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Research Papers: Heat and Mass Transfer

Heat Transfer and Fluid Flow Characteristics in a Sessile Droplet on Oil-Impregnated Surface Under Thermal Disturbance

[+] Author and Article Information
Abdullah Al-Sharafi

Department of Mechanical Engineering,
King Fahd University of Petroleum and Minerals,
Dhahran 31261, Saudi Arabia
e-mail: alsharafi@kfupm.edu.sa

Bekir S. Yilbas

Department of Mechanical Engineering,
Centre of Excellence for Renewable Energy,
King Fahd University of Petroleum and Minerals,
Dhahran 31261, Saudi Arabia
e-mail: bsyilbas@kfupm.edu.sa

Haider Ali

Department of Mechanical Engineering,
King Fahd University of Petroleum and Minerals,
Dhahran 31261, Saudi Arabia
e-mail: haiali@kfupm.edu.sa

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received November 19, 2016; final manuscript received March 31, 2017; published online May 9, 2017. Assoc. Editor: Guihua Tang.

J. Heat Transfer 139(9), 092004 (May 09, 2017) (14 pages) Paper No: HT-16-1757; doi: 10.1115/1.4036388 History: Received November 19, 2016; Revised March 31, 2017

The present study examines the flow field and heat transfer inside a sessile droplet on oil-impregnated surface when subjected to a small temperature difference at the droplet–oil interface. Temperature and flow fields inside the droplet are predicted and the flow velocities predicted are validated through the data obtained from a particle image velocimetry (PIV). Several images of droplets in varying sizes are analyzed and the droplet geometric features and experimental conditions are incorporated in the simulations. A polycarbonate wafer is used to texture the surface via incorporating a solvent-induced crystallization method. Silicon oil is used for impregnation of the textured surfaces. It is found that two counter-rotating circulation cells are formed in the droplet because of the combined effect of the Marangoni and buoyant currents on the flow field. A new dimensionless number (Merve number (MN)) is introduced to assess the behavior of the Nusselt and the Bond numbers with the droplet size. The Merve number represents the ratio of the gravitational force over the surface tension force associated with the sessile droplet and it differs from the Weber number. The Nusselt number demonstrates three distinct behaviors with the Merve number; in which case, the Nusselt number increases sharply for the range 0.8 ≤ MN ≤ 1. The Bond number increases with increasing the Merve number, provided that its values remain less than unity, which indicates the Marangoni current is dominant in the flow field.

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References

Figures

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Fig. 1

Optical image and mesh produced for 20 μL water droplet: (a) schematic of silicon oil encapsulation on the textured surface, (b) water droplet optical image on impregnated silicon oil surface with the presence of wetting ridge, and (c) water droplet geometry meshed for numerical simulations

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Fig. 2

Grid-independent test. Numerical predictions along the central rake predicted for various mesh sizes incorporated in the simulations. The droplet volume is 40 μL and the heating duration is 30 s: (a) velocity distribution and (b) temperature distribution.

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Fig. 3

Flow velocity inside the droplet after 30 s of heating duration: (a) and (b) velocity field obtained from three-dimensional simulation for the water droplet on an oil-impregnated surface for 20 μL and 80 μL droplet volumes, respectively, (c) and (d) velocity field obtained from two-dimensional simulation inside water droplet on an oil-impregnated surface for 20 μL and 80 μL droplet volumes, respectively, and (e) velocity field obtained from two-dimensional simulation of water droplet on a smooth solid surface

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Fig. 4

Micrographs of solution crystallized polycarbonate surface prior to silicon oil impregnation: (a) 3D image of crystallized surface obtained from atomic force microscope, (b) line scan of crystallized surface obtained from atomic force microscopy, (c) scanning electron microscope micrograph of crystallized surface, and (d) scanning electron microscope micrograph showing close view of whiskerslike structures on crystallized surface

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Fig. 5

Optical image of water droplet on oil-impregnated surface: (a) water droplet volume is 20 μL, (b) water droplet volume is 40 μL, (c) water droplet volume is 60 μL, and (d) water droplet volume is 80 μL

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Fig. 6

Particle velocimetry images of hollow glass particles inside 40 μL droplet (a) particle tracking of “A” particle and (b) tracking of “B” particle. The time in between the images are 0.12 s and the images captured at 100 fps.

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Fig. 7

Optical image of 90 μL water droplet on impregnated surface and partial cloaking of oil on droplet surface

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Fig. 8

Temperature variation along the central rake for different heating durations. The droplet volume is 40 μL.

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Fig. 9

Velocity contours inside water droplet and oil ridge for various droplet volumes. The heating duration is 30 s.

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Fig. 10

Temperature contours inside water droplet and oil ridge for various droplet volumes. The heating duration is 30 s.

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Fig. 11

Temperature variation along the central rake of droplet with two volumes after 30 s of heating duration

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Fig. 12

Temporal variation of Bond and Merve numbers in the droplet

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Fig. 13

The Nusselt and Bond numbers variation with the Merve number

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Fig. 14

A schematic view of ray-tracing diagram for a spherical liquid lens [23]

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