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Research Papers: Evaporation, Boiling, and Condensation

Wetting Mode Evolution of Steam Dropwise Condensation on Superhydrophobic Surface in the Presence of Noncondensable Gas

[+] Author and Article Information
Xuehu Ma1

Institute of Chemical Engineering,  Dalian University of Technology, Dalian 116024, Chinaxuehuma@dlut.edu.cn

Sifang Wang, Zhong Lan, Benli Peng

Institute of Chemical Engineering,  Dalian University of Technology, Dalian 116024, Chinaxuehuma@dlut.edu.cn

H. B. Ma, P. Cheng

Department of Mechanical and Aerospace Engineering,  University of Missouri-Columbia, Columbia, MO 65211

1

Corresponding author.

J. Heat Transfer 134(2), 021501 (Dec 09, 2011) (9 pages) doi:10.1115/1.4005094 History: Received May 18, 2010; Revised August 23, 2011; Published December 09, 2011; Online December 09, 2011

It is well known that heat transfer in dropwise condensation (DWC) is superior to that in filmwise condensation (FWC) by at least one order of magnitude. Surfaces with larger contact angle (CA) can promote DWC heat transfer due to the formation of “bare” condensation surface caused by the rapid removal of large condensate droplets and high surface replenishment frequency. Superhydrophobic surfaces with high contact angle (> 150°) of water and low contact angle hysteresis (< 5°) seem to be an ideal condensing surface to promote DWC and enhance heat transfer, in particular, for the steam-air mixture vapor. In the present paper, steam DWC heat transfer characteristics in the presence of noncondensable gas (NCG) were investigated experimentally on superhydrophobic and hydrophobic surfaces including the wetting mode evolution on the roughness-induced superhydrophobic surface. It was found that with increasing NCG concentration, the droplet conducts a transition from the Wenzel to Cassie-Baxter mode. And a new condensate wetting mode—a condensate sinkage mode—was observed, which can help to explain the effect of NCG on the condensation heat transfer performance of steam-air mixture on a roughness-induced superhydrophobic SAM-1 surface.

Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 14

Cassie-to-Wenzel transition. (WNCG  = 90.0%, Tb  = 43–48 °C, Tw  = 64–68 °C, P = 100.9 kPa, heating temperature: 68 °C)

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Figure 15

Condensate droplets in Wenzel and Cassie-Baxter modes (C—Cassie-Baxter; W—Wenzel. WNCG  = 93%, Tb  = 40 °C, Tw  = 18 °C, P = 105.2 kPa, q = 3.25 kW/m2 )

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Figure 16

Coalescence behavior of condensate droplets on superhydrophobic SAM-1 surface during steam-air vapor mixture condensation process (WNCG  = 75%, Tw  = 60.2 °C, ΔT = 5 K, P = 107.0 kPa, q = 4.61 kW/m2 )

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Figure 17

Coalescence model of the condensate droplets

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Figure 18

Photos of the condensate droplets on hydrophobic SAM-0 surface before and after heating the condensing block: (a) before (b) after (WNCG  = 90.2%, Tb  = 45–48 °C, Tw  = 60–67 °C, P = 101.0 kPa, heating temperature: 70 °C)

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Figure 19

Coalescence behavior of condensate droplets on hydrophobic SAM-0 surface during steam-air vapor mixture condensation process (WNCG  = 75%, Tw  = 59.6 °C, ΔT = 5 K, P = 107.0 kPa, q = 4.36 kW/m2 )

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Figure 20

Wetting mode for superhydrophobic surfaces. (a) perfect Cassie-Baxter state; (b) condensate wetting mode during pure vapor condensation; (c) condensate sinkage mode during vapor condensation in the presence of NCG.

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Figure 1

Wetting mode of droplet on rough surfaces

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Figure 2

Schematic diagram of the experimental system

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Figure 3

Schematic diagram of the test section and the measurement system. 1—bolts; 2—O-rings; 3—test surface; 4—observation window; 5—condensation chamber shell (stainless steel); 6—condensate drainage; 7—cuboid copper condensing block for visualization; 8—stainless steel plate; 9—cylindrical copper condensing block for measurement of condensation heat transfer rate; 10—PTFE insulation flange; 11—stainless steel flange; 12—condensation chamber back shell (stainless steel); 13—coolant inlet; 15—coolant outlet; 14,16,17—thermocouples.

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Figure 4

Schematic diagram of the measurement system for the condensing block. (a) Cylindrical copper condensing block for measurement of condensation heat transfer rate; (b) cuboid copper condensing block for visualization.

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Figure 5

SEM images of the SAM-1 surface. (a) 1000 × ; (b) 2500 ×.

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Figure 6

Reflection of light: (a) specular reflection; (b) diffused reflection; (c) reflection on a surface where liquid directly contacts solid surface; (d) reflection on a surface by air trapped between liquid and solid surface

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Figure 7

Construction and comparison of Wenzel–Cassie-Baxter droplet

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Figure 8

Formation of Wenzel droplet by Cassie–Wenzel mode transition during the ultrasonic vibration. (a) Cassie-Baxter mode before vibration; (b) and (c) transition process after vibration; (d) final Wenzel mode after vibration.

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Figure 9

Wenzel droplet constructed by ethanol and growth by injecting more water (C—Cassie-Baxter; W—Wenzel)

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Figure 10

Departure of the large droplet in the mixed C–W mode (C—Cassie-Baxter; W—Wenzel)

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Figure 11

Condensate behaviors during condensation with and without NCG. (a) WNCG  = 0, q = 149 kW/m2 , Tw  = 96.3 °C; (b) WNCG  = 35%, q = 9.70 kW/m2 , Tw  = 83.5 °C; (c) WNCG  = 50%, q = 7.93 kW/m2 , Tw  = 76.8 °C; (d) WNCG  = 75%, q = 4.61 kW/m2 , Tw  = 60.2 °C; (e) WNCG  = 90%, q = 1.44 kW/m2 , Tw  = 40.6 °C (ΔT = 5 K; P = 110–112 kPa).

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Figure 12

Condensate state before and after cutting off the cooling system

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Figure 13

Images of the condensate droplets when heating the condensing block (WNCG  = 90.0%, Tb  = 43–48 °C, Tw  = 64–68 °C, P = 100.9 kPa, heating temperature: 68 °C)

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