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Research Papers: Jets, Wakes, and Impingment Cooling

The Statistical Analysis of Droplet Train Splashing After Impinging on a Superheated Surface

[+] Author and Article Information
Lu Qiu, Swapnil Dubey, Fook Hoong Choo

Energy Research Institute @ NTU,
Nanyang Technological University,
1 Cleantech Loop, 06-04 Cleantech One,
Singapore 637141, Singapore

Fei Duan

School of Mechanical and
Aerospace Engineering,
Nanyang Technological University,
50 Nanyang Avenue,
Singapore 639798, Singapore
e-mail: feiduan@ntu.edu.sg

1Corresponding author.

Presented at the 2016 ASME 5th Micro/Nanoscale Heat & Mass Transfer International Conference. Paper No. MNHMT2016-6436. Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received June 4, 2016; final manuscript received December 24, 2016; published online February 23, 2017. Assoc. Editor: Zhuomin Zhang.

J. Heat Transfer 139(5), 052201 (Feb 23, 2017) (8 pages) Paper No: HT-16-1354; doi: 10.1115/1.4035661 History: Received June 04, 2016; Revised December 24, 2016

An orderly droplet splashing is established when a water droplet train impinges onto a superheated copper surface. The droplets continuously impinge onto the surface with a rate of 40,000 Hz, a diameter of 96 μm or 120 μm, and a velocity of 8.4 m/s or 14.5 m/s. The heat transfers under different wall temperatures are measured, and the corresponding droplet splashing is recorded and analyzed. The effects of wall temperature, droplet Weber number, and surface roughness on the transition of the droplet splashing are investigated. The results suggest that the transferred energy is kept a constant in the transition regime, but a sudden drop of around 25% is observed when it steps into post-transition regime, indicating that the Leidenfrost point is reached. A higher Weber number of droplet train results in a more stable splashing angle and a wider range of splashed droplet diameter. The surface roughness plays no significant role in influencing the splashing angle in the high Weber number case, but the rougher surface elevates the fluctuation of the splashing angle in the low Weber number case. On the rougher surface, the temporary accumulation of the impact droplets is observed, a “huge” secondary droplet can be formed and released. The continuous generation of the huge droplets is observed at a higher wall temperature. Based on the result of droplet tracking of the splashed secondary droplets, the diameter and velocity are correlated.

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Figures

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Fig. 2

The image is analyzed with matlab, in which (a) the sharp edges are detected, (b) the centroid and diameter of each white color area are calculated after filling the closed curves, and then (c) the splashing angle is fitted from the detected secondary droplets. After the initial fitting, the far-field secondary droplets are ruled out, and a second fitting is conducted with the survived droplets.

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Fig. 5

The splashing angle (a) before and (b) after the transition are significantly different. (c) The continuous recording shows that there is a fluctuation in splashing angle, but the mean splashing angle decreases linearly with an increase in wall temperature in the transition zone (100 fps).

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Fig. 4

ΔTtc and Q˙ as a function of surface temperature. A sudden drop of the heating power is observed at the end of the transition regime. The inset shows the calibration results.

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Fig. 3

The velocity vectors of the splashed droplets in 200 frames. The background is a typical frame as the reference. Every three consecutive frames are employed to track the secondary droplets.

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Fig. 9

The variation of the splashing angle in 1000 frames (2857 droplets impinge onto the surface). The splashing angle is statistically time-independent (14,000 fps).

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Fig. 1

(a) The schematics of experimental system, which includes (1) compressed air supply, (2) pressure vessel filled with DI water, (3) flow meter and pressure gauge, (4) nozzle, (5) function generator, (6) lamp, (7) high-speed camera, (8) copper rod, (9) temperature measurements and data acquisition unit, (10) cartridge heaters, (11) AC power supply. The measured surface profile of the (b) rougher surface (furnished by sandpaper #120) and the (c) smoother surface (furnished by sandpaper #1200).

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Fig. 6

The variation of the maximum diameter of the splashed droplet in each frame (100 fps)

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Fig. 7

Aside from the normal splashing (a), the huge droplets are created (b) and (c). On the rougher surface, the huge droplet is occasionally formed but departures very quick, after which the splashing is re-established (b). At higher wall temperature, the extreme droplets are continuously generated and there is no splashing at all (c). However, on the smoother surface, the pattern (b) is not observed, and pattern (c) is observed at a higher temperature (100 fps).

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Fig. 8

The location of the largest splashed droplet in eachframe. The low Weber number cases are more chaotic (100 fps).

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Fig. 10

The distribution of the splashing angle (14,000 fps)

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Fig. 11

The distribution of (a) the splashed droplet diameter and (b) the splashed droplet velocity (14,000 fps). (c) The diameter of the splashed droplet against the splashed velocity of the same secondary droplet. A clear boundary of the distribution could be distinguished, indicating that those two parameters are not independent.

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