Experimental Study on Heat Transfer and Thermo-PhysicalProperties of Covalently Functionalized Carbon NanotubesNanofluids in an Annular Heat Exchanger: A Green and NovelSynthesisMaryam Hosseini,*,† Rad Sadri,*,† Salim Newaz Kazi,*,† Samira Bagheri,‡ Nashrul Zubir,†

Chew Bee Teng,† and Tuan Zaharinie†

†Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia‡Nanotechnology & Catalysis Research Centre (NANOCAT), IPS Building, University of Malaya, 50603 Kuala Lumpur, Malaysia

ABSTRACT: In order to eliminate the use of toxic acids in common carbon nanomaterial functionalization methods, a facileand eco-friendly procedure is introduced in this study to synthesize highly dispersed, covalently functionalized multiwalledcarbon nanotubes (MWCNTs) for use in heat transfer fluids. The MWCNTs are treated covalently with clove buds in one potusing a free radical grafting reaction. The clove-treated MWCNTs (C-MWCNTs) are then dispersed in distilled water (DIwater) at three different concentrations of C-MWCNTs (0.075, 0.125, and 0.175 wt %), resulting in C-MWCNT-DI waternanofluids. The effectiveness of the functionalization process is then verified using Fourier transform infrared (FTIR)spectroscopy, thermogravimetric analysis (TGA), and transmission electron microscopy (TEM). UV−vis spectroscopy is alsoused to examine the stability of the C-MWCNTs in the base fluid. The thermal conductivity, density, and dynamic viscosity ofthe C-MWCNT-DI water nanofluids are studied experimentally, and the results show that there is significant thermalconductivity enhancement for the C-MWCNT-DI water nanofluids, whereas there is only a slight increase in the viscosity anddensity of these nanofluids. Lastly, convective heat transfer experiments are carried out for the C-MWCNT-DI water nanofluidsflowing through an annular heat exchanger at constant heat flux and fully developed turbulent flow conditions. The results showthat there is a significant enhancement in the convective heat transfer coefficient and Nusselt number for the C-MWCNT-DIwater nanofluids, whereas the increase in friction factor is almost negligible for these nanofluids. On the basis of the results, it canbe concluded that the eco-friendly C-MWCNT-DI water nanofluids have strong potential for use as effective working fluids invarious heat transfer applications.

1. INTRODUCTION

Heating or cooling fluids such as air, water, Freon, ethyleneglycol, and mineral oil play a vital role in various industrialsectors such as chemical production, power generation,microelectronics, transportation, and air-conditioning. How-ever, conventional heating or cooling fluids do not havesuperior capability to transfer or dissipate heat from industrialthermal equipment with high heat flux.1 One of the promisingsolutions to address this issue is to disperse nanoparticles withhigh thermal conductivity (e.g., metal, carbon, or metal oxidenanoparticles) into the base fluid in order to enhance thethermal performance of heat transfer systems.2−6 In this regard,Choi et al.7 found that the thermal conductivity of the CNT-water nanofluids is higher compared to that of water. Theymeasured the effective thermal conductivity of a nanofluidcontaining 1.0 vol % of multiwalled carbon nanotubes(MWCNTs) dispersed in synthetic poly(α-olefin) oil andreported a thermal conductivity enhancement of 160%.8 Sadriet al. studied the sonication time effect on the dynamicviscosity, thermal conductivity, and dispersion of MWCNTaqueous suspensions. The results represented that the viscositydecreases, whereas the thermal conductivity of the nanofluidsincreases with an increase in temperature and sonication time.The maximum thermal conductivity enhancement was found tobe 22.31% (ratio of 1.22) at a temperature and sonication time

of 45 °C and 40 min, respectively.9 Philip et al.10 investigatedthe thermal conductivity enhancement of kerosene-basedFe3O4 nanofluids under the influence of a magnetic field andthe results showed that the enhancement of thermalconductivity is 300% (knanof luid/kbase‑f luid = 4.0) at a concentrationof 6.3 vol % and particle size of 6.7 nm. Pak and Cho11

investigated the heat transfer performance of γ-Al2O3 and TiO2nanoparticles dispersed in water flowing through a horizontalcircular tube under turbulent flow and constant heat fluxconditions. The results represented that the Nusselt number ofthe nanoparticles aqueous suspensions increases by increasingthe Reynolds number and nanoparticle concentration. More-over, they discovered that the convective heat transfercoefficient of the nanofluids is lower than that for water by12% for a given condition when the particle loading is 3 vol %.They proposed a new heat transfer correlation to predict theconvective heat transfer coefficient of nanofluids in a turbulentflow regime. Annular heat exchangers are commonly used inindustrial applications, particularly in heat transfer equipment.These heat exchangers have gained much attention fromscientists and researchers, and they have been used in various

Received: November 7, 2016Revised: February 15, 2017Published: February 22, 2017

Article

pubs.acs.org/EF

© 2017 American Chemical Society 5635 DOI: 10.1021/acs.energyfuels.6b02928Energy Fuels 2017, 31, 5635−5644

thermal systems such as air-conditioning, electronic devices,heating and ventilation systems, turbomachinery, gas turbines,and nuclear reactors. Hence, investigating the heat transfercharacteristics of annular heat exchangers and devising a noveltechnique to improve their performance play a key role inpromoting energy savings.12,13 Weerapun Duangthongsuk etal.14 studied the friction factor and convective heat transfer ofTiO2 aqueous suspensions (concentration of 0.2 vol %) in ahorizontal double-tube counter flow heat exchanger underturbulent conditions. They discovered that the convective heattransfer coefficient of nanofluids is slightly greater than that ofthe base fluid by about 6−11% and increases by decreasing thefluid temperature. Moreover, they showed that the temperatureof the heating fluid has no significant effect on the heat transfercoefficient of the nanofluid. Izadi et al.15 simulated the forcedconvection flow of Al2O3/water aqueous suspensions through atwo-dimensional annular heat exchanger (in which the workingfluid was Al2O3-water nanofluid) using the single-phase methodunder turbulent conditions. The results showed that thedimensionless axial velocity profile does not remarkably varywith the nanoparticle volume fraction. However, the temper-ature profiles are affected by the nanoparticle concentration.They found that the convective heat transfer coefficientincreases by increasing the nanoparticle concentration. CNTshave garnered much interest among scientists and researchersbecause of their remarkable physical and chemical proper-ties16,17 as well as excellent mechanical properties.18−20 Indeed,it has been proven in previous studies that CNT nanofluidshave high thermal and electrical conductivities.21−24 However,the main disadvantage of CNTs is their poor solubility andprocessability since they easily become entangled, and more-over, they tend to aggregate in most organic and aqueoussolvents.25 It has been shown that noncovalent functionaliza-tion of CNTs based on the π−π stacking interaction andpolymer wrapping of surfactants such as sodium dodecylbenzenesulfonate (SDBS), sodium dodecyl sulfate (SDS), gumarabic (GA), and cetyltrimethylammonium bromide (CTAB)improves the solubility of CNTs in polar solvents.9,18,26

However, there are some undesirable effects of the above-mentioned surfactants on the thermophysial properties ofcarbon based nanofluids such as increasing viscosity and foamformation in the colloidal suspensions which have limitedsurfactant applications.27 On the other hand, various techniqueshave been employed for covalent modification of carbonnanotubes such as treatment with ozone, oxidation, fluorina-tion, free radical addition, 1,3-dipolar cycloaddition, nucleo-philic addition, alkylation, and plasma modification.28−30

Among these techniques, free radical grafting is a desirabletechnique for large-scale synthesis of CNTs with versatilefunctional groups due to its facile treatment and highreactivity.28 In this technique, peroxides, aryl diazonium salts,substituted anilines, or benzophenone are used as the startingmaterials.28,30 However, all of the aforementioned techniquestypically require very harsh and drastic reaction conditions. Inaddition, most of these techniques involve many reaction steps,and the reagents and chemicals employed in these techniquesare greatly prone to oxidation. These techniques can alsodamage the sp2 hybridized carbon atoms of nanotubes, which inturn, have an effect on the thermal, electrical, and opticalproperties of the carbon nanotubes.28 These techniques alsolead to other problems such as health hazards, equipmentcorrosion, and environmental pollution. Hence, there is acritical need to develop a functionalization method which is

environmentally friendly31 in order to tackle the above-mentioned issues. Clove, an aromatic flower bud, is one ofthe most widely cultivated spices in tropical countries. Clovebud is one of the main vegetal sources of phenolic compoundssuch as hydroxybenzoic acids, flavonoids, hydroxycinnamicacids, and hydroxyphenyl propane.32 Eugenol is the majorbioactive compound of cloves, which is typically found inconcentrations of around 82.6 wt %.33 Phenolic acid and gallicacid are also present in cloves at higher concentrations. Aprevious study34 has shown that ascorbic acid is found in cloveflower buds with concentrations up to 0.08 wt %. The uniqueproperties and structure of cloves make them a suitablecandidate to improve the functionalization of CNTs in aqueousmedia. Hence, the main objective of this study is to develop anenvironmentally friendly, cost-effective, and industrially scalablemethod for synthesizing MWCNTs covalently functionalizedwith cloves and to determine the effectiveness of this techniquein improving convective heat transfer in an annular heatexchanger relative to that for water. In order to verify thesuccess of the functionalization method, the clove-treatedMWCNTs are characterized using Fourier transform infrared(FTIR) spectroscopy, thermogravimetric analysis (TGA),transmission electron microscopy (TEM), and ultraviolet−visible (UV−vis) spectroscopy. Following this, the clove-treatedMWCNTs are dispersed into distilled water at three differentnanoparticle concentrations (0.075, 0.125, and 0.175 wt %) toprepare C-MWCNT-DI water nanofluids and the rheologicaland thermo-physical properties of these nanofluids alsoinvestigated at various temperatures. The friction factor andNusselt number values for the base fluid are determined fromthe experimental data, and the results are validated againstthose calculated from empirical correlations. Lastly, the Nusseltnumber, convective heat transfer coefficient, pressure drop,friction factor, and performance index are evaluated for the C-MWCNT-DI water nanofluids flowing through an annular heatexchanger in turbulent flow conditions.

2. MATERIALS AND METHODSPristine multiwalled carbon nanotubes (MWCNTs) with a diameterless than 30 nm and a purity of more than 95% were purchased fromNanostructured & Amorphous Materials Inc. (Houston, TX, USA).Hydrogen peroxide (H2O2, 30%) was purchased from Sigma-Aldrich(M) Sdn. Bhd., Selangor, Malaysia. Fresh cloves, which are a source ofeugenol and vitamin C (ascorbic acid), were purchased from a grocerystore in Iran.

2.1. Preparation of C-MWCNT-DI Water Nanofluids. A novel,facile, and eco-friendly technique for covalent functionalization ofMWCNTs with cloves by employing free radical grafting method wasintroduced for the first time in this study in order to attain highlydispersed MWCNTs in aqueous media. In this technique, eugenol(which is the main component of cloves33) was grafted onto theMWCNTs using hydrogen peroxide and ascorbic acid (also acomponent of cloves) as the redox initiator.34 In this method,ascorbic acid reacts with hydrogen peroxide (a free-radical oxidizerthat generates non-toxic by-products and leaves no chemical residue),producing hydroxyl radicals. Also at high temperatures, the hydrogenperoxide becomes unstable and decomposes spontaneously intohydroxyl radicals. The generated hydroxyl radicals will attack eugenoland MWCNTs to produce free radicals on their structure, which leadsto linkage of the activated eugenol onto the surface of MWCNTs. Theclove extract solution was prepared using the following procedure.First, 20 g of ground cloves were added into a beaker containing 1000mL of distilled water and preheated at 80 °C. The solution was thenhomogenized in heating mode at an agitation speed of 1000 rpm for

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30 min. Lastly, the clove extract solution was filtered using a 45 μmpolytetrafluoroethylene (PTFE) membrane in ambient conditions.The clove-treated MWCNTs (C-MWCNTs) were prepared using

the following procedure. First, 5 g of pristine MWCNTs were pouredinto a beaker filled with 1000 mL of clove extract solution, followed bycontinuous stirring for 15 min in order to achieve a uniform blacksuspension. Following this, 35 mL of concentrated hydrogen peroxidewas added gradually into the reaction mixture throughout thesonication time. The reaction mixture was ultrasonicated for 10 min.After sonication, the reaction mixture was heated to 80 °C under refluxfor 14 h. The resultant suspension was centrifuged repeatedly at 14000rpm and then washed with large amounts of distilled water until thesuspension attained a neutral pH. The functionalized sample was driedovernight in a vacuum oven set at 60 °C. It was observed that theclove-treated MWCNTs are highly stable in aqueous medium. In orderto synthesize the C-MWCNT-DI water nanofluids, the C-MWCNTswere ultrasonicated with distilled (DI) water as the base fluid for 10min. The nanofluids were prepared for three different concentrationsof C-MWCNTs: 0.075, 0.125, and 0.175 wt %. The samplepreparation procedure is summarized in Figure 1.

2.2. Experimental Procedure. The experiments were carried outin several stages. First, the pristine MWCNTs were covalently

functionalized with cloves, and the C-MWCNTs were characterizedusing various analytical instruments. Following this, the thermo-physical properties of the synthesized nanofluids and base fluid weredetermined. Lastly, the convective heat transfer and frictionalproperties of the C-MWCNT-DI water nanofluids flowing throughan annular heat exchanger were investigated. Characterization of theC-MWCNTs was carried out using Fourier transform infrared (FTIR)spectroscopy (Bruker, IFS-66/S, Germany) and thermogravimetricanalysis (TGA-50, Shimadzu, Japan). Hitachi HT7700 transmissionelectron microscope was used to examine the morphologicalcharacteristics of the samples. For the TEM analysis, the sampleswere prepared by ultrasonically dispersing the nanoparticles in ethanolprior to collection on Lacey carbon grids. We used a Shimadzu UV-1800 spectrophotometer to determine the dispersibility of C-MWCNT-DI water nanofluids. For the UV−vis analysis, we dilutedthe nanofluids with DI water at a dilution ratio of 1:20 in order toensure that the detectable wavelengths of the UV−vis spectrometer areable to pass through the samples. Following this, we poured oursamples into quartz cuvettes specialized for the transmission of UVwavelengths, and we measured the absorbance of the samples atpredefined time intervals over the course of 63 days. The thermalconductivity of the samples was measured using a KD-2 PRO portablefield and laboratory thermal property analyzer (Decagon Devices,USA). The KS-1 probe has a length and diameter of 60 and 1.3 mm,respectively. The accuracy of the thermal conductivity measurementsis around 5%. In order to ensure equilibrium of the nanofluids, anaverage of 20 measurements were recorded over a 5-h period for eachnanoparticle concentration and temperature. Calibration of theinstrument was also conducted with DI water prior to measurementsusing nanofluids. The viscosity of the nanofluids was measured using aPhysica MCR rheometer (Anton Paar, Austria). The rotationalrheometer consists of a moving cylindrical plate and a stationarycylindrical surface placed in parallel with a small gap between them.The density of the samples was measured using a DE-40 density meter(Mettler Toledo, Switzerland), with an accuracy of 10−4 g/cm3. Themeasurements were made in triplicate for each sample andtemperature. The overall experimental setup for convective heattransfer measurements includes an annular flow test rig, a reservoirtank, a pump, a data acquisition system, a cooling unit, a heated testsection, and measuring instruments including a differential pressuretransmitter (DPT) and a flow meter. The experimental setup is shown

Figure 1. Schematic of functionalization procedure of carbonnanotubes.

Figure 2. Schematic diagram of the experimental setup.

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in Figure 2. The aqueous suspensions were pumped using an ArakiEX-70 R magnet pump from a 10 L stainless steel jacketed tank at aflow rate of 0−14 L/min. The pressure loss and flow rate weremeasured using PX154 wet/wet low differential pressure transmitter(OMEGA Engineering Inc., USA) and an SE 32 inline paddle wheeltransmitter with display (Burkert Contromatic Corp., USA),respectively. The pump flow was regulated using a Hoffman Mullerinverter. The concentric annular test rig consists of a cartridge heater(OMEGA Engineering Inc., USA) with a length of 600 mm and adiameter of 15 mm, which was inserted in the center of a straight,seamless, and horizontal tube made from stainless steel with a lengthof 900 mm and outer and inner diameters of 33.4 and 26.7 mm,respectively. The heater was linked to a QPS VT2-1 variable voltagetransformer (Success Electronics & Transformer Manufacturer Sdn.Bhd., Malaysia), and voltage and current were measured to set thedesired heating power. Three built-in K-type thermocouples (OMEGAEngineering Inc., USA) were cautiously installed a little away from theoutside contact face of heater surface in order to prevent disturbancein the boundary layer caused by the thermocouple probes protrudinginto the internal surface of the tube. The bulk temperature of the flowwas measured using two platinum resistance temperature detectors(Pt-100 RTDs) which were placed inside the pipe at the outlet andinlet of the test section. The maximum error for thermocouples was±0.2 °C. The thermocouples were connected to a GL220 10-channelmidi logger (Graphtec Corporation, Japan) in order to monitor andrecord the temperature data. The test section was wrapped with thickfibreglass wool in order to reduce heat loss to the surroundings.2.3. Data Analysis. The experimental data were processed to

evaluate the hydrodynamic and heat transfer performance of theannular heat exchanger. Taking into account conduction in the heaterwall and convection heat transfer with the fluid in the test section,calibration was required to determine the temperature at the surface ofthe heater. Hence, an analysis based on the Wilson plot technique35

was done by equating the wall resistance. It shall be noted that thereare important parameters used to determine the effect of the C-MWCNTs on the thermal properties of DI water, namely, convectiveheat transfer coefficient (h), Nusselt number (Nu), and pressure drop.The experimental heat transfer coefficient was determined from the

measured bulk, surface, outlet, and inlet temperatures using Newton’slaw of cooling:

=″

−h

qT T( )w b (1)

where Tw, Tb, and q″ represent the wall temperature, bulk temperature,and heat flux, respectively. Tb is defined as

+T T2

o i , where To and Ti is the

outlet and inlet temperatures of the flow, respectively. The heat fluxwas determined using the following equation:

″ =qQA (2)

Here, Q is the input power (VI) generated by the voltage transformer,and A is the internal surface area of the tube. Note that A = πDhL.

Dh is the annulus hydraulic diameter, which is defined as thedifference between the inner tube diameter and outer heater diameter.L is the annulus heated length. The input power (VI) was kept fixed at900 W for the experiments. The Reynolds number (Re) was calculatedusing the following equation:

ρμ

=RevDh

(3)

Here, ρ, v, and μ represents the density, velocity, and dynamic viscosityof the working fluid, respectively. The Nusselt number is given by

=×

Nuh

KDh

(4)

where k and h represent the thermal conductivity and convective heattransfer coefficient, respectively. The friction factor ( f) of the DI waterand C-MWCNT-DI water nanofluids was determined from thepressure drop across the test section measured from experiments usingthe following eq (eq 5):

= Δρ( )( )

fP

L vD 2h

2

(5)

Figure 3. (A) FTIR spectra of pristine MWCNTs and C-MWCNTs, (B) TGA curves of pristine MWCNTs and C-MWCNTs, and (C) photographof C-MWCNTs dispersed in DI water after three months.

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Where, ΔP and v represent the pressure drop and flow velocity,respectively.

3. RESULTS AND DISCUSSION

3.1. Characterization of the C-MWCNTs. In this study,various characterization techniques were used to characterizethe C-MWCNTs. The morphology of the C-MWCNTs wasexamined using TEM. FTIR spectroscopy was used to identifythe functional groups attached to the C-MWCNTs. Figure 3Ashows the FTIR spectra for the pristine MWCNTs and C-MWCNTs, which are represented by the transmittance (%)versus the wavenumber (cm−1). It can be observed from Figure3A that there are noticeable cues of the eugenol molecule forthe C-MWCNTs compared to the pristine MWCNTs. Thebroad peak at 3448.19 cm−1 is ascribed to O−H stretchingvibrations, which may be caused by the reaction between theMWCNTs and hydroxyl groups of eugenol and/or hydrogenperoxide. The symmetric and asymmetric sharp vibrations ofC−H bonds are observed within a wavenumber range of 2850−3000 cm−1 for both MWCNTs and C-MWCNTs. A couple ofpeaks are observed within the wavenumber range of 1579−1639 cm−1, which arise from CC stretching vibrations ofMWCNTs after opening due to the addition of electrophilicreactions between the main structure of MWCNTs and the −OH band of eugenol. The functionalization of MWCNTs byeugenol is confirmed by the peaks at 1428, 1386, and1077−1113 cm−1, which are ascribed to CH2 bendingvibrations, out-of-plane CH vibrations, and C−O stretchingvibrations, respectively. The peaks within the wavenumberrange of 1722−1763 cm−1 and the peak centered at 1621 cm−1

are assigned to O−CO stretching vibrations, indicating theformation of carboxyl groups at the main structure ofMWCNTs.TGA is a thermal analysis technique, whereby modifications

in the structure of materials are evaluated as a function oftemperature. TGA provides information about the quantitativeamount of each functional groups on the surface of MWCNTs,which is why it was used to characterize the pristine MWCNTsand C-MWCNTs. Figure 3B shows the TGA curves for pristineMWCNTs and C-MWCNTs, and it can be seen that there isno weight loss in the pristine MWCNTs up to a temperature of∼700 °C, which is the temperature at which the main graphiticstructures begin to decompose. However, there is a gradualweight loss for the C-MWCNTs, which confirms thedecomposition of the functional groups. There are threedistinctive steps of weight loss for the clove-treated MWCNTswithin a temperature range of 0−600 °C. The first weight lossoccurs at ∼100 °C, which is attributed to water, whereas thesecond weight loss occurs between 200 and 300 °C, which isdue to the decomposition of the functional group (i.e.,eugenol). The third weight loss occurs after 600 °C, which iscaused by the degradation of the graphitic structures in air. Theresults indicate the successful covalent functionalization ofMWCNTs with eugenol, which is evidenced by the gradualweight loss observed in the TGA curve for C-MWCNTs. Adigital image of C-MWCNTs dispersed in DI water after 90days is shown in Figure 3C.Even though the functional groups cannot be inferred from

TEM images, these images can be used to examine the surfacedeterioration of MWCNTs, as evidence of functionalization.Figure 4A−C shows the TEM images of pristine MWCNTsand C-MWCNTs. It can be observed that the pristineMWCNTs have a relatively smooth surface and low wall

defects (Figure 4A). In contrast, there is an increase in surfaceroughness for the modified C-MWCNTs (Figures 4B and C).The cut and open ends of the MWCNTs are evident, whichmay be due to the partial damage of graphitic (sp2) carbon afterfunctionalization.UV−vis spectroscopy is a technique used to study the

dispersibility of nanofluids with respect to the sedimentationtime. Figure 5 shows the colloidal stability of the C-MWCNT-

DI water nanofluids with respect to the number of days afterpreparation. It is obvious that the relative concentration of theaqueous suspensions reduces with the number of days.However, all of the samples have a fairly constant concentrationafter day 45. The maximum magnitude of sedimentation is 7.6,9.2, and 11.3% for a nanoparticle concentration of 0.075, 0.125,and 0.175 wt %, which confirms the colloidal stability of thenanofluids containing C-MWCNTs.

3.2. Thermophysical Properties. The effective viscosity ofnanofluids containing C-MWCNTs and DI water weremeasured experimentally, and the results are shown in Figure6. The dynamic viscosity is shown as a function of thenanoparticle concentration and temperature at a fixed shear rateof 150 s−1. It can be observed that there is a slight increase in

Figure 4. TEM images of (A) pristine MWCNTs and (B and C) C-MWCNTs.

Figure 5. Colloidal stability of C-MWCNTs dispersed in DI water.

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the dynamic viscosity of the C-MWCNT-DI water nanofluidsrelative to that for DI water. This indicates that the C-MWCNTs remain suspended in the base fluid, which indicatessuccessful covalent functionalization. It can also be observedfrom Figure 6 that the dynamic viscosity of the C-MWCNTsnanofluids declines by increasing temperature, which may beattributed to the weakening of intermolecular forces.36 Thisobservation corroborates well the findings of Aravind et al.,36

Ko et al.,37 and Sadri et al.,38 who observed that there isdecrease in viscosity when the fluid temperature is increased.The rather mild increase in the effective viscosity withnanoparticle concentration is an important advantage sincethe increase in viscosity could undermine the overall positiveimpact of enhanced conductivity in heat transfer due to thepumping fluid penalty.The density was measured for the C-MWCNT aqueous

suspensions and base fluid at various temperatures, and theresults are presented in Table 1. It can be seen that there is a

decrease in the density of the C-MWCNT-DI water nanofluidsand DI water with an increase in fluid temperature, which canbe attributed to the liquid thermal expansion. There is a slightincrement in the density for the C-MWCNT-DI waternanofluids with an increase in nanoparticle concentration.The increase in density can be attributed to the density of theC-MWCNTs, which is higher than that of the base fluid.Hence, increasing the particle loading will increase the densityof the nanofluid. The maximum increase in density of the C-MWCNT-DI water nanofluid is 0.08% for a nanoparticleconcentration of 0.175 wt % at 20 °C. However, the densitydecreases by approximately 0.6% when the temperature is

increased from 20 to 40 °C for the same nanoparticleconcentration.The thermal conductivity of a coolant is one of the

parameters that play a significant role in increasing the heattransfer rate of heat exchangers. Hence, the thermalconductivity of the C-MWCNT-DI aqueous suspensions wasmeasured experimentally in this study, and the results areshown in Figure 7. The thermal conductivity of the C-

MWCNT-DI water nanofluids was plotted as a function oftemperature and nanoparticle concentration. It should be notedthat only nanofluids with low concentrations of C-MWCNTswere considered in this study in order to prevent a drasticincrease in viscosity. The thermal conductivity of the base fluidwas compared with that from the National Institute ofStandards and Technology (NIST) database, and there isgood agreement between these values,39 whereby the error isless than 1%. It is evident from Figure 7 that the thermalconductivity of the C-MWCNT-DI water nanofluids issignificantly greater than that for the base fluid. Moreover, itis evident that the thermal conductivity of the C-MWCNTaqueous suspensions and base fluid increases with an increasein fluid temperature. Nonetheless, the thermal conductivityenhancement is higher for the C-MWCNT-DI water nanofluidsat higher nanoparticle concentrations. On the basis of Figure 7,it can be deduced that temperature plays a vital role inenhancing the effective thermal conductivity of nanofluids,which is due to the Brownian motion of the C-MWCNTsdispersed in the base fluid.36 The maximum thermalconductivity enhancement obtained is 20.15% for a nano-particle concentration of 0.175 wt % at 50 °C.

3.3. Thermal Performance of C-MWCNT-DI WaterNanofluids. To evaluate the reliability and accuracy of theexperimental setup, preliminary experiments were carried outfor the base fluid prior to systematic experiments involvingnanofluids in the annular heat exchanger. The results for thebase fluid flowing through the experimental setup at constantheat flux boundary conditions were compared with the datacalculated using two empirical correlations proposed byPetukhov40 and Gnielinski41 for the turbulent flow regime.These empirical correlations are given by eqs 6 and (7),respectively.

=+ −

( )( )

NuRePr

Pr1.07 12.7 ( 1)

f

f

8

8

0.5 2/3(6)

Figure 6. Dynamic viscosity of nanofluids containing C-MWCNTsand DI water at a shear rate of 150 s−1 (mPa·s).

Table 1. Density of C-MWCNT-DI Water Nanofluids and DIWater as a Function of Temperature

density (kg/m3)

concn/temp (°C) 20 25 30 35 40

DI water 998.00 996.85 995.50 993.90 992.000.075 wt % 998.30 997.20 995.85 994.15 992.250.125 wt % 998.55 997.50 996.15 994.45 992.500.175 wt % 998.80 997.75 996.45 994.65 992.70

Figure 7. Thermal conductivity of C-MWCNT-DI water nanofluidsand DI water.

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where the friction factor ( f) is defined as. f = (0.79Ln Re −1.64)−2

It should be noted that eq 6 is applicable for a Reynoldsnumber range of 4000 < Re < 5(106) and Prandtl numberrange of 0.5 < Pr < 2000.

=+ −

+ ⎜ ⎟⎡⎣⎢⎢

⎛⎝

⎞⎠

⎤⎦⎥⎥Nu

f RePr

k f Pr

DL

F( /8)

12.7 /8( 1)1 Kann

ann

hann

12/3

2/3

(7)

with

= + −+

kRe Pr

1.07900 0.63

(1 10 )1(8)

Eq 7 is used for a Reynolds number range of Re > 4000, aPrandtl number range of 0.1 ≤ Pr ≤ 1000, and (Dh/L) ≤ 1.The factor Fann in eq 7 representing the boundary condition

of heat transfer at the inner wall with the outer wall insulatedand the factor K for liquids as variation of fluid properties withtemperature can be calculated by eqs 9 and 10, respectively.

= −F a0.75ann0.17

(9)

=⎛⎝⎜

⎞⎠⎟K

PrPr

b

w

0.11

(10)

The friction factor ( fann) in eq 7 is given by Gnielinski41 as

*= − −f Re(1.8 log 1.5)ann 102

(11)

where

* = + + −−

Re Rea a a

a a(1 )ln (1 )

(1 ) ln

2 2

2 (12)

Figure 8 shows the comparison between the experimentallymeasured average Nusselt number and those calculated using

the aforementioned empirical correlations for the base fluid at afixed water inlet temperature of 30 °C. Indeed, the Nusseltnumber increases with an increase in the Reynolds number, asexpected. The maximum error between the values obtainedfrom experiments and Petukhov40 and Gnielinski41 empiricalcorrelation is found to be approximately 3.5 and 9.5%,respectively. In general, the results obtained from theexperiments are in good agreement with those from empirical

correlations within the range of the Reynolds numberinvestigated in this study. Hence, it can be deduced that theexperimental setup is reliable and that it can be utilized toassess the heat transfer properties of the C-MWCNT aqueoussuspensions.A series of experiments were conducted for the C-MWCNT-

DI water nanofluids (nanoparticle concentration: 0.075, 0.125,and 0.175 wt %) by varying the Reynolds number from 3055 ±5 to 7944 ± 5 in order to determine the convective heattransfer of the aqueous suspensions in turbulent flow regime.The input power was kept fixed at 900 W. The results areshown in Figure 9 for a constant inlet temperature of 30 °C. It

can be observed that the convective heat transfer coefficientincreases when the Reynolds number is increased for both thebase fluid and C-MWCNT-DI water nanofluids. It can also beobserved that the nanoparticle concentration affects the heattransfer coefficient of the nanofluids. The remarkable enhance-ment in convective heat transfer coefficient of the C-MWCNT-DI water nanofluids is mainly due to the narrow thermalboundary layer resulting from the thermal conductivityenhancement of C-MWCNT-DI water nanofluids as well as adecrease in thermal resistance between the flowing nanofluidsand inner wall surface of the annular tube at higher Reynoldsnumbers. According to refs 1 and 36, carbon nanomaterials(e.g., carbon nanotubes and graphene nanoplatelets) tend toreduce the thickness of the thermal boundary layer. The specificsurface area and Brownian motion of the nanoparticles also playa role in influencing the convective heat transfer coefficient. Inthis study, the convective heat transfer coefficient of thenanofluids increases by 12.38, 24.1, and 35.89% for ananoparticle concentrations of 0.075, 0.125, and 0.175 wt %,respectively.To assess the convective-to-conductive heat transfer ratio of

C-MWCNT aqueous suspensions, the average Nusselt numberdetermined from eq 4 was plotted as a function of Reynoldsnumber and nanoparticle concentration. The results are shownin Figure 10, and it can be seen that there is a significantincrease in the Nusselt number with an increase in theReynolds number as well as the concentration of C-MWCNTsrelative to that for the base fluid. The greater Nusselt numberfor the C-MWCNT nanofluids is attributed to the higherthermal conductivity of the nanofluids, which is associated withthe Brownian motion of the C-MWCNTs.42 The maximum

Figure 8. Comparison between the measured Nusselt numbers of DIwater with those calculated from empirical correlations.

Figure 9. Variation of the average heat transfer coefficient for C-MWCNT/DI water nanofluids versus the Reynolds number.

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increase in the Nusselt number of the C-MWCNT-DI waternanofluids is found to be 6.96, 13.66, and 20.15% for ananoparticle concentration of 0.075, 0.125, and 0.175 wt %,respectively. This maximum increase is achieved when Re =7944 ± 5.In order to validate the pressure drop accuracy in the

experimental setup, the variation of the measured friction factorfor the DI water was compared with the values calculated fromempirical correlation proposed by Gnielinski41 (eq 11), and theresults are shown in Figure 11. It can be observed that there is

only a slight deviation between the data obtained fromexperiments and those calculated from empirical model. Thedeviation between the experimental friction factor values andthose calculated using the Gnielinski empirical correlation isless than 4.15%. Hence, it can be stated that the experimentalsetup used to measure the pressure drop is validated for therange of Reynolds number investigated in this study.The pressure drop of the C-MWCNT-DI water nanofluids

flowing through the test section recorded over a range ofReynolds number is shown in Figure 12A. The correspondingfriction factor values were calculated using eq 5, and the resultsare shown in Figure 12B. The results show that there is a smallincrement in both the pressure drop and friction factor for theC-MWCNT-DI water nanofluids relative to those for DI water.The maximum increase in the friction factor and pressure drop

is found to be ∼3% and 23%, respectively, for a nanoparticleconcentration of 0.175 wt %. This increment in pressure drop ismainly due to the slight increment in viscosity for all C-MWCNT-DI water nanofluids (Figure 6) which required aslight increment in fluid velocity since the correspondingReynolds number is constant (eq 3). Hence, it can be seen thatthe velocity of the working fluid plays a vital pattern inincreasing the pressure drop and friction factor compared tothat of DI water in convective heat transfer systems. Thismatter can be acknowledged by revisiting eq 5 for the pressuredrop and friction factor, and eq 3 for the Reynolds number.The performance index (ε) was calculated using eq 13 in

order to assess the economic performance of the novel, eco-friendly C-MWCNT-DI water nanofluids synthesized in thisstudy as potential fluids in heat transfer systems such as solarcollectors and heat exchangers.43

ε =Δ Δ

=Δ

h h

P PR

R

/

/nf bf

nf bf

h

P (13)

where RΔp is the ratio of pressure drop of the C-MWCNsaqueous suspensions to the base fluid, and Rh is the ratio of heattransfer enhancement of C-MWCNTs nanofluids relative to thebase fluid. Figure 13 indicates the performance index of the C-MWCNT aqueous suspensions over a range of Reynoldsnumber and various nanoparticle concentrations. It can beobserved that the performance index is higher than 1 for allsamples, which shows the advantage of using these novelnanofluids in heat transfer systems. It is apparent that theperformance index increases with increasing the concentrationof C-MWCNTs in the base fluid. This implies that these eco-friendly nanofluids have higher effectiveness when the particleloading is increased. Figure 13 shows that the performanceindex of the C-MWCNT nanofluids tends to increase with anincrease in the Reynolds number (regardless of the nanoparticleconcentration), indicating that the C-MWCNT-DI waternanofluids are suitable alternative coolants for heat transferapplications.

4. CONCLUSIONAn environmentally friendly and cost-effective procedure forsynthesizing covalently functionalized multiwalled carbonnanotubes with cloves (C-MWCNTs) has been developed inthis study. The C-MWCNT-DI water nanofluids are thenprepared by ultrasonicating C-MWCNTs with distilled water asthe base fluid for three different nanoparticle concentrations(i.e., 0.075, 0.125, and 0.175 wt %). The potential of thesenanofluids as heat transfer fluids is investigated by convectiveheat transfer experiments through an annular heat exchanger ata turbulent flow regime. The exceptional performance of thefunctionalization method is confirmed by TGA, FTIR spec-troscopy, UV−vis spectroscopy, and TEM. On the basis of theUV−vis absorption spectra, the C-MWCNT-DI water nano-fluids have remarkable stability over the course of 63 days. Onthe basis of the thermo-physical properties of the C-MWCNT-DI water nanofluids, it can be deduced that these nanofluids arepotential working fluids in heat transfer systems. It is found thatthere is significant enhancement in the convective heat transfercoefficient and Nusselt number up to 35.89 and 20.15%,respectively. This significant enhancement is attained at aReynolds number of 7944, nanoparticle concentration of 0.175wt %, and constant heat flux of 38346 W/m2. The maximumincrease in friction factor of the C-MWCNT-DI water

Figure 10. Variation of the Nusselt number for C-MWCNT-DI waternanofluids as a function of the Reynolds number.

Figure 11. Plot of the friction factor as a function of Reynolds numberfor DI water obtained from experiments and empirical correlation.

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nanofluids is about 3%, relative to that for the base fluid, for ananoparticle concentration of 0.175 wt %. In addition, it wasfound that the performance index of the C-MWCNT-DI waternanofluids is more than 1 (regardless of the nanoparticleconcentration investigated in this study), indicating that thesenanofluids are potential working fluids in heat transfer systemsin terms of overall thermal performance and energy savings.

■ AUTHOR INFORMATIONCorresponding Authors*(M.H.) Tel: +60 3 79674582. Fax: +60 3 79675317. E-mail:hoseini.sma@gmail.com.*(R.S.) E-mail: rod.sadri@gmail.com.*(S.N.K.) E-mail: salimnewaz@um.edu.my.ORCIDRad Sadri: 0000-0002-8260-7994NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe gratefully acknowledge the financial support provided byUniversity of Malaya under the Fundamental Research GrantScheme (Project no. FP060-2015A) and University of Malaya

Research Grant Scheme (project no. RP012B-13AET). We alsogreatly appreciate the assistance provided by the technical andadministrative staff of the Faculty of Engineering, University ofMalaya, in carrying out this study.

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