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Research Papers: Forced Convection

The Reasons of Heat Transfer Enhancement in a Laminar Channel Flow With Uniform Heat Flux on the Wall Under a Pair of Longitudinal Vortex Generators Mounted on the Bottom Wall

[+] Author and Article Information
Qiang Zhang

School of Mechanical Engineering,
Lanzhou Jiaotong University,
Lanzhou 730070, Gansu, China;
Key Laboratory of Railway Vehicle
Thermal Engineering,
Lanzhou Jiaotong University,
Ministry of Education,
Lanzhou 730070, Gansu, China
e-mail: zhangqiang@mail.lzjtu.cn

Liang-Bi Wang

School of Mechanical Engineering,
Lanzhou Jiaotong University,
Lanzhou 730070, Gansu, China;
Key Laboratory of Railway Vehicle
Thermal Engineering,
Lanzhou Jiaotong University,
Ministry of Education,
Lanzhou 730070, Gansu, China
e-mail: lbwang@mail.lzjtu.cn

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received August 28, 2016; final manuscript received May 20, 2017; published online July 25, 2017. Assoc. Editor: Jim A. Liburdy.

J. Heat Transfer 139(12), 121701 (Jul 25, 2017) (15 pages) Paper No: HT-16-1538; doi: 10.1115/1.4037080 History: Received August 28, 2016; Revised May 20, 2017

To find the reasons of heat transfer enhancement of a laminar convective heat transfer process in a channel at a uniform heat flux boundary when a pair of longitudinal vortex generators (VGs) is mounted on the bottom wall, the laminar convective heat transfer process in the channel is investigated numerically in a frame built up by the convective transport equation of the heat flux. The results show that longitudinal vortices greatly increase the local convection contribution terms that determine the local intensity of the convective transport of the heat flux component in the span direction, and that the increased local contribution terms intensify the local convective transport of the heat flux component in the same direction. This process increases the convection contribution terms that determine the convective transports of the heat flux components in the main stream direction and in the normal direction of the channel walls. The increase in these convection contribution terms results in an enhancement of the convective heat transfer ability on the channel walls, and then, the heat transfer is enhanced by longitudinal vortices. When the span-averaged characteristic is numerically counted, longitudinal vortices are found to have no contribution on the span-averaged convective transport of the heat flux in the span direction.

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Figures

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Fig. 1

Schematic view of studied model: (a) the studied channel, (b) the computational domain, (c) position of y = B/6, y = B/3, y = B/2, and (d) position of y = B/2, x = x1, x = x2, x = x3, x = x4, and four inspected lines

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Fig. 2

Schematic view of the grid system used in simulation

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Fig. 3

Velocity fields on the sections for the cases with VGs and without VGs: (a) and (e) x = 0.015 m, (b) and (f) x = 0.019 m, (c) and (g) x = 0.024 m, (d) and (h) x = 0.032 m, (a)–(d) the case without VGs, and (e)–(h) the case with VGs

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Fig. 4

Streamlines on the sections for the cases with VGs and without VGs: (a) x = 0.015 m, (b) x = 0.019 m, (c) x = 0.024 m, and (d) x = 0.032 m

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Fig. 5

Comparison of the velocity fields in the channels with and without VGs: (a) the channel without VGs, (b)–(d) the channels with VGs, (a) and (b) y = B/2, (c) y = B/3, and (d) y = B/6

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Fig. 6

Nulocal on the top and the bottom surfaces: (a) the top surface for the case without VGs, (b) the bottom surface for the case with VGs, and (c) the top surface for the case with VGs

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Fig. 7

The distributions of Nulocal and Nulocal/Nulocal,plain on the intersection lines by the planes of y = B/6, y = B/3, y = B/2 and the planes of z = 0 and z = H: (a) Nulocal on the top surface, (b) Nulocal on the bottom surface, (c) Nulocal/Nulocal,plain on the top surface, and (d) Nulocal/Nulocal,plain on the bottom surface

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Fig. 8

The distribution of Nus and Nus/Nus,plain along the x direction: (a) Nus and (b) Nus/Nus,plain

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Fig. 9

We-xT, Wc-xT, (We-x + Wc-x)/ΔT and qxT along the intersection lines of y = B/2 and x = x1, x2, x3, x4 in cases with VG and without VG: (a) We-xT, (b) Wc-xT, (c) (We-x + Wc-x)/ΔT, and (d) qxT

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Fig. 10

We-yT, Wc-yT, (We-y + Wc-y)/ΔT and qyT along the intersection lines of y = B/2 and x = x1, x2, x3, x4 in the cases with VGs and without VGs: (a) We-yT, (b) Wc-yT, (c) (We-y + Wc-y)/ΔT, and (d) qyT

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Fig. 11

We-zT, Wc-zT, (We-z + Wc-z)/ΔT and qzT along the intersection lines of y = B/2 and x = x1, x2, x3, x4 in the cases with VG and without VG: (a) We-zT, (b) Wc-zT, (c) (We-z + Wc-z)/ΔT, and (d) qzT

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Fig. 12

SWe-xT, SWc-xT, S(We-x + Wc-x)/ΔT and SqxT along the z-direction at the different x in the cases with VG and without VG: (a) SWe-xT, (b) SWc-xT, (c) S(We-x + Wc-x)/ΔT, and (d) SqxT

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Fig. 13

SWe-yT, SWc-yT, S(We-y + Wc-y)/ΔT and SqyT along the y-direction at the different x in the cases with VG and without VG: (a) SWe-yT, (b) SWc-yT, (c) S(We-y + Wc-y)/ΔT, and (d) SqyT

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Fig. 14

SWe-zT, SWc-zT, S(We-z + Wc-z)/ΔT and SqzT along the z-direction at the different x in the cases with VG and without VG: (a) SWe-zT, (b) SWc-zT, (c) S(We-z + Wc-z)/ΔT, and (d)SqzT

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