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Technical Briefs

On the Mechanism of Dropwise Condensation of Steam on Ion Implanted Metallic Surfaces

[+] Author and Article Information
Michael H. Rausch

Lehrstuhl für Technische Thermodynamik (LTT), Universität Erlangen-Nürnberg, Am Weichselgarten 8, D-91058 Erlangen, Germany

Alfred Leipertz

Lehrstuhl für Technische Thermodynamik (LTT), Universität Erlangen-Nürnberg, Am Weichselgarten 8, D-91058 Erlangen, Germanysek@ltt.uni-erlangen.de

Andreas P. Fröba1

Lehrstuhl für Technische Thermodynamik (LTT), Universität Erlangen-Nürnberg, Am Weichselgarten 8, D-91058 Erlangen, Germanyapf@ltt.uni-erlangen.de

1

Corresponding author.

J. Heat Transfer 132(9), 094503 (Jul 15, 2010) (3 pages) doi:10.1115/1.4001646 History: Received December 01, 2009; Revised April 13, 2010; Published July 15, 2010; Online July 15, 2010

Our recent experimental studies indicate that nanostructured, chemically inhomogeneous surfaces are the origin of dropwise condensation of steam on ion implanted metals. Yet, the underlying microscopic mechanism governing this special condensation form is still not clear. We suggest a condensation model based on droplet nucleation and growth on elevated precipitates, resulting in short-term steam entrapment after droplet coalescence. According to the wetting theory, this transition state yields increased macroscopic contact angles. Condensation phenomena such as enlarging dropwise condensation areas in spite of increasing condensation rate become comprehensible by our model. Furthermore, it points out that for this special surface type, contact angles and surface free energies measured under ambient air conditions are not usable for predicting the condensation form of steam. Although the suggested microscopic model cannot be directly proved by experiment, its validity is supported by its capability of explaining experimental observations colliding with previous theoretical approaches.

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Copyright © 2010 by American Society of Mechanical Engineers
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Figures

Grahic Jump Location
Figure 1

Condensation forms of steam at about 1 bar and small subcooling (<1 K) and contact angles Θ for water measured under ambient conditions: (a) stainless steel AISI 321, ion beam technology, C+ ions, 1016 cm−2, and 20 keV; (b) Hastelloy C-2000, plasma ion implantation, nitrogen ions, 1017 cm−2, and 40 keV

Grahic Jump Location
Figure 2

Steam entrapment in cavities between elevated nucleation sites due to fast droplet coalescence

Grahic Jump Location
Figure 3

Effect of gas entrapment and surface roughness on the macroscopic contact angle according to the model suggested by Johnson and Dettre (10)

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