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Research Papers: Evaporation, Boiling, and Condensation

The Formation of Banded Condensate Films in Weak Ammonia–Water Mixtures

[+] Author and Article Information
Joe Deans1

 Department of Mechanical Engineering University of Auckland Private Bag 92019 Auckland 1142New Zealand e-mail:j.deans@auckland.ac.nz Dokuz Eylul University Faculty of Engineering Department of Mechanical Engineering 35100 Bornova IzmirTurkey e-mail:serhan.kucuka@deu.edu.tr

Serhan Kucuka

 Department of Mechanical Engineering University of Auckland Private Bag 92019 Auckland 1142New Zealand e-mail:j.deans@auckland.ac.nz Dokuz Eylul University Faculty of Engineering Department of Mechanical Engineering 35100 Bornova IzmirTurkey e-mail:serhan.kucuka@deu.edu.tr

1

Corresponding author.

J. Heat Transfer 133(10), 101505 (Aug 15, 2011) (10 pages) doi:10.1115/1.4004019 History: Received January 12, 2010; Revised April 14, 2011; Published August 15, 2011; Online August 15, 2011

This experimental and analytical study focuses on the formation of banded condensate films on a horizontal tube, when the ammonia mass fraction of the vapor is less than 0.0126. The experimental results show that there is a step change in the heat transfer coefficient when a banded film is formed. This change develops when the vapor mass fraction is approximately 0.004 and the heat transfer coefficient at the end of the change is 15% greater than the value expected for steam. The banded films are initiated at the base of the tube; midway between the regions where droplets detach. Photographs taken during the tests show that the development starts with the formation of a single band; when this band is complete, secondary bands are formed on either side and then further bands are added until the complete tube surface is covered with a banded film. The reason for the delay in the formation of the bands is examined using a simple two-dimensional force balance. The success of this examination provides an insight to the mechanisms employed in the formation of banded films.

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Copyright © 2011 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Condensate film morphologies found during ammonia water condensation

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Figure 2

The condensing tube assembly

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Figure 3

Variation in heat transfer coefficient with ammonia concentration and cooling water temperature

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Figure 4

Photographs showing the evolution of banded films. (a) Disturbance initiation. (b) Disturbance growth. (c) Collection of bands. (d) Completely banded film.

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Figure 5

The circumferential variation of surface temperature and the calculated film thickness during the condensation of steam

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Figure 6

Surface tension at 20 °C for ammonia water mixtures with low ammonia mass fractions (a) Experimental values (b) Experimental and calculated values

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Figure 7

Surface tension force balance

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Figure 8

Phase diagram at interface

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Figure 9

Incision geometry at the base of the tube

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Figure 10

Forces on the incision wall

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