[ASME ASME 2009 Second International Conference on Micro/Nanoscale Heat and Mass Transfer -...

MNHMT2009-18130

EFFECT OF NANOPARTICLES ON FORMING PROCESS OF BUBBLES IN A NANOFLUID

Fengmin Su1, Xuehu Ma2, Jiabin Chen2

1. Marine Engineering College, Dalian Maritime University, 116026 Dalian, China

2. Institute of Chemical Engineering, Dalian University of Technology, 116012 Dalian, China

ABSTRACT An experimental investigation was conducted to

determine the effect of nanoparticles on the forming process of bubbles in the gas-liquid-nanoparticle three-phase fluidization. In the experiment, a SiO 2 -water nanofluid was prepared without any surfactants, and the forming process of bubbles in the nanofluid was observed by the high-speed CCD camera. The experimental results show that the diameter of the formed bubble in the nanofluid is smaller 5% than that in water, and the frequency forming bubbles in the nanofluid is higher than that in water. The investigation can provide an insight into the functionary mechanism of the bubble forming process in the gas-liquid-nanoparticles nanofluids. Keywords: nanofluids, bubble forming process, fluidization

1. INTRODUCTION

The operation of slurry bubble columns or gas-liquid-solid three-phase fluidized beds has been used in industrial practices such as biochemical reaction, petroleum fractions, coal liquefaction, gas absorption and removal of air pollutants[1~3]. The gas holdup is one of the most important operating parameters in bubble columns. And, the addition of particles has many effects on the gas holdup in bubble column. Sada[4] reported that the gas holdup in the slurry column with particles (diameter <10μm) increases over that in the column without particles. Kluytmans[5] observed also that the gas holdup in the slurry column with carbon particles(diameter 30μm) is higher than that in the column without particles.

With the wide application of nanometer particles, the nanoparticles have been also used as the suspended particles in the gas-nanofluids bubbling system. Feng [6]

reported the gas holdup in the nanofluid with TiO 2

(diameter =10nm) nanoparticles decreases with the volume fraction of nanoparticles increasing. However, Fan[7] reported that the gas holdup in the nanofluid with SiO 2 nanoparticles (diameter=20nm) increases with the volume fraction of nanoparticles increasing. The author thought that the dispersant, which was used to suspend the particles in Feng’s experiment, affected the formation and motion of the bubble in the nanofluid and caused the decrease of the gas holdup.

At present, the increase of gas holdup in suspensions is attributed to the spacer effect. The fine particles, adsorbed on the gas-liquid surface of bubbles, impede the coalescence of the bubbles in the rising process. This mechanism pays only attention to the bubble rising process, but doesn’t considers the possible effect of particles to the forming process of bubble. If the particles could decrease the initial size of bubbles, the gas holdup also increased.

In this paper, this assumption will be validated and the effect of the nanoparticles on the forming process of the bubbles in the bubble column is analyzed systematically. A SiO 2 -water nanofluid will be prepared without any surfactants, and the forming process of the bubbles in the nanofluid will be recorded using the high-speed CCD camera. The initial radius and the detaching frequency of the bubbles will be measured.

1 Copyright © 2009 by ASME

Proceedings of the ASME 2009 2nd Micro/Nanoscale Heat & Mass Transfer International Conference MNHMT2009

December 18-21, 2009, Shanghai, China

Downloaded From: http://proceedings.asmedigitalcollection.asme.org/ on 04/04/2014 Terms of Use: http://asme.org/terms

Fig. 2 TEM image of the SiO2 nanoparticles in the nanofluid

Fig.1 Image of the SiO2 -Water nanofluid

Fig.3 Schematic diagram of the bubble column device

2. EXPERIMENTAL APPARATUS AND PROCEDURES

The hydrophilic Degussa Aerosil 90 nanoparticles are chosen to prepare the nanofluid. The particles are 20nm in diameter and are composed of silicon dioxide (SiO2). The SiO2 nanoparticles are added directly into the distilled water. The suspension is agitated by ultrasonic applicator for two hours. The stable SiO 2 nanofluid is successfully prepared without any dispersants. Fig.1 shows the nanofluid with 0.4 wt% SiO 2 . It has settled for

six days, and can be observed that these hydrophilic nanoparticles are well dispersed in water. Fig.2 shows the transmission electron microscopy (TEM) image of the SiO2 nanoparticles in the nanofluid.

Fig. 3 shows the schematic diagram of the bubble column device. It consists of a bubble column with an orifice, a buffer vessel with a pressure gauge, a nitrogen gas vessel with a pressure reductor and a high-speed CCD camera (Fastcam-APX RS CCD camera, made in



Photron INC). The bubble column is a plastic rectangular column (250mm in height, 25mm in length, and 25mm in wide). The inner diameter of the orifice at the bottom of the bubble column is 2mm and its tip is 15mm above the bottom. In order to conveniently adjust the gas flow rate, the column is connected with a tee valve. The nitrogen gas is discharged from the environment through the tee valve when its flow rate is adjusted. After the flow rate adjusted, the tee valve is switched and the nitrogen gas is injected in the column. The high-speed CCD camera is used to record the forming process of the bubbles, with a rate of 1000 frames per second and a resolution of 1024×1024. The temperature of the experiment is room temperature about 18℃.

3. RESULTS AND DISCUSSIONS

Fig.4 shows the bubbles, which are just disengaging from the orifice, in water and in 0.1wt% nanofluid at the flow rate of 150ml/min. The bubble in the nanofluid is smaller than that in water at the same flow rate. To analyze quantificationally the variety of the bubble size, the projective areas of the bubbles images in water and nanofluids are respectively measured using the image editor (Image-Pro Plus) and are equated to an effective circular area, Ab, of radius rb. In order to improve the measuring precision, the images of six bubbles in the same experimental condition are measured and the mean value of these radiuses is as the final result. Fig.5 reveals the variation of the radius of the bubbles in the SiO 2 -water nanofluid with the flow rate of the gas. The line in the figure is the trend of the bubble radius in water. It is clearly seen that the radius of the bubble in the nanofluid is lower than that in water. Moreover, the radius of the bubble decreases with the mass fraction of the nanoparticles increasing.

A smaller bubble volume yields a higher detaching frequency of the bubbles because the flowrate of the gas, which flows into the bubble column, equals to the product between the volume and the detaching frequency of the bubbles. In order to validate the above experimental result, the detaching frequency of the bubbles is measured. Fig.6 shows the variation of the detaching frequency of the bubble in the SiO 2 -water nanofluid with the flowrate of the gas. The result reveals that the detaching frequency of the bubbles in the nanofluid is higher assuredly than that in water, and it increases with the mass fraction of the nanoparticles increasing.

100 150 200 250 300 350 400

12

16

20

24

28

32

Cnp

(wt%) 0% 0.1% 0.2% 0.3% 0.4% 0.5%

f(1/

s)

QN(ml/min)

100 150 200 250 300 350 4002.0

2.4

2.8

3.2

3.6

Cnp

(wt%)

0% 0.1% 0.2% 0.3% 0.4% 0.5%

r b (

mm

)

QN (ml/min)

Fig.5 Variation of the radius of the bubble in the nanofluid with the flowrate of the gas

Fig.4 Bubbles in (a) water and in (b) 0.1wt% nanofluid at the flow rate of

150ml/min

Fig. 6 Variation of the detaching frequency of the bubble in the nanofluid with the flowrate

of the gas



Table 1 the model of the initial size of a bubble emerging from a single nozzle [8]

Stage Equation

Expanded stage 3

2

3

1

3

1

3

2

2

3

2

4

32

3

4

3192

11e

leN

l

lNe V

g

dVQ

gg

QV

(1)

Detached stage

3

2

e3

2

eb2

e2

be

3

12

3

12VV

AQ

GVV

AQ

CVV

AQ

Br b

NNN

(2)

Where:

3

1

4

3

e

e

Vr

Nl

le

Q

rA

11

)25.1(961

NQ

gB

11

16

NlQ

dC

11

16

l

lG

3

1

4

311

24

It is evident from above experimental results that the addition of the SiO2 nanoparticles affects assuredly the forming process of the bubbles in the column and decreases their volume. The smaller bubbles imply the higher gas holdup at in the same flow rate of the gas. Thereby, it is also proved that besides the spacer effect between bubbles, which is induced by the particles, the effect of the nanoparticles on the forming process of the bubble is one of the important reasons for the increase of the gas holdup in nanofluids.

There are possibly two kinds of the reasons for the decrease of the bubble initial size in the nanofluid: the change of physical Properties of the nanofluid and the other effects of nanoparticles to the forming process of the bubbles.

It is assumed that the nanofluid is a homogeneous single-phase fluid. According to the bubble dynamics[8], a forming process of a bubble divides two stages: expanded stage and detached stage, and the initial size of a bubble emerging from a single nozzle in the liquid can be described by the model, which is shown in Tab.1. The volume of the bubble at expanded stage, Ve , is firstly computed with the iterative method, then substituted to Eq.(2). The initial volume of the bubble, V b , is finally gained with the trial and error method.

According to the model, the density, surface tension, and viscosity of the liquid affect the initial size of the bubble. It is observed that the surface tension for water and the nanofluids with 0.1, 0.3, 0.5wt% are 73.26, 76.44,

76.83 and 78.06mN/m respectively, and the kinetic viscosities are 1.08, 1.12, 1.13, and 1.14×10-6m2/s respectively. They all increase slightly with the mass fraction of the nanoparticles. The densities of the nanofluids change hardly because few particles are added.

120 140 160 1802

3

4

5

QN(ml min-1)

r b(mm

)

Simulated result of water Simulated result of nanofluid with 0.5wt% Experamental result of water Experamental result of nanofluid with 0.5wt%

Fig.7 Simulation of the bubble radius in the SiO 2 -H 2 O nanofluid

The physical properties of water and the nanofluids with 0.5wt% are respectively substituted to the model. As shown in Fig.7, the simulated results of the bubble in



water are in good coincidence with the experimental result. It indicates that the model can calculate the initial radius of the bubble in water. However, the simulated results in the nanofluid are very different with the experimental result, and are near the results in water. The difference shows that the changes of the physical properties aren’t the main reasons for the decrease of the radius of the bubble in the nanofluid. The nanofluid can not be simply assumed as a homogeneous single-phase fluid. The nanoparticles in the nanofluid have other effects to the forming process of the bubbles.

At the expanded stage, the forming process of a bubble is controlled by four forces: buoyancy, surface tension, viscous force and inertial force. The four forces are only interrelated with the physical properties of the liquid, the diameter of the nozzle and the flow rate of the gas. The foregoing calculated results have indicates that the changes of the physical properties affect hardly the radius at this stage.

0.0 0.1 0.2 0.3 0.4 0.51.40

1.45

1.50

1.55

1.60

1.65

1.70

1.75

L (

mm

)

Cnp

(wt%)

QN=150mlmin-1

Fig. 8 Rising distance of the bubble in the SiO 2 -H 2 O

nanofluid

At the detached stage, the up force which the bubble bears is larger than the down force. The bubble is gradually rising. The bubble detaches with the nozzle passing by a distance. The gas from the nozzle enters into the bubble by a very thin channel in the period. The nanoparticles, adsorbed at the gas –liquid surface of the channel, possibly weaken the stabilization of the thin channel and decrease the rising distance of the bubble. Thereby, they make the size of the bubbles in the nanofluid decrease. However, the distance is assumed as a constant and is the radius of the bubble at expanded stage, r e in the foregoing model. It is possibly the reason why the calculated radius of the bubble in nanofluid is near that in water. To validate it, the rising distances of the bubbles in the SiO2 nanofluids are measured by the bubbles images, which were used when the radius of the

bubbles are measured. In order to improve the measuring precision, the images of six bubbles in the same experimental condition are measured and their mean value is as the final result. As shown in Fig.8, the distances in the nanofluids are smaller than those in water, and decrease with the mass fraction of nanoparticles increasing. The results indicate that the effect of the nanoparticles to the rising distance at the detached stage is very possibly the main reason for the decrease of the initial size of the bubbles in the nanofluids.

4. CONCLUSION

The effect of the nanoparticles on the forming process of the bubbles in the nanofluids is studied experimentally. The present experimental results reveal that the initial radius of the bubble in the nanofluid is lower than that in water and the detaching frequency of the bubbles in the nanofluid is higher than that in water. The smaller bubbles imply the higher gas holdup at the same flow rate of the gas. Thereby, it can be proved that the nanoparticles affect assuredly the forming process of the bubble and are one of the important reasons for the increase of the gas holdup in the nanofluid.

NOMENCLATURE Cnp Mass fraction of nanoparticles, wt% D0 Inner diameter of nozzle, mm f Detaching frequency of bubble, 1/s L Rising distance of the bubble mm QN Flow rate of nitrogen gas, ml/min rb Radius of initial bubble, mm re Radius of bubble at expanded stage, mm Ve Volume of bubble at expanded stage, m3

Vb Volume of initial bubble, m3

l Viscosity of liquid, Pa·s

l Density of liquid, kg/m3

Surface tension of liquid, N/m

ACKNOWLEDGMENTS The authors are grateful to the financial support

provides by New Century Excellent Talent in University (NCET-05-0280) and National Natural Science Foundation of China (No. 50476072).

REFERENCES

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2. E. Alper, S. Ozturk, The effect of activated carbon loading on oxygen absorption into aqueous sodium sulphide solutions in a slurry reactor, Chem. Eng. J. 1986,32,127~130.

3. A. Duybray, J. Vanderschuren, Mass transfer phenomena during sorption of hydrophilic volatile organic compounds into aqueous suspensions of activated carbon, Sep. Purif. Technol. 2004, 38, 215~223.

4. E. Sada, H. Kumazawa, C.H. Lee, Influences of suspended fine particles on gas holdup and mass transfer characteristics in a slurry bubble column, AIChE J, 1986, 32(5), 853~856.

5. J.H.J. Kluytmans, B.G.M.V. Wachem, B.F.M. Kuster, Gas holdup in a slurry bubble column: Influence of Electrolyte and Carbon Particles, Ind. Eng. Chem. Res. 2001, 40, 5326~5333.

6. W. Feng, J. Wen, J. Fan, Localhydrodynamics of gas-liquid-nanoparticles three-phase fluidization, Chem. Eng. Sci. 2005, 60, 6887~6898.

7. L. S. Fan, O. Hemminger, Z. Yu, F. Wang, Bubbles in nanofluids, Ind. Eng. Chem. Rev. 2007, 46, 4341~4346.

8. G. C. Dai, M. H. Chen, Hydrodynamics in Chemical Engineering, Chemical Industry Press, 1988, Beijing, China.


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