• Title/Summary/Keyword: Nusselt number

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Natural Convection from a Horisontal Conducting Tube with One Vertical Axial Fin. (단일수직(單一垂直)한 축방향(軸方向)의 핀을 가진 수평전도관(水平傳導管)으로 부터의 자연대류(自然對流))

  • Chung, H.S.;Kwon, S.S.
    • Solar Energy
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    • v.6 no.1
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    • pp.60-69
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    • 1986
  • Two-dimensional finite difference numerical analysis is used to study conjugate natural convection heat transfer from a horizontal conducting tube with one vertical axial fin. By increasing dimensionless fin length ($L_F$), the mean total Nusselt number of the upward fin is slightly less than that of the downward fin at $L_F\;{\le}\;0.18$ and is higher than that of the downward fin at $L_F\;>\;0.18$. However comparing the upward fin and the downward fin with the no fin, the mean total Nusselt numbers of downward fin and upward fin at $L_F=0.30$ are increased approximately 4.01% and 5.51%, respectively. As Rayleigh number, Prandtl number and Biot number increase, the mean total and the mean tube Nusselt numbers are increased, but as wall thickness increases, the mean total and the mean tube Nusselt numbers are decreased. As the fin conduction parameter increases, the mean total Nusselt number is slightly increased because of increasing the mean fin Nusselt number.

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Convective heat transfer characteristics of a two-dimensional turbulent wall attaching offset jet (2차원 난류 벽부착제트의 대류열전달 특성)

  • Yun, Sun-Hyeon;Lee, Dae-Hui;Song, Heung-Bok;Kim, Dae-Seong
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.20 no.10
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    • pp.3304-3312
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    • 1996
  • An experimental study on the convective heat transfer characteristics was performed for a two-dimensional wall attaching offset jet(WAOJ). Thermochromic liquid crystal was used to measure the plate wall temperature. The Nusselt number was measured for Reynolds numbers from 6, 500 to 39, 000, and the offset ratios from 0.5 to 15. The maximum Nusselt number point coincides with the time-averaged reattachment point and Nusselt number decreases monotonically after the jet reattaches on the wall. In the recirculation region Nusselt number minimize near the upstream corner and then increases as X/D decreases to vanishes. This suggests the existence of secondary vortices, causing an additional mixing of the flow in the corner. The correlations between the local Nusselt number and Reynolds number, Re, offset ratio, H/D, and streamwise distance, X/D are presented.

Natural convection heat transfer in a horizontal annulus from an inner tube with two vertical fins (2개의 수직휜이 부착된 내관으로부터 환상공간내의 자연대류 열전달)

  • 정태현;정한식;권순석
    • Transactions of the Korean Society of Mechanical Engineers
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    • v.15 no.2
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    • pp.654-660
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    • 1991
  • Natural convection heat transfer in a horizontal annulus from an inner tube with two vertical fins has been studied for the effects of dimensionless fin length and Rayleigh number. The maximum local Nusselt number of inner tube was obtained at .theta. = 145.deg. and that of outer cylinder at .theta. = 0.deg. for the case of $l_{F}$=0.3 Local Nusselt number distributions for the lower fins show higher values than that of the upper fins. The mean Nusselt number of inner tube was increased with the values of dimensionless fin length. The mean Nusselt number can be represented in an exponential function of Grashof number at various fin lengths. As compared with experimental and numerical results, isotherms and local Nusselt number show good agreement.t.

Local heat transfer measurement from a concave surface to an oblique impinging jet (오목한 표면위에 분사되는 경사충돌제트에 대한 국소열전달계수의 측정)

  • 임경빈;김학주
    • Korean Journal of Air-Conditioning and Refrigeration Engineering
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    • v.10 no.3
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    • pp.324-333
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    • 1998
  • Measurements of the local heat transfer coefficients on a hemispherically concave surface with a round oblique impinging jet were made. The liquid crystal transient method was used for these measurements. This method, which is a variation of the transient method, suddenly exposes a preheated wall to an impinging jet while video recording the response of liquid crystal for the surface temperature measurements. The Reynolds number used was 23,000 and the nozzle -to -jet distance was L/d=2, 4, 6, 8 and 10 and the jet angle was $\alpha$=0$^{\circ}$, 15$^{\circ}$, 30$^{\circ}$and 40$^{\circ}$. In the experiment, the maximum Nusselt number at all region occurred at L/d(equation omitted)6 and Nusselt number decreases as the inclined jet angle increases. For the normal jet the contours of constant Nusselt number are circular and as the jet is inclined closer and closer to the surface the contours become elliptical shape. The decreasing rate of the Nusselt number at X/d> 0(upstream) on a surface curvature are higher than those on a flate plate and the decreasing rate of the Nusselt number at X/d <0(downstream) on a surface curvature are lower than those on a flate plate. And also, the decreasing rate of local Nusselt number distribution at X/d <0(upstream) exhibit lower than with X/d <0(downstream) as jet angle increases. The second maximum Nusselt number occurred at long distance from stagnation point as jet angle increases.

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A Study on the Convection Heat Transfer on the Side-wall with a Offset (오프셋이 있는 경우 측벽에서의 대류열전달에 관한연구)

  • Park, Yong-Il
    • Solar Energy
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    • v.8 no.2
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    • pp.57-65
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    • 1988
  • The coefficients of convective heat transfer were investigated when air is jetted to surface of the heated side-wall. The temperature on the side-wall was measured when the offsets changed from 1.5 to 10.5 as 7 steps at the state of fixed Reynolds numbers that were 35000, 29000 and 23000. The experimental results are as follows: 1. The mean Nusselt number is very high on the surface of reattached flow region. 2. The offset and the recirculation flow region decreased, while the mean Nusselt number increased between the outlet of nozzle and the region of reattachment flow. 3. The local Nusselt number is not concerned with Reynolds number on the recirculation flow and on the reattached flow region when the offset decrease. But the Nusselt number increased only when Reynolds numbers on the wall jet flow region increased. 4. The mean and the maximum Nusselt number decreases linearly, and in particular its values rapidly decrease in accordance with changing of the offset from 1.5 to 3 in inverse proportion.

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Thermal and Flow Analysis of the Flat Tube with Micro-Channels (미세유로를 갖는 납작관의 열·유동 해석)

  • Chung, Kilyoan;Lee, Kwan-Soo;Kim, Woo-Seung
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.23 no.8
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    • pp.978-986
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    • 1999
  • In this study, the general thermal and flow characteristics of flat tube with micro-channels has been studied and the correlation of Nusselt number and friction factor is proposed. The optimal flat tube geometry is determined by optimal design process. It is assumed to be a three dimensional laminar flow in the analysis of thermal and flow characteristics. The periodic boundary condition is applied since the geometry of flat tube with micro-channels shows uniform cross-section in primary flow direction. Local Nusselt number is examined for thermal characteristics of each membrane, and module average Nusselt number and friction factor are calculated to determine the characteristics of the heat transfer and pressure drop in overall flat tube with microchannels. The correlations between Nusselt number and friction factor are given by Reynolds number, aspect ratio of membranes, and the width of flat tube. ALM (Augmented Lagrangian Multiplier) method is applied to the correlations to determine an optimal shape of flat tube. It is shown that the optimal aspect ratio of flat tube is approximately 1.0, irrespective of the width of flat tube and Reynolds number.

Experimental Studies on Single Phase Flow and Heat Transfer in Microchannels (미세유로의 단상 유동 및 열전달에 대한 실험적 연구)

  • Kim, Byong-Joo;Kim, Geon-Il
    • Korean Journal of Air-Conditioning and Refrigeration Engineering
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    • v.20 no.12
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    • pp.795-801
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    • 2008
  • An experimental study has been performed on the single phase flow and convective heat transfer in trapezoidal microchannels. The microchannel was about $270{\mu}m$ wide, $800{\mu}m$ deep. and 7 mm long, which might ensure hydrodynamically fully-developed laminar flow at a low Reynolds number. The experiments were conducted with R1l3 and water, with the Reynolds number ranging from approximately 30 to 5000 for friction factor and 30 to 700 for the Nusselt number. Friction factors in laminar are found to be in good agreement with the predictions of existing correlation suggesting that a conventional analysis approach can be employed in predicting flow friction behavior in microchannels. However turbulent friction factors are hardly predictable by the existing correlations. The experimental results show that the Nusselt number is not a constant but increases almost linearly with the Reynolds number even the flow is fully developed (Re < 100). The dependence of the Nusslet number on the Reynolds number is contradictory to the conventional theory. At a Reynolds number greater than 100, the Nusselt number increases slowly with the Reynolds number, where thennally developing flow is responsible for the increase of the Nusselt number with the Reynolds number.

Mass transfer study of double diffusive natural convection in a two-dimensional enclosure during the physical vapor transport of mercurous bromide (Hg2Br2): Part II. Mass transfer (브로민화 수은(I)(Hg2Br2) 물리적 증착공정의 2차원 밀폐공간에서 이중확산 자연 대류에서의 물질전달 연구: Part II. 물질전달)

  • Sung Ho Ha
    • Journal of the Korean Crystal Growth and Crystal Technology
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    • v.33 no.4
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    • pp.145-152
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    • 2023
  • The average Nusselt numbers in the source and crystal region for the variation of thermal Grashof number (Grt) in the range of 2.31 × 104 ≤ Grt ≤ 4.68 × 104 are obtained through numerical simulations. It is shown the average Nusselt number in the crystal region is more than twice as large as the average Nusselt number in the source region. The average Nusselt number in the source region shows an increasing tendency with increasing the thermal Grashof number, Grt, while the average Nusselt number in the crystal region shows a decreasing tendency with increasing thermal Grashof number, Grt. For the variation of the solutal Grashof number (Grs) in the ran ge of 3.28 × 105 ≤ Grs ≤ 4.43 × 105, the average Sherwood number in the source region and crystal region tends to decrease as the solutal Grashof number, Grs increases. The average Sherwood number in the crystal region is about four times greater than the average Sherwood number in the source region.

Numerical Study on Heat Transfer and Pressure Drop Characteristics in a Horizontal Channel with Dimple and Protrusion Arrays (딤플과 돌출이 설치된 수평채널의 열전달 및 압력강하 특성에 관한 수치해석적 연구)

  • Kim, Ji-Hoon;Heo, Joo-Nyoung;Shin, Jee-Young;Son, Young-Seok
    • Journal of Advanced Marine Engineering and Technology
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    • v.36 no.1
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    • pp.57-63
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    • 2012
  • In this study, numerical analyses were performed on pressure drop and heat transfer characteristics in a rectangular horizontal channel with dimple and protrusion arrays of different height. The dimples/protrusions were installed at both top and bottom walls of the rectangular channel. The dimple and protrusion depths are 0.125, 0.2, 0.25, 0.3, and 0.375 times diameter. In case of the dimple, the highest Nusselt number occurred at the rear side of the dimple, and the average Nusselt number tended to decrease slightly with increase of depth. In case of protrusion, on the other hand, the highest Nusselt number occurred at the front side of the protrusion, and the average Nusselt number was increased with the increase of height. In both dimple and protrusion, the average Nusselt number and pressure drop were increased with the increase of velocity. Performance factor was decreased with the increase of velocity, and it was found that the best performance factor was obtained in the low velocity region.

A Numerical Study on the Heat Transfer Characteristics of the Multiple Slot Impinging Jet (다양한 노즐 수 변화에 따른 충돌 제트의 열전달 특성에 관한 수치적 연구)

  • Kim, Sang-Keun;Ha, Man-Yeong;Son, Chang-Min
    • Korean Journal of Air-Conditioning and Refrigeration Engineering
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    • v.23 no.11
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    • pp.754-761
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    • 2011
  • The present study numerically investigates two-dimensional flow and heat transfer in the multiple confined impinging slot jet. Numerical simulations are performed for the different Reynolds numbers(Re=100 and 200) in the range of nozzles from 1 to 9 and height ratios(H/D) from 2 to 5, where H/D is the ratio of the channel height to the slot width. The vector plots of velocity profile, stagnation and averaged Nusselt number distributions are presented in this paper. The dependency of thermal fields on the Reynolds number, nozzle number and height ratio can be clarified by observing the Nusselt number as heat transfer characteristic at the stagnation point and impingement surface. The Nusselt number at the stagnation point of the central slot shows unsteadiness at H/D=3 and Re=200. The value of Nusselt number at the stagnation point of the central slot decreases with higher Reynolds number and number of nozzle although overall area averaged Nusselt number increases. Hence careful selection of geometrical parameters and number of nozzle are necessary for optimization of the heat transfer performance of multiple slot impinging jet.