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Research Papers: Evaporation, Boiling, and Condensation

On Temporal Biphilicity: Definition, Relevance, and Technical Implementation in Boiling Heat Transfer

[+] Author and Article Information
Christophe Frankiewicz

Mem. ASME
Mechanical Engineering,
Iowa State University,
Ames, IA 50011
e-mail: franki@iastate.edu

Daniel Attinger

Fellow ASME
Mechanical Engineering,
Iowa State University,
Ames, IA 50011
e-mail: attinger@iastate.edu

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received August 1, 2016; final manuscript received June 23, 2017; published online August 1, 2017. Assoc. Editor: Satish G. Kandlikar.

J. Heat Transfer 139(11), 111511 (Aug 01, 2017) (14 pages) Paper No: HT-16-1484; doi: 10.1115/1.4037162 History: Received August 01, 2016; Revised June 23, 2017

Solid–fluid interfaces switching from a superhydrophilic to a superhydrophobic wetting state are desired for their ability to control and enhance phase-change heat transfer. Typically, these functional surfaces are fabricated from polymers and modify their chemistry or texture upon the application of a stimulus. For integration in relevant phase-change heat transfer applications, several challenges need to be overcome, of chemical stability, mechanical and thermal robustness, as well as large scale manufacturing. Here, we describe the design and fabrication of metallic surfaces that reversibly switch between hydrophilic and superhydrophobic states, in response to pressure and temperature stimuli. Characterization of the surfaces in pool boiling experiments verifies their thermal and mechanical robustness, and the fabrication method is scalable to large areas. During pool boiling experiments, it is experimentally demonstrated that the functional surfaces can be actively switched between a high-efficiency mode suitable at low heat flux, and a high-power mode suitable for high heat flux applications.

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Figures

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Fig. 1

The hypothesis underlying this work is that an engineered surface (on the left) optimized for phase-change heat transfer matches the optimum features of phase-change heat transfer (on the right, commented in introduction), in a similar way as a key matches a lock (center). For example, the left picture, reproduced with permission from Ref. [18], is a multiscale surface with spatial patterns of wettability, which optimizes nucleate boiling. Image of key licensed under the creative commons attribution-share Alike 3.0 unported license.

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Fig. 2

Multiscale texture and chemistry of the engineered E2 and EA surfaces. The surfaces EA and E2 are composed of three and two tiers of roughness, respectively. The chemistry of the surface EA is CuO and the chemistry of the surface E2 is Cu, as reported in a previous work [41].

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Fig. 3

The surface (EA on these pictures) can reversibly transition from a superhydrophobic metastable CB state to a hydrophilic stable W state. The stimulus triggering this transition can be reversibly controlled. Flooding of the cavities can be controlled by either diffusion of air inside the bulk liquid if the surface is immersed or by increasing the pressure in the liquid above the surface beyond the value of the break-in pressure. Filling the cavities with air or vapor can either be controlled by evaporation in air or by succinctly reaching the CHF.

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Fig. 4

The functional surfaces (here EA, see the Appendix for similar results on E2) were integrated in a pressure-controlled boiler (see figure on the left for details about the apparatus). The pool boiling curves on the right show the possibility to adapt the surface performances to the power needs depending on wetting state of the surface (W for high CHF, CB for high HTC). No hysteresis in the boiling curve has been observed on all surfaces as long as the heat flux provided to the surface is maintained below the CHF. Error bars are based on an uncertainty analysis; see Sec. A.3 of Appendix, which also describes the packaging of the boiling surface.

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Fig. 5

Surfaces EA and E2 can repeatedly and reversibly respond to a change in pressure or temperature conditions and adapt their wettability, as described in the table, right. Note that the wettability transitions are fast and can be done in approximately 30 s. The transitions from the W to the CB state and from the CB to the W state were triggered by a change in temperature and pressure conditions, respectively. The error bars indicate the range of contact angle measured on the surface.

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Fig. 6

Summary (left table) of the performances obtained in pool boiling. The CHF is correlated to the ability of the surface to wick. The wicked volume flux was obtained by measuring the initial velocity of the meniscus inside the capillary tube (external diameter 500 μm) and the wetted area Aw. The surfaces EA and E2 reached a CHF close to the maximum value of the CHF predicted by the value of the wicked volume flux [53]. By using functional surfaces, the CHF can be enhanced by a factor 1.4 and the HTC by up to 18 times compared to the values of a bare copper surface. Functional surfaces can both result in a significant energy saving and help delaying the occurrence of the CHF.

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Fig. 7

Boiling curve of the sample E2. The figure shows the possibility to adapt the performance of surface E2 to the power needs depending on the wetting state of the surface (W for high CHF, CB for high HTC).

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Fig. 8

Evolution of the transition from the metastable hydrophobic CB to the stable W hydrophilic state. The hydrophobic surface is naturally covered by a vapor layer (highly reflective to light) trapped into the surface texture and which progressively diffuses into water by either natural diffusion into the bulk (see top sequence of image on surface E2) or by applying on the surface a sufficient pressure, superior to the break-in pressure, until complete flooding of the cavities.

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Fig. 9

Reversible transition in boiling from the metastable hydrophobic CB to the stable W hydrophilic state. In the superhydrophobic state, the surface is almost entirely covered with a vapor layer that facilitates the nucleation process. In the hydrophilic wetting state, cavities are filled with water and are less active. Therefore, the size of the bubbles formed on the hydrophobic surfaces in the CB state is larger compared to the size (or volume) of the bubbles produced on the hydrophilic surface in the W state. The time of growth of the bubble to departure does not seem to be correlated with the wetting state.

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Fig. 10

Mechanistic model used to predict the break-in pressure and the contact angle as a function of the number of tiers on the surface and by assuming that only tier 0 is wetted

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Fig. 11

Evolution of the contact angle and break-in pressure as a function of the number of tiers wetted and assuming that only tier 0 is wetted

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Fig. 12

Repeated boiling curve of the sample EA, run 3 times following the sequence: Dry run 1, wet run 1, dry run 2, wet run 2, dry run 3. These boiling curves show the excellent reproducibility of the results in pool boiling conditions. This experiment also is, to some extent, a gauge of the durability of the sample.

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Fig. 13

Schematic (cross-sectional side view) of the sample packaging as prepared for pool boiling experiments. The engineered copper samples typically are ≈10 mm (length) × 10 mm (width) × 3 mm (depth). The thermocouple hole is located 1.5 mm below the boiling surface and is 1.6 mm deep inside the copper sample. A tiny air gap (about 1 mm thick) is intentionally kept underneath the heater to prevent any heat loss though conduction to the Teflon block. Note that Teflon is also a relatively good insulating material with a thermal conductivity of ≈0.25 W/m K. The epoxy also has a low thermal conductivity of ≈0.180 W/m K as per 3 M data sheets (DP 420 black epoxy).

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