ORIGINAL

Heat transfer characteristics of multi-walled carbon nanotubessuspension in a developing channel flow

Emad Elnajjar • Yousef Haik • Mohammad O. Hamdan •

Saud Khashan

Received: 29 October 2012 / Accepted: 27 July 2013 / Published online: 7 August 2013

� Springer-Verlag Berlin Heidelberg 2013

Abstract The present study experimentally investigates

the effect of multi wall carbon nanotubes (MWCNT) sus-

pensions on the convective heat transfer coefficients. The

MWCNT suspensions used in this study were prepared by

dispersing MWCNTs in deionized water 0.25 wt% arab

gum solution. The heat transfer characteristics were mea-

sured for thermally developing laminar flow in a finite

length horizontal circular pipe under isothermal wall con-

ditions. The study was conducted over a range of Reynolds

number of 300–2,300, based on 0.8 mm tube diameter.

Results indicate enhancements of the convective heat

transfer coefficient as a function of Reynolds number and

volume fractions. An average enhancement of heat transfer

coefficient of 50 % was observed over the base fluid. An

overall increase of pumping force varying from 20 to 30 %

over the flowing range is observed. The results suggest an

optimum MWCNT volume fraction point of 0.1 % which

gives the best heat transfer enhancement.

1 Introduction

Fluids in general have lower thermal conductivity com-

pared to solids; therefore, one of the promising methods of

enhancing heat exchanger efficiency is by improving the

thermo-physical properties of flowing fluids. Suspending

solid particles in a base fluid at very low concentrations is

used by different investigators to increase the overall

thermal conductivity [1]. The drawback of using suspended

millimeter particles in the fluid, is the development of a

high-pressure drop, which requires more pumping force.

Other shortcomings depending on the particles size and

concentration levels, include: clogging, abrasion and the

instability of the suspended particles. In the early nineties,

and after the recent development of nanotechnology, many

researchers [2–7, 15–18], studied the possibility of using

suspended nanoparticles in the base fluid (nanofluids), to

enhance the heat exchanger’s performance and reduce the

effect of these nanoparticles on the flow.

A number of theoretical and experimental studies that

investigate the heat transfer augmentation, thermo-physical

properties, and the increase of pumping force of nanofluids

are reported in the coming paragraphs.

Mohammed et al. [2], reviewed the theoretical and the

experimental work of the heat transfer using nanofluids,

and the flow characteristics in microchannel heat

exchangers. The study discussed the effects of channel

geometry, the fluid inlet and outlet arrangement, the

method of construction, type of nanofluids, method of

preparation, and mechanisms of enhancement. They stated

that the amount of work related to such an important and

promising topic and its applications is limited. They

stressed that the mechanism behind the heat transfer phe-

nomena in nanofluids is still not fully understood. And the

main reason behind the deviation in the researcher’s results

is due to the lack of standardization among different

studies.

Lee et al. [3], studied the effectiveness of using of

nanofluids in electronic cooling applications; where they

defined it as the scaling between the increase in the con-

vection augmentation factor and the adversary increase in

pumping power. By a direct measurement of thermal

conductivity and viscosity of different nanofluids types:

Al2O3, ZnO, and CuO and Carbon nanotubes, at different

volume percentage concentration levels, they reported an

E. Elnajjar (&) � Y. Haik � M. O. Hamdan � S. Khashan

Mechanical Engineering Department, United Arab Emirates

University, Al Ain, United Arab Emirates

e-mail: eelnajjar@uaeu.ac.ae

123

Heat Mass Transfer (2013) 49:1681–1687

DOI 10.1007/s00231-013-1212-1

increase in the effective convection coefficient of 5 % for

the 3 % volume fraction of Al2O3 nanoparticles in deion-

ized water (DW) nanofluid, 13.3 % for the 4 % volume

fraction of CuO nanoparticles dispersed in water nanofluid,

and 11.6 % for 0.2 % volume fraction of CNT suspended

in DW nanofluid.

Yang et al. [4], studied experimentally the heat transfer

of several nanoparticles dispersed in the transmission fluid,

and synthetic base-oils. The used nanoparticles were gra-

phitic in nature, within the aspect ratio of (l/d = 0.02), and

low loading ratio ranging from 2 to 2.5 wt%, flowing in a

horizontal laminar flow heat exchanger. The study shows a

low increase in the convective heat transfer coefficient as

compared to that predicted by other conventional heat

transfer correlations for homogenous nanofluids. They

conclude the need for more studies to develop an appro-

priate heat transfer correlation for the non-spherical nano-

particle dispersions.

Fotukian and Esfahany [5], experimentally studied the

convective heat transfer enhancement and pressure drop of

suspended CuO nanoparticles in a water base nanofluid.

The study was carried out with a turbulent flow inside a

5 mm circular tube. The study covered a range of flow

Reynolds number from 5,000 to 35,000, and nanoparticles

concentration range of 0.015–0.236 %. The study reported

a 25 % increase in the convective heat transfer coefficient

with a 20 % penalty in pressure drop.

Ding et al. [6], experimentally studied the heat transfer

of the aqueous suspension of multiwall carbon nanotubes

flowing through a horizontal tube. Arabic gum was used as

the dispersant in this work. The study suggested a signifi-

cant improvement of the convective heat transfer coeffi-

cient, the enhancements were related to the flow

conditions: Reynolds number (800–1,200), CNT concen-

tration (0, 0.1, 0.25, and 0.5 %.), and pH levels. The author

reported that the pH level shows the smallest effect on the

convective heat transfer coefficient. An enhancement of

350 % in the convective heat transfer coefficient for the

case of Reynolds number of 800 at CNT concentration of

0.5 %. Under similar operation condition the present work

reported an enhancement of 150 % of the heat transfer

coefficient. It is believed that the enhancement is due to the

increase of the thermal conductivity of the MWCNT sus-

pension, the particles re-arrangement, the shear which

induced thermal conduction enhancement, and the reduc-

tion in the thermal boundary layer due to the presence of

nanoparticles/MWCNT.

The present work is concerned on studying experimen-

tally the effect of different loading ratios of MWCNT

dispersed in a base fluid prepared using DW with 0.25 wt%

Arab Gum solution on the enhancement of the convective

heat transfer coefficient flowing in a thermally developing

flow in a straight horizontal finite length circular tube

subjected to constant surface temperature. This study

covers a range of flow Reynolds number of 300–2,300,

based on 0.8 mm tube diameter.

2 Experimental work

2.1 MWCNT suspension solution

The multi wall carbon nanotubes (MWCNT) used in the

present study were made by Carbon nanotubes Powder

Nano Lab, Inc. The nanotubes had a specified average

outside diameter of 10–30 nm, length of 0.5–20 lm and

purity of above 85 wt%, which was purchased from

NanoLab, Incorporation. In order to help dispersing the

MWCNT in DW and due to its hydrophobic nature [6],

surfactant agent is added to the MWCNT suspension,

which is used to change the wetting and the adhesion

behavior of the solution. Two types of surfactants were

used calcium chloride (CaCl), and arabic gum (AG), the

CaCl failed to produce a homogeneous MWCNT suspen-

sion for long period of time, where the MWCNT is sepa-

rated from the DW after a few minutes of preparation time.

Meanwhile, adding AG powder to the MWCNT suspension

with controlled ultrasonication and magnetic stirrers for

specific periods of time produces a stable, homogeneous

suspended solution for more than a month long period;

similar behavior is reported by other researchers [6, 7].

Ultrasonication and magnetic stirrer for controlled periods

of time were used to guarantee having untangled non

clustered MWCNT in the solution. The next paragraph

explains in details how the MWCNT suspension was

prepared.

In order to prepare 1 l of the MWCNT suspension with

0.1 wt% MWCNT, a 0.25 wt% solution of AG was added

to the DW where: 1 g of MWCNT and 2.5 g of AG were

dispersed in 996.5 g of DW. The mixture was put in a

magnetic stirrer for 10 min using (WiseStir MSH 20D) at

800 rpm, then the resulting composition was ultrasonicated

for 10 min using a 150 W, 20 kHz ultrasonic processor

(Branson SoniFier 450). The process of alternating

between the magnetic stirrer and ultrasonication is 10 min

periods within a total of a 40 min period. For more than a

month long period of preparing the MWCNT suspension

samples, using the above described technique for MWCNT

loading ratio of 0.05, 0.1, 0.15 and 0.2 % the MWCNT

suspension exhibited a stable and homogeneous mixture

without any signs of settling.

Figure 1 shows the zeta potential measurement using

Zetasizer Nano-ZS by Malvern Instruments for the base

fluid (DW), and the MWCNT suspension samples. The

results indicate that the base solution (DW) has a negative

charge, and the MWCNT suspension sample has a similar

1682 Heat Mass Transfer (2013) 49:1681–1687

123

charge which will create a repulsive force that keep the

MWCNT suspended in the base fluid. Figure 2 is an SEM

image for dried samples of 0.1 wt% MWCNT suspension

using JEOL JSM-5600 Scanning Electron Microscope at

10 kV for 15,000 magnifications. The SEM image suggests

that the MWCNTs are adsorbed with AG solution, which

prevents the agglomeration of the MWCNT and keeps it

suspended in the solution.

2.2 Experiment setup

To study the heat transfer performance of the MWCNT

suspension, a test setup was developed. A straight

0.8 mm diameter circular copper tube with a total length

of 52 cm is used. The tube was heated in a constant

temperature water bath of 100 �C constant temperature.

The water bath was controlled with a set of heaters and a

temperature controller to maintain a boiling water bath

temperature of 100 �C. A DC-pump is used to deliver the

fluid into the test tube; the pump has capacity range of

flow from 2 to 70 ml/s, which corresponds to a range of

Reynolds number of 800–2,500 respectively. The Rey-

nolds Number is defined as (U dp/mnf), where the U is the

average pipe flow velocity, dp is pipe diameter which is

equal to 0.8 mm and mnf is nanofluids kinematic viscos-

ity. The flow rate was metered directly by measuring the

time required for filling 100 ml glass vessel, the flow rate

was measured three times: during stabilization, at the

beginning of the run and at the end of the run. A k-type

thermocouple (Omega KT-12-39) was used to measure

the temperature of the hot bath, the inlet, and the outlet

flow. In order to minimize the error of the thermocouple,

all temperature measurements are performed using the

same thermocouple. The outlet flow is cooled by con-

necting the flow to a helical heat exchanger, to guarantee

a constant inlet flow temperature of 27 �C during the

duration of the runs. Figure 3 shows a schematic diagram

of the experiment setup, with all used components. The

mean bulk properties of the base fluid and the MWCNT

are listed in Table 1. It is worth mentioning that the

reported uncertainty of the K-type OmegaTM thermo-

couples is ±0.3 �C, whereas, the uncertainty of the flow

rate measurements is estimated to be around

±0.001 LPM. Accordingly, the uncertainty with calcu-

lated Nusselt number averages to be ±5 %.

In this paragraph some the calculations and definitions

are described. The absorbed heat energy by the fluid is

calculated using Eq. (1):

Fig. 1 The zeta potential

measurement for a the base fluid

sample and b 0.05 % MWCNT

suspensions sample

Fig. 2 SEM image 10 kV, 915,000 of 0.05 % MWCNT, 2.5 wt%

AG/DI solution

Heat Mass Transfer (2013) 49:1681–1687 1683

123

qout ¼ _mcðTout � TinÞ ð1Þ

The average overall convection heat transfer coefficient

is calculated by using Eq. (2):

havg ¼qout�

ADTLM

ð2Þ

And finally the Nusselt Number is calculated by using

Eq. (2):

Nu ¼ havgDh

kf

ð3Þ

where, Tin and Tout are the inlet and outlet temperatures are

calculated from the measured temperatures at the inlet and

outlet respectively for specific flow rate. Ts,avg is the

surface temperature. DTLM is the logarithmic mean

temperature difference, which is defined as follows:

DTLM ¼ðTs;avg � ToutÞ=ðTs;avg � TinÞ

ln½ðTs;avg � ToutÞ=ðTs;avg � TinÞ�ð4Þ

2.3 MWCNT suspension thermo physical properties

The thermophysical properties of the MWCNT suspensions

are the key parameters in evaluating the heat transfer

performance of the MWCNT suspensions flowing in the

horizontal tube. The thermophysical properties of the

MWCNT suspensions will be discussed in this section. All

properties were either measured in the lab such as the

density, the viscosity, the pH, or estimated using models

from the literature such as thermal conductivity, and spe-

cific heat. The mixture was prepared in weight percentage

(x%), meanwhile typically properties of MWCNT sus-

pensions are expressed in volume percent (/%). The con-

version between weight and volume fraction was done

through the bulk density [8]:

; ¼xqf

xqf þ ð1� xÞqCNT

ð5Þ

The MWCNT suspensions density measurements did

not show much change compared to the base fluid which is

anticipated to the low loading ratio of the MWCNT used.

Measuring the thermal conductivity was out of the

present work scope, the values of the effective thermal

conductivity of the MWCNT suspensions were taken after

the measurement done by Rashmi et al. [7], for similar base

fluid and volume fraction of MWCNT. Adopting the above

referenced measurements showed an average increase in

the effective thermal conductivity of the MWCNT sus-

pensions by a factor of 100 %, using the thermal conduc-

tivity of 0.631 (W/m K) for the base fluid. Other

researchers reported similar increase of the MWCNT sus-

pensions effective thermal conductivity [7, 8].

Fig. 3 The experiment setup

Table 1 Fluid properties and pipe dimensions

Property Value

Base fluid density (kg/m3) at 27 �C 967

MWCNT suspensions density (kg/m3) 968

Base fluid specific heat (W/kg K) at 27 �C 4,180

MWCNT specific heat (W/kg K) 509

Base fluid thermal conductivity (W/m K) 0.631

MWCNT suspensions thermal conductivity (W/m K) 0.751

Base fluid dynamic viscosity (Pa/s) at 27 �C 0.631

Hot bathe constant temperature (�C) 100

Internal pipe diameter (mm) 0.8

Pipe length (cm) 52

Wall thickness (mm) 0.5

Pipe material Copper

1684 Heat Mass Transfer (2013) 49:1681–1687

123

The dynamic and kinematic viscosity of the base fluid

were measured by a SVM 300 Stabinger Viscometer at

three temperatures 30, 50 and 80 �C, the results suggest

that the base fluid viscosity is significantly dependent on

temperature. Unfortunately due to clogging problems in the

Viscometer, the MWCNT suspension’s viscosity was not

measured. It is approximated by adopting the model

developed by Xie and Chen [9]. It is assumed that the

effective MWCNT suspensions viscosity is a function of

the base fluid viscosity and the MWCNT volume fraction

which is given by the relation (6).

The effective heat capacity for the MWCNT suspen-

sions is calculated using the model developed by Liu et al.

[10], as expressed in Eq. (7), where / the MWCNT volume

fraction percentage and densities are measured for both

basic fluid and the MWCNT suspensions.

leff ¼lf

ð1� ;Þ2:5ð6Þ

Ceff ¼ Cf ð1� ;Þ þ ;CCNT ð7Þ

3 Results and discussion

Different researchers reported an improvement in the

convective heat coefficient, with insignificant increase of

the required pumping force, when using nanoparticles/CNT

with very small volume percentage to the base fluid. The

enhancement was mainly due to significant increases in the

thermal conductivity of the mixture (nanoparticles/CNT–

base fluid) which varies from 10 to 100 depending on the

type and the concentration of the nanoparticles/CNT

[3, 11].

An average increase in pumping force is estimated at

about 20–30 % for all flow cases covering a Reynolds

number range 300–2,300. The increase in pumping force is

evaluated by measuring the increase in input DC power of

the DC-pump.

The tube’s wall temperature, the inlet and the outlet flow

mixed cup temperatures for all the cases are showing in

Fig. 4. The results illustrate that to a large extent, the

micro-tube surface temperature is kept constant at 100 �C

during the experiment. The outlet flow temperatures of the

MWCNT suspensions are larger than the one for the basic

fluid for the same mass flow rate, and same wall temper-

ature. This demonstrates clearly that the MWCNT sus-

pensions absorbed more heat than when compared to the

basic fluid, and as a result, having higher effective thermal

conductivity and higher heat capacity of the MWCNT

suspensions than the ones for the basic fluid, this is

expected and reported with other investigators [6, 12, 13].

The results suggested a consistent enhancement of the

MWCNT suspensions compared to the basic fluid.

The average convective heat transfer coefficient results

as a function of different Reynolds number for the

MWCNT suspensions cases and the base fluid case are

displayed in Fig. 5. The average convective heat transfer

coefficient is calculated as a function of following mea-

sured parameters: the mass flow rate, the absorbed heat,

and the inlet, the outlet, the wall temperature and the sur-

face area of the micro-tube. The results show a clear

enhancement of the convective heat transfer above 45 %

compared to the base fluid case for all Reynolds number

range. Despite the fact that enhancement of the heat

transfer of MWCNT suspensions is documented by many

investigators, until now, no clear mechanism is claimed to

justify this enhancement; many factors may lead to such

behavior. To begin with, the change in effective thermal

conductivity can be a direct reason of this increase. Even

after stating this finding, the thermal conductivity of the

mixture is controlled by many parameters (the type, the

shape and the aspect ratio of nanoparticles/MWCNT,

the agglomeration, the liquid layering around the

0.0

0.2

0.4

0.6

0.8

1.0

300 700 1,100 1,500 1,900 2,300

T ou

t/T

wal

l

Re

Twall/Twall Tin/TwallBase Fluid 0.05%CNT0.1%CNT 0.15%CNT

Fig. 4 The micro-tube mixed cup outlet, inlet and surface normalized

temperature for different Reynolds numbers MWCNT suspensions

flow

0

1000

2000

3000

4000

5000

6000

7000

300 700 1,100 1,500 1,900 2,300

h (

W/m

2k)

Re

Base Fluid 0.05%CNT 0.1%CNT

0.15%CNT 0.2%CNT

Fig. 5 The experimental average convective heat transfer coefficient

versus Reynolds numbers for the MWCNT suspensions flow

Heat Mass Transfer (2013) 49:1681–1687 1685

123

nanoparticles/MWCNT, and the Browning motion) which

can have a positive or negative effect on its value.

The average Nusselt number as a function of different

Reynolds number for all cases is reported in Fig. 6. The

results suggest an enhancement of the Nusselt number for

certain MWCNT suspensions loading ratios, the increase of

the Nusselt number is due to the balance between an

average increase of 45 % in convective heat transfer

coefficient and an increase in 22 % in the effective thermal

conductivity.

In order to analyze the measured data and study the

effect of MWCNT suspensions volume fraction, and the

Reynolds number on heat transfer enhancement, the mea-

sured data was curve fitted using the least square method to

give the best fitted results and the data is manipulated to get

the results as illustrated in Figs. 7 and 8 respectively.

Figure 7 shows the normalized convective heat transfer

coefficient defined as the ratio of the convective heat

transfer coefficient of the MWCNT suspensions to the base

fluid, as a function of different Reynolds number The

results suggest an enhancement of convective heat transfer

coefficient ranging from 20 to 60 % for all volume frac-

tions except for the case of 0.05 %. At Reynolds number

above 1,200 all volume fraction demonstrate an improve-

ment of convective heat transfer coefficient. Different

researchers [1, 14], reported a reduction in convective heat

transfer coefficient when using MWCNT suspensions and

they relate this behavior to nature of the interactions of the

MWCNT with themselves and the fluid flow.

Figure 8 displays the normalized convective heat

transfer coefficient as function of different MWCNT vol-

ume fractions. A volume fraction that yields the highest

heat transfer enhancement is found to be around /& 0.1 % at all Reynolds numbers. However, this

enhancement ranges from 42 to 55 % for all Reynolds

above 1,000. Below the optimum volume fraction, the

enhancement exhibits a clear dependence on the Reynolds

number. For Reynolds less than 1,000, the heat transfer

experiences degradation, in comparison with the clear flow,

up to a volume fraction of 0.05 % before it picks up toward

enhancement at the optimum volume fraction. At Reynolds

higher than 1,000, the heat transfer enhancement appears at

lower volume fractions, more profoundly at higher Rey-

nolds numbers. The enhancements diminish, almost iden-

tically for all Reynolds numbers, to 25 % at a volume

fraction of 0.15 %. Beyond which, the enhancement

slightly picks up but with less dependence on Reynolds

number.

4 Conclusions

The effect of MWCNT suspensions on convective heat

transfer was experimentally investigated in this present

work. Low volume concentrations ranging from 0.05 to

0.2 % of MWCNT were used for this study. The study

reported up to 50 % increase in the average convective heat

transfer coefficient of the MWCNT suspensions comparing

0

1

2

3

4

5

6

7

8

300 700 1,100 1,500 1,900 2,300

Nu

Re

Base Fluid 0.05%CNT 0.1%CNT

0.15%CNT 0.2%CNT

Fig. 6 The experimental average Nusselt number versus Reynolds

numbers for the MWCNT suspensions flow

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

300 700 1,100 1,500 1,900 2,300

h nf/

hf

Re

0.05%CNT 0.1%CNT0.15%CNT 0.2%CNT

Fig. 7 Normalize curve fitted convective heat transfer coefficient

versus Reynolds numbers for the MWCNT suspensions flow

0

4

0

4

0.

0.2

0.

0.6

0.8

1.

1.2

1.

1.6

1.8

h nf/

hf

0.00 0.05

Re=600Re=1800

0.10

%

Re=1000Re=2200

0.15 0.20

Re=1400

0.25

Fig. 8 Normalize curve fitted convective heat transfer coefficient

versus MWCNT volume fraction ratio for the MWCNT suspensions

flow

1686 Heat Mass Transfer (2013) 49:1681–1687

123

to the base fluid case, for flow Reynolds number above

1,200 for the case of 0.1 % volume fraction. The results

show that both volume fraction and Reynolds number are

major parameters affecting the augmentation of the heat

transfer in a laminar pipe flow. An optimum volume

fraction point of 0.1 % is observed for all used ranges of

Reynolds numbers which give the best heat transfer

enhancement. An average enhancement of 30 % is reported

for the Nusselt number of the MWCNT suspensions com-

paring to the base fluids case. Furthermore, our study

indicates for conditions at which there is enhancement and

others with degradation. For instance, the flow at Reynolds

number of 600 shows a degradation under dilute condi-

tions, while flows at Reynolds above 1,000 were showed

enhancement but with non-monotonic dependence with the

volume fraction.

References

1. Kakac S, Pramuanjaroenkij A (2009) Review of convective heat

transfer enhancement with nanofluids. Int J Heat Mass Transf

52:3187–3196

2. Mohammed HA, Bhaskaran G, Shuaib NH, Saidur R (2011) Heat

transfer and fluid flow characteristics in microchannels heat

exchanger using nanofluids: a review. Renew Sustain Energ Rev

15:1502–1512

3. Lee J, Gharagozloo PE, Kolade B, Eaton JK, Goodson KE (2010)

Nanofluid convection in microtubes. J Heat Transf 132:

092401–092405

4. Yang Y, Zhang G, Grulke E, Anderson W, Wu G (2005) Heat

transfer properties of nanoparticle-in-fluid dispersions (nanofl-

uids) in laminar flow. Int J Heat Mass Transf 48:1107–1116

5. Fotukian SM, Esfahany MN (2010) Experimental study of tur-

bulent convective heat transfer and pressure drop of dilute CuO/

water nanofluid inside a circular tube. Int Commun Heat Mass

Transf 37:214–219

6. Ding Y, Alias H, Wen D, Williams RA (2006) Heat transfer of

aqueous suspensions of carbon nanotubes (CNT nanofluids). Int J

Heat Mass Transf 49:240–250

7. Rashmi W, Ismail AF, Sopyan I, Jameel AT, Yusof F, Khalid M,

Mubarak NM (2011) Stability and thermal conductivity

enhancement of carbon nanotube nanofluid using gum Arabic.

J Exp Nanosci 6(6):567–579

8. Brinkman HC (1952) The viscosity of concentrated suspensions

and solutions. J Chem Phys 20:571–581

9. Xie H, Chen L (2009) Adjustable thermal conductivity in carbon

nanotube nanofluids. Phys Lett A 373:1861–1864

10. Liu M-S, Lin MC-C, Huang I-T, Wang C-C (2005) Enhancement

of thermal conductivity with carbon nanotube for nanofluids. Int

Commun Heat Mass Transf 32:1202–1210

11. Assael MJ, Chen CF, Metaxa I, Wakeham WA (2004) Thermal

conductivity of suspensions of carbon nanotubes in water. Int J

Thermophys 25(4):971–985

12. Amrollahi A, Rashidi AM, Lotfi R, Meibodi ME, Kashefi K

(2010) Convection heat transfer of functionalized MWNT in

aqueous fluids in laminar and turbulent flow at the entrance

region. Int Commun Heat Mass Transf 37:717–723

13. Keblinski P, Phillpot SR, Choi SUS, Eastman JA (2002) Mech-

anisms of heat transfer flow in suspensions of nano-sized particles

(nanofluids). Int J Heat Transf 45:855–863

14. Rea U, McKrell T, Hu L, Buongiorno J (2009) Laminar con-

vective heat transfer and viscous pressure loss of alumina–water

and zirconia–water nanofluids. Int J Heat Mass Transf 52:

2042–2048

15. Assael MJ, Metaxa I, Arvanitidis J, Christofilos D, Lioustas C

(2005) Thermal conductivity enhancement in aqueous suspen-

sions of carbon multi-walled and double-walled nanotubes in the

presence of two different dispersants. Int J Thermophys 26(3):

647–664

16. Choi SUS, Zhang ZG, Yu W, Lockwood FE, Grulke EA (2001)

Anomalous thermal conductivity enhancement in nanotube sus-

pensions. Appl Phys Lett 79(14):2252–2254

17. Garg P, Alvarado JL, Marsh C, Carlson TA, Kessler DA, An-

namalai K (2009) An experimental study on the effect of ultra-

sonication on viscosity and heat transfer performance of multi-

wall carbon nanotube-based aqueous nanofluids. Int J Heat Mass

Transf 52:5090–5101

18. Wen D, Ding Y (2004) Effective thermal conductivity of aqueous

suspensions of carbon nanotubes (carbon nanotube nanofluids).

J Thermophys Heat Transf 18(4):481–485

Heat Mass Transfer (2013) 49:1681–1687 1687

123