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Research Papers: Evaporation, Boiling, and Condensation

Droplet First and Second Consecutive Impacts and Droplet–Droplet Collision on a Hot Surface in the Film Boiling Region

[+] Author and Article Information
E. Kompinsky

Department of Mechanical Engineering,
Ben-Gurion University of the Negev,
Beer-Sheva 8410501, Israel
e-mail: eitank44@gmail.com

E. Sher

Faculty of Aerospace Engineering,
Technion—Israel Institute of Technology,
Haifa 3200003, Israel

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received April 26, 2014; final manuscript received February 1, 2015; published online February 25, 2015. Assoc. Editor: Sujoy Kumar Saha.

J. Heat Transfer 137(5), 051503 (May 01, 2015) (13 pages) Paper No: HT-14-1244; doi: 10.1115/1.4029743 History: Received April 26, 2014; Revised February 01, 2015; Online February 25, 2015

We studied experimentally the behavior of a single cold water droplet that falls onto a hot horizontal flat solid surface in the film boiling region. We found that when the droplet hits the surface (for the first time), three different regimes may occur. These regimes depend on the ratio of fluid's inertia and its surface tension (the Weber number, indicated as We) and on the surface temperature. For relatively low We numbers or surface temperatures, the droplet completely bounces back from the surface and no breakup occurs. For intermediate We numbers or surface temperatures, the spreading stage is faster and the droplet undergoes spreading and partial recession before it breaks up into bouncing small secondary droplets that leap inward and successively coalesce above the surface to form a single droplet once again. For high We numbers or surface temperatures, the spreading velocity is higher, the contact area with the surface is greater, and the liquid film thickness is smaller. Thus, during the expansion of the spreading stage, the droplet breaks up into bouncing small secondary droplets that uninterruptedly leap outward and travel independently. We also present the limiting conditions differentiating between the different behaviors found. This work shows droplet film boiling behaviors that are essentially different than droplet levitation on top of a thin vapor layer, as mainly assumed in theoretical models. We also observed that when a droplet hits the surface for the second, consecutive time (and on), the droplet behaves somewhat differently due to its preheating, very low impact velocity, different shape, spin, orientation, and the surface temperature. At the second impact on the surface (and on), the droplet can continue its bounce in a unique and different manner than in the first impact or it can explode violently to small secondary droplets. Both are unique and differentiating mainly by the droplet's shape and orientation at the exact moment of impact on the surface. Additionally, a rare and unique view of droplet–droplet collision during film boiling is presented. This type of collision behaves in a different way than other droplet–droplet collisions and compared to the adiabatic case of droplet–droplet collision on a nonheated surface. The behaviors found, presented, and discussed in this study change our view of the droplet–surface and droplet–droplet interactions that occur in spray cooling in the film boiling region.

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References

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Roisman, I. V., Prunet-Foch, B., Tropea, C., and Vignes-Adler, M., 2002, “Multiple Drop Impact Onto a Dry Solid Substrate,” J. Colloid Interface Sci., 256(2), pp. 396–410. [CrossRef]
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Kompinsky, E., and Sher, E., 2009, “Experimental Comparisons Between Droplet–Droplet Collision and Single-Droplet Impacts on a Solid Surface,” Atomization Sprays, 19(5), pp. 409–423. [CrossRef]
Kompinsky, E., Dolan, G., and Sher, E., 2013, “Experimental Study on the Dynamics of Binary Fuel Droplet Impacts on a Heated Surface,” Chem. Eng. Sci., 98, pp. 186–194. [CrossRef]
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Moita, A. S., and Moreira, A. L. N., 2007, “Drop Impacts Onto Cold and Heated Rigid Surfaces: Morphological Comparisons, Disintegration Limits and Secondary Atomization,” Int. J. Heat Fluid Flow, 28(4), pp. 735–752. [CrossRef]

Figures

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Fig. 1

Schematic experimental setup

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Fig. 2

Droplet impact on a hot surface and film boiling without breakup. Surface temperature—187 °C. Impact velocity—0.45 m/s. Droplet diameter—3.2 mm. We number—9. Time from impact is indicated below the images in milliseconds.

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Fig. 3

Schematic model of droplet film boiling without breakup

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Fig. 4

Droplet impact on a heated surface and film boiling direction onset. Surface temperature—147 °C. Impact velocity—2.5 m/s. Droplet diameter—3 mm. We number—262. Time from impact is indicated inside the figure in milliseconds.

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Fig. 5

Schematic model of droplet film boiling breakup and coalescence

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Fig. 6

Droplet impact and film boiling—inward (a) and outward (b) directions. Surface temperature—(a) 175 °C, (b) 201 °C. Impact velocity—1.6 m/s. Droplet diameter—3.1 mm. We number—111. Time from impact is indicated beneath the images milliseconds.

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Fig. 7

Schematic model of droplet film boiling breakup, no coalescence

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Fig. 8

Mapping of the conditions for droplet film boiling breakup. Surface temperature: 413–483 K. Impact velocity: 0.4–2.5 m/s. Droplet diameter: 3–3.3 mm. We number: 6–270. Initial droplet temperature: 303 K.

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Fig. 9

Droplet film boiling during the second surface contact. Surface temperature—145 °C. Impact velocity (for second impact)—0.1 m/s. Droplet diameter—3.1 mm. We number (for second impact)—0.5. Time from second surface impact is indicated below the images in milliseconds.

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Fig. 10

Droplet film boiling second surface contact. Surface temperature—173 °C. Impact velocity (for second impact)—0.2 m/s. Droplet diameter—3.1 mm. We number—2. Time from impact is indicated below the images in milliseconds.

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Fig. 11

Film boiling second surface contact of a secondary small droplet created during first surface contact breakup. Surface temperature—195 °C. Impact velocity (for second impact)—0.3 m/s. Droplet diameter—1.4 mm. We number—2.05. Time from impact is indicated below the images in milliseconds.

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Fig. 12

Schematic model of droplet film boiling, second impact on the hot surface and on

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Fig. 13

Droplet film boiling second surface contact violent breakup phenomenon. Surface temperature—169 °C. Impact velocity—0.3 m/s. Droplet diameter—3.1 mm. We number—4.5. Time from first surface impact is indicated below the images in milliseconds.

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Fig. 14

Schematic model of droplet film boiling, second impact explosion on the hot surface

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Fig. 15

Droplet film boiling evolution. Surface temperature—152 °C. Impact velocity—1.4 m/s. Droplet diameter—3 mm. We number—82.2. Time from impact is indicated below the images in milliseconds.

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Fig. 16

Droplet–droplet collision during film boiling. Surface temperature—137 °C. Impact velocity—1.7 m/s. Droplet diameter—3.3 mm. We number—132. Second deposited droplet is at initial temperature of 30 °C. Time from impact is indicated below the images in milliseconds.

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Fig. 17

Droplet–droplet collision on a nonheated surface (a) and on a heated surface in the film boiling region (b). Surface temperature—(a) 30 °C, (b) 137 °C. Impact velocity—1.7 m/s. Droplet diameter—3.3 mm. We number—132. Second deposited droplets are at initial temperature of 30 °C. Time from impact is indicated inside below the images in milliseconds.

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