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Research Papers: Multiphase Flows

Air Bubble Entrainment, Breakup, and Interplay in Vertical Plunging Jets

[+] Author and Article Information
Numa Bertola, Hubert Chanson

School of Civil Engineering,
The University of Queensland,
Brisbane QLD 4072, Australia

Hang Wang

School of Civil Engineering,
The University of Queensland,
Brisbane QLD 4072, Australia
e-mail: hang.wang@uqconnect.edu.au

1Present address: ETH-Zürich, Future Cities Laboratory, Singapore 138602.

2Corresponding author.

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received October 2, 2017; final manuscript received March 16, 2018; published online May 2, 2018. Assoc. Editor: Matevz Dular.

J. Fluids Eng 140(9), 091301 (May 02, 2018) (13 pages) Paper No: FE-17-1633; doi: 10.1115/1.4039715 History: Received October 02, 2017; Revised March 16, 2018

The entrainment, breakup, and interplay of air bubbles were observed in a vertical, two-dimensional supported jet at low impact velocities. Ultra-high-speed movies were analyzed both qualitatively and quantitatively. The onset velocity of bubble entrainment was between 0.9 and 1.1 m/s. Most bubbles were entrained as detached bubbles from elongated air cavities at the impingement point. Explosion, stretching, and dejection mechanisms were observed for individual bubble breakup, and the bubble interaction behaviors encompassed bubble rebound, “kiss-and-go,” coalescence and breakup induced by approaching bubble(s). The effects of jet impact velocity on the bubble behaviors were investigated for impact velocities from 1.0 to 1.36 m/s, in the presence of a shear flow environment.

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Figures

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Fig. 1

Sketches of experimental setup: (a) relative position of jet nozzle and camera system and (b) side view of jet impingement and definition of relevant parameters

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Fig. 2

Dimensionless onset velocity μwVe/σ as a function of the jet turbulence intensity Tu = v′/V, with comparison to past planar jet results [3] and circular jet results [8,19,2123]

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Fig. 3

Photographic sequence of the formation and breakup of an elongated air cavity at a plunging jet. Flow conditions: V1 = 1.26 m/s, x1 = 0.05 m. (1) Development of the air cavity: (a) t = 0.000 s, (b) t = 0.012 s, and (c) t = 0.023 s. (2) Stretching of the air cavity: (d) t = 0.028 s, (e) t = 0.033 s, and (f) t = 0.035 s. (3) Cavity pinch-off, air pocket formation and breakup: (g) t = 0.036 s, (h) t = 0.038 s, and (i) t = 0.040 s. Arrows indicating entrained air cavity and bubble(s).

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Fig. 4

Proportion of air entrainment mechanisms as functions of the impact velocity V1 in a vertical supported plunging jet

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Fig. 5

Sketch of typical air entrainment mechanisms for low disturbance jet (a) and high disturbance jet (b)

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Fig. 6

Photographic sequence of explosion bubble breakup mechanism. Flow conditions: V1 = 1.26 m/s, x1 = 0.05 m. (1) Unstable bubble: (a) t = 0.000 s and (b) t = 0.015 s. (2) Explosion of the bubble: (c) t = 0.024 s and (d) t = 0.031 s. (3) Multiples daughters: (e) t = 0.044 s and (f) t = 0.058 s. Arrows indicating mother and daughter bubbles.

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Fig. 7

Photographic sequence of stretching bubble breakup mechanism. Flow conditions: V1 = 1.26 m/s, x1 = 0.05 m. (1) Bubble stretching: (a) t = 0.000 s, (b) t = 0.007 s, and (c) t = 0.012 s. (2) Independent daughters: (d) t = 0.015 s, (e) t = 0.020 s, and (f) t = 0.022 s. (3) Daughters with different behaviors: (g) t = 0.026 s, (h) t = 0.035 s, and (i) t = 0.077 s. Arrows indicating mother and daughter bubbles.

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Fig. 8

Photographic sequence of dejection bubble breakup mechanism. Flow conditions: V1 = 1.26 m/s, x1 = 0.05 m. (1) First dejection: (a) t = 0.000 s, (b) t = 0.004 s, and (c) t = 0.007 s. (2) Mother and daughter independent behaviors: (d) t = 0.012 s and (e) t = 0.036 s. (3) Second dejection: (f) t = 0.044 s and (g) t = 0.047 s. (4) Mother and daughter independent behaviors: (h) t = 0.051 s and (i) t = 0.064 s. Arrows indicating mother and daughter bubbles.

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Fig. 9

Dimensionless statistical properties of bubble breakup events as functions of plunging jet's dimensionless impact velocity μwV1/σ: (a) bubble breakup count rate σFbreak/(μwg) and proportion of bubbles experiencing primary breakup, (b) proportion of different breakup mechanisms, (c) average cross-sectional area of mother bubbles ρwgA/σ, and (d) average vertical position of breakup (x − x1)/d1

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Fig. 10

Photographic sequence of rebound interplay mechanism. Flow conditions: V1 = 1.26 m/s, x1 = 0.05 m. (1) Bubble attraction: (a) t = 0.000 s, (b) t = 0.005 s, and (c) t = 0.012 s. (2) Bubble repulsion: (d) t = 0.016 s, (e) t = 0.023 s, and (f) t = 0.038 s. Arrows indicating parent bubbles.

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Fig. 11

Photographic sequence of “kiss-and-go” interplay mechanism. Flow conditions: V1 = 1.12 m/s, x1 = 0.05 m. (1) Bubble attraction: (a) t = 0.000 s, (b) t = 0.005 s, and (c) t = 0.009 s. (2) Bubble “kiss”: (d) t = 0.015 s, (e) t = 0.020 s, and (f) t = 0.025 s. (3) Bubble repulsion: (g) t = 0.031 s, (h) t = 0.035 s, and (i) t = 0.039 s. Arrows indicating parent and mother bubbles.

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Fig. 12

Photographic sequence of “kiss-and-go” interplay mechanism. Flow conditions: V1 = 1.12 m/s, x1 = 0.05 m. (1) Bubble attraction: (a) t = 0.000 s, (b) t = 0.016 s, and (c) t = 0.028 s. (2) Bubble “kiss”: (d) t = 0.034 s, (e) t = 0.042 s, (f) t = 0.048 s, and (g) t = 0.053 s. (3) Bubble repulsion: (h) t = 0.058 s and (i) t = 0.086 s. Arrows indicating parent, mother, and daughter bubbles.

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Fig. 13

Photographic sequence of collapse due to bubble approaching behavior. Flow conditions: V1 = 1.12 m/s, x1 = 0.05 m. (1) Stable bubble attraction: (a) t = 0.000 s, (b) t = 0.010 s, and (c) t = 0.036 s. (2) Bubble breakup: (d) t = 0.042 s, (e) t = 0.050 s, and (f) t = 0.062 s. Arrows indicating parent bubbles.

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Fig. 14

Dimensionless statistical properties of bubble interplay events as functions of plunging jet's dimensionless impact velocity μwV1/σ: (a) bubble interplay count rate σFinterplay/(μwg) and proportion of bubbles experiencing interplay, (b) proportion of different coalescence mechanisms, (c) cross-sectional area of parent bubbles and mother bubbles ρwgA/σ, and (d) average vertical position of occurrence of coalescence and average longitudinal distance covered during coalescence (x − x1)/d1

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Fig. 15

Oscillation of impingement point and formation of large vortices in the plunging pool: (a) definition sketch and (b) dimensionless frequency data with comparison to observation of horizontal hydraulic jump toe oscillations by Wang and Chanson [29]

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