application of cnt nanofluids in a turbulent he

This article was downloaded by: [University of Nottingham]On: 02 March 2015, At: 21:01Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registeredoffice: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

Click for updates

Journal of Experimental NanosciencePublication details, including instructions for authors andsubscription information:http://www.tandfonline.com/loi/tjen20

Application of CNT nanofluids in aturbulent flow heat exchangerRashmi Walvekara, Mohammad Khalid Siddiquib, SeikSan Onga &Ahmad Faris Ismailca Department of Chemical Engineering, Energy Research Group,School of Engineering, Taylors University Lakeside Campus,Subang Jaya, Selangor 47500, Malaysiab Department of Chemical Engineering, Division of Manufacturingand Industrial Processes, Faculty of Engineering, University ofNottingham Malaysia Campus, Jalan Broga, Semenyih 43500,Selangor, Malaysiac Department of Mechanical Engineering, International IslamicUniversity Malaysia, Gombak, MalaysiaPublished online: 26 Feb 2015.

To cite this article: Rashmi Walvekar, Mohammad Khalid Siddiqui, SeikSan Ong & Ahmad Faris Ismail(2015): Application of CNT nanofluids in a turbulent flow heat exchanger, Journal of ExperimentalNanoscience, DOI: 10.1080/17458080.2015.1015461

To link to this article: http://dx.doi.org/10.1080/17458080.2015.1015461

PLEASE SCROLL DOWN FOR ARTICLE

Taylor & Francis makes every effort to ensure the accuracy of all the information (theContent) contained in the publications on our platform. However, Taylor & Francis,our agents, and our licensors make no representations or warranties whatsoever as tothe accuracy, completeness, or suitability for any purpose of the Content. Any opinionsand views expressed in this publication are the opinions and views of the authors,and are not the views of or endorsed by Taylor & Francis. The accuracy of the Contentshould not be relied upon and should be independently verified with primary sourcesof information. Taylor and Francis shall not be liable for any losses, actions, claims,proceedings, demands, costs, expenses, damages, and other liabilities whatsoever orhowsoever caused arising directly or indirectly in connection with, in relation to or arisingout of the use of the Content.

This article may be used for research, teaching, and private study purposes. Anysubstantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,systematic supply, or distribution in any form to anyone is expressly forbidden. Terms &

Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions

Dow

nloa

ded

by [U

nivers

ity of

Nott

ingha

m] at

21:01

02 M

arch 2

015

Application of CNT nanofluids in a turbulent flow heat exchanger

Rashmi Walvekara*, Mohammad Khalid Siddiquib, SeikSan Onga and

Ahmad Faris Ismailc

aDepartment of Chemical Engineering, Energy Research Group, School of Engineering, TaylorsUniversity Lakeside Campus, Subang Jaya, Selangor 47500, Malaysia; bDepartment of ChemicalEngineering, Division of Manufacturing and Industrial Processes, Faculty of Engineering, University ofNottinghamMalaysia Campus, Jalan Broga, Semenyih 43500, Selangor, Malaysia; cDepartment ofMechanical Engineering, International Islamic University Malaysia, Gombak, Malaysia

(Received 16 October 2014; final version received 1 February 2006)

Nanofluids have received much attention since its discovery owing to its enhancedthermal conductivity and heat transfer characteristics which makes them apromising coolant in heat transfer application. In this study, the enhancement inheat transfer of carbon nanotube (CNT) nanofluids under turbulent flowconditions is investigated experimentally. The CNT concentration was variedfrom 0.051 to 0.085 wt%, respectively. The nanofluid suspension was stabilised bygum arabic through a process of homogenisation and water bath sonication at25 C. The flow rate of cold fluid (water) is varied from 1.7 to 3 L/min, while flowrate of the hot fluid is varied between 2 and 3.5 L/min. Thermal conductivity,density, and viscosity of the nanofluids are also measured as a function oftemperature and CNT concentration. The experimental results were validatedwith theoretical correlations for turbulent flow available in the literature. Resultsshowed an enhancement in heat transfer between 9% and 67% as a function oftemperature and CNT concentration.

Keywords: nanofluids; carbon nanotubes; gum arabic; heat transfer enhancement;turbulent flow

1. Introduction

Heat transfer plays an important role in almost every industry. Heat generated from power

generation requires high amount of cooling. Most of the chemical processes are exothermic

reactions; therefore, cooling is vital at different stages in chemical processes. The heat

generated in our vehicles and air-conditioning system requires cooling as well and

therefore, cooling the system to remove heat becomes important. Heat transfer also plays

a critical role in other industries such as food industries and micro-sized applications. Theconventional coolants or the conventional heat transfer fluids include water, ethylene

glycol (EG), and oil. They are not very efficient for high heat flux applications due to their

poor conductivity.[1,2] One of the methods proposed in the past decade to increase the

heat transfer rate was by increasing the thermal conductivity of conventional base fluid

using solid micro-sized particles which possess high thermal conductivity.[2] Applications

*Corresponding author. Emails: [email protected]; [email protected]

2015 Taylor & Francis

Journal of Experimental Nanoscience, 2015

http://dx.doi.org/10.1080/17458080.2015.1015461

Dow

nloa

ded

by [U

nivers

ity of

Nott

ingha

m] at

21:01

02 M

arch 2

015

using these micro-sized particles result in problems such as clogging, erosion, andsedimentation. Recently, researchers have discovered the techniques to disperse ultrafine

particles or nano-sized particles in these conventional base fluids.[2] The dispersion of

these ultrafine particles into conventional base fluids results into nanofluids. Nanoparticles

are particles with sizes less than 100 nm. These particles have excellent mechanical,

electrical, thermal, and optical properties compared to their parent material. Examples of

nanoparticles are Al2O3, CuO, TiO2, Cu, Fe, and carbon nanotube (CNT).[2]

Nanoparticles are synthesised physically or chemically. Physical synthesis includes

mechanical grinding or inert-gas-condensation technique, whereas chemical synthesisconsists of chemical vapour deposition, precipitation, or thermal spraying.[2]

One of the most effective nanoparticles is CNT. Researchers managed to disperse small

concentration of CNTs having very high thermal conductivity (3000 W/mK) intotraditional fluids to prepare CNTs nanofluid.[2] The reason which makes CNT outstand

the other nanoparticles is because CNT have very high thermal conductivity close to that

of diamond with very large surface area. CNTs are allotropes of carbon with cylindrical

nanostructure and are constructed with L/D ratio of 132,000,000:1.[1, 3]Duangthongsuk and Wongwises [4] applied TiO2water nanofluids in a horizontal

double tube counter-flow heat exchanger under turbulent conditions. The heat transfer

enhancement reported ranged from 20% to 32% at 1.0 vol%. Chun et al. [5] performed

studies in a concentric double-pipe heat exchanger system using Al2O3-transformer oil as a

coolant and the convective heat transfer coefficient (HTC) was reported to increase by

10%13% with a 0.5% volume concentration. Luciu and Mateescu [6] used Al2O3waternanofluid as coolant in a coaxial heat exchanger. They observed 50% increase in the

convective HTC of 5 wt% at T D 90 C. Furthermore, Haghshenas et al. [7] also studied theconvective heat transfer enhancement using ZnOwater nanofluids in a concentric tubeheat exchanger and a plate heat exchanger. Pantzali and co-workers [8,9] conducted two

studies using CuOwater nanofluids in a miniature plate heat exchanger and a plate heatexchanger separately. Results showed an increment in thermal conductivity, viscosity, and

density; however, significant drop in heat capacity is seen. Mare et al. [10] applied

Al2O3water and CNTwater nanofluids in two plate heat exchangers under laminarconditions. A significant enhancement was observed in HTC of 42% and 50% for alumina

and CNT, respectively. One possible reason could be the higher thermal conductivity of

CNT compared to Al2O3 particles. Pandey and Nema [11] also performed similar study in aplate heat exchanger using Al2O3 nanoparticles in water. It was reported that heat transfer

characteristics improved with increasing Peclet number and Reynolds number but decreases

with nanoparticle concentration. Maximum heat transfer rate was observed at lowest

concentration of nanofluids. One possible reason could be the clustering of nanoparticles at

higher particle volume fraction. A study comprising CNTwater and CNTEG nnofluidsshowed enhancement in heat transfer when applied in car radiator.[12] CNT also proved to

be an excellent coolant in heat dissipating from CPU with a rate of 21 C/s.[13] Kole andDey [14] studied pool boiling HTC and critical heat flux (CHF) using Cuwater nanofluidsin three different heater surfaces (copper, brass and aluminum). Copper surface showed

highest enhancement with 45% in HTC and 60% in CHF at 0.5 wt% Cu loading,respectively. The enhancement is attributed to increase in surface roughness. Saleh et al. [15]

studied the heat performance of tilted mesh heat pipe using TiO2water nanofluids. Effectof tilt angle, % fill charge ratio and volume fraction was studied to demonstrate the heat

transfer effect. It was found that 45 angle, 60% fill charge ratio showed best performance.

2 R. Walvekar et al.

Dow

nloa

ded

by [U

nivers

ity of

Nott

ingha

m] at

21:01

02 M

arch 2

015

More detailed review on thermo-physical properties and heat transfer studies can be foundin recently published paper by Rashmi et al.[16]

Thus, this work aims to study the thermo-physical properties and heat transfer

enhancement of highly stable CNT nanofluids under turbulent flow conditions. The

results were further validated with correlations available in the literature.

2. Materials

CNTs of 20 nm diameter and 35 mm length were obtained from Chengdu Organic ChemicalsCo. Ltd., Chinese Academy of Science, China. Gum arabic (GA) used as a stabiliser was

obtained from Gum Arabic Co., Sudan. Distilled water was used as a base fluid.

2.1. Preparation of nanofluids

Three different concentrations (0.051, 0.068, and 0.85 wt%) of CNT nanofluids were usedin this experimental study. The nanofluids were prepared by adding the measured amount

of CNT and optimum amount of GA as suitable dispersant. Nanofluids were

homogenised for 10 minutes using a high-speed homogenizer at 25,000 rpm (IKA-T18,

ULTRA-TURRAX, Germany). Immediately after that, sonication of 4 hours was carried

out using ultrasonic bath (Crest Ultrasonics, USA) to ensure stability of the nanofluids.

The method of sample preparation and optimum GA concentration was scaled down

based on the optimum results by Rashmi et al.[1] Table 1 below shows the quantity of

CNT and GA concentrations used in this work.As observed from the results of Rashmi et al.,[1] the stability of nanofluids decreased

with increase in CNT concentration. However, increase in CNT concentration will further

demand more dispersant to stabilise the nanoparticles in conventional fluids which could

further increase viscosity and pumping power requirements. Thus, in this work, the CNT

concentration was limited up to 0.085 wt%, respectively.

2.2. Thermo-physical properties

Convective heat transfer performance is related to the thermo-physical properties of

nanofluids. The thermal conductivity, viscosity, and the density of nanofluids were studied

with respect to particle concentration and temperatures (25 C55 C).

2.2.1. Thermal conductivity

Thermal conductivity is measured using a thermal conductivity meter (KD2 Pro, Decagon

device, USA) with accuracy within 5%10%. Before measuring the desired samples, the

Table 1. CNT and GA concentration used in this study.[1]

CNT (wt%) GA (wt%)

0.02 1.00

0.08 2.50

0.10 2.50

Journal of Experimental Nanoscience 3

Dow

nloa

ded

by [U

nivers

ity of

Nott

ingha

m] at

21:01

02 M

arch 2

015

thermal conductivity must first be calibrated using standard glycerine provided bysupplier. For the thermal conductivity meter, a probe or needle will be inserted into

the nanofluid samples. Reading will be automatically taken at a 15 minutes interval. The

readings only with R2 > 0.9995 will be recorded and an average of five readings will

be taken to obtain a more accurate thermal conductivity value of the nanofluid sample.

A viscometer (DV-11CPro, Brookfield, USA) was used for viscosity measurement.Density of nanofluids was measured using a density meter (DA-130N, Kyoto Electronics,

Mexico).

2.2.2. Viscosity

A viscometer (DV-11CPro, Brookfield, USA) will be used to measure the viscosity of eachnanofluid concentration prepared. Readings from 10%100% are satisfactory butreadings out of range will not be taken. The behaviour of the viscosity changes withparticle concentration will be studied.

2.2.3. Density

The density of the nanofluid samples will be measured using a density meter (DA-130N,

Kyoto Electronics, Mexico). Calibration will be done using de-ionized (DI) water bysucking the water in and out of the cell. Samples will be filled into the cell by sucking the

samples into the cell. It is important to have cell completely filled by fluid sample.

Reading will be recorded and the behaviour of the density changes with particle

concentration will be studied.

2.3. Convective heat transfer experiment

Convective heat transfer experiment will be carried out in a turbulent flow concentric tube

heat exchanger (P.A. Hilton Ltd., UK) using the nanofluids prepared at different

concentrations. The experimental set-up is shown in Figure 1. Inlet of cold fluid (tap

water) is connected to the water tap and outlet is left to drain, whereas the hot fluid

(nanofluid) is stored in a built-in reservoir. Flow rates of both cold and hot streams were

controlled by two built-in flow meters. Set-up consists of six thermometers mounted onthe inlet, middle, and the outlet of both cold and hot tubes. Experiments were carried out

with respect to concentration by varying the flow rates of cold and hot fluids from 1.7 to

3 L/min and 2 to 3.5 L/min, respectively.

Below are the specifications of the heat exchanger:

Tube outer diameterD 15 0:7 mm wall;Shell outer diameter D 22 0:9 mm wall;Insulation thickness D 20 mm wall;

Heat transmission length D 1:5 m:


Dow

nloa

ded

by [U

nivers

ity of

Nott

ingha

m] at

21:01

02 M

arch 2

015

2.4. Theoretical correlations

Based on the temperature values collected at different variable conditions at the inlet and

outlet of the experimental set-up, the values can be analysed by calculating the Nusselt

number.[1]Assuming steady-state condition throughout the research, heat transfer rate in laminar

or turbulent flows is given by the steady flow thermal energy equation.

The rate of heat transfer ( _Q ) from hot fluid to the cold fluid can be expressed as

_Qh _D _mhCp;hT1 T2 (1)

_Qc _D _mcCp;cT5 T6 (2)

where T1 and T2 are inlet and outlet temperatures of hot fluid,C, whereas T5 and T6 are

outlet and inlet temperatures of cold fluid, C. _m represents mass flow rate of fluid, kg/s.Cp corresponds to specific heat of the specific fluid, kJ/kg.K. h and c represent hot fluid

and cold fluid, respectively.

Assuming small difference between heat transfer rates of hot and cold fluid, average

value of the heat transfer rate is used for the calculations

_QavgD_QhC _Qc2

: (3)

Figure 1. Experimental set-up for convective heat transfer experiment.


Dow

nloa

ded

by [U

nivers

ity of

Nott

ingha

m] at

21:01

02 M

arch 2

015

The HTC is calculated by

hD_Qavg

Ai DTLMTD : (4)

Nusselt number was calculated using

NuD hdkeff

: (5)

The reliability and accuracy of the experimental data are further validated with a

turbulent flow correlation available in the literature [17]

NuDD f =8ReD 1000Pr1C 12:7f =812 Pr23 1

; (6)

where ReD is the Reynolds number of the flow. Pr is the Prandtl number. f is the Darcy

friction factor that can be obtained from Moody chart or Petukov correlation. Petukov

correlation can determine the friction factor by

f D 0:79lnReD 1:64 2: (7)

The Gnielinski [17] correlation is valid between

0:5 Pr 2000;3000ReD 5106:

Gnielinski [17] correlation is chosen due to the range of Reynolds number used. In this

study, Reynolds number is being varied from 2750 to 4850, respectively. Prior to conducting

the experiments using nanofluids, the convective heat transfer experiment is first conducted

using tap water. The experimental results or data collected are validated using Gnielinski

correlation as stated above to test the reliability and accuracy of the experimental set-up.Error analysis will be conducted by comparing the Nusselt number of the experimental

results and Nusselt number calculated using the Gnielinski [17] correlation.

Experimental data collected from the experiments using CNT nanofluid will be fitted

into the basic Gnielinski correlation as well as other correlations proposed by other

researchers in order to validate the final experimental data. Table 2 below shows the list of

Nusselt number correlations used for results validation.

3. Results and discussion

3.1. Thermo-physical properties

3.1.1. Thermal conductivity

Figure 2 shows the thermal conductivity of nanofluids with respect to temperatures at

three different CNT concentrations. It can be concluded that thermal conductivity


Dow

nloa

ded

by [U

nivers

ity of

Nott

ingha

m] at

21:01

02 M

arch 2

015

increases with increasing temperature and CNT concentrations. Enhancement in thermal

conductivity is due to the presence of high thermal conductivity of CNT nanoparticles.

From Figure 2, it is observed that the enhancement in thermal conductivity is a strong

function of CNT concentration and temperature. The enhancement is in the range of

70%250%. Thus, small concentration of CNT can tremendously increase the heat transferperformance of the system. Enhancement in this study showed higher enhancementcompared to studies performed by other researchers in the past. For example, study

performed by Choi et al. [18] using CNToil mixture showed an enhancement ratio ofmore than 2.5 or 160% at 1 vol% of CNT. Study by Ding et al. [19] using CNTwatershowed thermal conductivity enhancement of up to 80% at 1 wt%, respectively. Wen and

Ding [20] reported thermal conductivity enhancement of only 25% at 0.8 wt% of

CNTwater nanofluids. Lower enhancement in thermal conductivity in previous published

Table 2. List of theoretical correlations used to validate the performance of heat transfer.

Author (year) Nanofluid Model equation

Gnielinski [17] Theoretical correlation for turbulent flow NuDDf

8 ReD 1000Pr1C 12:7 f

8 12 Pr

23 1

f D 0:79lnReD 1:64 2Pak and Cho [21] Al2O3/water, TiO2/water, Turbulent flow NuD 0:21Re0:8Pr0:4Duangthongsuk and

Wongwises [4]TiO2/water, Turbulent flow NuD 0:074Re0:707Pr0:3850:074

Petukov [14] Theoretical correlation for turbulent flow NuDDf

8 RePr1:07C 12:7 f

8 12 Pr

23 1

f D 0:79lnReD 1:64 2

0.00

0.50

1.00

1.50

2.00

2.50

20 30 40 50 60

The

rmal

con

duct

ivit

y ra

tio

kf/k

b

Temperature (C)

0.085 wt%0.068 wt%0.051 wt%Water

Figure 2. Thermal conductivity of nanofluid vs temperature and particle concentrations.


Dow

nloa

ded

by [U

nivers

ity of

Nott

ingha

m] at

21:01

02 M

arch 2

015

works could be due to poor stability of the nanofluid suspensions. However, the presentmethod of nanofluids preparation showed higher stability of CNT nanofluids. Increase in

thermal conductivity with temperature can be explained by theory of Brownian motion.

Brownian motion of nanoparticles is one of the key mechanisms to explain the thermal

behaviour of nanofluids.[1] At higher temperature, the suspended nanoparticles gain higher

amount of energy and thus increasing the random movement and bombardment of

nanoparticles. Furthermore, the enhancement in thermal conductivity with increasing

temperature is in good agreement with many researches in the past.

3.1.2. Viscosity

The results from the study of viscosity is shown in Figure 3 representing the behaviour ofCNT nanofluids with temperature at a shear rate of 100/s. Results show that presence of

CNT nanoparticles in the suspension increases the viscosity of fluid. CNT nanofluids at

three different concentrations (0.085, 0.068, and 0.051 wt%) have small differences with

one another. However, viscosity of CNT nanofluids is higher compared to water. Higher

concentration of nanofluids exhibits higher internal viscous shear stresses causing an

increase in viscosity.[1] Increase in viscosity directly increases the pumping power and

cost. However, the practicability of nanofluids as a heat transfer fluid cannot be

questioned due to the superior enhancement in thermal conductivity which significantlyreduces a large amount of cost. It can be concluded that the viscosity decreases slightly

with an increase in temperature. Increasing temperature weakens the intermolecular forces

of both nanoparticles and fluid in the suspension, hence reducing the viscosity.[1]

3.1.3. Density

The results from the study of density are tabulated in Figure 4. Negligible change in

density is observed for all concentration of nanofluids and water. Nanofluids possess

0.000

0.200

0.400

0.600

0.800

1.000

1.200

1.400

1.600

1.800

2.000

20 30 40 50 60

Vis

cosi

ty (

cP)

Temperature (C)

0.085 wt% 0.068 wt%

0.051 wt% Water

Figure 3. Viscosity of nanofluid vs temperature and particle concentration.


Dow

nloa

ded

by [U

nivers

ity of

Nott

ingha

m] at

21:01

02 M

arch 2

015

slightly higher density than water due to presence of small weight fraction of nanoparticles

and GA in the suspension. Therefore, negligible pressure drop can be assumed.

3.2. Convective heat transfer experiment

Convective heat transfer studies were carried out in a concentric tube heat exchanger underturbulent conditions. Stable CNT nanofluids of concentrations 0.085, 0.068, and 0.051

wt% were used as hot fluid, whereas water was used as cold fluid. Experiments were

carried out with respect to flow rates (Reynolds number) of both hot and cold fluids. The

performance from convective heat transfer studies are evaluated in terms of Nusselt

number.

Prior to conducting experiments using nanofluids, heat transfer experiments using the

similar set-up were carried out using water to test the accuracy and reliability of the set-up.

Figures 58 show the validation of experimental Nusselt number with theoretical Nusseltnumber obtained from Gnielinski correlation.[17]

To validate the accuracy of experimental results, experiments were first conducted with

water and validated with Gnielinski [17] correlation for turbulent flow. Experimental set-

up gave a good insurance on the reliability and accuracy as only 0%15% of error wasobserved.

Furthermore, the experiments were conducted using CNT nanofluids as hot fluid in the

heat exchanger. Experimental Nusselt number and Reynolds number of CNT nanofluids

are plotted from Figures 912 according to the cold fluid Reynolds number (Re D 2750,3230, 4087, 4845).

Referring to Figures 912 above, it can be concluded that enhancement of Nusseltnumber increases with CNT concentrations. Higher Nusselt number represents a higher

HTC and higher heat transfer rate. It can be further observed that the presence of CNT

nanoparticles enhances the heat transfer characteristic of base fluid. Enhancement in rate

of heat transfer could be explained by the enhancement of thermal conductivity of

0.985

0.987

0.989

0.991

0.993

0.995

0.997

0.999

1.001

1.003

1.005

20 30 40 50 60

Den

sity

(g/

cm3 )

Temperature (C)

0.085 wt%0.068 wt%0.051 wt%Water

Figure 4. Density of nanofluid vs temperature and particle concentration.


Dow

nloa

ded

by [U

nivers

ity of

Nott

ingha

m] at

21:01

02 M

arch 2

015

nanofluids. On the other hand, Nusselt number increases with increasing Reynolds number

of hot fluid. Increasing the flow rates of hot fluid increases the viscosity of the nanofluids

and directly increases the heat transfer rate. Linear relationship between Nusselt Number

and Reynolds number is also observed by some of the researchers.[13,21,22]

Overall enhancement in Nusselt number of 7%202% is observed from Figure 12,respectively. Observed enhancement in Nusselt number is much higher compared to otherresearchers in the past. For example, Zamzamian et al. [23] reported an enhancement of

only 349% using Al2O3 and CuO nanoparticles in EG. Huminic and Huminic [24]

0

2

4

6

8

10

12

14

16

18

20

0 2000 4000 6000

Nu

Re

Theoretical

Hot Fluid 2l/min

Figure 5. Comparison between experimental and theoretical correlation at hot fluid rate of2.0 L/min.

0

2

4

6

8

10

12

14

16

18

20

0 1000 2000 3000 4000 5000 6000

Nu

Re

Theoretical

Hot fluid 2.5 l/min



Dow

nloa

ded

by [U

nivers

ity of

Nott

ingha

m] at

21:01

02 M

arch 2

015

reported an enhancement of 1419% using CuO and TiO2 nanoparticles in water. Otherthan that, Asirvatham et al. [25] reported an enhancement of 69.3% at 0.9 vol% silver

nanoparticles in water.Stability of the nanofluids is paid least attention in the past compared to heat transfer

studies. One of the possible reason of higher heat transfer enhancement in present study

could be high stability of nanofluid suspensions. Due to agglomeration and clustering of

nanoparticles in the suspension, enhancement of heat transfer performance could be

0

2

4

6

8

10

12

14

16

18

20

0 1000 2000 3000 4000 5000 6000

Nu

Re

Theoretical

Hot Fluid 3 L/min


0

2

4

6

8

10

12

14

16

18

20

0 2000 4000 6000

Nu

Re

Theoretical

Hot Fluid 3.5 L/min



Dow

nloa

ded

by [U

nivers

ity of

Nott

ingha

m] at

21:01

02 M

arch 2

015

affected. Another possible reason of higher enhancement could be explained by difference

in experimental set-up and operating conditions used by other researchers.

Furthermore, other important parameters which could significantly affect the heat

transfer rate are type of nanoparticles, type of surfactant used, preparation method of

nanofluid, particle aspect ratio, range of temperature measured, particle specification,

purity of nanoparticles, and preparation method of nanofluids.[1] It is expected that CNTnanoparticles would results in greater enhancement than other nanoparticles. Thermal

conductivity of CNT nanoparticles is approximately 3000 W/mK, much higher than other

0

5

10

15

20

25

30

2000 2500 3000 3500 4000

Nu

Re

0.085 wt%0.068 wt%0.051 wt%Water

Figure 9. The relationship between Nu and hot fluid Re for CNT nanofluids and water (cold fluidRe D 2750).

0

5

10

15

20

25

30

2000 2500 3000 3500 4000

Nu

Re

0.085 wt%0.068 wt%0.051 wt%Water

Figure 10. The relationship between Nu and hot fluid Re for nanofluids and water (cold fluid Re D3230).


Dow

nloa

ded

by [U

nivers

ity of

Nott

ingha

m] at

21:01

02 M

arch 2

015

05

10

15

20

25

30

2000 2500 3000 3500 4000

Nu

Re

0.085 wt% 0.068 wt%

0.051 wt% Water


0

5

10

15

20

25

30

35

2000 2500 3000 3500 4000

Nu

Re

0.085 wt% 0.068 wt%

0.051 wt% Water



Dow

nloa

ded

by [U

nivers

ity of

Nott

ingha

m] at

21:01

02 M

arch 2

015

nanoparticles. Besides, the superior aspect ratio allows high amount of heat to betransported making CNT nanofluids an ideal heat transfer fluid.[1]

3.3. Theoretical studies

Results validation was also performed for results obtained from convective heat transferstudies. Nusselt number from experimental studies was validated with theoretical

Gnielinski and Petukov correlation as well as nanofluids correlation proposed by Pak and

Cho [21] and Duangthongsuk and Wongwises.[4]

Figures 1315 represents the graphical validation in terms of experimental Nusseltnumber with theoretical Nusselt number calculated from available correlations for

different concentration of CNT nanofluids used in this study.

Experimental Nusselt number showed good agreement with the Nusselt number

calculated from available correlations in the literature. Referring to the figures above,experimental Nusselt number obtained fell between the ranges of Nusselt number

calculated using other correlations. Hence, the practicability of applying CNT nanofluids

in a heat exchanger under turbulent conditions is proven.

Deviations occurred can be explained by the differences in operating conditions and

experimental set-up used in this study. Some of the available correlations were proposed

based on results obtained from different experimental set-up. For example, some of the

correlations were proposed based on results obtained from heat transfer studies in a

cooling radiator, shell and tube heat exchanger, or a long copper tube. Operatingconditions are also a factor causing deviations in results. For example, the temperature

range used in this study is 25 C to 55 C. Heat transfer studies performed by otherresearchers could be of different temperature range.

0

5

10

15

20

25

30

35

40

45

50

2000 2500 3000 3500 4000

Nu

Re

Exp Re=2750 Exp Re=3230


Gnielinski Duang.-Wongwises

Pak & Cho Petukov

Figure 13. Validation of experimental Nusselt number with theoretical correlations for nanofluidconcentration of 0.051 wt%.


Dow

nloa

ded

by [U

nivers

ity of

Nott

ingha

m] at

21:01

02 M

arch 2

015

Others possible parameters which contribute to results deviation were mentioned in theprevious section under the validation of thermal conductivity results. These parameters

are type of nanoparticles, type of surfactant used, preparation method of nanofluids,

particle aspect ratio, range of temperature measured, particle specification, purity of

nanoparticles, and preparation method of nanofluids.[1]

0

5

10

15

20

25

30

35

40

45

2000 2500 3000 3500 4000

Nu

Re



Gnielinski Duang.-Wongwises

Pak & Cho Petukov

Figure 14. Validation of experimental Nusselt number with theoretical correlations for nanofluidconcentration of 0.068 wt%.

0

5

10

15

20

25

30

35

40

45

2000 2500 3000 3500 4000

Nu

Re

Exp Re=2750 Exp Re=3230Exp Re=4087 Exp Re=4845Gnielinski Duang.-WongwisesPak & Cho Petukov

Figure 15. Validation of experimental Nusselt number with theoretical correlations in the literaturefor nanofluid concentration of 0.085 wt%.


Dow

nloa

ded

by [U

nivers

ity of

Nott

ingha

m] at

21:01

02 M

arch 2

015

4. Conclusion

In this study, three sets of stable CNT nanofluids of concentration 0.051, 0.068, and 0.085

wt% were prepared to study the enhancement of heat transfer in a concentric tube heat

exchanger under turbulent conditions. CNT nanofluids prepared were stabilised using GA

provide sufficient repulsive force in order to ensure that the nanoparticles are well dispersed

in the base fluid. Thermo-physical properties were measured with respect to particle

concentration and temperature. Thermal conductivity results showed an enhancement of

67%250% compared to water, for temperature ranging from 25C to 55 C.Experimental results on heat transfer enhancement studies at turbulent flow conditions

showed good agreement with theoretical Gnielinski correlation. Convective heat transfer

results showed an enhancement of 7%202% compared to water which is attributed tohigh thermal conductivity and surface area of CNT nanoparticles. Furthermore, the

experimental results are validated with theoretical correlations and the small deviations

could be explained by difference in operating conditions, type of nanoparticles, base fluid,

surfactant, structure of nanoparticles and its interaction with base fluid, etc. Thus, the

results obtained indicate the application of CNT nanofluids as futures potential heattransfer fluid.

Disclosure statement

No potential conflict of interest was reported by the authors.

Funding

The authors would like to acknowledge the Ministry of Higher Education (MOHE) for the financialfunding of this research through Fundamental Research Grant Scheme [FRGS- FRGS/2/2013/TK06/TAYLOR/03/1].

References

[1] Rashmi W, Ismail AF, Sopyan I, Jameel AT, Yusof F, Khalid M. Stability and thermal

conductivity enhancement of carbon nanotube (CNT) nanofluid using gum arabic. J Exp

Nanosci. 2011;6(6):567579.[2] Ozerinc S. Heat transfer enhancement with nanofluids. [MS thesis]. Dept. Mechanical Eng., The

Graduate School of Natural & Applied Science of Middle East Technical University; 2010.

Available from: http://www.metu.edu.tr/~dbaker/downloads/Sezer_Ozerinc_Thesis.pdf

[3] Wang X, Qunqing L, Jing X, Zhong J, Jinyong W, Yan L, Kaili J, Shoushan F. Fabrication of

ultralong and electrically uniform single-walled carbon nanotubes on clean substrates. Nano

Lett. 2006;9(9):31373141.[4] Duangthongsuk W, Wongwises S. An experimental study on the heat transfer performance and

pressure drop of TiO2-water nanofluids flowing under a turbulent flow regime. Int J Heat Mass

Transfer. 2010;53:334344.[5] Chun BH, Kang HU, Kim SH. Effect of alumina nanoparticles in the fluid on heat transfer in

double pipe heat exchanger system. Korean J Chem Eng. 2008;25(5):966971.[6] Luciu RS, Mateescu T. Heat transfer study in a coaxial heat exchanger using nanofluids.

Buletinul Institutului Politehnic Din Iasi (LX) Fasc. 2010;59(2):180.


Dow

nloa

ded

by [U

nivers

ity of

Nott

ingha

m] at

21:01

02 M

arch 2

015

[7] Haghshenas Fard M, Talaie MR, Nasr S. Numerical and experimental investigation of heat

transfer of ZnO/water nanofluid in the concentric tube and plate heat exchanger. Thermal Sci.

2011;15(1):183194.[8] Pantzali MN, Kanaris AG, Antoniadis KD, Mouza AA, Paras SV. Effect of nanofluids on the

performance of a miniature plate heat exchanger with modulated surface. Int J Heat Fluid

Flow. 2009;30:691699.[9] Pantzali MN, Mouza AA, Paras SV. Investigating the efficiency of nanofluids as coolants in

plate heat exchangers (PHE). Chem Eng Sci. 2009;64(14):32903300.[10] Mare T, Halelfadl S, Sow O, Estelle P, Duret S, Bazantay F. Comparison of the thermal

performances of two nanofluids at low temperature in a plate heat exchanger. Exp Thermal

Fluid Sci. 2011;35(8):15351543.[11] Pandey SD, Nema VK. Experimental analysis of heat transfer and friction factor of nanofluid

as a coolant in a corrugated plate heat exchanger. Exp Thermal Fluid Sci. 2012;38:248256.[12] Rashmi W, Ismail AF, Khalid M, Anuar A, Yusaf T. Investigating corrosion effects and heat

transfer enhancement in smaller size radiators using CNT-nanofluids. J Mater Sci. 2014;49

(13):45444551.[13] Siddique AB, Prabhu KN. Assessment of wetting characteristics and cooling performance of

aluminium and MWCNT nanofluids in PC environment. J Nanofluids. 2014;3(3):189199[14] Kole M, Dey TK. Pool boiling heat transfer and critical heat flux enhancement of copper

nanoparticles dispersed in distilled water. J Nanofluids. 2014;3(2):8596.[15] Saleh R, Putra N, Wibowo RE, Septiadi WN, Prakoso SP. Titanium dioxide nanofluids for heat

transfer applications. Exp Thermal Fluid Sci. 2014;52:1929.[16] Rashmi W, Khalid M, Ong SS, Saidur R. Preparation, thermo-physical properties and heat

transfer enhancement of nanofluids. Mater Res Express. 2014;1(3):032001.

[17] Gnielinski V. New equations for heat and mass transfer in turbulent pipe and channel flow. Int

Chem Eng. 1976;16:359368.[18] Choi SUS, Zhang ZG, Yu W, Lockwood FE, Grulke EA. Anomalous thermal conductivity

enhancement in nano-tube suspensions. Appl Phys. Lett. 2001;79:22522254.[19] Ding Y, Alias H, Wen D, Williams RA. Heat transfer of aqueous suspensions of carbon

nanotubes (CNT nanofluids). Int J Heat Mass Transfer. 2005;49:240250.[20] Wen D, Ding Y. Effective thermal conductivity of aqueous suspensions of carbon nanotubes

(carbon nanotube nanofluids). J Thermophy Heat Transfer. 2004;18(4):481485.[21] Pak BC, Cho YI. Hydrodynamic and heat transfer study of dispersed fluids with submicron

metallic oxide particles. Exp Heat Transfer. 1998;11(2):151170.[22] Petukov SS. Heat transfer and friction in turbulent pipe flow with variable physical properties.

In: Hartnett JP, Irvine TF, editors. Advances in heat transfer. New York, NY: Academic Press;

1970. p. 504564.[23] Zamzamian A, Oskouie SN, Doosthoseini A, Joneidi A, Pazouki M. Experimental investigation

of forced convective heat transfer coefficient in nanofluids of Al2O3/EG and CuO/EG in a

double pipe and plate heat exchangers under turbulent flow. Exp Thermal Fluid Sci. 2011;35

(3):495502.[24] Huminic G, Huminic A. Heat transfer characteristics in double tube helical heat exchangers

using nanofluids. Int J Heat Mass Transfer 2011;54(1920):42804287.[25] Asirvatham LG, Raja B, Lal DM, Wongwises S. Convective heat transfer of nanofluids with

correlations. Particuology 2011;9:626631.


Dow

nloa

ded

by [U

nivers

ity of

Nott

ingha

m] at

21:01

02 M

arch 2

015

Abstract1. Introduction2. Materials2.1. Preparation of nanofluids2.2. Thermo-physical properties2.2.1. Thermal conductivity2.2.2. Viscosity2.2.3. Density

2.3. Convective heat transfer experiment2.4. Theoretical correlations

3. Results and discussion3.1. Thermo-physical properties3.1.1. Thermal conductivity3.1.2. Viscosity3.1.3. Density

3.2. Convective heat transfer experiment3.3. Theoretical studies

4. ConclusionDisclosure statementFundingReferences

application of cnt nanofluids in a turbulent he

Documents