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Research Papers: Evaporation, Boiling, and Condensation

Droplet Departure Characteristics and Dropwise Condensation Heat Transfer at Low Steam Pressure

[+] Author and Article Information
Rongfu Wen, Zhong Lan, Benli Peng, Wei Xu, Yaqi Cheng

Liaoning Provincial Key Laboratory of
Clean Utilization of Chemical Resources,
Institute of Chemical Engineering,
Dalian University of Technology,
No. 2 Linggong Rd.,
High-Tech District,
Dalian 116024, China

Xuehu Ma

Liaoning Provincial Key Laboratory of
Clean Utilization of Chemical Resources,
Institute of Chemical Engineering,
Dalian University of Technology,
No. 2 Linggong Rd.,
High-Tech District,
Dalian 116024, China
e-mail: xuehuma@dlut.edu.cn

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received May 21, 2015; final manuscript received March 7, 2016; published online April 12, 2016. Assoc. Editor: P. K. Das.

J. Heat Transfer 138(7), 071501 (Apr 12, 2016) (8 pages) Paper No: HT-15-1354; doi: 10.1115/1.4032956 History: Received May 21, 2015; Revised March 07, 2016

Dropwise condensation has received significant attention due to its great potential to enhance heat transfer by the rapid droplet removal. In this work, droplet departure characteristics on a vertical surface, especially the droplet departure retention at low steam pressure and its effect on the heat transfer performance are investigated experimentally. The energy dissipation increases during droplet movement due to the increased viscosity at low pressure. Droplet oscillation caused by excess kinetic energy weakens and the dynamic contact angle (CA) hysteresis becomes apparent, which is not beneficial to droplet departure. Condensed droplets grow larger and fall more slowly at low pressure compared to that at atmospheric pressure. The droplet moves smoothly downward once it grows to departure size at atmospheric pressure while the droplet exhibits an intermittent motion at low pressure. Based on the droplet departure characteristics, a unified heat transfer model for dropwise condensation is developed by introducing the pressure-dependent departure velocity. The modified model very well predicts heat transfer performances at various pressures and the nonlinearity of heat flux varying with surface subcooling is quantitatively explained. This work provides insights into the heat transfer mechanism of dropwise condensation and offers a new avenue to further enhance heat transfer at low steam pressure.

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Figures

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Fig. 1

Schematic of experimental setup and condensing chamber

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Fig. 2

Surface morphology and wettability. (a) SEM image and CA measure and (b) surface roughness.

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Fig. 3

Droplet morphologies and dynamic behaviors. Images of falling droplets showing departure characteristics at different pressures. (a) Smooth liquid–gas interface and hemispherical droplet. Conditions: Pv = 1.5 kPa, ΔT = 4 ± 0.2 K. (b) Obvious surface waves and intense oscillating droplets. Conditions: Pv = 101 kPa, ΔT = 4 ± 0.3 K.

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Fig. 4

Schematics of the dynamic CA response to contact line velocity according to Manor [41]. (a) Falling droplet at low pressure and CA without oscillation. (b) Falling droplet at atmospheric pressure and dynamic CA with intense oscillation of amplitude δθ around its mean value, θm. (c) and (d) The apparent CA respond to contact line velocity, uCL, with and without oscillation.

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Fig. 5

Droplet departure characteristics. Time-lapse images of falling droplet at pressures of 101 kPa (a) and 1.5 kPa (b). Falling distance and falling velocity calculated by the slope of distance at the pressure of 101 kPa (c) and 1.5 kPa (d). The hollow triangles represent the falling distances and the solid triangles represent the falling velocities.

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Fig. 6

Droplet departure velocity as a function of steam pressure and surface subcooling. The droplet departure velocity is determined by the departure distance and departure time, vde = Δlt.

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Fig. 7

Schematic of typical stages through the droplet life cycle. (a) Nucleation at the exposed surface, t = 0. (b) Droplet growth by direct steam condensation, 0 < t < τdi. (c) Droplet growth by coalescences, τdi < t < τdi + τco. (d) Droplet departure, τdi + τco < t < τdi + τco + τdep.

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Fig. 8

Droplet life cycle time and departure period as a function of steam pressure and surface subcooling. (a) Droplets life cycle time includes nucleation, growth, coalescence, and departure. (b) Departure time is not sensitive to the surface subcooling. It is highly dependent on the steam pressure.

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Fig. 9

The reduced proportion of heat flux as a function of steam pressure and surface subcooling

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Fig. 10

Comparison of experimental results with the predictions by previous model and modified model. (a) Previous heat transfer model [29] ignoring droplet departure velocity. (b) Modified heat transfer model by introducing droplet departure characteristic.

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