bubble behavior and flow structure on bubble collapse

10
567 * 2012.11. 24 ** ⟃ἼᏛᏛ㝔ࢸࢫࢩሗᕤᏛ◊✲⛉ᵓ㐀ᕤᏛᑓᨷ 305-8573 Ⲉᇛ┴ࡤࡃࡘᕷኳ⋤ 1–-1–-1 TEL: (029)853-5487 FAX: (029)853-5487 E-mail: [email protected] ⟶ෆẼἻᚤ⣽⌧㇟ࡅ࠾ẼἻᣲὶ≉ᛶ Bubble Behavior and Flow Structure on Bubble Collapse Phenomena in a Venturi Tube UESAWA Shin-ichiro KANEKO Akiko NOMURA Yasumichi ABE Yutaka Abstract A micro-bubble generator with a venturi tube generates a large number of micro-bubbles with a diameter of 10 Pm – 1 mm by bubble collapse. The bubble collapse is caused by pressure recovery in the diverging region of the venturi tube. The pressure recovery is expected to be a shock for supersonic flow in gas-liquid two-phase flow. However, a profile of Mach numbers to flow direction of the venturi tube has not been estimated experimentally. The present study reveals mechanisms of a bubble collapse. In order to achieve the objectives, we observe bubble behavior with the bubble collapse. In addition, we measure pressure and volumetric void fraction profiles in the flow direction. Pressure is measured by a differential pressure gauge. Void fraction is measured by a constant electric current method and Maxwell’s theory. From these measurements, gas-liquid mixture velocity, sonic speed and Mach number are estimated. In experimental results, bubble collapse is observed with high liquid inlet velocity. When bubble collapse is caused, bubbles expand once into a divergence region of the venturi tube. After that, they contracted rapidly and broken up into a great number of tiny bubbles. Pressure decreases sharply around the throat and increase at the bubble collapse point. On the other hand, the void fraction increase downstream from the throat and decrease around the bubble collapse point. From these results, it is confirmed that the flow is supersonic flow between the throat and the bubble collapse point, while it becomes subsonic flow downstream of the point. Therefore, it is proposed that a shock is present at the bubble collapse point. Keywords: Micro-bubble, Bubbly flow, Venturi tube, Void fraction measurement, Shock 1. ௨ୗẼἻᚄᣢᚤ⣽ ẼἻẼἻ⏺㠃྾╔ᛶ ẼἻෆ⁐ࡢࢫゎ㏿ᗘ㧗ᅽ≉ᚩ᭷ࡇࠋ≉ᚩ⏕ࠊࡋ Ỉ㉁ί➼ᕤᴗⓗᛂ⏝ ࢹࢢࢸࢫࢩ㐀ᙳ௦⾲⒪ᢏ⾡➼⮳ᵝ࡞ࠎศ㔝ᛂ⏝⾜ࡀ✲ࡢࡃⓎ⏕⨨⟶ᘧ⏕ᡂ⨨ෆ㉳ẼἻᚤ⣽⌧㇟⏝ ⏕ᡂ᪉ἲẼ┦ᐜ✚ὶ㔞 ẚస[1]ࠊࡓᚤ⣽⌧㇟ ⏕ࡢẼἻᔂቯ ᅽຊἼⓎ⏕ẼἻᚤ⣽ᛴ⦰ ẼἻෆࡢࢫ㧗ᅽ㧗  ࢱࢡຠᯝᮇᚅࡢࡇ࡞࠺ẼἻᚤ⣽⌧㇟࠸ࡘᐇ㦂 ゎᯒ୧᪉◊✲Ivany, Hammitt and Mitchell[2]⟶ෆ ẼἻᔂቯᅽຊィ  ࢫࠊ࠸ୗὶ⏕㧗ᅽ㒊࠸࠾ᡥᖹ≧Thang and Davis[3]ᆶ┤᪉⟶ෆẼᾮ┦ 㟼ᅽィ ⾜ࠊ࠸⟶ᣑ㒊ᅽຊᅇ☜ㄆࠋࡓࡋࡢࡑࡓᅽຊศᕸẼᾮ ┦ὶࡅ࠾⾪ᧁἼ♧၀ ࠋࡓࡋWang

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Page 1: Bubble Behavior and Flow Structure on Bubble Collapse

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* 2012.11. 24 ** 305-8573 1–-1–-1 TEL: (029)853-5487 FAX: (029)853-5487 E-mail: [email protected]

Bubble Behavior and Flow Structure on Bubble Collapse Phenomena in a Venturi Tube

UESAWA Shin-ichiro KANEKO Akiko NOMURA Yasumichi ABE Yutaka

Abstract A micro-bubble generator with a venturi tube generates a large number of micro-bubbles with a diameter of 10 m – 1 mm by bubble collapse. The bubble collapse is caused by pressure recovery in the diverging region of the venturi tube. The pressure recovery is expected to be a shock for supersonic flow in gas-liquid two-phase flow. However, a profile of Mach numbers to flow direction of the venturi tube has not been estimated experimentally. The present study reveals mechanisms of a bubble collapse. In order to achieve the objectives, we observe bubble behavior with the bubble collapse. In addition, we measure pressure and volumetric void fraction profiles in the flow direction. Pressure is measured by a differential pressure gauge. Void fraction is measured by a constant electric current method and Maxwell’s theory. From these measurements, gas-liquid mixture velocity, sonic speed and Mach number are estimated. In experimental results, bubble collapse is observed with high liquid inlet velocity. When bubble collapse is caused, bubbles expand once into a divergence region of the venturi tube. After that, they contracted rapidly and broken up into a great number of tiny bubbles. Pressure decreases sharply around the throat and increase at the bubble collapse point. On the other hand, the void fraction increase downstream from the throat and decrease around the bubble collapse point. From these results, it is confirmed that the flow is supersonic flow between the throat and the bubble collapse point, while it becomes subsonic flow downstream of the point. Therefore, it is proposed that a shock is present at the bubble collapse point.

Keywords: Micro-bubble, Bubbly flow, Venturi tube, Void fraction measurement, Shock

1.

[1]

Ivany, Hammitt and Mitchell[2]

Thang and Davis[3]

Wang

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Page 3: Bubble Behavior and Flow Structure on Bubble Collapse

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Bubble collapse

Throat Diverging region

Bubble collapseBubble

0 17 37 57 77 97 117-23 z

(b) jLin = 2.49 m/s

(a) jLin = 1.66 m/s

(c) jLin = 3.32 m/s

Fig. 5 Snapshots of the bubbly flow in the venturi tube with each superficial liquid velocity.

0.25

0.20

0.15

0.10

0.05

0.00

Poss

ibili

ty [-

]

3 4 5 6 70.1

2 3 4 5 6 71

2

Bubble diameter D [mm]

= 0.004jLin = 1.66 m/s(Inlet)jLin = 1.66 m/s(Outlet)jLin = 2.50 m/s(Inlet)jLin = 2.50 m/s(Outlet)jLin = 3.33 m/s(Inlet)jLin = 3.33 m/s(Outlet)

PDF

[-]

1.66 m/s (inlet)

jLin = 3.32 m/s (outlet)

3.32 m/s (inlet)

2.49 m/s (outlet)

2.49 m/s (inlet)

1.66 m/s (outlet)

Fig. 6 Bubble distribution in inlet and outlet of venturi tube.

LG

G

QQQ (3)

QG [L/min]

3.3.1

Fig. 5 = 0.01jLin = 1.66 m/s 2.49 m/s

3.32 m/sjLin = 1.66 m/s

jLin = 2.49 m/s

21 mmjLin = 3.32 m/s

42 mm jLin = 2.49 m/s

Fig. 6

jLin = 1.66 m/s

jLin = 2.49 m/s 3.32 m/s

3.2

Fig. 7 jLin = 1.66 m/s

Page 5: Bubble Behavior and Flow Structure on Bubble Collapse

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20

15

10

5

0

Liqu

id v

eloc

ity u

L [m

/s]

2520151050z [mm](Throat)

20 m/s

PIV Bernoulli’s principle

Fig. 7 Flow fields and liquid velocity in the venturi tube measured by PIV with jLin = 1.66 m/s.

20

15

10

5

0

Liqu

id v

eloc

ity u

L [m

/s]

353025201510z [mm]

Bubble collapse

Bubbles

20 m/s

Fig. 8 Flow fields and liquid velocity in the venturi tube measured by PIV with jLin = 2.49 m/s.

PIV

PIV

Fig. 8

10 m/s

10 m/s

const.1 2L L Cpu (4)

00

z = 17 mm = 0.289 0.082 [-] uL = 15.7 m/s p = 10.1 1.1 kPa (4)

1 175 20 kJ/m3 2 10.1 1.1 kJ/m3 C = 185 21 kJ/m3

z = 17 mmz =

27 mm = 0.165 0.068 [-] uL = 10.2 m/sp = 77.6 1.4 kPa (4)1 87.4 7 kJ/m3 2 77.6 1.4 kJ/m3

C = 165 9 kJ/m3

CC

3.3PIV

Fig. 9

Page 6: Bubble Behavior and Flow Structure on Bubble Collapse

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Fig. 9 (a) jLin = 1.66 m/s

jLin = 1.66 m/s60 kPa

Fig. 9 (b) jLin = 2.49 m/s

10 kPa

20 mm 10 kPa

0.4 [-]

Fig. 9(c) jLin = 3.32 m/sjLin = 2.49 m/s

jLin = 2.49 m/s 3.32 m/s300 kPa jLin = 3.32 m/s

jLin = 2.49 m/s

jLin = 3.32 m/s0.8 [-] z =

10 mm 0.4 [-]200 kPa jLin = 2.49 m/s

z = 10 mm

jLin = 3.32 m/s0.8 [-]

400

300

200

100

0Abs

olut

e pr

essu

re p

[kPa

]

12080400z [mm]

1.00.80.60.40.20.0

-0.2

Voi

dfr

actio

n

Void fractionPressure

Throat

16 m

mFlowDiverging region

(Throat)

(a) jLin = 1.66 m/s

400

300

200

100

0Abs

olut

e pr

essu

re p

[kPa

]

12080400z [mm]

1.00.80.60.40.20.0

-0.2

Voi

d fr

actio

n

Void fractionPressure

Bubble collapse

(Throat)

(b) jLin = 2.49 m/s

400

300

200

100

0Abs

olut

e pr

essu

re p

[kPa

]

12080400z [mm]

1.00.80.60.40.20.0

-0.2

Voi

d fr

actio

n

Void fraction

Pressure Bubble collapse

(Throat)

(c) jLin = 3.32 m/s Fig. 9 Pressure profiles and void fraction profiles

to the flow direction in the venturi tube for each liquid superficial velocity.

Page 7: Bubble Behavior and Flow Structure on Bubble Collapse

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60

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Um

D [m

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6040200z [mm]

60

40

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0 Soni

c sp

eed

c m [m

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40

20

0

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vel

ocity

Um

D [m

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6040200z [mm]

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c sp

eed

c m [m

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40

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Um

D [m

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c sp

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c m [m

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PIV

Bubble collapse

(b) jLin = 2.49 m/s

PIV

(a) jLin = 1.66 m/s c jLin = 3.32 m/s(Throat) (Throat) (Throat)

Fig. 10 Gas-liquid mixture velocity and sonic speed profiles around the throat and the bubble collapse points for each liquid superficial velocity.

3.4

Mach

UmD [m/s] cm [m/s] Mach[3]

LG

2LL

2GG

mD 11 uuU (5)

LGm 1

pc (6)

m

mD

cUM (7)

G, L[kg/m3] u [m/s]

(6)

G00

G pp (8)

0

G

GG

AQ

u 0G

(9)

AQ

u L

1L (10)

Q A

G (8)

(5) (6)

Fig. 10PIV

jLin = 1.66 m/s

jLin = 2.49 m/s 3.32 m/s

Page 8: Bubble Behavior and Flow Structure on Bubble Collapse

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5

4

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Bubble collapse (jLin= 3.32 m/s)

2.49 m/s

1.66 m/s

Bubble collapse(jLin= 2.49 m/s)

Fig. 11 Mach number profiles to the flow direction in the venturi tube for each liquid superficial velocity.

(7)Mach Fig. 11

jLin = 1.66 m/s Mach1

jLin = 2.49 m/s 3.32 m/s1

Mach1

Mach 1

4.

Mach

Mach

NomenclatureA : cross-sectional area ]m[ 2

cm : sonic speed ]sm[j : superficial velocity ]sm[M : Mach number ][p : pressure ]Pa[Q : volume flow rate min]L/[u : cross-sectional average velocity ]sm[UmD : gas-liquid mixture velocity ]sm[v : electric voltage ratio ][z : vertical coordinate in the venturi tube ]m[Greek letters

: bulk void fraction ][ : gas volume flow ratio ][ : mass density ]mkg[ 3

SubscriptsG : gas phase L : liquid phase in : inlet of a venturi tube 0 : atmosphere

[1] Fujiwara, A., Takagi, S., Watanabe, K. and Matsumoto, Y., Experimental Study on the New

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F)G ÕH ØIóÕÖ7­ô575

Micro-bubble Generator and Its Application to Water Purification System, Proceedings of ASME FEDSM’03 4th ASME/JSME Joint Fluid Engineering Conference, CD-ROM, FEDSM2003-45162 (2003).

[2] Ivany, R. D., Hammit, F. G. and Mitchell, T. M., Cavitation Bubble Collapse Observations in a Venturi, J. Basic Eng., Vol. 88(3), 649-657 (1966).

[3] Thang, N.T. and Davis, M.R., Pressure Distribution in Bubbly Flow Through Venturis, Int. J. Multiphase Flow, Vol. 7(2), 191-210 (1981).

[4] Wang, Y. and Chen, E., Effects of Phase Relative Motion on Critical Bubbly Flows through a Converging-diverging Nozzle, Phys. Fluids, Vol. 14(9), 3215-3223 (2002).

[5] Kaneko, A., Nomura, Y., Takagi, S., Matsumoto, Y. and Abe, Y., Bubble break-up phenomena in a venturi tube, Trans. JSME Ser. B, Vol. 78(786), 207-217 (2012).

[6] Fukano, T., Measurement of Time Varying Thickness of Liquid Film flowing with High Speed Gas Flow by a Constant Electric Current Method (CECM), Nucl. Eng. Des., Vol. 184(2-3), 363-377 (1998).

[7] Uesawa, S., Kaneko, A., and Abe, Y., Measurement of Void Fraction in Dispersed Bubbly Flow Containing Micro-bubbles with Constant Electrilc Current Method, Flow Meas. Instrum., Vol. 24, 50-62 (2012).

[8] Maxwell, J. C., A Treatise on Electricity & Magnetism. Vol. 1, 435-441, Dover Publications, New York (1954).

____________________________________________________________________________________________

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