Research Papers: Evaporation, Boiling, and Condensation

Enhanced Macroconvection Mechanism With Separate Liquid–Vapor Pathways to Improve Pool Boiling Performance

[+] Author and Article Information
Satish G. Kandlikar

Fellow ASME
Mechanical Engineering Department,
Rochester Institute of Technology,
76 Lomb Memorial Drive,
Rochester, NY 14623
e-mail: sgkeme@rit.edu

Presented at the 2016 ASME 5th Micro/Nanoscale Heat & Mass Transfer International Conference. Paper No. MNHMT2016-6371.Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received May 10, 2016; final manuscript received September 19, 2016; published online February 7, 2017. Assoc. Editor: Robert D. Tzou.

J. Heat Transfer 139(5), 051501 (Feb 07, 2017) (11 pages) Paper No: HT-16-1263; doi: 10.1115/1.4035247 History: Received May 10, 2016; Revised September 19, 2016

Understanding heat transfer mechanisms is crucial in developing new enhancement techniques in pool boiling. In this paper, the available literature on fundamental mechanisms and their role in some of the outstanding enhancement techniques is critically evaluated. Such an understanding is essential in our quest to extend the critical heat flux (CHF) while maintaining low wall superheats. A new heat transfer mechanism related to macroconvection is introduced and its ability to simultaneously enhance both CHF and heat transfer coefficient (HTC) is presented. In the earlier works, increasing nucleation site density by coating a porous layer, providing hierarchical multiscale structures with different surface energies, and nanoscale surface modifications were some of the widely used techniques which relied on enhancing transient conduction, microconvection, microlayer evaporation, or contact line evaporation mechanisms. The microconvection around a bubble is related to convection currents in its immediate vicinity, referred to as the influence region (within one to two times the departing bubble diameter). Bubble-induced convection, which is active beyond the influence region on a heater surface, is introduced in this paper as a new macroconvection mechanism. It results from the macroconvection currents created by the motion of bubbles as they grow and depart from the nucleating sites along a specific trajectory. Directing these bubble-induced macroconvection currents so as to create separate vapor–liquid pathways provides a highly effective enhancement mechanism, improving both CHF and HTC. The incoming liquid as well as the departing bubbles in some cases play a major role in enhancing the heat transfer. Significant performance improvements have been reported in the literature based on enhanced macroconvection contribution. One such microstructure has yielded a CHF of 420 W/cm2 with a wall superheat of only 1.7 °C in pool boiling with water at atmospheric pressure. Further enhancements that can be expected through geometrical refinements and integration of different techniques with macroconvection enhancement mechanism are discussed here.

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Fig. 1

Schematic representation of heat transfer mechanisms: a nucleating bubble reported in literature during pool boiling

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Fig. 2

Schematic of a pore and tunnel structure. Redrawn from Ref. [38].

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Fig. 3

Surfaces with hydrophilic network with hydrophilic islands and reverse configurations, images courtesy of Professor Attinger [44]

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Fig. 4

(a) Hierarchical micronanostructures and (b) boiling curves for hierarchical enhanced pool boiling surfaces developed by Chu et al. [46], images courtesy of Professor Wang

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Fig. 5

Schematic representation of two types of macroconvection heat transfer outside the influence region of a departing bubble. (a) Bubble departing normal to the surface and liquid flow toward the nucleation site and (b) bubble departing along the heater surface and liquid–vapor flow along the surface away from the nucleation site.

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Fig. 6

Contoured fin design based on evaporation momentum force to create separate liquid–vapor pathways: (a) contoured fin geometry and (b) boiling curve with water at atmospheric pressure. Redrawn from Ref. [55].

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Fig. 7

Heat transfer mechanism of bubble-induced impinging liquid jet into the microchannel: (a) type 1–sintered fin tops and (b) type 2–sintered channel. Redrawn from Ref. [61].

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Fig. 8

Separate liquid–vapor pathways induced by nucleating regions separated by feeder channels: (a) photograph showing the departing bubbles in the nucleation region and the feeder channels, (b) schematic showing the macroconvection enhancement mechanism with separate liquid–vapor pathways [63], (c) to-view of the feeder microchannels directing liquid to nucleating regions, (d) use of pin fins inside the feeder microchannels for further enhancement, and (e) offset strip fins replacing the feeder microchannels

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Fig. 9

Comparison of different enhancement techniques; Tall microstructures–Li et al. [36], Mori and Okayuma [37]; Bi-conductive–Rahman et al. [57]; Nanomicro ridges–Zou and Maroo [49], Jaikumar et al. [50]; Wicking microstructures–Rahman et al. [48], Chu et al. [46]; Nakayama et al. [38]; Separate liquid–vapor pathways (enhanced microconvective mechanism), Cooke and Kandlikar [58], Kandlikar [55], Patil and Kandlikar [60], Jaikumar and Kandlikar [63]: (a) CHF versus wall superheat, (b) HTC versus CHF, and (c) images of the respective enhanced surfaces




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