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A review of forced convection heat transfer enhancementand hydrodynamic characteristics of a nanouid

Adnan M. Hussein a,b,n, K.V. Sharma c, R.A. Bakar a, K. Kadirgama a

a Faculty of Mechanical Engineering, University Malaysia Pahang, 26600 Pekan, Pahang, MalaysiabAl-Haweeja Institute, Foundation of Technical Education, Iraqc Department of Mechanical Engineering, JNTUH College of Engineering, Manthani, India

a r t i c l e i n f o

Article history:Received 22 May 2013

Received in revised form

5 August 2013

Accepted 11 August 2013Available online 27 September 2013

Keywords:

Nanouid

Heat transfer

Nusselt number

Friction factorHeat exchanger

a b s t r a c t

The low thermal properties of liquids have led to investigations into additives of small size (less than100 nm solid particles) to enhance their heat transfer properties and hydrodynamic ow. To summarise

the experimental and numerical studies, this paper reviews these computational simulations and nds

that most of them are in agreement with the results of experimental work. Many of the studies reportenhancements in the heat transfer coefcient with an increase in the concentration of solid particles.Certain studies with a smaller particle size indicated an increase in the heat transfer enhancement when

compared to values obtained with a larger size. Additionally, the effect of the shape of the ow area onthe heat transfer enhancement has been explored by a number of studies. All of the studies showed a

nominal increase in pressure drop. The signicant applications in the engineering eld explain why somany investigators have studied heat transfer with augmentation by a nanouid in the heat exchanger.

This article presents a review of the heat transfer applications of nanouids to develop directions forfuture work. The high volume fraction of various nanouids will be useful in car radiators to enhance the

heat transfer numerically and experimentally. Correlation equations can expose relationships betweenthe Nusselt number, the Reynolds number, the concentration and the diameter of the nanoparticles. Onthe other hand, more work is needed to compare the shapes (e.g., circular, elliptical and at tube) that

might enhance the heat transfer with a minimal pressure drop.&2013 Elsevier Ltd. All rights reserved.

Contents

1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7352. Thermal properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 735

3. Forced convection heat transfer in a tube . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 735

3.1. Experimental studies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7353.1.1. Laminar ow in a tube . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7353.1.2. Laminar ow with twisted tape inserts in a tube . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 737

3.1.3. Turbulent ow in a tube . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 737

3.1.4. Turbulent

ow with twisted tape inserts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7383.2. Numerical studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7383.2.1. Laminar ow in a tube . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 738

3.2.2. Laminar ow in a tube with inserts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7383.2.3. Turbulent ow in a tube . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 738

4. Forced convection heat transfer in a heat exchanger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7394.1. Experimental studies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7394.2. Numerical studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 739

5. Forced convection heat transfer in other shapes of tubes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 739

5.1. Experimental studies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 739

Contents lists available at ScienceDirect

journal homepage: www.elsevier.com/locate/rser

Renewable and Sustainable Energy Reviews

1364-0321/$ - see front matter & 2013 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.rser.2013.08.014

n Corresponding author at: Faculty of Mechanical Engineering, University Malaysia Pahang, 26600 Pekan, Pahang, Malaysia. Tel.: 60 179809114.

E-mail address:[email protected] (A.M. Hussein).

Renewable and Sustainable Energy Reviews 29 (2014) 734743
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5.2. Numerical studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7406. Experimental and numerical studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 740

7. Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7418. Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 741

9. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 741Acknowledgements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 742References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 742

1. Introduction

A nanouid is a new class of heat transfer uids engineered by

dispersing metallic or non-metallic nanoparticles with a typical sizeof less than 100 nm in the base uid (e.g., water, ethylene glycol andoil). The poor thermophysical properties of liquids may have led tothe use of suspended solid particles as an additive to enhance the

thermal properties and improve the heat transfer characteristics ofliquids. The key idea is to improve the thermal conductivity.Because the solid particles have a larger thermal conductivity thanthe liquids, solid particles suspended in the liquid will improve the

thermal conductivity of liquid. For many years, suspensions of

millimetre- or micrometre-sized solid particles have been testedto enhance the thermal conductivity of conventional uids, but

problems of sedimentation led to increased pressure drop in theow channel, as reported by Lee et al. [1]. Recent advances inmaterial technology made it possible to produce innovative heattransfer uids by suspending nanometre-sized particles that change

the transport and thermal properties in base uids. A solid liquidcomposite materials consisting of solid nanoparticles with size notlarge than 100 nm suspended in liquid dened as nanouids [2].Nanouids have attracted signicant interest recently because of

reports of the enhancement of Thermophysical properties for manyindustrial applications[37].

This review will focus mainly on the hydrodynamic and heattransfer enhancement potential of nanouids with forced convec-

tion ow types (laminar

turbulent) and on the effect of theconcentration and diameter of nanoparticles and the shape ofcross sectional tubes without much detail about Thermophysicalproperties. Additionally, this review will give a signicant plan for

accurate future work.

2. Thermal properties

The thermal conductivity of ultrane suspensions of alumina, silicaand other oxides in a base uid water was proven to increase up to30% at a concentration by volume of 4.3% by Masuda et al. [8]. Many

researchers have used the mixture relation for estimating the densityand specic heat capacity[920]of nanouids with

nf 100

p 1 100

f 1

Cnf=100Cp 1=100Cf

nf2

The convection heat transfer coefcient doubled when addingnanoparticles suspended in water as a base uid in a study by Choi[21]. The thermal conductivity is a signicant thermal property for

enhancing the heat transfer of liquids by suspending metal or non-metal as shown inTable 1.

The thermal conductivity of nanouids has been determinedexperimentally[918], and the data of the thermal conductivity

for nanouids of metal and metal oxides, such as Al2O3, Fe3O4,TiO2, ZnO, ZrO2 and CuO, consist of many data points available in

the literature for use in the development of the regression Eq. (3)

by Sharma et al. [23]

kr knfkf

0:8938 1

100

1:371

Tnf70

0:27771

dp150

( 0:0336p

f

0:01737

3

Viscosity is another parameter that inuences heat transfeand pressure drop. The experimental study of alumina and copper

oxide nanouid viscosity under ambient conditions with differentvolume fractions and particle diameters have been conducted byNguyen et al. [22]. The viscosity of a waterAl2O3 nanouid withparticle diameters of 36 and 47 nm and with CuO solid particles o

29 nm diameters were studied by Sharma et al. [23]. The experi-mental data for viscosity gathered by[2431]for up to 4% volumeconcentration consisted of many data points subjected to regres-sion by[23]to obtain the following correlation:

r nf

f 1

100

11:31

Tnf70

0:0381

dp170

0:0614

3. Forced convection heat transfer in a tube

3.1. Experimental studies

The heat transfer coefcients of nanouids were calculatedfrom the following equations:

Nu h d

k 5

h Q

TwTf6

3.1.1. Laminarow in a tube

Experimental results illustrated that the convective heat trans-fer coefcients of nanouids varied with the ow velocity and

Table 1

Thermal conductivities of various materials [95].

Materials Thermal conductivity atroom temperature (W/m-K)

Silver 429

Copper 401Aluminium 237

Diamond 3300

Silicon 148

Alumina 40

Water 0.61

Ethylene glycol 0.25

Motor oil 0.15

A.M. Hussein et al. / Renewable and Sustainable Energy Reviews 29 (2014) 734743 735


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volume fraction and were higher than the base uid under the

same conditions as shown inTable 2.A hybrid nanouid used to enhance the heat transfer and

pressure drop in fully developed laminar ow through a uniformlyheated circular tube was studied experimentally by Suresh et al.[32]. The nanouid was composed of CuAl2O3 in water synthe-sised with a 0.1% concentration by volume. Experimental results

showed a maximum enhancement of 13.56% in the Nusseltnumber at a Reynolds number of 1730 when compared to theNusselt number of water. The results also showed that 0.1% CuAl2O3water nanouids have a slightly higher friction factor when

compared to a 0.1% Al2O3water nanouid. The correlations of theNusselt number and the friction factor were reported, and therewas good agreement with the experimental data reported else-

where. Experimental results by Yang et al. [33] illustrated theconvection heat transfer coefcient of graphite nanoparticlesdispersed in a liquid for laminar ow in a horizontal tube heatexchanger. A study of laminar ow convective heat transfer of

alumina nanoparticles in water with a constant surface tempera-ture and various volume fractions was performed by Heris et al.[34]. The schematic of the experimental setup is shown inFig. 1.The experimental results showed that the heat transfer coefcient

of nanouids increases with the Peclet number as well as with thenanoparticle concentration. An experimental study of convectiveheat transfer of de-ionised water at steady state with a low volumeconcentration of 0.003% of suspended CuO nanoparticles was

conducted by Asirvatham et al. [35]. The effect of the mass owrate ranging from (0.0113 kg/s to 0.0139 kg/s) and the effect ofthe inlet temperature at 100 1C and 170 1C on the heat transfer

coefcient were also studied. The results showed the enhance-

ment of convective heat transfer compared with other papers. Onthe other hand, the governing equations were simulated in parallel

with the experimental work and the results of comparison showedgood agreement.

Table 2

The heat transfer enhancement by a nanouid under laminar ow[96].

Ref. Nanouid Re Nunf/Nuf

Lee and Choi (1996) Cuwater 2 vol% Laminar 100%

Li and Xuan[7] Cuwater 0.32 vol% 80023,000 60%Xuan and Li (2003) Al2O3water 0.21.6% Laminar 30%

Yang et al.[33] Al2O3water 0.22.5 vol% 6502050 350%

Heris et al.[34] Titanium nanotubewater 110 Enhancement of a with u and PeChen et al. (2008) (aspect ratio10) 0.50.5% 7002050 Increases with aspect ratio (nanoparticle shape) increase

Rea et al.[62] Al2O3water 0.66.0 vol% ZrO2water

0.323.5 vol%

1700 No abnormal heat transfer enhancement using measured properties

of the nanouid

Hwang et al. [24] Al2O3water 0.010.3 vol% Laminar 8% at 0.3 vol%

Mansour et al. (2011) Al2O3/water nanouids 550800 Heat tr ansfer enhancement

Shariet al.[32] Al2O3/water nanouids Laminar Heat transfer enhancement

Suresh et al.[32] Al2O3Cu/water hybrid Laminar Enhancement of 13.56% in Nusselt

Fig. 1. Schematic of the experimental setup of[26].

Nomenclature

m mass ow rate (kg/s)

A area (m2)

Cp specic heat capacity (W/kg k)d diameter of tube (m)

dp particle diameter (m)

e height of corrugation (m)f friction factor

h heat transfer coefcient (W/m2 k)

k thermal conductivity (W/m k)

Nu Nusselt number (Nuh d/k)

P pitch of corrugation (m)

Pe Peclet number (PeRe Pr)

Pr Prandtl number (PrC/k)

Q heat transfer (W)

Re Reynolds number (Reu d /m)

T temperature (1C)

u velocity ofuid (m/s)

p pressure drop (pa)

concentration of solid particles

viscosity

density

Subscripts

av average value

f uid

nf nanouidr ratio

s solid

w wall

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3.1.2. Laminarow with twisted tape inserts in a tube

Fully developed laminar convection heat transfer and frictionfactor characteristics of different volume fractions of Al2O3 nano-

uid in a straight tube with different twist ratios of twisted tape

inserts have been investigated experimentally by Sundar andSharma[36]. The schematic of the test rig is shown inFig. 2. Theresults showed that the heat transfer coefcient of the nanouidwas higher than the water; however, the pressure drop increases

slightly with the inserts.

3.1.3. Turbulentow in a tube

The convection heat transfer of a nanouid through a circulartube under turbulent ow with Reynolds numbers rangingup to 2300 is summarised inTable 3. Many researchers used the

DittusBoelter Eq.(7) for pure water as follows:

Nu hfkf

d 0:023Re0:8Pr0:4 7

The reason for using this equation is to verify all of the resultsfrom the experimental setup before adding nanoparticles. Theexperimental study of the heat transfer of Al2O3and TiO2in water

through a heated horizontal straight tube has been conducted byPak and Cho[9].

The correlation equation of enhanced heat transfer was foundto be

Nu 0:021 Re0:8 Pr0:5 8

Li and Xuan [37] concluded that the Nusselt number of ananouid with a 2% volume fraction of Cu in water was 60%

higher than water. The DittusBoelter Eq.(7)was not valid for theprediction of the Nusselt number of different volume concentra-tions of nanouids. Therefore, new heat transfer correlations were

established for the prediction of the Nusselt number of nanouidsowing in a tube based on experimental data as follows:

Nu 0:43281:0 11:2850:754Pe0:218Re0:333Pr0:4 9

Nu 0:00591:0 7:62860:6886Pe0:001Re0:9238Pr0:4 10

The correlation of the friction factor used for more accuratepredictions might be determined by the Blasius Eq.(11a)and from

the pressure drop with the following equation:

f0:316

Re0:25 11a

fP d

L

2g

u2 11b

The heat transfer of the nanouids inside a tube evaluatedexperimentally as

Q m Cp ToutTin 12

An experimental study of the effect of nanoparticle volumefraction on the convection heat transfer characteristics and pres-sure drop of TiO2water nanouid was conducted by [3842]

This study demonstrated that by increasing the Reynolds numberor nanoparticle volume fraction, the Nusselt number alsoincreases. On the other hand, using a nanouid with a highReynolds number requires more power than one with a low

Reynolds number. This power compensates the pressure drop ofnanouid. The correlation equation of the friction factor and theNusselt number developed by Duangthongsuk and Wongwise[43]is

f 0:961 0:052 Re0:375 13

Nu 0:074 Re0:707 Pr0:385 0:074 14

An experimental study of the turbulent convective hea

transfer behaviour of alumina and zirconia nanoparticle dispersions in water inside a horizontal tube was conducted byWilliams et al. [10]. The experimental data for single-phaseconvection heat transfer and viscous pressure loss correlations

for fully developed turbulent ow are compared with DittusBoelter and Blasius equations, respectively. There was noabnormal heat transfer enhancement observed. The convectiveheat transfer and friction factor of Al2O3water nanouids in a

circular tube with a constant wall temperature under turbulen

ow conditions were investigated experimentally by [44]. Theresults showed that the heat transfer coefcient of the nanouidwas higher than that of the base uid and increased with the

particle concentration. Moreover, the Reynolds number hadlittle effect on the heat transfer enhancement. The effect othe alumina nanoparticle size on the thermophysical propertiesheat transfer performance and pressure loss characteristics o

Aviation Turbine Fuel (ATF)-Al2O3 nanouids were studiedexperimentally by Sonawane et al. [45]. The alumina particleswith mean diameters of 50 nm or 150 nm were dispersed in ATF

at 500 1C and 0.3% particle volume concentration. The resultsshowed the heat transfer performance and pressure loss char-acteristics at the same pumping power; the maximum enhance-ment in the heat transfer coefcient at 500 1C and 0.3%

concentration was approximately 47% using bigger particleswhereas it was only 36% using smaller particles. The effect of the

Table 3

The heat transfer enhancement by a nanouid under turbulent ow[97].

Ref. Nanouid Re Nunf/Nuf

Pak and Cho[9] Al2O3water TiO2water 3% Turbulent 3% to 12%Asirvatham et al. (2009) CuOwater Turbulent 8% enhancement

Duangthongsuk and Wongwises[43] TiO2/water nanouids Turbulent Heat transf er is 26% greater than that of water

Vajjha et al.[67] 0.2 to 2.0% by vol. Turbulent Heat transfer coef cient and friction factor increased with

increasing volumetric concentration of the nanouids

Sajadi and Kazemi[47] Al2O3/water nanouids Turbulent Nusselt number and pressure drop increased

CuO/water nanouids Turbulent

SiO2/water nanouids Turbulent

Akbari et al.[53] TiO2/water nanouids Turbulent Heat transfer enhancement

0.2 to 0.25% by vol. Turbulent

Fig. 2. Full-length twisted tape inserted inside a tube[36].



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mean diameter of nanoparticles on the convective heat transferand pressure drop of TiO2water has been studied experimen-tally by Abbasian[46]. The particles of size diameters of 10, 20,30 and 50 nm were dispersed in distilled water at volume

fractions of 0.01 to 0.02. The Nusselt number was compared tothe base uid, and it did not increase by decreasing the diameterof nanoparticles. The results showed that nanouids with a20 nm particle size diameter had the highest Nusselt number

and pressure drop in the same range of Reynolds number andvolume concentrations as shown in Fig. 3. A comparison of theexperimental data of the Nusselt number with the existingconvective heat transfer correlations is shown by Sajadi and

Kazemi[47]i nFig. 4.

3.1.4. Turbulentow with twisted tape inserts

The effect of the volume fraction of waterAl2O3on the Nusseltnumber and friction factor in a circular tube with a twisted tape

under turbulent ow has been studied by Sundar and Sharma[48].Their results provided the heat transfer enhancement with theReynolds number and the volume fraction of nanoparticles inwater. In addition, the comparison with other researchers showed

good agreement.

3.2. Numerical studies

3.2.1. Laminarow in a tube

A numerical study was conducted by Hyder et al. [49]for Al2O3,CuO and TiO2 nanoparticles in water under laminar ow in acircular tube. It predicted that the pressure drops and Nusseltnumber increases with increasing of volume fraction and Reynolds

number. Additionally, a comparison of the numerical results withexperimental data is available and there was good agreementbetween them.

3.2.2. Laminarow in a tube with inserts

Siva and Sivashanmugam[50] numerically solved the govern-ing equations for heat transfer of nanouids inside a circular tubewith helical inserts under laminar ow. The results showed that

the heat transfer increases with the Reynolds number and withdecreasing twist ratio with a maximum at 2.93. Additionally,a comparison of the heat transfer rates of water and nanouidsshowed an increase in the Nusselt number of 534% for different

twist inserts and different volume concentrations.

3.2.3. Turbulentow in a tube

A number of investigators have studied the heat transfer

and the pressure drop in a circular tube numerically [5155].A mathematical formulation and numerical method to determinethe forced convection heat transfer and wall shear stress for thelaminar and turbulent regions of Al2O3water and Al2O3ethylene

glycol inside a uniform heated tube was introduced by Maiga et al.[56]. For the turbulent ow region, the averaged Reynolds numberunder the NavierStokes equation and the k turbulent modelwere adopted to describe the shear stress and heat ux of the

nanouids. The grid size that was determined to be appropriate forthis problem is shown in Fig. 5. In the area of laminar ow, the

Reynolds number was xed at 250 with different heat ux from 10to 250 W/m2. For turbulent ow, the constant heat ux was

500,000 W/m2 and the Reynolds number varied in the range of1 1035 104. Fig. 6 shows the effect of the particle volumefraction on the heat transfer coefcients for both ethylene glycoland water as base uids. They reported that ethylene glycol was

better than water in hydrodynamic and heat transfer enhance-ment. The numerical results indicated that the heat transfer andthe wall friction of nanouids increased with increasing particlefraction and that the Al2O3ethylene glycol gave a greater heat

transfer enhancement than the Al2O3water. For the turbulentow region, the heat transfer performance of the nanouids was

Fig. 4. Comparison of the experimental Nusselt number with existing convective

heat transfer correlations at 0.2% volume fraction[47].

Fig. 5. Conguration of the problem in the study of[56].

Fig. 3. The effect of size diameter on the Nusselt number and the pressure drop

by[46].



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more pronounced with the increase of the Reynolds number.A theoretical model of turbulent ow in a tube was proposed by[23]to predict the friction factor and the heat transfer for a widerange of nanouids containing Cu, CuO, TiO2, SiC, ZrO and Al2O3nanoparticles of different sizes, concentration and temperaturesdispersed in water. There was good agreement between theprediction values and the experimental data in the literature. Fullydeveloped forced convection of a nanouid (waterAl2O3) was

studied numerically by Mirmasoumi and Behzadmehr [57].The results showed that the convection heat transfer coefcientsignicantly increases with decreasing mean diameter of thenanoparticles; in addition, the hydrodynamics parameters do not

change signicantly. A simulation study of convection heat trans-fer enhancement in a circular pipe under turbulent ow wasperformed by Kumar et al.[58]. Forced convection under turbulent

ow of an alumina nanouid in a circular tube with a constant and

uniform wall temperature was studied numerically by Bianco et al.[59]. These authors found that the nanouid's convective heattransfer coefcient was greater than that of water. The resultsshowed that the heat transfer enhancement increased with the

Reynolds number and the volume fraction of the nanoparticles.Additionally, the friction factor and heat transfer coefcient results

agreed with[9].

4. Forced convection heat transfer in a heat exchanger


Many papers have focused on heat exchanger applications withnanouids because of the wide range of applications for heat

exchangers in the practical and industrial elds [6069]. Forcedconvection heat transfer in a double pipe with turbulent nanouid

ow and plate heat exchangers was studied experimentally byZamzamian et al.[70]. The nanouid consisted of aluminium oxideand copper oxide in ethylene glycol separately. The effects ovolume fraction and operating temperature on the forced convec-

tion heat transfer coefcient of the nanouids were evaluated. Theresults showed that the heat transfer coefcient of the nanouidincreased with increasing nanoparticles fraction and the tempera-ture of the nanouid.


Computational and numerical studies of nanouid applicationsof heat exchangers were performed by [7179]; all of themconcluded that the heat transfer was enhanced in the heat exchan-ger when the solid particles were suspended in a base uid. The

potential mass ow rate reduction in an exchanger with a givenheat exchange capacity using Al2O3water nanouids was studiedby Bozorgan et al.[80]. The numerical study focused on turbulent

ow in a horizontal double-tube counter ow heat exchanger. The

results showed that the nanouid ow rate decreased as thevolume fraction in the exchanger increased; on the other handthe pressure drop of the nanouid was slightly higher than that o

water and increased with the increase of volume concentration. Thelouvered strip inserts in a circular double pipe heat exchanger werestudied numerically by Mohammed et al. [81]. The nite volumemethod (FVM) was used to solve the governing equations and

determine the thermal and ow characteristics. Four different typesof nanoparticles, Al2O3, CuO, SiO2, and ZnO with different nanopar-ticle diameters and different volume fractions in the range o(2050 nm) and (14%), respectively, were dispersed in water

The numerical results found that the heat transfer increased byapproximately 367% to 411%, but the friction factor of the enhancedtube was approximately 10 times that of the smooth tube. Thiresult indicated that the Nusselt number of the SiO2nanouid had

the highest value, followed by Al2O3, ZnO, and CuO when comparedwith pure water. The results showed that the Nusselt number

increased with decreasing nanoparticle diameter, and it increasedslightly with increasing volume fraction the streamline and

isothermal line, as shown inFig. 7.

5. Forced convection heat transfer in other shapes of tubes


Because of the pressure drop limitations, the need for noncir-cular ducts arises in many heat transfer applications according toTauscher and Mayinger[82]. A study of the forced convection heat

transfer and friction factor in the laminar to turbulent transitionregion of a vertical heated rectangular channel was conducted

Fig. 7. Streamline and isotherms of the forward louvered strip [81].

Fig. 6. Effect of the particle volume fraction on heat transfer coefcient ratios[56].



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experimentally by Wang et al.[83]. The effect of the Prandtl numberon the heat transfer and friction factor was analysed in thetransition region in seven groups of experiments performed in theReynolds number range of 1000 to 20,000. The results showed that

the friction factors increase with increasing Prandtl number for axed Reynolds number in the transition region. In addition, thecomparisons between the present experimental data and classicalcorrelations were indicated based on the experimental data. The

empirical correlations for the calculation of the friction factor andthe heat transfer coefcient in the rectangular channel ow transi-tion region during forced circulation were developed. Sasmito et al.and Castiglia et al.[84,85]present studies of forced convection heat

transfer enhancement by nanouids in a helical tube. An experi-mental study of turbulent pressure drop and heat transfer incorrugated tubes to nd the effect of SiO2water nanouid havebeen performed by Darzi et al. [86]. The experimental work setup

included a straight tube and ve roughened tubes with differentcorrugation pitches. They obtained a correlation for the Nusseltnumber depending on the Reynolds number, Prandtl number andheight of corrugation and pitch, which is expressed as

Nu 44:26 e

d 0:89 P

d 0:96

Re15000:27Pr0:26 15


A simulation study of laminar forced convection between twoparallel plates with a new model including a bi-partitionedsolution domain was introduced by Zhou et al. [87]. One sectionof the solution domain was modelled with bright meshes to solve

the multicomponent ow and the other had a coarse mesh tocharacterise the single component ow as shown inFig. 8. It seemsthat the validity and accuracy of this model compared well with

LBM using only one type of modelling for the entireow. Laminarconvection under constant heat ux boundary conditions usingthe nite volume method to nd the effects of the solid volume

fraction on thermal and hydraulic behaviours of nanouid ow inelliptical ducts have been presented by [8890]. The resultsshowed that for a given Reynolds number (Re), the Nusseltnumber (Nu) increased with the volume fraction of solid nano-particles while the friction factor decreased. The effect of aspect

ratio (b/a) in elliptic tubes reduced the local friction factor,whereas it had no effect on the local Nusselt number. The laminar

ow forced convection heat transfer of a CuOwater nanouid in atriangular duct under a constant wall temperature condition was

investigated numerically by Heris et al. [91]. This study evaluatedthe effect of the nanoparticle volume fraction, size diameter, andtype on heat transfer and compared the results between thenanouid and the pure uid. A comparison of the convection heat

transfer of a nanouid in isosceles triangular ducts with various

apex angles was also presented. The results showed that an

equilateral triangular duct has a maximum heat transfer comparedwith other types of isosceles triangular ducts. A numerical study ofthe heat transfer enhancement by internal longitudinal ribs andalumina water nanouid in a stationary curved square duct was

performed by Soltanipour et al. [92].The geometry of a ribbed curved duct and the coordinate

system assumed constant properties: laminar, three-dimensional

ow, steady, and incompressible. The governing equations were

solved with the nite volume method. The effects of the Deannumber, rib size and volume concentration on the coefcient ofheat transfer and pressure drop were measured.

De Re

ffiffiffiffiffiffiDhR

r 16

Numerical results showed that the heat transfer coefcientincreased with increasing nanoparticle concentration. Addition-ally, the addition of a nanoparticle to the base uid is more usefulfor low Dean number. In the case of water ow, results indicate

that the ratio of heat transfer rate of a ribbed duct to a smoothduct is nearly independent of Dean number (De).Fig. 9shows theeffect of the particle volume fraction on the heat transfer andpressure drop ratio in a ribbed curved duct. The forced convection

heat transfer of a nanouid under laminar and turbulent owthrough a nned tube was studied by Pongsoi et al. [93].

6. Experimental and numerical studies

Numerical simulation and experimental investigation of laminarforced convection heat transfer of an Al2O3water nanouid wasintroduced by Shariet al.[94]. Depending on the temperature, the

single phase model was employed in the numerical simulation of

Fig. 8. Different regions of meshing for a Couette ow[87].

Fig. 9. The effect of the particle volume fraction on: (a) the heat transfer coefcient

ratio with Dean number and, (b) the pressure drop ratio in a ribbed curved duct

[93].



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the nanouid transport phenomena and grid meshing was used inthe numerical part. Experimental and simulation results indicatedgood enhancement of convection heat transfer of the base uid byadding small amounts of Al2O3nanoparticles to the base uid. The

heat transfer was enhanced with increasing nanoparticle concen-tration and the Reynolds number of ow, as shown in Fig. 10.Additionally, the contour of temperature and velocity with tubelength is shown inFig. 11.

7. Outlook

Regarding all of these papers describing experimental studies, theheat transfer in a uid may be enhanced with suspended small solid

metal or non-metallic particles less than 100 nm in diameter, evenwith a small percentage volume fraction. Meanwhile, the numericalstudies using commercial simulation software show enhanced heattransfer in uid through a circular tube, a heat exchanger and tubes

of other shapes. The author will focus on heat transfer enhancementby adding a high volume fraction of solid particles to waterexperimentally. Additionally, numerical studies of heat transfer

enhancement using CFD commercial FLUENT software comparedwith experimental data and correlated equations of the Nusseltnumber and friction factor in an automobile radiator cooling systemwould be useful. In addition, the tube shape of a car radiator isrepresented as a at tube, so a comparison among other shapes

(e.g., circular, at and elliptical tubes) will be necessary in futurestudies. The type of nanouid in these literature studies are metallicor non-metallic nanoparticles suspended in a base uid, but the

author will prepare hybrid nanouids suspended in mixed baseuids to enhance the heat transfer of an automotive cooling system

8. Future work

It is necessary to study how to develop correlations of friction

factor and heat transfer through tubes lled with nanouids. Hence

further studies are needed to develop generalised hydrodynamic andheat transfer characteristic correlations for nanouid in a tubeFurthermore, the effect of the tube shape (e.g., at, elliptical or

circular) on the hydrodynamics and heat transfer ina tube and heat exchanger need to be understood. Additionallya comparison among tube shapes for use in a car radiator can beperformed experimentally and numerically. High volume concentra-

tions of various types of nanouids may be used as a coolant in a carradiator. A hybrid nanouid prepared from mixing types with samevolume fraction can be studied in the future for augmentation of theheat transfer and friction factor. Additionally, the mixing of base uidscan be used in an automobile radiator to enhance the heat transfer

and hydrodynamic characteristics as future work.

9. Conclusions

Nanouids are a new class of heat transfer uid engineered bydispersing metallic or non-metallic nanoparticles less than 100 nm

in size in a liquid. The thermal properties of solid nanoparticlesgive the base uid better hydrodynamic and heat transfer attri-butes. The great potential of nanouids for heat transfer enhance-ment in highly suited practical heat transfer applications will lead

to the development of compact and effective heat transfer equip-ment by engineers. The enhanced heat transfer potential of thebase liquids will offer an opportunity for engineers to develop

highly compact and effective heat transfer equipment for manyindustrial applications, including transportation, nuclear reactors

electronics, biomedicine, and food. According to the majority oexperimental and numerical studies, suspensions of solid nano-

particles signicantly enhance heat transfer and the heat transfercoefcient of nanouids is found to be larger than that of its base

uid at the same Reynolds number. Additionally, increasing thevolume fraction of solid nanoparticles increases the heat transfer

of nanouids. On the other hand, the friction factor and pressuredrop of nanouids are larger than the base uids. The enhance-ment of the heat transfer of the nanouids may be caused by thesuspended nanoparticles increasing the thermal conductivity o

uids, and the chaotic movement of ultrane particles increasesuctuation and turbulence of the uids, accelerating the energyexchange process. Furthermore, the numerical correlations o

Fig. 11. (a) The temperature eld of nanouid ow (b) the velocity eld of nanouid ow, at Re 1000, 5% by vol. of Al2O3nanoparticles and heat ux5000 w/m2 [94]

Fig.10. The effect of the nanouidow Reynolds number on the Nusselt number at

a concentration of 5% by vol. of Al2O3 nanoparticles[94].



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single-phase uid have clearly failed to predict the heat transfercoefcients of nanouids. The Nusselt number is predicted for thedifferent volume concentrations of nanouids in a circular tube asin Eq.(7)for laminar and Eq.(8)for turbulent, and the correlation

of friction factor is used for more accurate predictions in Eq. (9).The relationships among the Nusselt number, the Reynolds num-ber, the Prandtl number, and the height of corrugation and pitchhas been correlated in Eq. (15).

Future studies are very important for determining a plan ofresearch and any work that will be proposed. It is necessary to studythe development of correlations of friction factor and heat transferthrough tubes with nanouids. Hence, further studies are needed to

develop a generalised hydrodynamic and heat transfer characteristiccorrelation for nanouid in a tube. Furthermore, the effect of the tubeshape (e.g., at, elliptical or circular) on the hydrodynamics and heattransfer in a tube and heat exchanger are needed. Additionally, a

comparison among tube shapes for use in a car radiator can beperformed experimentally and numerically. High volume concentra-tions of various types of nanouid may be used as coolant in a carradiator. The hybrid nanouid prepared from mixing types with the

same volume fraction can be studied in the future for augmentation ofthe heat transfer and friction factor. Additionally, the mixing of base

uids can be used in an automobile radiator to enhance heat transferand hydrodynamic characteristics as future work.

Acknowledgements

Thenancial support to the authors by the University of MalaysiaPahang is gratefully acknowledged.

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water nanouids owing under a

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