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Research Papers: Evaporation, Boiling, and Condensation

Physics of the Microchannel Flow Boiling Process and Comparison With the Existing Theories

[+] Author and Article Information
Sajjad Bigham

Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, FL 32611

Saeed Moghaddam

Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, FL 32611 e-mail: saeedmog@ufl.edu

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received September 21, 2016; final manuscript received February 23, 2017; published online June 21, 2017. Assoc. Editor: Peter Stephan.

J. Heat Transfer 139(11), 111503 (Jun 21, 2017) (10 pages) Paper No: HT-16-1594; doi: 10.1115/1.4036655 History: Received September 21, 2016; Revised February 23, 2017

In this study, six benchmark experiments are conducted on bubbles at different growth stages to evaluate the assumptions of the existing microchannel flow boiling heat transfer models/hypothesis. The results show that the bubble ebullition process triggers a spike in the local surface heat flux due to the thin film evaporation and transient conduction heat transfer mechanisms. This enhancement in the surface heat flux is limited to a very small area at the bubble–surface contact region at the nucleation site limiting the overall heat transfer contribution of the bubble ebullition process. The contribution of these two mechanisms of heat transfer increases as the bubble–surface contact area becomes larger. As the bubbles length increases, the time period of activation of the microlayer evaporation mechanism substantially increases while that of the transient conduction mechanism remains relatively unchanged. When the microchannel is mostly occupied by bubbles, the thin film evaporation mechanism becomes the dominant heat transfer mode. The results clearly indicate that single-phase heat transfer mechanism active at surface regions not covered by bubbles is governed by the laminar flow theory (for the test conditions presented here). In essence, a measureable enhancement effect in the liquid phase due to bubbles growth and flow has not been observed. A comparison with the existing microchannel flow boiling models suggests that the three-zone flow boiling model can qualitatively describe the heat transfer events observed in this experiment but fails to accurately predict the magnitude of the heat transfer mechanisms.

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Figures

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Fig. 1

(a) An image of the microfluidic chip (preheater and test sections are labeled), (b) a close view of the test section, (c) a zoomed view of the pulsed function microheater, (d) an SEM image of the artificial cavity, and (e) a schematic view of the composite substrate cross section

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Fig. 2

(a) A zoomed out view of the microchannel and (b) a close view of the test section and the sensors chosen for the single-phase flow tests

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Fig. 3

Comparison of the experimental, theoretical, and numerical Nusselt values

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Fig. 4

A schematic representation of different stages of bubble growth in a microchannel

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Fig. 5

(a) Images of a long vapor slug, (b) surface temperature history, and (c) respective local heat flux data. Test is conducted at a mass flux of 68.4 kg/m2 s (average temperature at the SU8–Si interface and vapor slug passing time over the sensor are 66 °C and 14 ms, respectively).

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Fig. 6

(a) Images of a short vapor slug (average temperature at SU8–Si interface and vapor slug passing time over the sensor are 66 °C and 5.3 ms, respectively), (b) surface temperature history, and (c) respective local heat flux data

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Fig. 7

(a) Images, (b) surface temperature history, and (c) local heat flux data corresponding to a bubble with a passing time of 1.4 ms. Test is conducted at a mass flux of 68.4 kg/m2 s (average temperature at the SU8–Si interface is 66 °C).

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Fig. 8

Images of a nucleating bubble in microchannel flow boiling process (average temperature at the SU8–Si interface is 64.8 °C)

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Fig. 9

(a) Surface temperature history associated with a nucleating bubble shown in Fig. 8 (average temperature at the SU8–Si interface is 68.4 °C), (b) respective local heat flux data, and (c) bubble growth and departure as a function of time

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Fig. 10

Local heat flux history of two subsequent bubbles at a bubble generation frequency of 56 bubbles/s

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Fig. 11

(a)–(d) Images and (e) local heat flux history corresponding to successive moving bubbles at a bubble generation rate of approximately 190 bubbles/s and a mass flux of 93.3 kg/m2 s

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Fig. 12

(a) Surface temperature history and (b) heat flux data corresponding to a long liquid slug at a bubble generation frequency of ∼12 bubbles/s. Test is conducted at a mass flux of 93.3 kg/m2 s (average temperature at the SU8–Si interface is 64.2 °C).

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Fig. 13

Images of a nucleating bubble in pool boiling process at surface temperature 80.2 °C [17]

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Fig. 14

Local heat flux variations corresponding to the bubble shown in Fig. 13 [17]

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Fig. 15

Two-phase heat transfer coefficient-time histories measured at sensor 26 and calculated by the three-zone model [22]

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