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Evaporation, Boiling, and Condensation

Thermophysical Phenomena Associated With Nano-Droplet Impingement on a Solid Surface

[+] Author and Article Information
Geoffrey M. Haas, Aaron P. Wemhoff

Department of Mechanical Engineering,  Villanova University, Villanova, PA 19085geoffrey.m.haas@gmail.com

J. Heat Transfer 134(7), 071503 (May 24, 2012) (7 pages) doi:10.1115/1.4006099 History: Received July 05, 2011; Revised November 01, 2011; Published May 24, 2012; Online May 24, 2012

The thermophysical properties pertaining to the impingement of a nano-droplet onto a solid surface were investigated using molecular dynamics (MD) simulations. The MD simulations used data collection for an entire group of molecules to investigate the propagation of energy in the system. Simulations of a moving nano-droplet colliding with a stationary solid were performed to determine the heat transfer between the droplet and the surface. It was discovered that the droplet-substrate collision caused the droplet temperature to rise significantly upon impact. The substrate also experiences a temperature jump with a slower response time. A theoretical relation for the substrate temperature jump is also developed that shows reasonable agreement with the MD simulations for small droplet diameters. Increasing the diameter of the droplet from 2.0 nm to 4.5 nm showed a gain in the total added substrate kinetic energy. Varying the initial speed of the droplet from 10 m/s to 40 m/s showed no significant difference in the applied kinetic energy onto the substrate, suggesting that the acceleration of the droplet toward the surface due to intermolecular interactions produces an impact speed relatively independent of the initial droplet bulk speed. These trends were also reflected in a thermodynamically based simple theoretical prediction of collision energy, which was shown to be accurate for droplet diameters up to 3.5 nm. The collision energy was estimated to be on the order of 1–10 eV, and the applied heat flux is on the order of GW/m2 .

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Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 1

Initialized domain featuring a 3 nm diameter spherical liquid droplet above a solid surface

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Figure 2

Overall droplet transient temperature distribution

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Figure 3

Overall substrate transient temperature distribution

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Figure 4

Transient droplet downward speed distributions for various initial speeds

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Figure 5

Impingement temperature difference for droplets with various initial speeds

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Figure 6

Transient droplet downward speed distributions for various droplet diameters

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Figure 7

Impingement temperature rise for various droplet sizes

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Figure 8

Final configuration of layered fluid molecules on the substrate for a 4.0 nm diameter droplet

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Figure 9

Predicted substrate collision heat flux for various initial droplet speeds

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Figure 10

Predicted substrate collision heat flux for various droplet diameter values

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