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Research Papers: Evaporation, Boiling, and Condensation

Computational Study of Saturated Flow Boiling Within a Microchannel in the Slug Flow Regime

[+] Author and Article Information
Mirco Magnini

Heat and Mass Transfer Laboratory (LTCM),
École Polytechnique Fédérale
de Lausanne (EPFL),
EPFL-STI-IGM-LTCM, Station 9,
Lausanne 1015, Switzerland
e-mail: mirco.magnini@epfl.ch

John R. Thome

Heat and Mass Transfer Laboratory (LTCM),
École Polytechnique
Fédérale de Lausanne (EPFL),
EPFL-STI-IGM-LTCM, Station 9,
Lausanne 1015, Switzerland
e-mail: john.thome@epfl.ch

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received July 3, 2014; final manuscript received July 28, 2015; published online September 2, 2015. Assoc. Editor: Terry Simon.

J. Heat Transfer 138(2), 021502 (Sep 02, 2015) (12 pages) Paper No: HT-14-1439; doi: 10.1115/1.4031234 History: Received July 03, 2014; Revised July 28, 2015

This paper presents a fundamental study of the flow dynamics and heat transfer induced by a slug flow under saturated flow boiling in a circular microchannel. Numerical simulations are carried out by utilizing the commercial CFD solver ansys fluent v. 14.5, with its built-in volume of fluid (VOF) method to advect the interface, which was improved here by implementing self-developed functions to model the phase change and the surface tension force. A continuous stream of bubbles is generated (by additional user-defined functions) by patching vapor bubbles at the channel upstream with a constant generation frequency. This modeling framework can capture the essential features of heat transfer in slug flows for a continuous stream of bubbles which are here investigated in detail, e.g., the mutual influence among the growing bubbles, the fluid mechanics in the liquid slug trapped between two consecutive bubbles, the effect of bubble acceleration on the thickness of the thin liquid film trapped against the channel wall and on other bubbles, and the transient growth of the heat transfer coefficient and then its periodic variation at the terminal steady-periodic regime, which is reached after the transit of a few bubble–liquid slug pairs. Furthermore, the results for a continuous stream of bubbles are found to be quite different than that of a single bubble, emphasizing the importance of modeling multiple bubbles to study this process. Finally, the outcomes of this analysis are utilized to advance a theoretical model for heat transfer in microchannel slug flow that best reproduces the present simulation data.

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References

Figures

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Fig. 1

Schematic of the initial flow configuration and contours of the initial temperature field. Flow conditions are: fluid R245fa, D = 0.5 mm, Tsat = 304 K, G = 550 kg/m2 s, and q = 5 kW/m2. The bubble interface is represented by the white line profile at the upstream of the channel. The dashed red lines locate the upstream and downstream ends of the heated region.

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Fig. 2

Heat transfer coefficients versus vapor quality for experiments [12] and CFD simulations. Flow conditions are: fluid R245fa, D = 0.5 mm, and Tsat = 304 K. The black dashed line identifies the transition between the IB and CB regime according to Ref. [38].

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Fig. 3

Bubbles profiles (white solid lines), contours of temperature (top) and velocity (bottom) fields, at different time instants, obtained by the simulation of a slug flow. Temperature and velocity contours are conveniently rescaled to a maximum of 1.2 K for T − Tsat, and 1.2 m/s for |u|. The black dashed lines indicate the limits (start and end) of the heated region.

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Fig. 4

Length of the bubbles against dimensionless position of their nose. The black dashed lines indicate the limits (start and end) of the heated region.

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Fig. 5

Evolution of the vapor quality along the microchannel. The black dashed lines indicate the limits (start and end) of the heated region.

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Fig. 6

Velocity of the bubbles nose against elapsed time

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Fig. 7

Velocity of the nose of the seventh flowing bubble, as a function of the dimensionless position of its nose, given by the CFD simulation and by selected predicting models

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Fig. 8

Evolution of the profile of the seventh bubble as it flows and grows within the heated section of the microchannel. The profiles are shifted in order to match the bubble nose position zN.

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Fig. 9

Thickness of the liquid film surrounding the seventh flowing bubble, as a function of the dimensionless position of its nose, given by the CFD simulation and by selected predicting models

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Fig. 10

Length of the liquid slugs against dimensionless position of the nose of the upstream bubble

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Fig. 11

Contours of the enhancement of the heat transfer coefficient given by the two-phase flow (htp) with respect to the liquid-only single-phase case (hsp), and time strip plot [46] of bubbles nose (black solid lines) and tail (dashed lines) positions

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Fig. 12

Two-phase flow heat transfer coefficient against elapsed time after 21 heated diameters for CFD (blue) and prediction method of Magnini et al. [22] (red) with embedded submodels for bubble velocity and liquid film thickness

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