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Research Papers: Evaporation, Boiling, and Condensation

Effect of Length Scales on the Boiling Enhancement of Structured Copper Surfaces

[+] Author and Article Information
Md Mahamudur Rahman

Mem. ASME
Department of Mechanical Engineering
and Mechanics,
Drexel University,
3141 Chestnut Street,
Philadelphia, PA 19104
e-mail: mr698@drexel.edu

Matthew McCarthy

Mem. ASME
Department of Mechanical Engineering
and Mechanics,
Drexel University,
3141 Chestnut Street,
Philadelphia, PA 19104
e-mail: mccarthy@coe.drexel.edu

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received October 15, 2016; final manuscript received December 16, 2016; published online June 21, 2017. Assoc. Editor: Satish G. Kandlikar.

J. Heat Transfer 139(11), 111508 (Jun 21, 2017) (9 pages) Paper No: HT-16-1668; doi: 10.1115/1.4036693 History: Received October 15, 2016; Revised December 16, 2016

Boiling heat transfer can be substantially altered with the addition of surface structures. While significant enhancements in critical heat flux (CHF) and heat transfer coefficient (HTC) have been demonstrated using this approach, fundamental questions remain about the nature of enhancement and the role of structure length scale. This work presents a systematic investigation of structures from 100's of nanometers to several millimeters. Specifically, copper substrates were fabricated with five different microchannel geometries (characteristic lengths of 300 μm to 3 mm) and four different copper oxide nanostructured coatings (characteristic lengths of 50 nm to 50 μm). Additionally, twenty different multiscale structures were fabricated coinciding with each permutation of the various microchannels and nanostructures. Each surface was tested up to CHF during pool boiling of saturated water at atmospheric conditions. The nanostructured coatings were observed to increase CHF via surface wicking, consistent with existing models, but decrease HTC due to the suppression of the nucleation process. The microchannels were observed to increase both CHF and HTC, generally outperforming the nanostructured coatings. The multiscale surfaces exhibited superior performance, with CHF and HTC values as high as 313 W/cm2 and 461 kW/m2 K, respectively. Most importantly, multiscale surfaces were observed to exhibit the individual enhancement mechanisms seen from each length scale, namely, increased nucleation and bubble dynamics from the microchannels and wicking-enhanced CHF from the nanostructures. Additionally, two of the surfaces tested here exhibited uncharacteristically high HTC values due to a decreasing wall superheat at increasing heat fluxes. While the potential mechanisms producing this counterintuitive behavior are discussed, further research is needed to definitively determine its cause.

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References

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Figures

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Fig. 1

Microchannel copper surfaces, showing optical images of (a) two samples fabricated using wire EDM with four and ten channels each and (b) a close-up of one microchannel cross section identifying the channel width, WC, and depth, D. (c) Geometric details of all five microchannel surfaces fabricated and tested in this work, including the names of the surfaces, number of channels per centimeter, the measured width and depth of the channels, the measure width of the fins, WF, and the ratio of the true surface area to the footprint area.

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Fig. 2

Copper oxide nanostructured coatings, showing (a)–(d) the names of each nanostructure type, the solution chemistry and bath conditions used to fabricate them, and SEM images of each resulting coating at two magnifications. SEM images of two multiscale surfaces comprised of CuO nanostructures grown directly onto microchannel copper samples are shown for the (e) CuO-3 and (f) CuO-4 nanocoatings.

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Fig. 3

Boiling results for single length scale samples, showing microchannel surfaces with no nanostructures (open square symbols) and nanostructured samples with no microchannels (closed circle symbols) as compared to bare copper (open triangles). Results for (a) heat flux as a function of superheat and (b) heat transfer coefficient as a function of heat flux indicating that microchannel surfaces largely outperform nanostructure surfaces, showing comparable CHF enhancements and consistently higher HTC values.

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Fig. 4

Wicking-enhanced critical heat flux on nanostructured surfaces, showing the measured CHF as a function of the measured wicked volume flux for each CuO nanostructure surface and compared against the previously reported model from Rahman et al. [7]

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Fig. 5

Boiling results for multiscale surfaces comprised of all five microchannel designs coated with CuO-1 nanostructures (Fig. 2(a)), showing (a) heat flux as a function of superheat and (b) HTC as a function of heat flux as compared against a flat CuO-1 surface with no microchannels

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Fig. 6

Boiling results for multiscale surfaces comprised of all five microchannel designs coated with CuO-2 nanostructures (Fig. 2(b)), showing (a) heat flux as a function of superheat and (b) HTC as a function of heat flux as compared against a flat CuO-2 surface with no microchannels

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Fig. 7

Boiling results for multiscale surfaces comprised of all five microchannel designs coated with CuO-3 nanostructures (Fig. 2(c)), showing (a) heat flux as a function of superheat and (b) HTC as a function of heat flux as compared against a flat CuO-3 surface with no microchannels

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Fig. 8

Boiling results for multiscale surfaces comprised of all five microchannel designs coated with CuO-4 nanostructures (Fig. 2(d)), showing (a) heat flux as a function of superheat and (b) HTC as a function of heat flux as compared against a flat CuO-4 surface with no microchannels

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Fig. 9

Repeatability and the role of length scales on boiling performance for multiscale surfaces with CuO-3 nanostructures. Repeatability testing of (a) a multiscale CuO-3, M10 surface and (b) a multiscale CuO-3, M8 surface showing consistent boiling curves. These multiscale surfaces exhibit characteristics of their individual components, where the microchannels promote increased nucleation and bubble dynamics at low superheats (labeled as “1”) and the CuO nanostructures delay CHF due to wicking (labeled as “2”) at high heat fluxes.

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Fig. 10

Repeatability of two different multiscale CuO-4, M10 samples fabricated and tested using the same methods. The two multiscale surfaces show differences at low heat fluxes, which is attributed to variations in the onset of nucleate boiling, but show consistent boiling curves at high heat fluxes. Additionally, the curves show distinct similarities to those reported by Kandlikar [4], suggesting the potential role of spatial ordering of liquid and vapor flow fields.

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Fig. 11

The effect of microchannels on (a) critical heat flux and (b) maximum heat transfer coefficient for all four CuO nanostructured coatings (closed circles) investigated in this work, as compared to surfaces with no CuO nanostructures (open triangles)

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