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Research Papers: Evaporation, Boiling, and Condensation

Directional Passive Condensate Film Drainage on a Horizontal Surface With Periodic Asymmetrical Structures

[+] Author and Article Information
Shashank Natesh, Eric Truong

Mechanical and Aerospace Engineering, University of California-Davis, One Shields Avenue, Davis, CA 95616

Vinod Narayanan

Mechanical and Aerospace Engineering, University of California-Davis, One Shields Avenue, Davis, CA 95616 e-mail: vnarayanan@ucdavis.edu

Sushil Bhavnani

Department of Mechanical Engineering, Auburn University, Mary Martin Hall, Auburn, AL 36849

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received October 14, 2016; final manuscript received May 8, 2017; published online June 21, 2017. Assoc. Editor: C. A. Dorao.

J. Heat Transfer 139(11), 111507 (Jun 21, 2017) (15 pages) Paper No: HT-16-1662; doi: 10.1115/1.4036708 History: Received October 14, 2016; Revised May 08, 2017

Condensation of a highly wetting fluid on a horizontal surface with asymmetric millimeter-sized ratchets and periodically located film drainage pathways (DPs) in the spanwise direction is characterized. The hypothesis to be tested is whether the geometry would result in a net steady-state preferential drainage of the condensate film. Experiments are performed using PF5060 on a brass surface with ratchets of 3 mm pitch and 75–15 deg asymmetry. Drainage pathways are varied in density as nondimensional drainage pathways per meter depth ranging from 133 to 400. Experiments are performed at varied wall subcooling temperatures from 1 to 10 °C. Results of the asymmetric ratchet are compared against a control test surface with 45–45 deg symmetric ratchets. Both global and film visualization experiments are performed to characterize the differences in condensation between the symmetric and asymmetric surfaces. Global mass collection results indicate that all characterized asymmetric ratchet surfaces exhibit a net directional drainage of condensate while the symmetric control surface exhibited no preferential drainage. Among the asymmetric ratchets, the total mass flux rate increase with decrease in drainage pathway density, while the net mass flux rate increased with pathway density. Visualization of the condensate film was performed to explain the trends in net drainage with subcooling for different drainage pathway densities. For small drainage path density surfaces, a two-dimensional analytical model was developed to further characterize the effect of ratchet angle and Bond number on the net preferential drainage.

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Figures

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Fig. 1

Liquid flow over an asymmetric millimeter-sized textured condensing surface, assisted by a network of drainage pathways

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Fig. 2

(a) Isometric views of the test section with a network of six drainage pathways, (b) close up of an asymmetric ratchet, and (c) close up of a symmetric ratchet

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Fig. 3

An exploded-view of the experimental chamber

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Fig. 4

Total mass flux rate as a function of wall subcooling with respect to saturation temperature. Asymmetric 75 deg–15 deg ratchets with (a) symmetric ratchet test surface with six-drainage pathways WDP = 400, (b) two-drainage pathways, WDP = 133, (c) four-drainage pathways, WDP = 267, and (d) six-drainage pathways, WDP = 400.

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Fig. 5

Trend lines of condensate film average heat transfer coefficient with surface subcooling for the asymmetric ratchet surfaces with WDP of 133, 267, and 400, and symmetric ratchet with WDP of 400

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Fig. 6

Net mass flux rate as a function of wall subcooling with respect to saturation temperature. Asymmetric 75 deg–15 deg ratchets with (a) symmetric ratchet test surface with six-drainage pathways, WDP = 400, (b) two-drainage pathways, WDP = 133, (c) four-drainage pathways, WDP = 267, and (d) six-drainage pathways, WDP = 400. The net mass flux rate is in the direction of the steeper slope of the ratchets.

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Fig. 7

Condensate film profile for a symmetric ratchet (with six-drainage pathways) at varying subcooling. Dashed–dotted line represents the center of the ratchet and bold lines represent the walls of the ratchet. (a) 2.81 °C, (b) 5.69 °C, and (c) 8.96 °C.

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Fig. 8

Condensate film profile for an asymmetric ratchet with two-drainage pathways/ WDP = 133 (left) and six-drainage pathways/ WDP = 400 (right) at varying subcooling. Dashed–dotted line represents the center of the ratchet, bold lines represent the walls of the ratchet, and the arrow indicates the position of minima of the drainage edge of the film. (a) 1.58 °C, (b) 8.38 °C, (d) 2.42 °C, (e) 9.57 °C.

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Fig. 9

Lines of constant- ϕ (solid) and constant-η (dashed) and a sample liquid film profile (bold-dashed) on a Cartesian coordinate system

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Fig. 10

Comparison of condensate film edge profiles from experiment (WDP = 133) and liquid film profile from the model at various subcooling levels

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Fig. 11

Liquid film thickness variation in (a) hyperbolic coordinates and (b) Cartesian coordinates. ϵ=0.1, D=2.5.

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Fig. 12

Liquid film profile as a function of Bond number for (a) symmetric and (b) asymmetric ratchet

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Fig. 13

Interfacial velocity as a function of ϕ for symmetric and asymmetric ratchet

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