applications of swirling-type micro-bubble generator to...

2012/12/12 Special Seminar on Micro/Nano-bubble

Technology in Thailand 1

Applications of Swirling-type Micro-bubble Generator

to Engineering Problems

Design:

Pt. 1 Review of the Swirling-type Bubble Generator

Applications:

Pt. 2 Absorption of Pressure by Micro-bubbles

Pt. 3 Forth Flotation of Suspended Soil Particles by Micro-bubbles

Pt. 4 Dissolved Oxygen (DO) by Micro-bubble Aeration

Harumichi Kyotoh

University of Tsukuba,

Tsukuba, Ibaraki 305-8573, Japan

Pt.1 Review of the Swirling-type

Bubble Generator

2 2012/12/12 Special Seminar on Micro/Nano-bubble

Technology in Thailand

3

Review of

Devises using a swirling flow

1. Vortex tube（1933）

2. Burner (Swirling jet flame)

(for instance, Tangirala (1987))

3. Swirling type micro-bubbler

Ohnari(1995)



4

Ranque-Hilsch vortex tube(1)

http://www.phys.tue.nl/lt/projects/vortex.html

断熱膨張・圧縮過程を利用して暖気と冷気に分離する。

1933, 1947

Adiabatic expansion and compression

1.



5

http://www.mech.kuleuven.be/combust/research/burners/

burner

To Enhance the mixing of air and fuel to reduce the NOx

2.

2012/12/12 Special Seminar on Micro/Nano-bubble Technology in Thailand

6

Micro-bubble generator（Ohnari(1995), pioneer）

http://www.nanoplanet.co.jp/NP_coinfo.html

3.

gas

liquid



There is enough clearance for contaminated water to go through this devise.

7

Micro-bubble generator

Vortex breakdown nozzle for micro-bubble generator

Vane swirler for micro-bubble generator

Vane swirler Vortex breakdown nozzle

hD eD

f

The characteristics of the micro-bubble generator 1. A series arrangement → compact

2. The number of vanes, vane angle, vane channel depth→ control of the swirling flow

3. Vortex breakdown nozzle→ Enhancement of the micro-bubble generation

liquid



Gas

MB-nozzle

⇒Design

8

Vane swirler

Air supply tube

Vortex breakdown nozzle

Ou

ter face of th

e VB

-no

zzle

Liquid

h

θf

Gas

D De

δe

Rim



MB-nozzle



Motor

Gas

Suction cover

Rotor vane

Stator vane

MB pump

Pump

Flow

meter

Inverter

wQ

Pressure gage

Gas

Gas is injected at the inlet of the pump

and is pressurized in the outlet pipe.

Experimental facilities

MB nozzle

50cm

50cm

10

50cm

Pump discharge=16L/min, Head 55m, Air discharge=400cc/min,

Pump power 1000W (Air is injected at the inlet of the pump.), ζ=1200. 2012/12/12

Special Seminar on Micro/Nano-bubble Technology in Thailand

11

t=2min 4min later from the start

t=0

t=4min t=6min 2012/12/12 Special Seminar on Micro/Nano-bubble


12

Pump discharge=21L/min, Head= 33m, Air discharge=400cc/min,

Pump power 580W (Air is injected at the inlet of the pump.), ζ=400. 2012/12/12


13 2012/12/12 Special Seminar on Micro/Nano-bubble Technology in Thailand

t=2min

t=4min

3min later from the start

t=0

14

Side view

Cylindrical bubble

(swirling flow)

Flow attachment

(Coanda effect)

Fragmentation

of the cylindrical

bubble

Spiral type

vortex breakdown



Gas column is formed

because of the lower

pressure at the center

of the swirling flow.

15

Front view Discharge 700cc/sec

1 frame/1ms

cm4


16

Ligaments

&

Bubble

breakup



17

A) Mechanism of the vortex-breakdown induced by

the Coanda effect

B) Mechanisms for the bubble breakup

C) Design of the swirling-type micro-bubble generator

D) Spiral-type vortex breakdown and sound

E) Pressure and sound control at the exit of the vortex

breakdown nozzle

F) Micro-bubble generation in a circular pipe

Topics



Spiral type

Bubble type

An Album of Fluid Motion, The Parabolic Press, 1982

Vortex-breakdown

Delta wing

Circular pipe

The abrupt change of the vortex core

2012/12/12 18 Special Seminar on Micro/Nano-bubble


Flow separation at the leading edge

generates concentrated vortices.

The breakdown of these vortices

causes the trouble of the airplane

manipulation.

Coanda effect

Wall jet

日本機械学会論文集（B編）54巻500号、p.784

1) Put a spoon near tap water jet. 2) The tap water jet is attracted to the spoon.

Tap

water jet

Wall jet deflection along the curved surface

2012/12/12 19 Special Seminar on Micro/Nano-bubble Technology in Thailand

Tap

water jet

Spoon

Cylinder

Coanda effect

A. Mechanism of the vortex-breakdown

induced by the Coanda effect

Low

er pressu

re

Hig

her p

ressure

Criteria： 1．Subcritical swirling flow

（ Se≡ Azimuthal velocity / Axial velocity ≥ 2）

2．Coanda effect

Hig

h p

ressure

20 2012/12/12


v

1. Flow is Subcritical

602, , fcre

e

e

e Su

vS

ev

ee

e

e

e

e DQ

DvSv

r

Rv

r

Qu

4,,

2

D

Swirl number

R2 er2

eu

2eSSubcritical flow →

eD

Mean velocity

Mass

conservation

Angular momentum

conservation

Suction flow at the center of the channel

Subcritical flow is necessary for the Coanda effect.

21

Circumferential velocity



Subcritical flow

Jet

Coanda effect

2. Coanda effect occurs

Strong subcritical flow → wall jet

Curvature of the nozzle edge Low curvature

→ flow attachment but small pressure gradient

High curvature

→ large pressure gradient but flow detachment

Suction flow

Wall jet

High pressure

Low pressure

2 eee Sr

e

er

Condition for flow attachment


1. Generation of secondary bubble (ligaments)

from cylindrical bubble

2. Bubble breakup

by pressure gradient and shear flow

3. Sorting of bubbles

by swirling flow

B. Mechanisms for the bubble breakup



1. Generation of secondary bubble (ligaments)

from the cylindrical bubble

Low pressure High pressure

Breakup

Collapsing of low pressure bubble

under high pressure environment (Fragmentation of cavitations bubble)

10~20kPa 100kPa


2. Bubble breakup

by pressure gradient and shear flow

ee

e

e

H

rLvu

r

v

L

u

d

,

,3.1

33

5/1

32

3

γ：surface tension coefficient

450

udRe

(J. Fluid Mech., vol. 401, pp. 157-182(1999))

Ligament

breakup

(Fan & Tsuchiya(1990))

・Hinze scale(Inertia dominated) ？



3. Sorting of bubbles by swirling flow

2/2

1 2

wawaDolwaolw qqqqafds

dPVqqV

dt

d

D

ol

w

wa

f

V

s

qq

：relative velocity

：liquid density

：bubble volume

：drag coefficient

3aVol

：stream direction

Dynamic equation for translational motion of a bubble

Higher pressure

a

sLower pressure

e

ew

r

v

ds

dP2

χ>1

χ<1



Sound from primary bubble

oscillations

Secondary bubble

(ligament)

Sound from secondary bubble

oscillations

Primary bubble

(a) Generation of secondary bubble

from the cylindrical bubble

Hinze scale

(Inertia dominated)

Secondary bubble

time

(b) Bubble breakup

by pressure gradient

(c) Sorting of bubbles

by swirling flow

Viscosity dominated

small bubbles

Summary of the mechanisms of the bubble breakup


28

1) The radius of curvature at the nozzle edge should satisfy

2) The diameter of the vortex breakdown nozzle is determined so

that the Coanda effect occurs, i.e., Se>2. (Determine the vane angle θf which should be greater than 64 degrees.)

C. Design

of the swirling-type micro-bubble generator

Conservation of the angular momentum

Conservation of the angular momentum flux

)2/(6,2/

ˆ D

hh

Geometric parameters

h/D, De/D, θf, τ

1. Flow attachment


.2

eee Sr

29

2. Resistance



θf=78°，h=3mm，D=19mm

De/D

θf=78°，h=4mm，D=19mm

De/D

fDhDDeFUp

,/,/,2

2

To generate many fine bubbles,

vane angle θf should be larger.

As a result, the resistance of MB-

nozzle becomes larger.

6072,2/1,10/7

1056,2/1,10/7

F

F

30

Miscellaneous

D. Spiral-type vortex breakdown and sound

E. Pressure control

at the exit of the vortex breakdown nozzle

F. Micro-bubble generation in a circular pipe



(a) Flow attachment

(b) Flow detachment

D. Spiral-type vortex breakdown and sound

Aco

ust

ic p

ress

ure

A

coust

ic p

ress

ure

31 2012/12/12


(Vortex whistle)

1. Spectrum of the sound

(a) Flow attachment (b) Flow detachment

fw: Angular frequency of the swirl

fa: Natural frequency of the cylindrical bubble 32 2012/12/12


wcylindercylinder

sphere

cafaa

Pf

aa

Pf

/2ln/2

2

1

23

2

1

32

32

2. Natural frequency of the spherical and cylindrical bubbles

(Dispersion relation)



a: bubble radius

ρ:liquid density

σ:surface tension

γ:ratio of specific heat

P:gas pressure

E. Pressure control

at the exit of the vortex breakdown nozzle

Purpose:

1. Efficiency of micro-bubble generation

2. Suppression of the sound

Front pressure becomes lower.

Pressure inside the cylindrical bubble decreases.

Pressure gradient increases.

Discharge increases.



Front nozzle

Distance between the Vortex breakdown nozzle and the plate

・Pressure inside

of the cylindrical bubble

・Resistance of the nozzle

・Acoustic pressure

t0

4/eD

S2

S1

S3

S4

S1 S2 S3 S4

eD t

Effect of the front nozzle

on the efficiency of the swirling-type micro-bubble generator



36

F. Micro-bubble generation in a circular pipe

1. Swirling flow propagates to

the downstream direction.

2. Bubble merging occurs at the

center of the swirling flow.

3. Flow shear above the front

side of the vortex breakdown

nozzle is weakened,

because the relative velocity

between the swirling flow and

the flow down-stream becomes

smaller.

Problems



22

Bubbler

10 mm

1. Front nozzle is installed to suppress the swirling flow downstream.

2. Multi-nozzle whose total circulation is zero

Solutions to the problem





Summary

A swirling-type micro-bubble generator is reviewed.

The Coanda effect and the vortex-breakdown are

the key words for the design of the micro-bubble

nozzle.

High-head pump is better for the generation of

small and high density bubbles.

Special care is necessary when the micro-bubble

nozzle is used in the channel.

Sound generation can be suppressed if the front

nozzle is installed.

Pt. 2 Absorption of pressure

A. Japan Proton Accelerator Research Complex

Materials and Life Science Experimental Facility (MLF) is aimed at promoting materials science and life science(protein) using the world highest

intensity pulsed neutron and muon beams which are produced using 3-GeV protons with a

current of 333micro-amps and a repetition rate of 25 Hz. Proton hits the mercury target.

Micro-bubble is used to reduce the heat shock waves.

http://j-parc.jp/MatLife/en/index.html

B. Air-Entrained Concrete The primary purpose of air entrainment is to increase the durability of the hardened

concrete, especially in climates subject to freeze-thaw. During the freeze-thaw, the air

bubble can be compressed a little, and so the bubbles act to reduce or absorb stresses

from freezing.

C. Soil liquefaction describes the behavior of soils that, when loaded, suddenly suffer

a transition from a solid state to a liquefied state. During earthquake loading, loose sands

tend to decrease in volume, which produces an increase in their pore-water pressures and

consequently a decrease in shear strength. It has been known that liquefaction is

refrained if there were cavities in the ground.



40

http://j-parc.jp/MatLife/ja/index.html

To generate high energy neutron beam,

proton beam is applied to liquid mercury,

where heat shock waves are scattered in

the liquid mercury. That shock waves

cause cavitations at the boundary of the

vessel, which will erode the vessel surface.

The bubbles which can react that strong

pressure rise need to be micron-size

because they have high resonant frequency.

A. Entrainment of micro-bubbles into mercury

1m



Joint research

with Japan Proton Accelerator Research Complex

Mercury

Heat shock

waves

Vessel for mercury

Pressure waves Cavitations

Proton pulse beam

(1MW、25Hz)

2012/12/12 Special Seminar on Micro/Nano-bubble Technology in Thailand 41

Reduction of the pressure by gas bubbles

Requirement: Micro-bubbles need to be generated by low pressure loss, since

the power of the mercury pump is limited.

Helium-gas does not dissolve into mercury.

Surface tension of mercury is around 10 times that of water.

The volumetric ratio of He-gas to Mercury is about 0.1%.

Solution: Swirling type micro-bubble generator with low resistance,

i.e., resistance coefficient ζ～10 (θf~60°).

Multi-nozzle whose total circulation is zero.




Minimum size of bubble generated in water and mercury

ee

e

e

H

rLvu

r

v

L

u

d

,

,3.1

33

5/1

32

3

γ：surface tension coefficient

450

udRe

(J. Fluid Mech., vol. 401, pp. 157-182(1999))

Ligament breakup

(Fan & Tsuchiya(1990))

water mercury

・Hinze scale(Inertia dominated) ？

43 2012/12/12

B. Generation of micro-bubbles in cement paste

Grout pump

Micro-bubble nozzle

Bucket

Cement paste

Joint Research

with Sato Kogyo co. Ltd, Wakachiku Construction co. Ltd

Kawada Construction co. Ltd



Spacing Factor

Micro-bubble

Plain



2012/12/12


46

MB cement

Plain cement

Incremental pore volume

Pore diameter(μm)

Incr

eme

nta

l po

re v

olu

me

(mL/

g)



Void fraction increases.

Pore distance becomes shorter.

Compressive strength becomes larger.

Durable to freeze-thaw

No decrease of the strength due to pores

Plain

Spacing factor Pore volume Density Strength

Summary



歌川紀之、金子典由、小俣文良、楠岡弘康、木俣陽一、関東継樹、京藤敏達：

マイクロバブルの建設・環境分野への適用に関する研究、佐藤工業㈱技術研究所報、No.31,pp.29-36,2005-2006.(Sato Kogyo Research Report)

C. Soil liquefaction

To suppress soil liquefaction in the saturated zone, air

may be injected into the soil so that the pore water

pressure will not rise during earthquakes.

Because micro-bubbles can easily permeate into voids

between sand particles, air is supplied as a form of

micro-bubble water.



Joint research

with Sato Kogyo co. Ltd.

Nagao K., Azegami Y., Yamada S., Suemasa N. and Katada T. : A micro-bubble injection method for a countermeasure against liquefaction, 4th international conference on earthquake geotechnical engineering, paper ID 1764, pp.392, 2007.

Soil(sand particles)

Micro-bubble water



VideoNo.1

No1Liquefaction.wmv

VideoNo.2

No2（50,100,150ｇａｌ）.wmv



A. Japan Proton Accelerator Research Complex

Materials and Life Science Experimental Facility (MLF) Micro-bubble can respond high frequency oscillations of pressure and reduce

the heat shock waves.

B. Air-Entrained Concrete The compressive strength of Micro-bubble-entrained cement increases even if

the pore volume increases.

C. Soil liquefaction Micro-bubble water can go through the spacing of sand particles.

Summary

歌川紀之、楠岡弘康、木俣陽一、京藤敏達：マイクロバブルを用いた高度濁水処理装置の開発ー低SS処理水の浮上分離実験土木学会第64回年次学術講演会、Ⅵ-183,2009.

Pt. 3 Forth flotation of suspended soil particles

by micro-bubbles

Purpose: The muddy water discharged from construction

works need to be disposed of to preserve

environments.

To remove soils, Poly-Aluminum Chlorohydrate

(PAC) and Polymer are used as a flocculating agents

for water purification.

After the water purification, however, suspended

flocs still remains in the water.

Micro-bubbles are used to remove these flocs.

Joint research

with Sato Kogyo co. Ltd. & Wakachiku Construction co. Ltd.





Experimental facility

Floc suspended water

PAC

Polymer

Micro-bubbles

Experimental procedure

Sampling

Sampling

15min

MB-nozzle

Soil mixed

water

honeycomb

Floated

flocs

pump

PAC(ml/l)

Before MB

After MB

Before MB

After MB

Turb

idit

y

Floated flocs

Susp

en

de

d S

olid

PAC(ml/l) Microscope image of floc

50μm

Turbidity and Suspended Solid 2012/12/12 54 Special Seminar on Micro/Nano-bubble Technology in Thailand

1mm

Floated flocs

MB nozzle

Floated flocs

Before

treatment

After

treatment

Stream-type turbidity treatment




Summary

To remove soil particles, floc formation is necessary, i.e.,

PAC and Polymer need to be added to the muddy water.

The floc containing micro-bubble is the key point of the

present turbidity treatment, because the flocs containing

micro-bubbles float even if they are fragmented into small

parts by turbulent flows.

The value of Suspended Solid becomes less than 20 mg/L,

which is the present drainage standard of construction

works, after the present treatment.

Pt. 4 Dissolved oxygen (DO)

by Micro-bubble Aeration

Questions Smaller bubbles are better ？

Increase of DO due to air

←Nitrogen hinders the dissolution of oxygen？

Water quality influences DO ?

Purpose：To supply oxygen to aquatic environment,

A. Dissolution of gas into water B. Efficiency of DO-increase C. Water quality and DO



A. Dissolution of gas into water

Henry’s law

jp jP

jS

/2RURe

jcj DS /

Sherwood number

Reynolds number

Schmid number

Henry’s constant jH

R

jMMole of gas

・Residence time dt

・Diffusion of gas RSj

・Pressure inside bubble pj - Pj

jj

j

jjjpP

H

SRD

dt

dM

2


Dissolution rate of gas into liquid is

proportional to

Micro-bubbles

(Qa is small.)

Aeration tube

(Qa is large.)

1. The total surface-area of the bubbles ○ ○

2. The diffusion of the dissolved gas × ○(significant)

3. Pressure inside the bubble ○(insignificant) ×

Summary of the results according to Henry’s law

10 12 14 16 18 20 22 240

5. 10 10

1. 10 9

1.5 10 9

2. 10 9

temperature deg.

Dif

fusi

on

coef

fici

entm

2s

10 12 14 16 18 20 22 240

50 000

100 000

150 000

temperature deg .

Hen

ryco

nst

ant

mo

l1

m3

Pa

Oxygen

Nitrogen

Oxygen

Nitrogen

Oxygen is around two times more

dissolvable than nitrogen.

Turbulent diffusion is scaled by

uL,

hence, it is proportional to

（the Reynoluds number

×Molecular diffusion） .

Therefore, dissolution of gas is

strongly influenced by the flow

fields, i.e., turbulent or laminar.

Molecular diffusion



(1) Rise velocity of a single bubble

182

2 gRU

61/39

61/3961/8961/5038.0

RgU

Solid sphere

Fluid sphere

Residence time h/U

bRR

bRR

Small bubble is advantageous jj

j

jjjpP

H

SRD

dt

dM

2



(2) Diffusion of gas in water

41.0

3/1

11 e

e

cjj RR

SS

4001 eR

3/111 cjej SRS

1eR

Large bubble is advantageous. jj

j

jjjpP

H

SRD

dt

dM

2



(3) Pressure inside a single bubble

RPp

2

Small bubble is advantageous. jj

j

jjjpP

H

SRD

dt

dM

2

mh 5

mh 1

cmh 10S

urf

ace

tensi

on/w

ater

pre

ssure



<Numerical simulation> Boundary condition: DO=50％，DN=100％

A micro-bubble with 50μｍ in radius is released at the depth of 1m underwater.

Dissolution of air micro-bubble

(Numerical simulation)

55秒で12cm浮上後に消滅

Oxygen

dissolution Oxygen and Nitrogen

dissolution

•Oxygen rapidly dissolves at the initial stage.

•Nitrogen also dissolves completely.

O2/N

2

Rad

ius(

μm

)



Dissolution of air bubble in a tank

(Numerical simulation)

Nitrogen saturates before DO becomes 100%.

• For micro-bubble,

The super-saturation of nitrogen occurs.

The component of dissolved gas tends to be similar to that of air.

• For milli-bubble, oxygen dissolves dominantly.

Micro-bubble Milli-bubble

Separation of Nitrogen

The micro-bubble aeration might be inefficient for DO-increase. 2012/12/12 64 Special Seminar on Micro/Nano-bubble Technology in Thailand

B. Efficiency of DO-increase

200cm

98cm

94cm

Experimental facility

MB pump Porous stone (pore diameter 100μm)

Purpose: To compare the efficiency of DO-increase between

• Aeration tube (porous stone with diameter 100μm) 74W

• Micro-bubble pump 1000W

MB pump


MB pump

Oxygen discharge is 1L/min.

Aeration tube

Oxygen discharge is 10L/min.

Oxygen dissolution experiment





Oxygen aeration by Micro-bubble and Aeration tube

0 20 40 60 80 100 120 140100

200

300

400

500

Time min

Ox

yg

en

Aeration tube:10L min , MB pump :1L min

Dissolved Oxygen

MB-pump

1L/min

Aeration tube

10L/min

0 20 40 60 80 100 120

0.0

0.2

0.4

0.6

0.8

1.0

Time min

Dis

solv

edO

xy

gen

Su

pp

lied

Ox

yg

en

Aeration tube:10L min , MB pump :1L min

The ratio of Dissolved Oxygen

to Supplied Oxygen

Aeration tube

10L/min

MB-pump

1L/min



0 20 40 60 80 100

100

200

300

400

500

Time(min)

Oxy

gen

(%)

Aeration tube

10L/min

MB-pump

1L/min

MB-Nozzle

1L/min

Oxygen aeration by Micro-bubble and Aeration tube

20 30 40 50 60 70 80

0.0

0.2

0.4

0.6

0.8

1.0

Time min

Ox

yg

en

MB pump ,1L min Oxygen ,770W

0%

20%

40%

60%

80%

100%

120%

0 20 40 60 80 100 120

Dis

solv

ed

Oxy

gen

Time（min）


MB pump(Air 2L/min)

Aeration tube(Air 31L/min)

DO=78%

Aeration tube:

5% of oxygen in air is dissolved

MB pump

33% of oxygen in air is dissolved

Air aeration by Micro-bubble and Aeration tube

Super-saturation

The efficiency defined by DO-increase per Watt

(electricity consumption)

The efficiency of the micro-bubble pump is roughly 1/20-

times that of the aeration tube.

The efficiencies become comparable at around DO=95%

for air aeration.

Micro-bubble will be useful to dissolve special gases, such

as ozone, since it is dangerous and expensive.

Note that the efficiency depends on the depth of water.

DO is influenced not only by the total surface area of the

injected gas, but also by the diffusion of the dissolved gas

due to turbulent flows. Therefore, the oxygen inside the

large air bubbles also effectively dissolves into water.

Summary



1.3cm

Water discharge=6ℓ/min

Purpose：

The influence of water quality on DO-increase is studied experimentally.

tap water, refined water, distilled water, salt water, pond water

Micro-bubble nozzle

Experiment：

Oxygen micro-bubbles are generated in a tank, and DO is measured.

C. Water quality and DO due to oxygen micro-bubbles

(Experiment)

Gas discharge=10cc/min



Photograph

Tap water Refined water Distilled water



Salt water Pond water



DO

0

5

10

15

20

25

0 500 1000 1500 2000

t[s]

DO

[mg/L] 水道水

純水

蒸留水

3%食塩水

池水

Pond

Tap

Refined

Distilled

Salt

Pond

Salt



Summary

DO-increase of pond water in summer is slow.

The rate of DO-increase of salt water abruptly diminishes at some critical point.

Bubble density of pond water and salt water is high, but DO is low.

1. To supply oxygen to pond water, large air bubbles are

more efficient than air micro-bubbles.

2. Aeration due to air micro-bubbles makes the component

of the dissolved gas into the same component of air, so

that the DO in water will not saturate.

3. The gas inside the micro-bubble in contaminant water is

difficult to dissolve since the surface of the bubble is

covered by contaminants.





Conclusion

for swirling-type bubbler,

1. DO-increase by micro-bubble aeration is inefficient.

2. Micro-bubble may be useful for the dissolution of

expensive gas.

3. Pressure absorption by micro-bubbles is essential

to the reduction of shock waves.

4. Micro-bubble air entrained concrete needs more

study, for instance, the tracking of micro-bubbles

in cement milk and the more efficient method to

mix micro-bubbles into concrete.

5. Micro-bubbles are useful for the flotation of

contaminants.

6. To generate micro-bubbles is easy, but to find their

effectiveness is not easy.

applications of swirling-type micro-bubble generator to...

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