turbulence statistics for the Couette-Poiseuille flow, such as mean velocity, root mean square of the velocity fluctuations, Reynolds shear stress and pre-multiplied energy spectra of the velocity fluctuations, are compared with those of canonical turbulent channel flows. The comparison suggests that although the turbulent activity on the top wall increases with increasing the Reynolds number, that on the bottom wall decreases, contrary to the previous finding for the canonical turbulent channel flows. The increase of the turbulent energy on the top wall is attributed to not only the increase of the Reynolds number but also elongation of the logarithmic layer due to increase of the wall layer on the top wall. However, because the logarithmic layer is shortened on the bottom wall due to the decrease of the wall layer, the turbulence energy on the bottom wall decreases despite of the increase of the Reynolds number.

A Comparison of Flow Condensation HTCs of R22 Alternatives in the Multi-Channel Tube (알루미늄 다채널 평판관내 R22의 흐름응축 열전달 성능 비교)

  • Seo, Young-Ho;Lim, Dae-Taeg;Park, Ki-Jung;Jung, Dong-Soo
    • Proceedings of the KSME Conference
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    • 2004.11a
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    • pp.1270-1275
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    • 2004
  • Flow condensation heat transfer coefficients(HTCs) of R22 and R134a were measured on horizontal aluminum multi-channel tube. The experimental apparatus was composed of three main parts ; a refrigerant loop, a water loop and a water-ethylene glycol loop. The test section in the refrigerant loop was made of aluminum multi-channel tube of 1.4 mm hydraulic diameter and 0.53 m length. The refrigerant was cooled by passing cold water through an annulus surrounding the test section. The data scan vapor qualities $(0.1{\sim}0.9)$, mass flux ($200{\sim}400$ $kg/m^{2}s$) and heat flux ($7.3{\sim}7.7$ $kW/m^{2}$) at $40{\times}0.2^{\circ}C$ saturation temperature in small hydraulic diameter tube. It was found that some well-known previous correlations were not suitable for multichannel tube. So, It must develop new correlations for multi-channel tubes.

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A Numerical Analysis of Convective Heat Transfer in Air Flow Channels of a Plate Fin-tube Matrix for Heat Pipe Heat Sinks (히트파이프 히트싱크에서 평판 휜-관으로 구성된 공기유동 냉각채널의 대류 열전달 특성에 관한 수치해석)

  • Kim Sung-Hoon;Shin Hyun-Myung;Kim Chul-Ju
    • Journal of Advanced Marine Engineering and Technology
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    • v.29 no.8
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    • pp.862-869
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    • 2005
  • A study on convective cooling characteristics has been done in the channels with heat pipes and associated Plane fins Analysis with FLUENT V5.0 lies its Purpose on the possible enhancement of heat transfer capability between an existing three in-line arrayed heatpipes and an extending four in-line arrayed heatpipes with increasing channel width. Numerical analysis is limited to the laminar flow in an isolated flow channel by employing cyclic boundary conditions for calculation purposes. Friction factors for three and four in-line arrayed heatpipes are compared with experimental results. In addition, temperature behavior at the plate fin for the three in-line arrayed heatpipes is compared with experiment. Friction factors and overall channel heat transfer coefficients (and/or Nusselt numbers) are presented as a function of Reynolds number. An increase of number of heatpipes and channel width reults in a decrease of the friction factor and doesn't not result in an increase of heat transfer performance. However. considering the 25$\%$ increase of heat load accompanies with maximum 8$^{\circ}C$ rise of average temperature of heat pipes, the four in-line array with the increase of channel width of heat pipe heat sink can be considered appropriate.