Accepted Manuscript

Comparative study on thermal performance of twisted tape and wire coil inserts

in turbulent flow using CuO/water nanofluid

M.T. Naik, Syed Sha Fahad, L. Syam Sundar, Manoj K. Singh

PII: S0894-1777(14)00094-6

DOI: http://dx.doi.org/10.1016/j.expthermflusci.2014.04.006

Reference: ETF 8197

To appear in: Experimental Thermal and Fluid Science

Received Date: 13 November 2013

Revised Date: 28 March 2014

Accepted Date: 2 April 2014

Please cite this article as: M.T. Naik, S.S. Fahad, L. Syam Sundar, M.K. Singh, Comparative study on thermal

performance of twisted tape and wire coil inserts in turbulent flow using CuO/water nanofluid, Experimental

Thermal and Fluid Science (2014), doi: http://dx.doi.org/10.1016/j.expthermflusci.2014.04.006

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Comparative study on thermal performance of twisted tape and wire coil inserts in

turbulent flow using CuO/water nanofluid

M.T. Naik1,*, Syed Sha Fahad2, L. Syam Sundar3,*, Manoj K. Singh3 1Centre for Energy Studies, JNTU College of Engineering, Kukatpally, Hyderabad, India 2Muffakhamjah College of Engineering and Technology, Banjara Hills, Hyderabad. India 3Centre for Mechanical Technology and Automation (TEMA-UA), Department of Mechanical

Engineering, University of Aveiro, 3810-193 Aveiro, Portugal

*Authors: mtnaik56@gmail.com (M.T. Naik), sslingala@gmail.com (L.S. Sundar)

Abstract

Heat transfer and friction factor analysis of CuO/water nanofluid flowing through a tube

under turbulent flow conditions and with twisted tape (TT) and wire coil (WC) inserts were

presented in this paper. The experimental investigations were performed in the Reynolds

number range from 4000 to 20000, volume concentrations of 0.1% and 0.3%, twisted tape

inserts of and wire coil inserts of . The experimental

results indicated that under same operating conditions and flow rates, heat transfer

coefficient, friction factor and thermal performance factor associated with nanofluid in a tube

with wire coil inserts are higher than those with the twisted tape inserts. The Nusselt number

enhancement for 0.3% nanofluid in a tube without inserts is 17.62%, 0.3% nanofluid in a tube

with TT-2 is 31.88% and 0.3% nanofluid in a tube with WC-2 is 44.45% at a Reynolds

number of 20000 compared to water. Whereas, the friction factor enhancement for 0.3%

nanofluid in a tube without inserts is 1.149-times, 0.3% nanofluid in a tube with TT-2 is

1.179-times and 0.3% nanofluid in a tube with WC-2 is 1.198-times at a Reynolds number of

20000 compared to water. The thermal performance factor of 0.3% nanofluid in tube with

twisted tape and wire coil inserts are 1.24 and 1.36 compared against water data respectively.

Keywords: CuO nanofluid, twisted tape, wire coil, heat transfer, friction factor.

2

1. Introduction

Conventional single phase fluids such as water, engine oil, ethylene glycol and propylene

glycol and transformer oil etc., plays an important role in thermal management of industries

such as process industries, chemical plants, thermal power plants, but they have poor thermal

characteristics, in particular thermal conductivity. To reach the industrial requirements heat

transfer intensification of the fluids is very essential. Since solid materials posses higher

thermal conductivities compared to conventional fluids. Many studies have been carried out

on thermal properties of suspension of solid particles in conventional heat transfer fluids.

Ahuja [1] and Liu et al. [2] have studied experimentally on heat transfer intensification of

fluids by dispersing of millimeter or micrometer sized particles. They observed heat transfer

enhancement but suffered from sedimentation of the particles in the fluids.

Nanotechnology provides to manufacture solid particles down to millimetre or

micrometer size to nanometer meter size. Fluids containing dispersion of nanometer sized

particles are called nanofluids (Choi [3]). The heat transfer performance of these fluids is

superior to suspended millimeter or micrometer particles in fluid and also other potential

benefits such as large relative surface area, higher heat conduction, excellent stability and

minimal clogging. Many researchers have explained thermal conductivity enhancement of

nanofluids with influence of particle concentrations and temperatures [4-6]. The applicability

of nanofluids in industrial sector can be analyzed by its convective heat transfer coefficient.

Pak and Cho [7] experimentally studied the convective heat transfer of Al2O3/water and

TiO2/water nanofluid in a tube under turbulent flow conditions and obtained approximately

75% heat transfer enhancement with 2.78% volume concentration of Al2O3 nanofluid. Sundar

et al. [8] estimated the convective heat transfer of Fe3O4/water nanofluid and obtained

Nusselt number and friction factor enhancements of 30.96% and 10.01% at 0.6% volume

concentration compared to water at similar operating conditions. Ding et al. [9] observed

3

350% heat transfer enhancement with carbon nanotubes (CNT’s)/water flowing in a

horizontal tube at 0.5% weight concentration at Reynolds number is 800. Chandraprabu [10]

experimentally investigated the convective heat transfer of CuO/water nanofluid in the

condensing unit of an air conditioner with particle concentrations of 1%, 2%, 3%, and 4 vol.

% and they found heat transfer rate of CuO nanofluid improved up to 35%. Asirvatham [11]

experimentally investigated the heat transfer of CuO/water nanofluid with low volume

fraction of 0.003% in a copper tube with mass flow rate range from 0.0113 kg/s to 0.0139

kg/s and in the inlet temperatures of 10oC and 17oC and found that 8% heat transfer

enhancement. Suresh et al. [12] experimentally investigated convective heat transfer of

CuO/water nanofluids with particle concentrations of 0.1%, 0.2% and 0.3% and found that

Nusselt number enhancements of 6%, 9.9% and 12.6%, respectively, compared to water.

Hashemi and Akhavan-Behabadi [13] prepared nanofluids with different particle weight

concentrations of 0.5%, 1% and 2% and observed highest heat transfer enhancement with

helically coiled tube instead of straight tube. Kannadasan et al. [14] experiments were

conducted in the turbulent flow for 0.1% and 0.2% CuO nanofluid in horizontal and vertical

arrangements and observed the higher heat transfer rates for helically coiled heat exchanger.

Suresh et al. [15] have conducted experiments by using helically dimpled tube with

CuO/water nanofluid of 0.1%, 0.2% and 0.3% volume concentrations and the Nusselt number

with dimpled tube and nanofluids under turbulent flow is about 19%, 27% and 39% higher

than the Nusselt number obtained with plain tube and water. Most of the researchers have

obtained enhancement in heat transfer with the use nanofluids flowing in a tube.

Another passive technique to enhance the convective heat transfer of fluid flowing in

a tube by inserting inserts. Generally used inserts such as twisted tape, wire coil, longitudinal

strip and helical screw tape inserts etc. The rate of heat transfer enhancement is depending on

flow conditions and geometry of the insert. Single phase fluids flowing in a tube with twisted

4

tape inserts have been analyzed by Smithberg and Landis [16], Lopina and Bergles [17],

Manglik and Bergles [18]. Sarma et al. [19,20] also reported heat transfer enhancements of

single phase fluids flowing in a tube with twisted tape inserts. In the similar way single phase

fluid in a tube with wire coil inserts have been analyzed by Uttarwar and Rao [21], Vicente et

al. [22,23], Wang et al. [24], Garcia et al. [25] and Akhavan-Behabadi et al. [26]. Most of the

researchers have obtained enhancement in heat transfer for single phase fluid flowing in a

tube with inserts. This indicates that, the similar passive technique may also be helpful to

further heat transfer enhancement of nanofluids flowing in a tube.

Nanofluid flowing in a tube with twisted tape inserts has been analyzed by Sundar and

Sharma [27]. They conducted experiments with Al2O3/water nanofluid flowing in a tube and

with twisted tape inserts and obtained 33.51% enhancement with twist ratio of at

0.5% volume concentration and proposed Nusselt number and friction factor correlations. In

the another study of Sundar and Sharma [28], they used Al2O3/water nanofluid flowing in a

tube with longitudinal strip inserts and obtained 55.73% heat transfer enhancement at

longitudinal strip insert of AR = 1 for 0.5% volume concentration. Wongcharee and Eiamsa-

ard [29] observed Nusselt number increase of 12.8 and 7.2-times with CuO/water nanofluid

in a tube with modified twisted tape and alternative twisted tape inserts under laminar flow.

Wongcharee and Eiamsa-ard [30] also observed 1.57-times thermal performance factor for

0.7% of CuO/water nanofluid in a corrugated tube with twisted tape inserts. Eiamsa-ard and

Wongcharee [31] studied the combined effects of nanofluids, dual twisted tapes and micro fin

tube on the heat transfer rate, friction factor and thermal performance factor characteristics

for 0.3% and 1.0% by volume in the Reynolds number between 5650 and 17000. Suresh et al.

[32, 33] found maximum enhancement of 166.8% for Al2O3/water nanofluid and 179.8% for

CuO/water nanofluid at helical screw tape ratio, under the same flow conditions

by using 0.1% volume concentration.

5

Nanofluid in a tube with wire coil inserts has been analyzed by Chandrasekar et al.

[34]. They considered 0.1% of Al2O3/water nanofluid in a tube under fully developed laminar

flow with wire coil inserts and observed 21.5% heat transfer enhancement with wire coiled

insert pitch of 3 and also developed Nusselt number correlation. Naik and Sundar [35]

developed Nusselt number and friction factor correlations for propylene glycol and water

based CuO nanofluid flowing in a tube with helical coil inserts. Saeedinia et al. [36] have

prepared heat transfer experiments with CuO/base oil nanofluid in a tube with wire coil

inserts. They considered particle concentrations from 0.07%-0.3% and five wire coil pitches

of 25-35 mm and wire diameters of 0.9-1.5 mm and obtained 45% heat transfer enhancement

with 63% penalty in pressure drop at high wire diameter coil inserts. Kahani et al. [37]

investigated heat transfer behavior of Al2O3/water and TiO2/water nanofluid flowing in a tube

with helical coil inserts in the volume concentrations from 0.25%-1.0% in the Reynolds

number of 500 to 4500. Sundar and Singh [38] have provided the available correlations for

nanofluid flowing in a tube with different kind of inserts in their review paper. The authors

individually obtained further heat transfer enhancement for nanofluids flowing in a tube with

inserts without any penalty in pumping power.

Performance analysis between various inserted inserts in a tube for nanofluid flow is

very essential. Till now, there is no such analysis is available in the literature. In this regard,

the present work focuses on the estimation of convective heat transfer and friction factor of

CuO/water nanofluid flowing in a tube with twisted tape and wire coil inserts under turbulent

flow conditions. Thermal performance between twisted tape and wire coil inserts in a tube

together with CuO/water nanofluid has been analyzed. Nusselt number and friction factor

correlations were proposed based on the experimental data.

6

2. Preparation of CuO nanofluid

In this study, nanofluids were prepared by dispersing CuO nanoparticles in distilled water.

The physical properties of CuO nanoparticles and the equations for the estimation of thermal

conductivity, density, specific heat and viscosity of distilled water were shown in Table 1.

The nanofluids of 0.1% and 0.3% volume concentrations were prepared. Fig. 1a shows the

transmission electron microscopy image of CuO nanoparticles dispersed in water and reveals

that nanoparticles are in spherical shape. Sample nanofluid and bulk nanofluid preparations

were represented in Fig. 1b and Fig. 1c. There are several procedures to prepare stable

nanofluids, which include using dispersant, a stabilizer, a surface activator and an ultrasonic

vibrator. Among these methods, surface modification method attracts more attention because

of its unique, less cost and technological advantages [40]. Nanofluids could be stable a

maximum of 30 min without using suitable surfactants. A surfactant can improve the stability

of nanofluid dramatically. In this study, Cetyltrimethyl Ammonium Bromide (C-TAB) was

used as surfactant for CuO nanoparticles. The required quantity of nanoparticles for given

volume concentration was estimated from Eq. (1). The amount of surfactant is nearly equal to

1/10th of weight of nanoparticles for particular concentration was mixed with distilled water

and stirred by high speed stirrer. The sonication was done continuously by ultrasonic

processor (Hielscher, Germany) for at least 60 min to obtain a stable nanofluid. No settlement

of nanoparticles was observed after 45 days.

Volume concentration, (1)

Where is the percentage of volume concentration, = 6300 kg/m3, = 998.5

kg/m3, = 100 g and is the weight of the nanoparticles. The thermophysical

properties of prepared nanofluids were estimated from the properties of water (Table 1) and

7

properties of CuO nanoparticles at bulk temperature of fluid using the below equations for

density, specific heat and viscosity [41-43].

(2)

Where and are the particle volume concentration and viscosity and the subscripts

and refer to particle, base fluid and nanofluid. The above equation is valid for

a very low particle volume concentration .

(3)

(4)

The theoretical model to predict the thermal conductivity of solid liquid mixture, Maxwell

[44] model can be described as follows:

(5)

3. Experimental setup and procedure

The schematic representation of experimental setup was shown in Fig. 2. The test section

contains copper tube with 1750 mm long, 14 mm inner diameter (ID) and 16 mm outer

diameter (OD). The other parts involved in the experimental setup are chiller, collecting tank,

storage tank, variable pump and by-pass valve arrangement. Constant heat flux boundary

condition was maintained by winding the nichrome heater with a gauge of 20 mm, resistance

of 53.3Ω/m and a maximum capacity of 1000 W on outer surface of the copper tube. The test

section was placed in a straight square duct in order to maintain horizontal position. The gap

between the test tube and square duct was filled with rock wool insulation to reduce the heat

loss to atmosphere. Seven PT-100 resistance temperature detector (RTD) sensors are

provided; in which two were used to record the inlet and outlet temperatures, five were

brazed on the outer surface of the test tube at distances of 187.5, 375, 750, 1125 and 1312

8

mm from the inlet of the test section to measure the wall temperatures of the tube. The

resolution of all the thermocouples was ±0.1oC and they are calibrated before fixing at the

specified locations. The aspect ratio, of the test section was sufficiently large for

the flow to be hydrodynamically developed. The working fluid is circulated through the test

section with an aid of pump; the suction side is connected to a storage tank. In order to

measure the mass flow rate of working fluid a flow meter was used and it is connected

between the pump and the test section. The storage tank is made of stainless steel of 30 liters

capacity. The liquid which is heated in the test section is allowed to cool by passing it

through a chiller. The liquid then flows to the storage tank by gravity. The provision of chiller

helped in achieving steady state condition faster.

The photographic representation of twisted tape (TT) and wire coil (WC) inserts were

shown in Fig. 3. The twisted tape inserts were made in the laboratory from 1mm thick and 13

mm width of aluminum strip and the dimensions of twisted tape were shown in Table 2. A

gap of 1 mm is provided between inner diameter of the tube and the width of the twisted tape

for smooth insertion of inserts into a test section. The two ends of the aluminum strip were

inserted into lathe; one end at the headstock and the other end at the tail stock, by rotating the

head stock manually, the helix lengths of 70 mm and 140 mm were achieved. The twist ratios

of twisted tape inserts such as TT-2 ( and TT-1 ( were obtained. The

twisted tapes are snug fit into the test tube and the tube fin effect is neglected. The convective

heat transfer between twisted tape material and the adjacent fluid was neglected. The mass

flow rate of nanofluid flowing through a tube with twisted tape inserts were estimated based

on the inner diameter of the tube. The hydraulic diameter of tube with twisted tape inserts

was considered as inner diameter of the tube, because the twisted tape has very negligible

thickness i.e. 1 mm.

9

The wire coil inserts were made by winding uniformly aluminium wire of 2 mm

diameter over copper core rod of 8 mm diameter with a length of 1750 mm. The aluminium

wire is tightly wound on the copper core rod. The convective heat transfer between wire coil

material and the adjacent fluid was neglected. The mass flow rate of nanofluid flowing in a

tube with wire coil inserts were calculated based on the hydraulic diameter (Eq. (6)). The

dimensions of the wire coil inserts WC-2 and WC-1 were listed

in Table 3. In this table, is wire coil pitch, is wire diameter, and are inside and

hydraulic diameters of test tube, respectively. The hydraulic diameter is defined as given

below [36]:

(6)

Where is a wire coil diameter. The friction factor of nanofluid in a tube with twisted tape

and wire coil inserts were estimated based on the pressure drop across the test tube. The

pressure drop was measured by placing the U-tube manometer between two ends of the test

tube. For this purpose, two 4 mm holes were drilled at two ends of the test tube and U-tube

manometer was connected with flexible tube. The manometer fluid was used as carbon

tetrachloride and its equivalent height is recorded at different mass flow rates. Initial

experiments were conducted with base fluid, after that different volume concentration of CuO

nanofluid was considered one after the other. The flow rate of base fluid and nanofluid was

measured with high precision flow meter supplied by Chambal Magnects, Ltd, India with an

accuracy of ±0.1 liters/sec. The tube is cleaned with pure water between the experiments

conducted with different concentrations of nanofluids. After reaching the steady state the

inlet, outlet and wall temperatures are notes and the properties of the working is evaluated at

bulk temperature of fluid. Similar procedure and data is collected for nanofluids flowing in a

tube with twisted and wire coil inserts. The convective heat transfer coefficient was estimated

based on the Newton’s law of cooling.

10

4. Data reduction

4.1. Experimental Nusselt number

The amount of heat supplied to the test section and heat gained by the working fluid was

estimated from Eq. (7) and Eq. (8) and found a maximum difference of ±2.5%. This indicates

that negligible amount of heat loss takes place from test section to atmosphere. Experimental

heat transfer coefficient and Nusselt number was estimated based on the given expressions:

(Energy supplied) (7)

(Energy absorbed) (8)

(9)

Where , ,

(10)

The correlations for estimation of Nusselt number for single phase fluid are given below:

(i) Gnielinski [45] correlation for turbulent flow

(11)

, ,

(ii) Notter-Rouse [46] equation for turbulent flow

(12)

4.2. Experimental friction factor

The experimental friction was estimated by considering the pressure drop across the test

section and the equation is given below:

11

(13)

The friction factor correlations for single phase fluid were given below:

(i) Blasius [47] equation for turbulent region

(14)

(ii) P

etukov [48] equation for turbulent region

(15)

The available correlations for Nusselt number and friction factor of nanofluids flowing in a

tube with twisted tape and wire coil inserts were presented in Tables 4 & 5.

5. Results and discussion

5.1. Nusselt number

5.1.1. Nanofluid in a plain tube

Heat transfer and friction factor experiments were initially conducted with water as working

fluid. The experimental heat transfer coefficient and experimental Nusselt number was

estimated from Eq. (9) and Eq. (10). Thermal conductivity of water and nanofluid was used

for the estimation of experimental Nusselt number. The estimated Nusselt number from Eq.

(10) was shown in Fig. 4 in comparison with the data obtained from Eq. (11) of Gnielinski

[45] and Eq. (12) of Notter-Rouse [46]. The difference between experimental and theoretical

Nusselt number for water was obtained a maximum of ±3%. Nanofluids of different

concentrations were introduced into test section one by one for the estimation of heat transfer

12

coefficient. The Eq. (9) is used to analyze the experimental heat transfer coefficient of CuO

nanofluid in a tube. Thermal conductivity of nanofluid estimated from Eq. (5); which is used

to estimate the experimental Nusselt number of CuO nanofluid (Eq. (10)).

The estimated Nusselt number for CuO nanofluid from Eq. (10) was represented in

Fig. 5 along with base fluid data. Under similar operating parameters, the Nusselt number of

nanofluid is higher than that of the base fluid (water), as the presence of nanoparticles

directly results in an increase of thermal conductivity. Besides, the heat transfer improvement

is also associated by the collision among nanoparticles and also that between the

nanoparticles and tube wall, leading to an increase in the energy exchange rate. Nusselt

number increases with increasing Reynolds number due the intensification of the nanofluid

mixing fluctuation. In concentration range studied, Nusselt number slightly increases with the

increase of nanoparticle concentration. In general, the increase of nanoparticle concentration

in base fluid results the increases of thermal conductivity and collision of nanoparticles which

are favourite factors for heat transfer enhancement and an increase of fluid viscosity which

diminishes the fluid movement and thus heat transfer rate. The obtained result implies that for

the present range, the effect of the increase in thermal conductivity and the collision of

nanoparticles are more prominent than the increase of the fluid viscosity. According to the

experimental results the Nusselt number of CuO nanofluid increases with increase of particle

concentration. At 0.1% volume concentration, the enhancement in Nusselt number is 11.87%

and 14.47% in the Reynolds number of 4000 and 20000. In the similar way, at 0.3% volume

concentration, the enhancement in Nusselt number is 15.70% and 17.62% in the Reynolds

number of 4000 and 20000 respectively. The enhancement is more in high Reynolds number

compare to low Reynolds number, because of the effective mixing of fluid in fully developed

flow condition.

13

5.1.2. Nanofluid in a plain tube with twisted tape inserts

Further heat transfer and friction factor experiments were conducted with nanofluids flowing

in a tube with different ratios of twisted tape inserts. The experimental Nusselt number for

nanofluid in a tube with twisted tape inserts are estimated from Eq. (10) and the data was

represented in Fig. 6. Compared to the same concentration of 0.3% nanofluid in a tube with

TT-1, the Nusselt number enhancement is 7% and 8.7% in the Reynolds number of 4000 and

20000, respectively. In the similar fashion, compared to same concentration of 0.3%

nanofluid in a tube with TT-2, the Nusselt number enhancement is 10.57% and 12.21% in the

Reynolds number of 4000 and 20000, respectively. Compared to the plain tube, the tubes

with twisted tapes exhibit higher Nusselt number, because the tape inserts generate swirl flow

offering a longer flowing path of fluid flow through the tube and also better fluid mixing,

resulting in a thinner thermal boundary layer along the tube wall and thus superior convective

heat transfer. Compared to water in a tube and 0.3% nanofluid in a tube with TT-1, the

Nusselt number enhancement is 23.85% and 27.89% in the Reynolds number of 4000 and

20000, respectively. In the same way, compared to water in a tube and 0.3% nanofluid in a

tube with TT-2, the Nusselt number enhancement is 27.94% and 31.88% in the Reynolds

number of 4000 and 20000, respectively. The same trend has been observed by Sundar et al.

[50] by using Fe3O4/water nanofluid in a tube with twisted tape inserts and Wongcharee and

Eiamsa-ard [29] by using CuO/water nanofluid in a tube with twisted tape inserts. The

experimental Nusselt number of CuO/water nanofluid in a tube with twisted tape inserts was

shown in Fig. 7 in comparison with the data of Sundar et al. [50] for Fe3O4/water nanofluid in

a tube with twisted tape inserts.

5.1.3. Nanofluid in a plain tube with wire coil inserts

14

Heat transfer and friction factor experiments were conducted with nanofluid in a tube

together with wire coil inserts. The Eq. (10) is used to estimate the experimental Nusselt

number and the data was represented in Fig. 8. From the figure, it was observed that the

contribution of wire coil insert to the enhancement of Nusselt number is larger than the

enhancement produced by CuO/water nanofluids. The convective heat transfer enhancement

of nanofluids may be because of several factors such as improved effective thermal

conductivity of the nanofluid over the base fluid (water), Brownian motion of nanoparticles,

and particle migration as reported in literature [28,29]. The reason for heat transfer

enhancement is due to the wire coil inserts which increase the irregular and random

movement of the particles and the energy exchange rates in the nanofluid. The higher

turbulence intensity of the fluid close to the tube wall and the wire coil insert is responsible

promote thorough mixing of nanofluid and an efficient redevelopment of the thermal or

hydrodynamic boundary layer which consequently results in the improvement of convective

heat transfer. Compared to the same concentration of 0.3% nanofluid in a tube with WC-1,

the Nusselt number enhancement is 11.47% and 16.26% in the Reynolds number of 4000 and

20000, respectively. In the similar manner, compared to same concentration of 0.3%

nanofluid in a tube with WC-2, the Nusselt number enhancement is 14.33% and 22.81% in

the Reynolds number of 4000 and 20000, respectively. Compared to water in a tube and 0.3%

nanofluid in a tube with WC-1, the Nusselt number enhancement is 28.99% and 37.07% in

the Reynolds number of 4000 and 20000, respectively. Same manner, compared to water in a

tube and 0.3% nanofluid in a tube with WC-2, the Nusselt number enhancement is 32.28%

and 44.45% in the Reynolds number of 4000 and 20000, respectively. Compared to wire coil

and twisted tape inserts under same flow and volume concentrations, the heat transfer

augmentation is caused with wire coil inserts because of secondary flow. Therefore, the

Nusselt number effect of nanofluids in wire coil inserted tubes is more noticeable.

15

5.2. Friction factor

5.2.1. Nanofluid in a plain tube

Friction factor experiments for water were conducted initially and the values are estimated

from Eq. (16). Fig. 9 representing the experimental friction factor of water is in comparison

with the data obtained from Eq. (17) Blasius [47] and Eq. (18) of Petukov [48] and found to

be a maximum of ±2.5% deviation. Experimental friction factor of different volume

concentrations of CuO nanofluid was estimated from Eq. (16) and the data was indicted in

Fig. 10. The friction factor of CuO nanofluid increases with increase of Reynolds number and

particle concentration. The viscosity of CuO nanofluid is also one of the major parameters for

friction factor enhancement. Based on the Eq. (2), the viscosity enhancement for 0.1% and

0.3% volume concentrations of nanofluid are 2.5% and 7.5% respectively over the water. The

friction factor of the nanofluid depends on the flow rate and viscosity of nanofluids. The

friction factor enhancement for 0.1% volume concentration of CuO nanofluid is about 1.10-

times and 1.11-times at a Reynolds number of 4000 and 20000, respectively compared to

water under same flow conditions. Similarly, the friction factor enhancement for 0.3%

volume concentration of CuO nanofluid is about 1.10-times and 1.15-times at a Reynolds

number of 4000 and 20000, respectively compared to water under the same flow conditions.

5.2.2. Nanofluid in a plain tube with twisted tape inserts

Experimental friction factor of different volume concentrations of CuO nanofluid in a tube

with different twisted tape inserts were calculated based on Eq. (16) and the data was

16

presented in Fig. 11. It observed that friction factor increases with increase of Reynolds

number, volume concentration and decreases with decrease of twist ratio. It is clear that the

use of twisted tape inserts results in a very high friction factor than that of plain tube. The

friction factor of 0.3% nanofluid flowing in a tube with TT-2 enhances 1.139-times at a

Reynolds number of 4000 and 1.026-times at a Reynolds number of 20000 compared to same

concentration fluid without twisted tape insert. Compared to water flowing in a tube, the

enhancement in heat transfer coefficient is about 1.26-times and 1.179-times under the same

Reynolds number.

5.2.3. Nanofluid in a plain tube with wire coil inserts

Experimental friction factor of CuO nanofluid flowing in a tube together with wire coil

inserts were calculated from Eq. (16) and the data was shown in Fig. 12. It indicates that, the

friction factor increases with increase of Reynolds number, particle concentrations and

decrease of wire coil pitch. It is also observed that, under same range of Reynolds numbers,

the effect of decreasing the pitch of wire coil insert is more prominent in friction factor

enhancement. Therefore, the highest friction factor is obtained for the wire coil with the

decreased pitch (WC-2). The trend of change in friction factor is in coherence with that for

the plain tube at low Reynolds number, but with rising Reynolds number, the friction factor

of wire coil inserted tubes is more increased compared to that of plain tube. It is also found

that the increase of friction factor for 0.3% volume concentration of CuO nanofluid at higher

Reynolds number is more. In fact, with the increase of Reynolds number, the coiled wire

induces a secondary flow, which in turn, promotes turbulence that leads in friction factor

increase [30]. The friction factor of 0.3% nanofluid flowing in a tube with WC-2 is enhances

1.196-times at a Reynolds number of 4000 and 1.042-times at a Reynolds number of 20000

compared to same concentration fluid without twisted tape insert. Compared to water flowing

17

in a tube, the enhancement in heat transfer coefficient is about 1.324-times and 1.198-times

under the same Reynolds number. Compared to heat transfer coefficient, the magnitude of

nanofluid friction factor with wire coil inserts is negligible and this will not affect any penalty

on the pumping of nanofluid into the test section. The percentage enhancement in Nusselt

number and friction factor of different volume concentrations of CuO/water nanofluid in a

tube with twisted and wire coil inserts were summarized in Table 6.

5.3. Correlation for Nusselt number and friction factor

The experimental Nusselt number of water, nanofluid, nanofluid with twisted and nanofluid

with wire coil inserts (135 data points) are fit into general equation with an average deviation

of 5% and standard deviation of 6% and the equation is given below:

(16)

, ,

,

In the similar way, the experimental friction factor of water, nanofluid, nanofluid with twisted

and nanofluid with wire coil inserts (135 data points) are fit into general equation with an

average deviation of 5.12% and standard deviation of 6.33% and the equation is given below:

(17)

,

The data obtained from Eq. (16) and Eq. (17) is shown in Fig. 13 and Fig. 14 along with the

experimental data.

5.4. Thermal performance factor

18

The thermal performance of twisted tape and wire coil inserts in turbulent flow of CuO/water

nanofluids is evaluated in terms of thermal performance factor for constant pumping power

condition. The thermal performance factor (η) can be defined as the ratio of the heat transfer

coefficient (or Nusselt number) ratio to the friction factor (or pressure drop) ratio at the same

pumping power:

(18)

Where is the index. The larger the values of the thermal performance factor, the more

suitable the enhancement heat transfer technique. The index is experienced different

values in previous literatures. In the laminar flow condition, Usui et al. [51] and Suresh et al.

[33] have considered = 0.1666 and Hashemi and Akhavan-Behabadi [13] have taken

. In the turbulent flow condition, Wongcharee et al. [30] and Abbasian-Arani and

Amani [52] have considered, . In order to investigate the influence of both twisted

tape and wire coil inserts and nanofluid techniques on thermal performance of the

equipments, the following equation was used in this study:

(19)

The variation of thermal performance factor with Reynolds number for 0.1% and 0.3%

nanofluid through twisted tape and wire coil inserts is illustrated in Fig. 15 and Fig. 16. The

thermal performance factor for all twisted and wire coil inserts are greater than unity. It

means that using both of the heat transfer enhancement techniques studied in this

investigation is a good choice in practical application. Also, WC-2 shows the best thermal

performance among other twisted tape and wire coil tubes. For example, at the highest

Reynolds number the thermal performance factor for nanofluids with TT-2 is 1.24-times and

19

for WC-2 is 1.36 respectively. So, under same particle loading and flow rates, the thermal

performance factor for wire coil inserts are more compared to twisted tape inserts.

6. Conclusions

The present work focuses on the estimation of heat transfer and friction factor CuO/water

nanofluid flowing in a plain tube with twisted tape and wire coil inserts. With the use of

nanoparticles in base fluid, the heat transfer coefficient is increases. A maximum of 17.62%

enhancement is obtained at 0.3% nanofluid at a Reynolds number of 20000. The heat transfer

coefficient is further enhances for nanofluid in a tube with twisted tape insert. The

enhancement is also depends on the twisted tape twist ratio. Higher heat transfer rates are

obtained with decrease in twist ratio. It is noticed that, with the use of TT-2, a maximum of

31.88% enhancement at 0.3% nanofluid in Reynolds number of 20000. The heat transfer

coefficient is also further enhances for nanofluid in a tube with wire coil inserts. A maximum

of 44.45% heat transfer enhancement for 0.3% nanofluid in a tube with WC-2 at a Reynolds

number of 20000 is observed. Nanofluid (0.3%) in a tube with twisted tape and wire coil

inserts causes higher friction factors in the order of 1.17-times and 1.19-times compared to

water flowing in a tube at Re = 20000. Compared to heat transfer enhancement, the

enhancement in friction factor is negligible. The thermal performance factor for nanofluids

with wire coil inserts is more effective than twisted tape inserts under same particle loading

and flow rate.

20

Nomenclature

Area,

Specific heat,

Inner diameter of the tube,

Friction factor

Twist tape pitch,

Heat transfer coefficient,

Current,

Thermal conductivity,

Length of the tube,

Mass flow rate,

Nusselt number,

Power,

Prandtl number,

Heat flow,

Heat flux,

Reynolds number,

Temperature, o

Thickness of the tape,

Voltage,

Velocity,

Greek symbols

21

Uncertainty

Pressure drop

Volume concentration of nanoparticles, %

Dynamic viscosity,

Density,

Subscripts

Bulk temperature

Experimental

Inlet

Outlet

Regression

Wall temperature

Appendix

Uncertainties associated with various parameters such as Reynolds number, heat flux, heat

transfer coefficient, Nusselt number and friction factor is estimated based on the procedure of

Beckwith et al. [49]. Uncertainties of various instruments are shown in Table 7.

Reynolds number, (A1)

Heat flux,

(A2)

22

Heat transfer coefficient, (A3)

Nusselt number, (A4)

Friction factor, (A5)

Acknowledgment

The authors would like to acknowledge the Portuguese Foundation of Science and

Technology (FCT) and J N T University-Hyderabad for performing the research work and the

author (L.S.S.) would like to thank FCT for his Post-Doctoral research grant

(SFRH/BPD/79104/2011).

References

[1] A.S. Ahuja, Augmentation of heat transport in laminar flow of polystyrene

suspension: I. Experimental and results. Journal of Applied Physics 46 (1975) 3408-

3416.

[2] K.V. Liu, S.U.S. Choi, K.E. Kasza, Measurements of pressure drop and heat transfer

in turbulent pipe flows of particulate slurries. Report, Argonne National Laboratory

ANL-88-15 (1988) 1–91.

23

[3] S.U.S. Choi, Enhancing thermal conductivity of fluids with nanoparticles. In

Proceedings of the 1995 ASME International Mechanical Engineering Congress and

Exposition San Francisco, CA, USA, 1995.

[4] S.U.S. Choi, Z.G. Zhang, F.E. Lockwood, E.A. Grulke, Anomalous thermal

conductivity enhancement in nanotube suspensions, Applied Physics Letters 79

(2001) 2252-2254.

[5] S. Lee, S.U.S. Choi, S. Li, J.A. Eastman, Measuring thermal conductivity of fluids

containing oxide Nanoparticles, Journal of Heat Transfer 121 (1999) 280-289.

[6] H. Masuda, A. Ebata, K. Teramae, N. Hishinuma, Alteration of thermal conductivity

and viscosity of liquid by dispersing ultra-fine particles (dispersion of Al2O3, SiO2

and TiO2 ultra-fine particles), Netsu Bussei 4 (1993) 227-233.

[7] B.C. Pak, Y.I. Cho, Hydrodynamic and heat transfer study of dispersed fluids with

submicron metallic oxide particles, Experimental Heat Transfer. 11 (1998) 151-170.

[8] L.S. Sundar, M.T. Naik, K.V. Sharma, M.K. Singh, T.Ch. Siva Reddy, Experimental

investigation of forced convection heat transfer and friction factor in a tube with

Fe3O4 magnetic nanofluid, Experimental Thermal and Fluid Science 37 (2012) 65-71

[9] Y. Ding, H. Alias, D. Wen, R.A. Williams, Heat transfer of aqueous suspensions of

carbon nanotubes (CNT nanofluids), International Journal of Heat and Mass Transfer.

49 (2006) 240-250.

[10] V. Chandraprabu, G. Sankaranarayanan, S. Iniyan, S. Suresh, Performance of

CuO/water nanofluid as outer fluid in the tube in tube condensing unit of air

conditioner: Experimental Study, Journal of Nanofluids. 2 (2013) 213-220.

[11] L.G. Asirvatham, N. Vishal, S. K. Gangatharan, D.M. Lal, Experimental study on

forced convective heat transfer with low volume fraction of CuO/water nanofluid,

Energies. 2 (2009) 97-119.

24

[12] S. Suresh, M. Chandrasekar, S. Chandra Sekhar, Experimental studies on heat

transfer and friction factor characteristics of CuO/water nanofluid under turbulent

flow in a helically dimpled tube, Experimental Thermal and Fluid Science 35 (2011)

542-549.

[13] S.M. Hashemi, M.A. Akhavan-Behabadi An empirical study on heat transfer and

pressure drop characteristics of CuO–base oil nanofluid flow in a horizontal helically

coiled tube under constant heat flux, International Communications in Heat and Mass

Transfer 39 (2012) 144-151

[14] N. Kannadasan, K. Ramanathan, S. Suresh, Comparison of heat transfer and pressure

drop in horizontal and vertical helically coiled heat exchanger with CuO/water based

nanofluids Experimental Thermal and Fluid Science. 42 (2012) 64-70.

[15] S. Suresh, M. Chandrasekar, P. Selva kumar, Experimental studies on heat

transfer and friction factor characteristics of CuO/water nanofluid under laminar

flow in a helically dimpled tube, Heat and Mass Transfer. 48 (2012) 683-694.

[16] E. Smithberg, F. Landis, Friction and forced convective heat transfer characteristics

in tube with twisted-tape swirl generators. Journal of Heat Transfer 86 (1964) 39-49.

[17] R.F. Lopina, A.E. Bergles, Heat transfer and pressure drop in tape-generated swirl

flow of single phase water. Journal of Heat Transfer 91 (1969) 434-442.

[18] R.M. Manglik, A.E. Bergles, Heat transfer and pressure drop correlations for twisted-

tape inserts in isothermal tubes: part II–transition and turbulent flows, Journal of Heat

Transfer 115 (1993) 890-896.

[19] P.K. Sarma, T. Subramanyam, P.S. Kishore, V. Dharma Rao, Sadik Kaka, Laminar

convective heat transfer with twisted tape inserts in a tube, International Journal of

Thermal Science. 42 (2003) 821-828.

25

[20] P.K. Sarma, T. Subramanyam, P.S. Kishore, V. Dharma Rao, Sadik Kakac, A new

method to redict convective heat transfer in a tube with twisted tape inserts for

turbulent flow, International Journal of Thermal Science 41 (2002) 955-960.

[21] S.B. Uttarwar, M. Raja Rao, Augmentation of laminar flow heat transfer in tubes by

means of wire coil inserts, Journal of Heat Transfer. 107 (1985) 930-935.

[22] P.G. Vicente, A. Garcia, A. Viedma, Experimental study of mixed convection and

pressure drop in helically dimpled tubes for laminar and transition flow. International

Journal of Heat and Mass Transfer 45 (2002) 5091-5105.

[23] P.G. Vicente, A. Garcia, A. Viedma, Mixed convection heat transfer and isothermal

pressure drop in corrugated tubes for laminar and transition flow. International

Communications of Heat and Mass Transfer 31 (2004) 651-662.

[24] L. Wang, B. Sunden, Performance comparison of some tube inserts. International

Communications of Heat and Mass Transfer 29 (2002) 45-56.

[25] A. Garcia, J.P. Solano, P.G. Vicente, A. Viedm, Enhancement of laminar and

transitional flow heat transfer in tubes by means of wire coil inserts, International

Journal of Heat and Mass Transfer 50 (2007) 3176-3189.

[26] M.A. Akhavan-Behabadi, R. Kumar, M.R. Salimpour, R. Azimi, Pressure drop and

heat transfer augmentation due to coiled wire inserts during laminar flow of oil inside

a horizontal tube, International Journal of Thermal Sciences. 49 (2010) 373-379.

[27] L.S. Sundar, K.V. Sharma, Turbulent heat transfer and friction factor of Al2O3

nanofluid in circular tube with twisted tape inserts, International Journal Heat and

Mass Transfer. 53 (2010) 1409-1416.

[28] L.S. Sundar, K.V. Sharma, Heat transfer enhancements of low volume concentration

Al2O3 nanofluid and with longitudinal strip inserts in a circular tube, International

Journal of Heat and Mass Transfer. 53 (2010) 4280-4286.

26

[29] K. Wongcharee, S. Eiamsa-ard, Heat transfer enhancement by using CuO/water

nanofluid in corrugated tube equipped with twisted tape, International

Communications in Heat and Mass Transfer. 39 (2012) 251-257.

[30] K. Wongcharee, S. Eiamsa-ard, Enhancement of heat transfer using CuO/water

nanofluid and twisted tape with alternate axis, International Communications in Heat

and Mass Transfer. 38 (2011) 742-748.

[31] S. Eiamsa-ard, K. Wongcharee, Single-phase heat transfer of CuO/water nanofluids

in micro-fin tube equipped with dual twisted-tapes, International Communications in

Heat and Mass Transfer 39 (2012) 1453-1459

[32] S. Suresh, K.P. Venkitaraj, P. Selvakumar, M. Chandrasekar, A comparison of

thermal characteristics of Al2O3/water and CuO/water nanofluids in transition flow

through a straight circular duct fitted with helical screw tape inserts, Experimental

Thermal and Fluid Science. 39 (2012) 37-44.

[33] S. Suresh, K.P. Venkitaraj, P. Selvakumar, Comparative study on thermal

performance of helical screw tape inserts in laminar flow using Al2O3/water and

CuO/water nanofluids, Superlattices and Microstructures. 49 (2011) 608-622.

[34] M. Chandrasekar, S. Suresh, A. Chandra Bose, Experimental studies on heat transfer

and friction factor characteristics of Al2O3/water nanofluid in a circular pipe under

laminar flow with wire coil inserts, Experimental Thermal and Fluid Science. 34

(2010) 122-130.

[35] M.T. Naik, L.S. Sundar, Heat transfer and friction factor with water/propylene

glycol-based CuO nanofluid in circular tube with helical inserts under transition flow

regime, Heat Transfer Engineering. 35 (2014) 53-62.

[36] M. Saeedinia, M.A. Akhavan-Behabadi, M. Nasr, Experimental study on heat

transfer and pressure drop of nanofluid flow in a horizontal coiled wire inserted tube

27

under constant heat flux, Experimental Thermal and Fluid Science. 36 (2012) 158-

168.

[37] M. Kahani, S. Zeinali Heris, S.M. Mousavi Comparative study between metal oxide

nanopowders on thermal characteristics of nanofluid flow through helical coils,

Powder Technology 246 (2013) 82-92

[38] L.S. Sundar, M.K. Singh, Convective heat transfer and friction factor correlations of

nanofluid in a tube and with inserts: A review, Renewable and Sustainable Energy

Reviews. 20 (2013) 23-35.

[39] C.O. Popiel, J. Wojtkowiak, Simple formulas for thermophysical properties of liquid

water for heat transfer calculation (from 0°C to 150°C), Heat Transfer Engineering

19 (1998) 87.

[40] M.R.B. Romdhane, T. Chartier, S. Baklouti, A new processing aid for dry-pressing: a

copolymer actin gas dispersant and binder, Journal of the European Ceramic Society

27 (2007) 2687.

[41] D.A. Drew, S.L. Passman, Theory of Multi Component Fluids, first ed. Springer,

Germany, 1999.

[42] A. Einstein, Investigation on Theory of Brownian Motion, first ed. Dover

publications, USA, 1956.

[43] H.C. Brinkman, The viscosity of concentrated suspensions and solutions, Chemical

Physics 20 (1952) 571.

[44] J.C. Maxwell, A treatise on electricity and magnetism, 2nd Edition, Oxford University

Press, Cambridge, UK, 1904.

[45] V. Gnielinski, New equations for heat and mass transfer in turbulent pipe and channel

flow, International Chemical Engineering. 16 (1976) 359-368.

28

[46] R.H. Notter, M.W. Rouse, A solution to the Graetz problem – III. Fully developed

region heat transfer rates, Chemical Engineering Science 27 (1972) 2073–2093.

[47] H.Blasius, Grenzschichten in Flussigkeiten mit kleiner Reibung (German), Z. Math.

Phys., 56 (1908) 1-37.

[48] B.S. Petukhov, Heat transfer and friction in turbulent pipe flow with variable physical

properties, J. P. Hartnett and T. F.Irvine, (eds), Advances in Heat Transfer, Academic

Press, New York, (1970) 504-564.

[49] T.G. Beckwith, R.D. Marangoni, L.H. Lienhard, L.H., Mechanical measurements,

5th Edition, Addison–Wesley publishing company, New York, (1990) 45-112.

[50] L.S. Sundar, N.T.R. Kumar, M.T. Naik, K.V. Sharma, Effect of full length twisted

tape inserts on heat transfer and friction factor enhancement with Fe3O4 magnetic

nanofluid inside a plain tube: An experimental study, International Journal of Heat

and Mass Transfer 55 (2012) 2761-2768.

[51] H. Usui, Y. Sano, K. Iwashita, A. Isozaki, Enhancement of heat transfer by a

combination of internally grooved rough tube and twisted tape, International

Chemical Engineering 26 (1996) 97.

[52] A.A. Abbasian Arani, J. Amani, Experimental study on the effect of TiO2–water

nanofluid on heat transfer and pressure drop, Experimental Thermal and Fluid

Science 42 (2012) 107.

Figure captions

Fig. 1 (a) TEM image of CuO nanoparticles (b) sample preparation (c) bulk CuO nanofluid

preparation.

Fig. 2 Schematic representation of experimental setup.

Fig. 3 Images of inserts (a) twisted tape (b) wire coil.

29

Fig. 4 Experimental Nusselt number of water is compared with the data of Gnielinski [45]

and Notter-Rouse [46].

Fig. 5 Experimental Nusselt number of nanofluid with effect of Reynolds number and particle

concentration.

Fig. 6 Experimental Nusselt number of nanofluid in a tube together with twisted tape inserts.

Fig. 7 Experimental Nusselt number of nanofluid in a tube with TT-1 and TT-2

is in comparison with Sundar et al. [27] for Fe3O4 nanofluid in a tube with twisted

tape insert.

Fig. 8 Experimental Nusselt number of nanofluid in a tube together with wire coil inserts.

Fig. 9 Experimental friction factor of water compared with Blasius [47] and Petukov [48].

Fig. 10 Experimental friction factor of CuO nanofluid with effect of Reynolds number and

particle concentration.

Fig. 11 Experimental friction factor of nanofluid in a tube together with twisted tape inserts.

Fig. 12 Experimental friction factor of nanofluid in a tube together with wire coil inserts.

Fig. 13 Experimental Nusselt number is in compared with values from Eq. (16).

Fig. 14 Experimental friction factor is in compared with values from Eq. (17).

Fig. 15 Variation of thermal performance factor with Reynolds number for 0.1% and 0.3%

concentration of nanofluid flows inside twisted tape tubes.

Fig. 16 Variation of thermal performance factor with Reynolds number for 0.1% and 0.3%

concentration of nanofluid flows inside wire coil tubes.

Table captions

Table 1 Thermophysical properties of CuO nanoparticles and base fluid.

Table 2 Dimensions of twisted tape inserts.

Table 3 Dimensions of wire coil inserts.

30

Table 4 Various proposed correlations for the estimation of Nusselt number for nanofluid.

flowing in a tube with twisted and wire coil inserts.

Table 5 Various proposed correlations for the estimation of friction factor for nanofluid.

flowing in a tube with twisted and wire coil inserts.

Table 6 Percentage of Nusselt number and friction factor enhancements.

Table 7 Uncertainties of instruments and properties.

Table 1 Thermophysical properties of CuO nanoparticles and base fluid.

Particle/Base fluid Diameter

(nm)

Purity (%)

(kg/m3)

Surface area to

mass, (m2/g)

(J/kg K)

(W/m K)

CuO <50 nm 99 6310 29 525 17.65

Distilled water*

*All temperatures are in degrees Celsius [39].

Table 2 Dimensions of twisted tape inserts

Tube set (mm) (mm)

TT-0 (Plain tube) 14 Plain tube (Without inserts)

TT-1 14 140 10

31

TT-2 14 70 5

Table 3 Dimensions of wire coil inserts

Tube set (mm) (mm) (mm) (mm)

WC-0 (Plain tube) 14 -- -- Plain tube (Without insert)

WC-1 14 41.4 12.158 2 2.95 0.1428

WC-2 14 27.6 11.389 2 1.97 0.1428

32

Table 4 Various proposed correlations for the estimation of Nusselt number for nanofluid

flowing in a tube with twisted and wire coil inserts

Nanofluid/(

Insert) Expression Range Ref.

Al2O3-water (Twisted tape)

10000 < Re < 22000 0-0.5%, 4.50 < Pr < 5.5 0 < H/D < 83

Sundar and Sharma [27]

CuO-water (Twisted tape)

830 < Re < 1990 0.3% - 0.7%, H/D = 3

Wongcharee and Eiamsa-ard [29]

CuO-70:30% W/PG (Twisted tape)

1000 < Re < 10000 0-0.5%, 4.50 < Pr < 5.5 0 < H/D < 83

Naik et al. [38]

Fe3O4-water (Twisted tape)

3000 < Re < 22000 0-0.6%, 3.19 < Pr < 6.5 0 < H/D < 15

Sundar et al. [50]

TiO2-water Al2O3-water (Wire coil)

500 < Re < 4500 0.25% and 0.1% 5.89 < Pr < 8.95 115.3 < He<1311.4

Kahani et al. [37]

CuO-Base oil (Wire coil)

20 < Re < 120 0-0.3% p/d = 1.79, 2.14 and 2.50

Saeedinia et al. [36]

Al2O3-water (Wire coil)

Re < 2300 =0.1%, 2 < p/d < 3

Chandrasekar et al. [34]

33

Table 5 Various proposed correlations for the estimation of friction factor for nanofluid

flowing in a tube with twisted and wire coil inserts

Nanofluid/

(Insert)

Expression Range Ref.

Al2O3-water (Twisted tape)

10000 < Re < 22000 0-0.5%, 4.50 < Pr < 5.5 0 < H/D < 83

Sundar and Sharma [27]

CuO-water (Twisted tape)

830 < Re < 1990 0.3% - 0.7%, H/D = 3

Wongcharee and Eiamsa-ard [29]

CuO-70:30% W/PG (Twisted tape)

1000 < Re < 10000 0-0.5%, 4.50 < Pr < 5.5 0 < H/D < 83

Naik et al. [38]

Fe3O4-water (Twisted tape)

3000 < Re < 22000 0-0.6%, 3.19 < Pr < 6.5 0 < H/D < 15

Sundar et al. [50]

TiO2-water Al2O3-water (Wire coil)

500 < Re < 4500 0.25% and 0.1% 5.89 < Pr < 8.95 115.3 < He<1311.4

Kahani et al. [37]

CuO-Base oil (Wire coil)

20 < Re < 120 0-0.3% p/d = 1.79, 2.14 and 2.50

Saeedinia et al. [36]

Al2O3-water (Wire coil)

Re < 2300 =0.1%, 2 < p/d < 3

Chandrasekar et al. [34]

34

Table 6 Percentage of Nusselt number and friction factor enhancements

Nanofluid

Insert Nusselt number, Nu Friction factor, f

Re = 4000 Re = 20000 Re = 4000 Re = 20000

0.3%

TT-0

(Plain tube)

15.70% 17.62% 1.10-times 1.15-times

TT-1 23.85% 27.89% 1.13-times 1.08-times

TT-2 27.94% 31.88% 1.12-times 1.17-times

WC-1 28.99% 37.07% 1.20-times 1.11-times

WC-2 32.28% 44.45% 1.32-times 1.19-times

Table 7 Uncertainties of instruments and properties

Instrument name Instrument range

Measured variable

Least division in measuring instrument

Min. and Max. values measured in experiment

Uncertainty,

Thermocouple, oC 0-120oC Wall temperature,

0.1oC 45.66-72.96 0.13706

Thermocouple, oC 0-120oC Bulk temperature,

0.1oC 31.25-42.9 0.23310

Voltage, V 0-220 V Voltage, 0.1 V 0-220 0.04545

Current, I 0-20 I Current , 0.01 I 0-20 0.05

Resistance, R 0-53.3 R Resistance, 0.1 R 0-53.3 0.1876

U-tube manometer, cm

0-50 cm Height of the CCl4 1 mm 2.0-38.3 cm 0.003

Totalizer, liters 0-9999 liters.

Mass flow rate, kg/sec

1 liters 1-15 liters 0.00001

Properties Thermal conductivity, density, specific heat, viscosity 0.1

35

Fig. 1 (a) TEM image of CuO nanoparticles (b) sample preparation (c) bulk CuO nanofluid

preparation.

36

Fig. 2 Schematic representation of experimental setup.

37

Fig. 3 Images of inserts (a) twisted tape (b) wire coil.

38

Fig. 4 Experimental Nusselt number of water is compared with the data of Gnielinski [45]

and Notter-Rouse [46].

39

Fig. 5 Experimental Nusselt number of nanofluid with effect of Reynolds number and particle

concentration.

40

Fig. 6 Experimental Nusselt number of nanofluid in a tube together with twisted tape inserts.

41

Fig. 7 Experimental Nusselt number of nanofluid in a tube with TT-1 and TT-2

is in comparison with Sundar et al. [27] for Fe3O4 nanofluid in a tube with twisted

tape insert.

42

Fig. 8 Experimental Nusselt number of nanofluid in a tube together with wire coil inserts.

43

Fig. 9 Experimental friction factor of water compared with Blasius [47] and Petukov [48].

44

Fig. 10 Experimental friction factor of CuO nanofluid with effect of Reynolds number and

particle concentration.

45

Fig. 11 Experimental friction factor of nanofluid in a tube together with twisted tape inserts.

46

Fig. 12 Experimental friction factor of nanofluid in a tube together with wire coil inserts.

47

Fig. 13 Experimental Nusselt number is in compared with values from Eq. (16).

48

Fig. 14 Experimental friction factor is in compared with values from Eq. (17).

49

Fig. 15 Variation of thermal performance factor with Reynolds number for 0.1% and 0.3%

concentration of nanofluid flows inside twisted tape tubes.

50

Fig. 16 Variation of thermal performance factor with Reynolds number for 0.1% and 0.3%

concentration of nanofluid flows inside wire coil tubes.

51

Highlights

Heat transfer and friction factor characteristics of nanofluid flowing in a tube with

inserts are studied experimentally.

Increasing nanofluid volume concentration enhances the heat transfer and friction

factor.

Further heat transfer and friction factor enhancements for nanofluid in a tube with

twisted tape and wire coil inserts.

Maximum increase in Nusselt number of 45% for 0.3% nanofluid in a tube with wire

coil insert-2 under turbulent flow.

Two empirical correlations are developed to predict Nusselt number and friction

factor.