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Multiphase Flows

An Experimental Study on the Oblique Collisions of Water Droplets With a Smooth Hot Solid

[+] Author and Article Information
Hitoshi Fujimoto

Graduate School of Energy Science,  Kyoto University, Kyoto 606-8501, Japanh-fujimoto@energy.kyoto-u.ac.jp

Ryota Doi, Hirohiko Takuda

Graduate School of Energy Science,  Kyoto University, Kyoto 606-8501, Japan

J. Fluids Eng 134(7), 071301 (Jun 21, 2012) (12 pages) doi:10.1115/1.4006926 History: Received June 22, 2011; Revised May 29, 2012; Published June 21, 2012; Online June 21, 2012

The motions of liquid droplets impinging on a solid substrate have been studied experimentally in fundamental research on various types of industrial applications, including spray cooling. The oblique collision of a single water droplet with a hot Inconel 625 alloy surface has been investigated by means of a two-directional flash photography technique that uses two digital still cameras and three flash units. The experiments were conducted under the following conditions. The preimpact diameter of the droplets was approximately 0.6 mm, the impact velocity was 1.9–3.1 m/s, and the temperature of the Inconel 625 alloy surface ranged from 170 °C to 500 °C. The impact angle of droplets on the solid surface was in the range 45 deg–90 deg. Experiments using 2.5 mm diameter droplets at an impact velocity of 0.84–1.4 m/s were also conducted at the surface temperature of 500 °C. At surface temperatures of 200 °C, 300 °C, and 400 °C, the droplet deforms into an asymmetric shape and moves downward along the tilted surface. Numerous secondary droplets jet upward from the deforming droplet as a result of the blowout of vapor bubbles into the atmosphere. At a surface temperature of 500 °C and a low Weber number Wen based on the normal velocity component to the solid surface, no secondary droplets are observed. The droplet rebounds off the solid without disintegrating. The droplet becomes almost axisymmetric in shape during the collision regardless of the impact angle. The dimensionless collision behaviors of large and small droplets were similar for the same Wen when the temperature was 500 °C. Using Wen , we investigated the deformation characteristics of droplets in oblique collisions.

Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 1

Schematic of experimental apparatus

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Figure 2

Schematic of photography setup

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Figure 3

Examples of the double-exposed images of the droplet

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Figure 4

Definition of coordinate and apparent contact length l of a droplet

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Figure 5

Deformation behavior of water droplets impacting on Inconel surface at Tw  = 170 ± 10 °C and α = 30 deg for dp  = 0.61 ± 0.01 mm and υ = 2.1 ± 0.05 m/s. (a) Double-exposure backlit images of droplet taken by Camera A shown in Fig. 2 and (b) bird’s-eye view images of droplets taken by Camera B.

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Figure 6

Deformation behavior of water droplets impacting on Inconel surface at Tw  = 200 ± 10 °C and α = 30 deg for dp  = 0.59 ± 0.01 mm and υ = 2.1 ± 0.05 m/s

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Figure 7

Deformation behavior of water droplets impacting on Inconel surface at Tw  = 300 ± 10 °C and α = 30 deg for dp  = 0.60 ± 0.01 mm and υ = 2.0 ± 0.05 m/s

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Figure 16

Relationship between dimensionless time required for the circular water film to reach the maximum spreading diameter and Weber number based on normal velocity component

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Figure 17

Relationship between dimensionless droplet resident time on a hot surface and Weber number based on normal velocity component

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Figure 8

Deformation behavior of water droplets impacting on Inconel surface at Tw  = 400 ± 10 °C and α = 30 deg for dp  = 0.60 ± 0.01 mm and υ = 2.1 ± 0.05 m/s

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Figure 9

Deformation behavior of water droplets impacting on Inconel surface at Tw  = 500 ± 10 °C and α = 30 deg for dp  = 0.58 ± 0.01 mm and υ = 1.9 ± 0.05 m/s

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Figure 10

Deformation behavior of water droplets impacting on Inconel surface at Tw  = 500 ± 10 °C and α = 45 deg for dp  = 0.61 ± 0.01 mm and υ = 2.6 ± 0.05 m/s

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Figure 11

Time evolution of spreading diameters of droplet and their definitions

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Figure 12

Deformation behavior of water droplets impacting on Inconel surface at Tw  = 500 ± 10 °C and α = 30 deg for dp  = 2.49 ± 0.08 mm and υ = 0.90 ± 0.02 m/s

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Figure 13

Time evolution of the contact length of a large and small droplet. The impact conditions are the same as in Figs.  912. Note that the measurement uncertainty of l/dp is within ± 0.04 for a small droplet and ± 0.08 for a large droplet.

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Figure 14

Time evolution of leading edges, trailing edges, and average values of large and small droplets. The impact conditions are the same as in Figs.  912. Note that the measurement uncertainty of x/dp is within ± 0.04 for small droplets and ± 0.08 for large droplets.

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Figure 15

Relationship between dimensionless maximum spreading length of a droplet and Weber number based on normal velocity component

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