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Research Papers: Evaporation, Boiling, and Condensation

# Investigation of Bubble Frequency for Slug Flow Regime in a Uniformly Heated Horizontal Microchannel

[+] Author and Article Information

Department of Mechanical and
Industrial Engineering,
Concordia University,
1455 de Maisonneuve Boulevard W,

Ibrahim Hassan

Mechanical Engineering Department,
Texas A&M University at Qatar,
P.O. Box 23874,
Doha, Qatar
e-mail: ibrahim.hassan@qatar.tamu.edu

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received August 8, 2015; final manuscript received December 1, 2016; published online February 28, 2017. Assoc. Editor: P. K. Das.

J. Heat Transfer 139(6), 061501 (Feb 28, 2017) (13 pages) Paper No: HT-15-1532; doi: 10.1115/1.4035562 History: Received August 08, 2015; Revised December 01, 2016

## Abstract

Slug flow is an essential flow pattern observed in microchannels where its transition boundaries in microchannels are characterized by two complex hydrodynamic phenomena, the bubble confinement and the bubble coalescence. Slug flow may be classified in terms of bubble size into two major zones: isolated bubble zone and coalescence bubble zone. In this paper, a semi-analytical model is developed for predicting the main characteristics of isolated bubble zone for flow boiling in a horizontal microchannel. The influences of surface tension, shear, and inertial forces have been taken into account. The model is developed on the basis of drift flux model, and a fully developed slug unit is chosen as a control volume for deriving the equations of motion. The effects of main operating conditions, mass and heat fluxes, on bubble length and bubble frequency have been investigated. The boundaries of slug flow regime have been identified based on the most proper diabatic flow pattern maps available in the literature for the chosen database. The model has been validated using the database available in the literature for flow boiling of R134a and R245fa in 0.509 mm and 3.0 mm inner diameter horizontal mini-tubes, respectively, and over wide range of mass fluxes ($300≤G≤1000 kg/m2 s$). This study has shown that the mass flux has a significant effect on the slug length and the bubble frequency. The model gave a good agreement with the experimental data of bubble length and bubble frequency with a mean absolute error (MAE) of 18.0% and 27.34%, respectively.

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## Figures

Fig. 1

Sketch of a fully developed slug flow unit for flow boiling in a horizontal microchannel

Fig. 2

A sketch of a control volume of a slug flow unit for deriving mass balance equation

Fig. 3

A sketch of control volume of a slug flow unit for deriving momentum conservation equation

Fig. 4

Shows the energy terms and works applied on the slug unit

Fig. 5

Bubble frequency predicted by the present model, and the one measured by Revellin et al. [4] versus vapor quality for R134a in 0.509 mm inner diameter microtube with a mass flux, 350 kg/m2 s, at Tsat=30 °C, and ΔTsub=3 °C

Fig. 6

Bubble frequency predicted by the present model and the one measured by Revellin et al. [4] versus vapor quality for R134a in 0.509 mm inner diameter microtube, 500 kg/m2 s, at Tsat=30 °C, and ΔTsub=3 °C

Fig. 7

Bubble frequency predicted by the present model and the one measured by Revellin et al. [4] versus vapor quality for R134a in 0.509 mm inner diameter microtube, 700 kg/m2 s, at Tsat=30 °C, and ΔTsub=3 °C

Fig. 8

Bubble frequency predicted by the present model and the one measured by Revellin et al. [4] versus vapor quality for R134a in 0.509 mm inner diameter microtube, 1000 kg/m2 s, at Tsat=30 °C, and ΔTsub=3 °C

Fig. 9

Shows bubble frequency versus vapor quality for flow boiling of R245fa in 3.0 mm inner diameter mini-tube at various mass fluxes compared with experimental data of Charnay et al.[7]

Fig. 10

A comparison between bubble frequency data measured by Revellin et al. [4], and Charney et al. [7] and those predicted by the present model for a range of mass fluxes of 300–1000 kg/m2 s

Fig. 11

Bubble length versus vapor quality for flow boiling of R134a in a 0.509 mm inner diameter microtube

Fig. 12

Bubble length data measured by Revellin et al. [4] versus those predicted by the present model for flow boiling of R134a in the isolated bubble zone

Fig. 13

Shows the variation of bubble length, liquid slug length, and slug unit length versus vapor quality for flow boiling of R134a in a 0.509 mm inner diameter microtube with a mass flux 500 kg/m2 s at an inlet saturation temperature Tsat=30 °C

Fig. 14

Shows the influence of distribution parameter on the bubble frequency obtained by the present model for flow boiling of R134a in a 0.509 mm inner diameter microtube with a mass flux 350 kg/m2 s at an inlet saturation temperature Tsat=30 °C

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