Downl

EXPERIMENTAL STUDY ON THE THERMAL PHYSICAL PROPERTIES OF A CNTS-AMMONIA BINARY NANOFLUID

Xuehu Ma, Fengmin Su, Zhong Lan, Jiabin Chen

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

Proceedings of MNHT2008 Micro/Nanoscale Heat Transfer International Conference

January 6-9, 2008, Tainan, Taiwan

MNHT2008-52153

ABSTRACT In this paper, carbon nanotubes—ammonia

nanofluids(the binary nanofluids) have been prepared by two steps method. And the thermal conductivity, surface tension and kinetic viscosity of the binary nanofluid have been measured. The effects of the mass fraction of carbon nanotubes, the concentration of ammonia and temperature on the thermal physical properties of the binary nanofluid have been systematically studied. On the base, the effective thermal conductivities of the binary nanofluids have been calculated using the models in the literatures, and have been compared with the experimental values. Key words: binary nanofluids, thermal conductivity, surface tension, kinetic viscosity, ammonia, carbon nanotubes, .

1. INTRODUCTION

The binary nanofluid is the suspension in which nanopartilces are suspended evenly in the aqueous ammonia. The base fluid, aqueous ammonia, is the binary mixture, so it is named as the binary nanoduids. The addition of nanoparticles can significantly improve the thermal conductivity of liquids. Since Choi et al.[1] first defined the concept of nanofluid in 1995, many researchers have measured the thermal conductivities of nanofluids with various nanoparticles[2~4]. Nanofluids, containing only a small amount of nanoparticles, have remarkably higher thermal conductivities than the base fluids. Especially, for the nanofluid with Carbon NanoTubes(CNTs) , its thermal conductivity can be increased by 38% at 0.6%（V/V）CNTs[3]. The nanofluid has become one of the most attractive enhancement heat transfer media. It has been found that nanofluids also illustrate significant enhancement for the convective heat transfer [5~7] and boiling heat transfer [8, 9]. Besides its enhancement for heat transfer process, nanofluid can enhance mass diffusion performance in liquid. Krishnamurthy et al.[10] reported that the diffusion coefficient of a dye in a nanofluid containing 20nm Al2O3

oaded From: http://proceedings.asmedigitalcollection.asme.org/ on 04/01/2014 Ter

nanoparticles is higher about 13 times than that in water without nanopartilces. The benefits of nanofluid can also be used to enhance the heat and mass transfer processes of absorption. Kim et al.[11] had firstly found that the binary nanofluids have remarkable enhancement for absorption, for the NH3/H2O system with/without Cu, CuO and Al2O3 particles.

The addition of ammonia and CNTs has the remarkable effects on the thermal physical properties of the binary nanofluid, such as thermal conductivity, kinetic viscosity, surface tension and so on. The thermal physical properties directly affect the enhancement of heat and mass transfer characteristics of the absorption process. In this paper, CNTs-ammonia binary nanofluids will be prepared without any surfactant addition. The thermal conductivity, kinetic viscosity and surface tension of the binary nanofluid will be measured experimentally, and the effects of the mass fraction of carbon nanotubes, the concentration of ammonia and operating temperature on the thermal physical properties will be investigated. Meanwhile, the thermal conductivities of the binary nanofluids will be calculated using the models in literature for comparison.

2．EXPERIMENTAL APPARATUS AND PROCEDURES 2.1 Preparation of CNTs-Ammonia binary nanofluids

In this paper, Aligned multi-wall carbon nanotubes(A-MCNTs) are used as the nanoparticles in the binary nanofluid, since there is no chemical reaction to take place between CNTs and ammonia. In order to obtain the even dispersion solution, Nitric acid is used to modify the surface of CNTs [12,13]. One gram of CNTs is suspended in 40 ml of concentrated nitric acid (≈68%) and refluxed for an hour at 120℃. After washing with deionized water until the supernatant attained a pH around 7, the CNTs are dried at 55℃. And then the chemically treated CNTs can be added directly into the base fluid. The suspension is agitated by ultrasonic applicator for two hours. Stable nanofluid is successfully produced without surfactant.

1 Copyright © 2008 by ASME

ms of Use: http://asme.org/terms

Dow

Fig.1 illustrates that the suspensions with 0.2 wt% pristine CNTs (PCNTs) and treated CNTs (TCNTs) in 15wt% aqueous ammonia after three days. As shown in Fig.1, almost all PCNTs deposited, having upper fluid transparent. In sharp contrast to PCNTs, TCNTs were well dispersed in aqueous ammonia. Fig.2 is the transmission electron microscopy (TEM) image of TCNTs in the binary nanofluid. 2.2 Thermal conductivity measurement

The transient hot-wire method is used to measure the effective thermal conductivity of the CNTs-Ammonia binary nanofluid. For the conventional transient hot-wire method, an infinitely long fine wire is needed to serve as the heat source and thermometer. But in the actual experiment, a finite length of platinum wire is usually used, so that the measurement error is inevitable. In this paper, Two fine platinum wires are embedded vertically in the fluid in this system. The two wires are at the same conditions except their lengths, so the wires have the identical end effect. The temperature difference of the two wires can be considered as the temperature increase of the finite part of an infinitely long wire. So the measurement error due to the end effect can be neglected. The detailed proceeding of the transient double hot-wire method were presented by Li et

Fig.1: Digitial pictures of CNTs suspensions (a) PCNTs (b) TCNTs

Fig.2: TEM image of CNTs in binary nanofluid

nloaded From: http://proceedings.asmedigitalcollection.asme.org/ on 04/01/2014 T

al[14]. To determine the uncertainty for the thermal conductivity measurement, the thermal conductivity of water is measured after the installation of the experimental setup. The uncertainty of the measured thermal conductivity is less than 3%.

In order to examine the enhanced effect of the binary nanofluids, the thermal conductivity ratio is defined as:

f

eff

kk

E =

(1)

Where E is the thermal conductivity ratio, keff and kf are the effective thermal conductivity of the binary nanofluid and the thermal conductivity of the base fluid, respectively. 2.3 Surface tension measurement

The pendant drop method is used to measure the surface tension of the CNTs-Ammonia binary nanoflids. The contact angle measuring system (OCAH200, DataPhysics, Germany) is employed for the measurement. To determine the uncertainty for surface tension measurement, the surface tension of water at 25℃ is measured. Compared the measured value with the data in literature, the measurement uncertainty is less than 0.5%. 2.4 Kinetic viscosity measurement

The capillary viscometer is used to measure the kinetic measurement of the CNTs-Ammonia binary nanofluids. The diameter of the capillary pipe in the measurement is 0.6mm. The capillary viscometer is calibrated using water. The constant of the capillary viscometer is 8.1x10-9m2s-1. The measurement uncertainty is less than 1%.

3. THERMAL PHYSICAL PROPERTIES OF CNTS-AMMONIA BINARY NANOFLUIDS 3.1 Effective thermal conductivity of binary nanofluid

Fig. 3 shows the effect of the the mass fraction of CNTs on the thermal conductivity ratio, E, of the CNTs-ammonia binary nanofluid. The base fluids used in this study as the reference liquids are deionized water, 12.5wt% ammonia solution and 25.0wt% ammonia solution. The temperature is 20℃ when the measurement is conducted. The result shows that the binary nanofluid, containing only a small amount of CNTs, has a remarkably higher effective thermal conductivity than the base fluid. For the CNTs-ammonia binary nanofluid, the thermal conductivity can be enhanced by more than 16% at a mass fraction of 0.5%.

It can also be found in Fig.3 thatThe thermal conductivity ratios of the CNTs-water nanofluids are different from those for the CNTs-ammonia binary nanofluids. The thermal conductivity ratios of the CNTs-ammonia binary nanofluid increase nonlinearly with the mass fraction of nanoparticles. At lower mass fraction of CNTs, the thermal conductivity ratios of the binary nanofluids are almost same with those of the water nanofluids. But with the mass fraction of CNTs increasing, the thermal conductivity ratios are gradually lower than those of

2 Copyright © 2008 by ASME

erms of Use: http://asme.org/terms

Do

water nanofluids. The ammonia is a weak electrolyte and ionizes partly in water. According to colloid stability theory, the ammonia ions in the ammonia solution facilitate the aggregation of CNTs and deteriorate the dispersion of CNTs in the fluid, especially at higher mass fraction of CNTs. As a result, the thermal conductivities of the binary nanofluids at higher mass fraction of CNTs are lower than those of water nanofluids.

As shown in Fig.3, the thermal conductivity ratios of the binary nanofluids with the ammonia concentrations of 12.5% and 25% respectively are almost same. Ammonia is a weak electrolyte. The concentration of ammonia ions hardly changes with the ammonia concentration in the solution. So the thermal conductivity ratios of the binary nanofluid would not change with the ammonia concentration.

Figure 4 demonstrates the variation of the thermal conductivity ratio of the CNTs-ammonia binary nanofluid with temperature. The mass fraction of CNTs is 0.05%. The base

1.2

1.3

0.0 0.1 0.2 0.3 0.4 0.51.0

1.1

0% 12.5% 25%

/kf

mass fraction of CNTs(%)

k eff

Fig.3: Variation of the thermal conductivity ratio of the CNTs-ammonia binary nanofluids with the mass fraction

of nanoparticles

20 30 401.00

1.05

1.10 25% 12.5%

k eff/k

f

Temperature(℃)

Fig.4: Variation of the thermal conductivity ratio of the CNTs-ammonia binary nanofluids with temperature

wnloaded From: http://proceedings.asmedigitalcollection.asme.org/ on 04/01/2014 Ter

fluids used are 12.5% and 25.0% ammonia solutions. The thermal conductivity ratios increase nonlinearly with temperature. According to the reference [15], the temperature dependence of thermal conductivity enhancement can be expressed as:

Where keff and kf are the effective thermal conductivity of

the binary nanofluid and the thermal conductivity of the base fluid, respectively, dp denotes the diameter of nanoparticles, T is the temperature of nanofluids, A, B and C are constants and are 1.314, 247.8 and 140, respectively.

The solid line and the dash line in Fig.4 are the calculated results of equation (2) for two kinds of binary nanofluids, respectively. It is seen that the experimental data and the calculated results agree very satisfactorily. 3.2 Surface tension of binary nanofluid

In the experiment, the effects of the mass fraction of CNTs and the concentration of ammonia in the binary

nanofluid on the surface tension are mainly studied. The surface tension of the CNTs-ammonia binary nanofluid is measured at 25 . Fig.5 shows the variation of the surface ℃tension of the nanofluid with the mass fraction of CNTs. The concentration of ammonia is 0. As shown in Fig.5, the surface tension increases slightly with increasing the mass fraction of CNTs. Das et al [9] also reported that the surface tension of the Al2O3-water nanofluid (without adding surfactants) was almost identical to that of water at small mass fraction of nanoparticles.

Fig.6 reveals the variation of the surface tension of binary nanofluids with the concentration of ammonia. The mass fractions of CNTs are 0 and 0.3% respectively. The surface

⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛

⎟⎟⎠

⎞⎜⎜⎝

⎛⋅+=

−CTB

pf

eff Td

Akk

4642.2

2321.1369.0

10

11(2)

0.0 0.2 0.4 0.6 0.868

70

72

74

σ(x1

0-3N

m-1)

Cnp(wt%)

Fig.5: variety of the surface tension of the binary nanofluids with the mass fraction of

CNTs

3 Copyright © 2008 by ASME

ms of Use: http://asme.org/terms

D

tension decreases with the concentration of ammonia increasing, whether CNTs are added or not. The surface tension of the binary nanofluid in which the mass fraction of CNTs is 0.3% is almost identical to that of aqueous ammonia. It can be concluded that the CNTs have only a slight effect on the surface tension of the binary nanofluid without adding surfactants at small mass fraction of CNTs.

3.3 kinetic viscosity of binary nanofluid Fig.7 shows the variation of the kinetic viscosity of the CNTs-water nanofluid with the mass fraction of CNTs. The temperature of the nanofluid in the experiment is 25 . The ℃concentration of ammonia is 0. As shown in Fig.7, the kinetic viscosity of the nanofluid increases linearly with the mass fraction of CNTs increasing, but the degree of increase is very small and is less than 4%. It can concluded that the CNTs have only the slightly effect on the kinetic viscosity of the nanofluid without surfactants at small mass fraction of CNTs.

0 5 10 15 20

60

65

70

0% 0.3%

σ(x1

0-3N

m-1)

CA(wt%)

Cnp(wt%)

Fig.6: variety of the surface tension of the binary nanofluids with the concentration of

ammonia

Fig.7: Variety of the kinetic viscosity of the CNTs-water nanofluid with the mass fraction of CNTs

0.0 0.1 0.2 0.3 0.4 0.50.7

0.8

0.9

1.0

1.1

1.2

ν(x1

0-6m

2 s-1)

Cnp(wt%)

ownloaded From: http://proceedings.asmedigitalcollection.asme.org/ on 04/01/2014 T

4. PREDICTION OF THERMAL CONDUCTIVITY OF BINARY NANOFLUID

The effective thermal conductivity of the CNTs-ammonia binary nanofluid is predicted using five conventional models reported in the literatures. The five models are shown in Tab.1.

0.0 0.1 0.2 0.3 0.4 0.51.00

1.05

1.10

1.15

1.20 present measurements Maxwell model H-C model M-C model 1 M-C model 2 M-C model 3

k eff/k

f

Cnp(wt%)

(c) 25%ammonia

0.0 0.1 0.2 0.3 0.4 0.51.00

1.05

1.10

1.15

1.20 present measurements Maxwell model H-C model M-C model 1 M-C model 2 M-C model 3

k eff/k

f

Cnp(wt%)

(b) 12.5%ammonia

0.0 0.1 0.2 0.3 0.4 0.51.0

1.1

1.2

1.3 present measurements

k eff/k

f

Cnp(wt%)

Maxwell model H-C model M-C model 1 M-C model 2 M-C model 3

(a) water

Fig.8: the simulated results of the effective thermal conductivity of the CNTs-ammonia binary nanofluid

4 Copyright © 2008 by ASME

erms of Use: http://asme.org/terms

Download

Table 1 models for effective thermal conductivity of nanofluids Models Equations

Maxwell[16]

( ) ( )φααφα

12)1(31−−+

−+=

f

eff

kk

Hamiton-Crosser[17] (H-C) ( ) ( )( )

( ) ( )φααφαα

−+−+−−−−+

=11

111n

nnkk

f

eff

micro-convection model 1[18]

(M-C model 1) ( )( ) (

( ) ())αφααφαφ

−−+−++

+=1211221PrRe1 333..04.2D

kk

f

eff

micro-convection model 2[15]

(M-C model 2) 2321.19955.07476.0

369.0

7460.0 RePr1 αφ ⎟⎟⎠

⎞⎜⎜⎝

⎛+=

p

f

f

eff

dd

Fkk

micro-convection model 3 (M-C model 3) ( ) ( ) ( )( )

( ) ( )φααφααφ

−+−+−−−−+

+=11

111PrRe1 333..04.2

nnnD

kk

f

eff

α=keff/kf; D and F are the constants, and are 15000 and 20 respectively; n is the particle shape factor;φ

The micro-convection model 3 considers mainly the effect of the micro-convection which is caused by CNTs and the shape of CNTs on the effective thermal conductivity of the binary nanofluid and is given on the basis of Hamiton-Crosser model and the micro-convection model 1. The volume fraction of particles is used in all models. However the mass fraction of particles is used in the experiment, so the mass fraction of particles is transformed into the volume fraction of particles in the prediction program.

Fig.5 shows the simulated results of the five models. The concentrations of ammonia in the binary nanofluids are 0, 12.5% and 25% respectively. As shown in the three figures, the results of Maxwell model and Hamiton-Crosser model are anomalously lower than the experimental values. Therefore, Maxwell model and Hamiton-Crosser model don’t fit for the predictions of the effective thermal conductivity of the CNT-ammonia binary nanofluid. The predicted results of the three micro-convection models are close. The result of the micro-convection model 2 is highest, and that of the micro-convection model 3 is higher than that of the micro-convection model 1. When the concentration of ammonia is 0 as shown in Fig. 5(a), the result of the micro-convection model 2 agreed much better to the experimental values. However, when the concentrations of ammonia are 12.5% [Fig. 5(b)] and 25% [Fig. 5(c)], the experimental values lie mainly between the micro-convection model 2 and the micro-convection model 3. The result of the micro-convection model 3 is closer than that of the micro-convection model 2. In a conclusion, the micro-convection model 2 and the micro-convection model 3 are fit for the prediction of the effective thermal conductivity of the CNTs-ammonia binary nanofluid.

5. CONCLUSIONS

is the volume fraction of particles.

ed From: http://proceedings.asmedigitalcollection.asme.org/ on 04/01/2014 Ter

In this paper, the thermal physical properties of the CNTs-Ammonia binary nanofluids are measured, and the effective thermal conductivity of the binary nanofluid is predicted. The following conclusions can be drawn from the results. 1. After the surface modification of CNTs, the CNTs-

Ammonia binary nanofluid can be prepared without surfactant.

2. The CNTs-Ammonia binary nanofluids have remarkably higher effective thermal conductivities than the base fluids. The concentration of ammonia has insignificant influence on the effective thermal conductivity of the binary nanofluid. The thermal conductivity ratios are lower than that of H2O-nanofluids and increase nonlinearly with the temperature of binary nanofluids

3. The adding of CNTs has only a slight effect on the surface tension and kinetic viscosity of the CNTs-ammonia binary nanofluid without surfactant at small mass fraction of CNTs.

4. The micro-convection model 2 and the micro-convection model 3 can predict well the effective thermal conductivity of the CNTs-ammonia binary nanofluid.

NOMENCLATURE Cnp mass fraction of CNTs wt% CA concentration of ammonia wt% dp diameter of nanoparticles. m E thermal conductivity ratio k thermal conductivity, W/m2°C n particle shape factor. T temperature. °C α knp/kf

5 Copyright © 2008 by ASME

ms of Use: http://asme.org/terms

Subscripts A ammonia eff nanofluid f ammonia solution np CNTs ACKNOWLEDGEMENTS The authors are grateful to the financial support provides by National Natural Science Foundation of China (Contracts No. 50476072) and Ministry of Education of China (NECT-05-02087). REFERENCES 1. U.S.Choi, Enhancing thermal conductivity of fluids with

nanoparticles , In development and application of non-newtonian flows, edited by D.A.Singer and H.P.Wang (ASME, New York,1995),FFD-VOL231/MD-VOL.66.

2. S. Kabelac, J. F Kuhuke, Heat transfer mechanism in nanofluids—experimental and theory, In: Proceedings of 13th International Heat Conference. Australia, KN-11 (2006).

3. P. Keblinski, J. A. Eastman, D. G. Cahill, Nanofluids for thermal transport, Materials Today, 36~44 (2005).

4. S. K. Das, U.S. Choi, H. E. Patel, Heat transfer in Nanofluids-a review, Heat Transfer Engineering, 27(10): 3~19 (2006).

5. Y. M. Xuan, Q. Li, Heat transfer enhancement of nanofluids, International Journal of heat and fluid flow, 21: 58~64 (2000).

6. D. S. Wen,Y. L. Ding, Experimental investigation into convective heat transfer of nanofluids at the entrance region under laminar flow conditions, International Journal of Heat and Mass Transfer, 47: 5181~5188 (2004).

7. Q. Li, Y. M. Xuan, Convective heat transfer and flow characteristics of Cu-water nanofluid, Science in China(Series E), 45(4): 408~416 (2002).

8. P. Vassallo, R. Kumar, S. D. Amico, Pool boiling heat

Downloaded From: http://proceedings.asmedigitalcollection.asme.org/ on 04/01/2014 Te

transfer experiments in silica-water nanofluids, International Journal of Heat and Mass Transfer, 47 :407~411 (2004).

9. S. K. Das, N. Putra, W. Roetzel, Pool boiling characteristics of nanofluids, International Journal of Heat and Mass Transfer, 46: 851~862 (2003).

10. S. Krishnamurthy, P. Bhattacharya, P. E. Phelan, R. S. Prasher, Enhanced mass transport in nanofluids, Nano Letter, 6(3): 419~423 (2006).

11. J. K. Kim, J. Y. Jung, Y. T. Kang, The effect of nano-particles on the bubble absorption performance in a binary nanofluid, International Journal of Refrigeration, 29: 22~29 (2006).

12. K.Esumi, M.Ishigami, A.Nakajima, K.Sawada, H.Honda, Chemical treatment of carbon CNTs, Carbon, 34: 279~281 (1996).

13. H. Q. Xie, H. H. Lee, W. J. Youn, M. Choi, Nanofluids containing multiwalled carbon CNTs and their enhanced thermal conductivities, Journal of Applied Physics, 94: 4967~4971 (2003).

14. Q. Li, Y. M. Xuan,, The measurement of the thermal conductivity of nanofluids, Journal of Chemical industry and Engineering (China), 54(1): 42~36 (2003).

15. C. H. Chon, K. D. Kihm, S. P. Lee, U. S. Choi, Empirical correlation finding the role of temperature and particle size for nanofluid(Al2O3) thermal conductivity enhancement, Applied Physics Letter, 87: 153107-1~3 (2005).

16. J. C. Maxwell, A Treatise on electricity and magnetism[M], 2nd ed. (Oxford University Press. Cambridge, U.K., 1904), 435-441.

17. R.L. Hamilton, O.K. Crosser, I & EC Fundamentals 1,187,1962

18. R. Prasher, P. Bhattacharya, P. E. Phelan, Thermal conductivity of nanoscale colloidal solutions(nanofluids), Physical Review letters, 025901-1~4. (2005)

6 Copyright © 2008 by ASME

rms of Use: http://asme.org/terms