• Title, Summary, Keyword: 종횡비

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Influence of Aspect Ratio on Friction Characteristics in Rectangular Gas Microchannel Flow (사각 미세채널 유동에서 마찰특성에 미치는 종횡비의 영향)

  • Islam, M.T.;Lee, Y.W.
    • Journal of the Korea Society For Power System Engineering
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    • v.13 no.2
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    • pp.18-29
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    • 2009
  • 미세유동에 대한 폭발적인 관심에 의해 이 분야의 연구는 다양한 측면에서 이루어지고 있다. 본 연구는 사각 미세채널에서의 슬립유동에 관한 연구 중 아직 제대로 이루어져 있지 않은 마찰특성에 관한 종횡비의 영향에 초점을 맞추어 3차원 수치해석을 행하였다. 그 결과 종횡비가 1.0 일 때 상하 벽면 및 좌우측 벽면에서의 전단음력은 동일하나, 종횡비가 감소함에 따라 전단응력은 상하 벽면과 좌우측 벽면이 상이한 강도로 증가함을 보였다. 또한 Knudsen 수의 증가에 따라서는 전단응력이 감소함을 알 수 있었다. 따라서 벽면에서의 전단응력은 종횡비를 증가시키거나 혹은 Knudsen 수를 증가시키면 감소시킬 수 있으며, 마찰계수(fRe)도 종횡비를 증가시키거나 혹은 Knudsen 수를 증가시키면 감소됨을 밝혔다.

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Mechanical Stability Analysis to Determine the Optimum Aspect Ratio of Rock Caverns for Thermal Energy Storage (열에너지 저장용 암반 공동의 최적 종횡비 결정을 위한 역학적 안정성 해석)

  • Park, Dohyun;Ryu, Dongwoo;Choi, Byung-Hee;Sunwoo, Choon;Han, Kong-Chang
    • Tunnel and Underground Space
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    • v.23 no.2
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    • pp.150-159
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    • 2013
  • It is generally well known that the stratification of thermal energy in heat stores can be improved by increasing the aspect ratio (the height-to-width ratio) of the stores. Accordingly, it will be desirable to apply a high aspect ratio so as to demonstrate the good thermal performance of heat stores. However, as the aspect ratio of a store increases, the height of the store become larger compared to its width, which may be unfavorable for the structural stability of the store. Therefore, to determine an optimum aspect ratio of heat stores, a quantitative mechanical stability assessment should be performed in addition to thermal performance evaluations. In the present study, we numerically investigated the mechanical stability of silo-shaped rock caverns for underground thermal energy storage at different aspect ratios. The applied aspect ratios ranged from 1 to 6 and the mechanical stability was examined based on factor of safety using a shear strength reduction method. The results from the present study showed that the factor of safety of rock caverns tended to decrease with the increase in aspect ratio and the stress ratio of the surrounding rock mass was influential to the stability of the caverns. In addition, the numerical results demonstrated that under the same conditions of rock mass properties and aspect ratio, mechanical stability could be improved by the reduction in cavern size (storage volume), which indicates that one can design high-aspect-ratio rock caverns by dividing a single large cavern into multiple small caverns.

Thermal Stratification and Heat Loss in Underground Thermal Storage Caverns with Different Aspect Ratios and Storage Volumes (지하 열저장 공동의 종횡비와 저장용량에 따른 열성층화 및 열손실)

  • Park, Dohyun;Ryu, Dong-Woo;Choi, Byung-Hee;Sunwoo, Choon;Han, Kong-Chang
    • Tunnel and Underground Space
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    • v.23 no.4
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    • pp.308-318
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    • 2013
  • Thermal stratification in heat stores is essential to improve the efficiency of energy storage systems and deliver more useful energy on demand. It is generally well known that the degree of thermal stratification in heat stores varies depending on the aspect ratio (the height-to-width ratio) and size of the stores. The present study aims to investigate the effect of the aspect ratio and storage volume of rock caverns for storing hot water on thermal stratification in the caverns and heat loss to the surroundings. Heat transfer simulations using a computational fluid dynamics code, FLUENT were performed at different aspect ratios and storage volumes of rock caverns. The variation of thermal stratification with respect to time was examined using an index to quantify the degree of stratification, and the heat loss to the surroundings was evaluated. The results of the numerical simulations demonstrated that the thermal stratification in rock caverns was improved by increasing the aspect ratio, but this effect was not remarkable beyond an aspect ratio of 3-4. When the storage volume of rock caverns was large, a higher thermal stratification was maintained for a relatively longer time compared to caverns with a small storage volume, but the difference in thermal stratification between the two cases tended to decrease as the aspect ratio became larger. In addition, the numerical results showed that the heat loss to the surrounding rock tended to increase with an increase in aspect ratio because the surface area of rock caverns increased as the aspect ratio became larger. The total heat loss from multiple small caverns with a reduced storage volume per cavern was larger compared to a single cavern with the same total storage volume as that of the multiple caverns.

Effect of Aspect Ratio of Enclosure with Inner Circular Cylinder on Three-Dimensional Natural Convection (원형 실린더가 존재하는 밀폐계의 종횡비 변화가 3차원 자연대류 현상에 미치는 영향)

  • Lee, Jeong Min;Seo, Young Min;Ha, Man Yeong
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.40 no.11
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    • pp.717-726
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    • 2016
  • This study evaluated the effect of aspect ratio of an enclosure with a heated inner circular cylinder on three-dimensional natural convection. The immersed boundary method was used to model the inner circular cylinder based on the finite volume method. The Rayleigh number was varied between $10^5$ and $10^6$, and the Prandtl number was maintained at 0.7. The aspect ratio of the three-dimensional enclosure was changed in steps of 1 within a range of 1-4 by increasing the width of the enclosure. In this study, the flow and thermal fields in the enclosure reached the steady state, and showed a mirror-symmetric pattern with respect to the center plane (x=0). In addition, the surface-averaged Nusselt number of the inner circular cylinder increased, while the total surface-averaged Nusselt number of the enclosure walls decreased with increase in the aspect ratio of the enclosure.

Prediction of Tensile Properties for Short-fiber-reinforced Composites (단섬유 보강 복합재료의 기계적 특성 평가에 관한 연구)

  • Jeong, Tae-Heon;Lee, Dong-Joo
    • Journal of the Korean Society of Industry Convergence
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    • v.3 no.1
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    • pp.53-59
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    • 2000
  • 단섬유 보강 복합재료의 종횡비(aspect ratio)를 변화시키며 기계적 특성(탄성계수, 인장강도)을 평가하였다. 2차원 다중 파이버(multi-fiber) 모델을 이용하여 엇갈린(staggered) 배열과 규칙적(aligned) 배열에 대해 유한요소 해석을 하였다. 단섬유 복합재료의 유효탄성계수 및 인장강도는 섬유와 기지의 탄성계수비, 섬유 배열상태, 그리고 단섬유 종횡비의 함수로 표현되었으며, 해석결과의 탄성계수와 인장강도는 이론 모델의 결과와 사출 성형된 PEEK 복합재료 시험편의 결과와 비교하였다. 시험결과는 낮은 종횡비에서 이론 모델 결과와 일치함을 보였다. 단섬유 보강 복합재료의 배열 및 종횡비 변화에 따른 섬유보강 효과에 따른 계면응력 상태는 기계적 특성 결정에 중요한 영향을 보였다.

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Effects of Aspect Ratio of a Fuel Injection Nozzle into a Supersonic Air Stream on Combustion Characteristics (초음속 공기유동으로의 연료 분사노즐 종횡비 변화에 대한 연소특성 연구)

  • 김경무;백승욱;김윤곤
    • Journal of the Korean Society of Propulsion Engineers
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    • v.8 no.1
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    • pp.44-53
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    • 2004
  • This paper is to investigate the combustion characteristics with a three dimensional chemical reacting flow on the aspect ratio of an exit configuration of the slit type nozzle for the fuel injection and to device the methods of combustion/mixing enhancement. The results show that both inside inflow and slit side vertices should be considered from a viewpoint of the mixing. The combustion efficiency becomes the smallest at aspect ratio, where the aspect ratio is less and more than unity, respectively. The total pressure loss becomes the largest at aspect ratio of unity due to the high penetration. All results imply that a streamwise very long slit is desirable with respect to the combustion and the pressure loss.

A Study on Puncture Properties of Short-fiber Reinforced Rubber (단섬유 강화고무의 관통 특성 연구)

  • Ryu, Sang-Ryeoul;Lee, Dong-Joo
    • Composites Research
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    • v.19 no.6
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    • pp.16-22
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    • 2006
  • The puncture properties under various conditions were investigated for the optimum conditions to yield the best properties. Fiber aspect ratio(AR: length of fiber/diameter of fiber), interphase condition and fiber content were considered as variables which impact the puncture force and friction force. The puncture force of short-fiber reinforced rubber increases up to 3.4 times compared to the virgin material. The better interphase condition shows the higher puncture force at given fiber AR and fiber content. The friction force of the matrix and reinforced rubber with a fiber AR below 155 does not exist. The friction force of the reinforced rubber with the good interphase condition and high fiber AR is higher than puncture force of matrix. Overall, it was found that the interphase condition, fiber AR and fiber content have an important effect on the puncture properties.

Condensation Heat Transfer and Pressure Drop in Flat Tubes with Different Aspect Ratios (종횡비가 다른 납작관 내 응축열전달 및 압력손실)

  • Kim, Nae-Hyun;Park, Ji-Hoon;Cha, Sang-Jin
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.34 no.12
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    • pp.1111-1119
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    • 2010
  • In this study, condensation heat transfer coefficients of R-410A were obtained in flattened tubes made from round tubes with an inner diameter of 5.0 mm. The saturation temperature was $45^{\circ}C$; the heat flux, 10 kW/$m^2K$; the mass flux, 100-400 kg/$m^2s$; and the quality, 0.2-0.8. The results showed that the effect of the aspect ratio on the condensation heat transfer coefficient depended on the flow pattern. For annular flow, the heat transfer coefficient increased as the aspect ratio increased. For stratified flow, however, the reverse was true: the pressure drop increased as the aspect ratio increased. Existing correlations adequately predicted the heat transfer coefficients and pressure drops of the flattened tubes.

Optimal Design of Laminate Composites with Gradient Structure (경사형 구조 적층복합재료의 최적설계에 관한 연구)

  • 백성기;강태진;이경우
    • Composites Research
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    • v.13 no.2
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    • pp.40-50
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    • 2000
  • In an effort to construct a structure under the design principle of minimal use of materials for maximum performances, a discrete gradient structure has been introduced in laminate composite systems. Using a sequential linear programming method, the gradient structure of composites to maximize the buckling load was optimized in terms of fiber volume fraction and thickness of each layer. The buckling load showed maximum value with the outmost [$0^{\circ}$] layer concentrated by almost all the fibers when the ratio of length to width(aspect ratio) was less than 1.0. But when the aspect ratio was 2.0, the optimum was determined in a structure where the thickness and fiber volume fraction were well-balanced in each layer. From the optimization of gradient structure, the optimal fiber volume fraction and thickness of each layer were proposed. Gradient structures have also shown an advantage in the weight reduction of composites compared with the conventional homogeneous structures.

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A study on grid aspect ratio of fire dynamics simulator (FDS의 격자 종횡비에 관한 연구)

  • Kim, Won Ouk;Park, Woe-Chul
    • Journal of the Korean Society of Marine Engineering
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    • v.39 no.9
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    • pp.923-928
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    • 2015
  • The FDS is one of the most used programs for fire analysis and needs an optimal grid selection for an accurate analysis. This study selected various grid aspect ratios (ARs) for selection of optimal grid and analyzed them with FDS v 6.1.2. A calculation time of 10 min. was used, which is enough to obtain the time average value of temperature changes. Temperature, visibility, and the time average value of mass balance are obtained from 200-600 s, which is a period of maintaining quasi-steady state. Two polyurethane fires of 1 [MW] and 2 [MW] in two enclosures of $10{\times}10{\times}3[m^3]$ and $20{\times}20{\times}3[m^3]$ were considered. Time variations of heat release rates, temperature, visibility, and mass balance were compared for ARs from 1-6. The heat release rates were accurate for all aspect ratios regardless of fire and enclosure sizes. The quasi-steady state temperature and visibility were well predicted for $AR{\leq}5$. Temperature drop and skewness of mass conservation, however, increased with increasing aspect ratio. Therefore, careful investigation of the grid size is recommended in performance-based design when $AR{\geq}3$, where temperature and visibility in early stage of a fire are important parameters. For accurate simulations of enclosure fires, grid sizes of 0.1~0.2 [m] and smaller in the vertical direction and $AR{\leq}2$ are recommended.