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Research Papers: Forced Convection

Influence of a Magnetic Obstacle on Forced Convection in a Three-Dimensional Duct With a Circular Cylinder

[+] Author and Article Information
Xidong Zhang

College of Energy and Power Engineering,
Nanjing Institute of Technology,
Nanjing 211167, China;
College of Astronautics,
Nanjing University of Aeronautics
and Astronautics,
Nanjing 210016, China

Hulin Huang

College of Astronautics,
Nanjing University of Aeronautics
and Astronautics,
Nanjing 210016, China
e-mail: hlhuang@nuaa.edu.cn

Yin Zhang

College of Astronautics,
Nanjing University of Aeronautics
and Astronautics,
Nanjing 210016, China

Hongyan Wang

College of Energy and Power Engineering,
Nanjing Institute of Technology,
Nanjing 211167, China

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received April 11, 2014; final manuscript received June 23, 2015; published online August 11, 2015. Assoc. Editor: Ali Khounsary.

J. Heat Transfer 138(1), 011703 (Aug 11, 2015) (11 pages) Paper No: HT-14-1185; doi: 10.1115/1.4031108 History: Received April 11, 2014

The predictions of flow structure, vortex shedding, and drag force around a circular cylinder are promoted by both academic interest and a wide range of practical situations. To control the flow around a circular cylinder, a magnetic obstacle is set upstream of the circular cylinder in this study for active controlling the separated flow behind bluff obstacle. Moreover, the changing of position, size, and intensity of magnetic obstacle is easy. The governing parameters are the magnetic obstacle width (d/D = 0.0333, 0.1, and 0.333) selected on cylinder diameter, D, and position (L/D) ranging from 2 to 11.667 at fixed Reynolds number Rel (based on the half-height of the duct) of 300 and the relative magnetic effect given by the Hartmann number Ha of 52. Results are presented in terms of instantaneous contours of vorticity, streamlines, drag coefficient, Strouhal number, pressure drop penalty, and local and average Nusselt numbers for various magnetic obstacle widths and positions. The computed results show that there are two flow patterns, one with vortex shedding from the magnetic obstacle and one without vortex shedding. The optimum conditions for drag reduction are L/D = 2 and d/D = 0.0333–0.333, and under these conditions, the pressure drop penalty is acceptable. However, the maximum value of the mean Nusselt number of the downstream cylinder is about 93% of that for a single cylinder.

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Figures

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Fig. 1

Schematic configuration of flow region and related geometrical parameters. The north (N) and south (S) magnetic poles with the same size (d) are separated by a distance H = 3. The origin of the coordinate axes is placed at the cylinder's geometrical center.

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Fig. 2

Flow past a magnetic obstacle: (a) present result and (b) Samsami et al. [29]

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Fig. 3

Instantaneous vorticity contours (ωz) at (a) L/D = 2, (b) 2.667, (c) 3.333, (d) 4.667, (e) 6.667, and (f) L/D = 11.667 for d/D = 0.333. Contour levels are displayed between −1 ≤ ωz ≤ 1, with the solid and dotted lines represent positive and negative values, respectively. Solid bold rectangle and circularity show borders of the external magnet and cylinder.

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Fig. 4

Flow streamlines in plane z = 0: (a) L/D = 2, (b) 2.667, (c) 3.333, and (d) L/D = 4.667 at d/D = 0.333. Solid bold rectangle and circularity show borders of the external magnet and cylinder.

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Fig. 5

Wake structure around two tandem cylinders at d/D = 0.25 (experiments of Wang et al. [12])

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Fig. 6

Flow streamlines in plane z = 0: (a) L/D = 2, (b) 2.667, (c) 3.333, and (d) L/D = 4.667 at d/D = 0.0333. Solid bold rectangle and circularity show borders of the external magnet and cylinder.

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Fig. 7

Time history of spanwise velocity v (solid line) and lift force coefficient CL (dashed line) for various spaces L/D: (a) 2, (b) 2.667, (c) 4.667, and (d) 11.667 at d/D = 0.333

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Fig. 8

Fourier spectra of spanwise velocity F(v) and lift coefficient F(CL) at d/D = 0.333 for spaces: (a) L/D = 2, (b) 2.667, (c) 4.667, and (d) L/D = 11.667. Spanwise velocity (v) and lift coefficient (CL) are as indicated in the figures.

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Fig. 9

Time-averaged drag coefficient CD/CDs of the circular cylinder versus space L/D for various magnetic obstacle widths d/D

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Fig. 10

Time-averaged pressure coefficient Cp as a function of θ for different spaces L/D: (a) and (b) d/D = 0.0333 and (c) and (d) d/D = 0.333

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Fig. 11

Time- and surface-averaged Nusselt number Nu/Nus as a function of space L/D for various magnetic obstacle widths d/D

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Fig. 12

Time-averaged local Nusselt number Nu¯ as a function of θ for different spaces L/D: (a) and (b) d/D = 0.0333 and (c) and (d) d/D = 0.333

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