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Research Papers: Heat and Mass Transfer

Flow Field Inside a Sessile Droplet on a Hydrophobic Surface in Relation to Self Cleaning Applications of Dust Particles

[+] 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

Ahmet Z. Sahin

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

H. 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 April 28, 2016; final manuscript received November 9, 2016; published online January 24, 2017. Assoc. Editor: Gennady Ziskind.

J. Heat Transfer 139(4), 042003 (Jan 24, 2017) (16 pages) Paper No: HT-16-1234; doi: 10.1115/1.4035281 History: Received April 28, 2016; Revised November 09, 2016

Internal fluidity of a sessile droplet on a hydrophobic surface and dynamics of fine size dust particles in the droplet interior are examined for various droplet contact angles. The geometric features of the droplet incorporated in the simulations resemble the actual droplet geometry of the experiments, and simulation conditions are set in line with the experimental conditions. The dust particles are analyzed, and the surface tension of the fluid, which composes of the dust particles and water, is measured and incorporated in the analysis. Particle tracking method is adopted experimentally to validate the numerical predictions of the flow field. It is found that heat transfer from the hydrophobic surface to the droplet gives rise to the formation of two counter rotating cells inside the droplet. The Nusselt and the Bond numbers increase with increasing droplet contact angle. The number of dust particles crossing over the horizontal rake, which corresponds to the top surface of the dust particles settled in the droplet bottom, toward the droplet interior increases as the particle density reduces, which is more pronounced in the early period. Experimental findings of flow velocity well agree with its counterparts obtained from the simulations.

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Figures

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Fig. 1

Optical image and droplet geometry used in the simulations: (a) side view of optical image on the hydrophobic surface (θ = 140 deg), (b) geometric configuration used in the simulations and resembling the actual optical image, and (c) top view of optical image of the droplet

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Fig. 2

Velocity field inside droplet for the contact angle of 100 deg: (a) three-dimensional simulation and (b) two-dimensional simulation

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Fig. 3

Surface tension of water–dust mixture for three different concentrations

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Fig. 4

Mesh used in the simulations for two-dimensional case (θ = 120 deg)

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Fig. 5

SEM micrograph and AFM images of crystalized polycarbonate surface: (a) SEM micrograph of textured surface, (b) SEM images of fibrils on the crystalized surface, (c) 3D AFM image of surface texture, and (d) line scan of AFM image

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Fig. 6

Optical image of water droplet and dust particles inside the water droplet, and contact angle images: (a) Optical image of water droplet with dust particles, Δl is the moving particles inside droplet and Δh is the sediment dust particles at droplet bottom, and (b) sessile droplet contact angle images

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Fig. 7

SEM micrographs of dust particles: (a) dust particles with different sizes and (b) small dust particles attach at large particle surface (dotted circles shows the attached some small particles)

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Fig. 8

X-ray diffractogram of dust particles

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Fig. 9

Experimental setup for particle analysis in the droplet

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Fig. 10

Microscopic images of particles and tracking for the velocity measurement

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Fig. 11

Velocity and temperature variation along the vertical rake inside the droplet for various durations: (a) velocity variation along the horizontal rake for different durations and (b) temperature variation along the horizontal rake for different durations

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Fig. 12

Velocity contours with and without particles inside the droplet for two contact angles after 30 s duration. The particle density is 857 kg/m3.

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Fig. 13

Temperature contours inside the droplet for two contact angles after 30 s duration

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Fig. 14

Distribution of dust particles inside the droplet for various dust particle densities and two droplet contact angles after 30 s duration

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Fig. 15

Temporal variation of total number of particles crossing over the horizontal rake and inside the droplet for three different densities of the dust particles. The particle diameter is kept 20 μm for all densities and 30 s duration.

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Fig. 16

Variation of the Nusselt and the Bond numbers with droplet contact angle

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Fig. 17

Variation of the Nusselt number with the Ayse number

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Fig. 18

A schematic view of ray-tracing diagram for a spherical liquid lens [5,29]

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