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Energy Procedia 52 (2014) 296 - 307

2013 International Conference on Alternative Energy in Developing Countries and

Emerging Economies

Turbulent forced convection heat transfer of nanofluids with twisted tape insert in a plain tube

W.H.Azmia*, K.V.Sharmab, Rizalman Mamata, and Shahrani Anuara

aFaculty of Mechanical Engineering, Universiti Malaysia Pahang, 26600 Pekan, Pahang, Malaysia bDepartment of Mechanical Engineering, University Technology Petronas, Bandar Seri Iskandar, 31 750 Tronoh, Perak, Malaysia

Abstract

Experimental determination of heat transfer coefficients of SiO2/water and TiO2/water nanofluid up to 3% volume concentration flowing in a circular tube is undertaken. The investigations are conducted in the Reynolds number range of 5000 to 25000 at a bulk temperature of 30oC. The experiments are undertaken for flow in a circular tube with twisted tapes of different twist ratios in the range of 5 < H/D < 93. The heat transfer enhancement is inversely increased with twist ratio. The heat transfer coefficient of SiO2/water nanofluid at 3.0% volume concentration is 27.9% higher than water flow for the same twist ratio of five. However, the value of heat transfer coefficient of TiO2/water nanofluid evaluated at the same concentration is 11.4% greater than water for twist ratio five. Regression equations for Nusselt number estimation are developed valid for water and nanofluid flow with twisted tape inserts under turbulent flow conditions.

© 2014 PublishedbyElsevier Ltd.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense (http://creativecommons.Org/licenses/by-nc-nd/3.0/).

Selection and peer-review under responsibility of the Organizing Committee of 2013 AEDCEE Keywords: forced convection; heat transfer coefficient; nanofluid; Nusselt number; twisted tape

* Corresponding author. Tel.: +6-012-947-8091; fax: +6-09-424-6222 E-mail address: wanazmi@ump.edu.my

1876-6102 © 2014 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/3.0/).

Selection and peer-review under responsibility of the Organizing Committee of 2013 AEDCEE doi:10.1016/j.egypro.2014.07.081

1. Introduction

The performance of thermal equipment with conventional fluids has reached their limit due to the miniaturization and compactness of electronic devices and their low thermal conductivities. The active and passive methods are taking place in order to enhance the ability of the conventional fluids to overcome the current heat dissipation problem.

Nomenclature

A area, m2

C specific heat, J/kg K

dp diameter of nanoparticle, nm

D tube inner diameter, m

H helical pitch of the twisted tape for 180' rotation, m

h heat transfer coefficient, W/m2 K

k thermal conductivity, W/m K

Nu Nusselt number

Q heat input, W

Pr Prandtl number

Re Reynolds number

T temperature, oC

V volume, L

a thermal diffusivity, m2/s

S thickness of strip, m

$ volume concentration, %

(p volume fraction, 0 = [p /100)

M absolute viscosity, kg/m s

« weight concentration, %

P density, kg/m3

b bulk

nf nanofluid

P particle

r ratio

w water

The active and passive methods of augmentation have been suggested by Ahuja [1] and Bergles [2]. Studies for the development of new thermal fluids for enhanced heat transfer capability have been initiated using an engineered fluid called nanofluid. The use of nanometer size particles for use as heat transfer fluid is initiated by a research group at the Argonne National Laboratory. Choi [3] coined the word 'nano fluids' who observed very high values of thermal conductivity compared to suspended particles of millimetre or micrometer dimension. The nanofluids showed better stability and rheological properties, dramatically higher thermal conductivities with no significant penalty on pressure drop. Most of the experiments in literature are concentrated on the theoretical prediction and measurement of thermal conductivity of the nanofluids. Recent interest in the use of nanofluids for possible heat transfer augmentation has drawn the attention of many investigators.

Estimation of turbulent forced convection heat transfer coefficients for flow in a tube has been made mostly through experimental investigations with Al2O3, Cu, CuO, SiC, TiO2, etc nanoparticles dispersed in water with volume concentration up to 3.7% [4-9]. The properties such as viscosity and thermal conductivity are estimated from experiments in the requisite range of the experimental analysis. Investigations are undertaken with particles of different materials, sizes, concentration and temperature range having a limitation for comparison of either properties or heat transfer coefficients.

Passive heat transfer augmentation using twisted tapes, longitudinal inserts, wire coil insert, etc for a wide range of Reynolds and Prandtl numbers have been reported by Bergles [10]. The twisted tape causes the flow to swirl, providing longer path length and residence time and thereby enhancing heat transfer. However, the pressure drop with insert is higher due to the resistance offered by the additional tape surface area when compared to flow in plain tubes.

The earliest study of heat transfer enhancement for nanofluids with twisted tape is presented by Sharma et al. [11] and Sundar and Sharma [12] in the low volume concentration. Experiments with nanofluid are conducted by them for the determination of heat transfer coefficients in a tube and with tape inserts using Al2O3 applicable in the transition and turbulent range of Reynolds number. Sharma et al. [11] found heat transfer enhancement of 37% at Reynolds number 3000 and 44.7% at Reynolds number 9000 compared to flow of nanofluid in a plain tube with twist ratio of five at 0.1% volume concentration. The heat transfer coefficient and friction factor of 0.5% volume concentration with twist ratio of five is 33.5% and 1.096 times respectively higher compared to flow of water in a tube presented by Sundar and Sharma [12].

Wongcharee and Eiamsa-ard [13] studied the heat transfer enhancement of using CuO/water nanofluid (concentration 0.3 - 0.7%) and twisted tape with alternate axis under laminar flow. The Nusselt number is slightly improved with the increase of concentration in their range of study. Again, Wongcharee and Eiamsa-ard [14] presented the study of CuO/water nanofluid in corrugated tube equipped with twisted tape under turbulent flow at concentration 0.3, 0.5 and 0.7 vol.%. They found that the convective heat transfer, friction factor of CuO/water nanofluid and twisted tape are higher than those associated with the individual techniques.

The earlier studies are limited to the determination of heat transfer coefficient of nanofluid with twisted tape in the low volume concentration. Therefore, the present objective is to determine the convective heat transfer coefficient of SiO2/water and TiO2/water nanofluids enhanced with twisted tape at high concentration up to 4% for a wide range of Reynolds numbers in the turbulent flow.

1.1. Preparation of nanofluids

Nanofluids procured from US Research Nanomaterials, Inc. are used in the present study. The water based nanofluid contained amorphous SiO2 nanoparticles with an average diameter of 30nm and original concentration of 25 wt% (13.1 vol.%). Anatase TiO2 nanoparticles of average diameter 50nm dispersed

in water is supplied at a weight concentration a = 40 wt.% (0 = 13.6 vol.%). The nanofluid available in weight concentration co is converted in terms of volumes 0 with Eq. (1) using the nanoparticle properties given in Table 1. The volume of distilled water to be added (AV) for attaining a desired concentration is estimated using Eq. (2) with the initial values of V1 and ^ known. A requisite quantity of 15 liters of nanofluid is prepared by dilution for the conduct of the experiment at different concentrations.

1 m I + m ~ löör p + 1ÖÖ Pw

AV = (v2 - V )= V,

ZL _ 1

Table 1. Physical properties of metal oxide nano materials

. Thermal Conductivity, Density, Specific heat,

Nanoparticle „,, „

p W/m.K kg/m3 J/ kg.K

SiO2 [15] 1.4 2220 745

TiO2 [7] 8.4 4175 692

Nanofluid of different concentrations is prepared in this manner. They are subjected to mechanical homogenization for about 2 hours and observed for dispersion stability. The samples are observed to be stable for more than a month. The pH of TiO2/water and SiO2/water diluted nanofluids varied between 7.2 - 7.7 and 8.9 -10.1, respectively in the range of concentration tested.

1.2. Thermo-physical properties of nanofluids

Applying the principle of mass conservation of the two species in a finite control volume of the nano fluid, the nanofluid density pnf can be obtained from the relation

Pnf =PPp +(l -p)pw (3)

The thermal conservation of energy of the two species in a finite control volume will yield the overall specific heat Cnf of the nanofluid as

C (1 -v)(pC )w+p{pC )p nf (l -9)Pw +VPp

The equations of Sharma et al. [16] are used in the present analysis for the estimation of viscosity and thermal conductivity for water based nanofluids. Equations (5) and (6) are applicable for spherical shape particles having diameters of 20 - 150nm, temperature of 20 - 70oC and volume concentration less than

* =|!+100

1 + ^ 170

k„ = 0.8938 I 1 +

1.37/ T > ' + if 70

( d ^ 1 +

1.3. Experimental setup

The experimental setup is integrated with a circulating pump, flow meter, heater, control panel, thermocouples, pressure transducer, chiller, collecting tank, and the test section. The heaters enclose a copper tube of 1.5m (ID=16mm and OD=19mm) which constitutes the test section. The total length of fluid flow in the tube is approximately 4.0m which ensures turbulent flow condition at the entry of the test section. The schematic diagram of the experimental set up is shown in Fig. 1.

Fig. 1. Schematic diagram of the experimental setup

A 0.5HP pump connected to a collecting tank of 0.03m capacity is used to circulate the working fluid through the test section. The outer diameter of the test section is wrapped with two nichrome heaters each of 1500W rating. The tube is enclosed with ceramic fiber insulation to minimize heat loss to the surroundings. Seven K-type thermocouples are fixed at different locations, five on the surface of the tube wall at 0.25, 0.5, 0.75, 1.0 and 1.25m from the inlet and the other two are located at the inlet and outlet to measure the temperature of the working fluid. A flow meter which works in the range of 5 to 16 LPM is connected between the pump and the inlet to test section. A chiller of 1.2 kW rating is located between the test section and the collecting tank.

A constant 600W power is supplied to the heater, while the chiller is adjusted to obtain a fluid bulk temperature of 30oC with a deviation of ±1oC. A pressure transducer connected across the test section

0.2777

-0.0336

records the pressure drop. A data logger records the surface and fluid temperatures every five seconds to determine the steady state nature of the experiment.

The twisted tapes are made using aluminum plate with 1 mm thick and 0.016 m width as shown in Fig. 2. The tape with twist ratio of 5, 10, 15 and 93 are prepared and the experiment with different twist ratio performed with them. The twist ratio of the tape is one of parameters considered in the present study for heat transfer enhancement. It is also assumed that heat conduction in the aluminum tape is negligible.

Copper

Fig. 2. Schematic diagram of the test section with twisted tape inside

2. Result and Discussion

2.1. TEM analysis

Transmission Electron Microscopy (TEM) is used to identify the rate of particle sedimentation and the relative stability of nanosuspension. TEM is also useful tool to distinguish the shape, size and distribution of nanoparticles. The aggregation of nanoparticles in water can be observed with TEM. The dispersion of nanoparticles, the size range and the shape of the nanoparticles also can be observed from the TEM photographs as shown in Figs. 3(a) and 3(b). The average size of TiO2/water and SiO2/water nanofluids is found 50nm and 22nm, respectively.

Fig. 3. TEM images of (a) TiO2 nanofluid with the scale of 200nm; (b) SiO2 nanofluid with the scale of 50nm

2.2. Properties evaluation

The thermo-physical properties of nanofluid such as thermal conductivity and viscosity at each concentration are determined experimentally using KD2 Pro thermal property analyzer and Brookfield LVDV-III Ultra Rheometer, respectively.

The viscosity of SiO2 nanofluid are in good agreement with the experimental data of other investigators [7, 8, 17, 18] as shown in Fig. 4(a). However, the viscosity of TiO2 nanofluid is higher by 6.0% than the values predicted with Eq. (5). This can be due to the non-spherical shape of the TiO2 particles determined from TEM analysis shown in Fig. 3(a). The thermal conductivity of TiO2 and SiO2 nanofluid obtained at different concentrations shown in Fig. 4(b) are in satisfactory agreement with the values estimated with the Eq. (6) and the data from literature [7, 8, 19].

Volume Concentration, Volume Concentration, <p(%)

Fig. 4. (a) Comparison of viscosity experimental values with Eq. (5); (b) Comparison of experimental values with Eq. (6)

2.3. Convective heat transfer coefficient

The heat transfer coefficient is evaluated with the energy balance equation, assuming no losses to the surroundings. The relevant equations used in the analysis are:

hexp =----(7)

exp As (Ts - Tb) ()

hexp D

NU exp (8)

The experiments are conducted with TiO2 and SiO2 nanofluids to determine the convective heat transfer coefficient and Nusselt number for volume concentrations up to 4% and twist ratio of 5, 10, 15 and 93. Distilled water is used as a testing fluid in order to estimate the reliability and accuracy of the measured data. Figure 5 shows the experimental Nusselt numbers of water plain tube and twisted tapes in comparison with values evaluated with Sarma [20] and Manglik and Bergles [21] equations. The Nusselt number increases with Reynolds number and decreases with twist ratio.

Fig. 5. Comparison of experimental values of twisted tape Nusselt numbers for water with other equations in literature

The Nusselt number of TiO2 and SiO2 nanofluids for a twist ratio of H/D=5 are presented in Figs. 6(a) and 6(b), respectively. It can be observed that the heat transfer enhancement of nanofluids with twisted tape is higher than the water flow for the same twist ratio. A comparison of Figs. 6(a) and 6(b) indicates higher values of heat transfer coefficients with twisted tape insert of SiO2 nanofluid compared to the values of TiO2 nanofluid at 3% concentration. The heat transfer coefficient of SiO2/water nanofluid at 3% concentration is 27.9% higher than water. However, the value of heat transfer coefficient of TiO2/water nanofluid evaluated at the same concentration is 11.4% greater than water for twist ratio five.

Fig. 6. Nusselt number of nanofluids at twist ratio of 5 (a) TiO2 nanofluid; (b) SiO2 nanofluid

The effect of twist ratio on the Nusselt number is shown in Figs. 7(a) and 7(b) for 3.0% volume concentration. It can be observed that the Nusselt number decreases with increase in twist ratio for both nanofluids.

^ 120-

H/D TiOt nanofluid

e 5 if) = 3.0 % Vol.

A 10 >»

* 15 e A

m 93 . - V « A * 9 A * ® , 9 i * A ® A ♦ « ®

A ® »

* ^ % f * ■ ■

■ ■ Water plain tube

10000 15000 20000

Reynolds number, Re

_ 160-

I 120^

H/D SiO, nanofluid

c» 5 <|> = 3.0 % Vol. « • A

15 9 A A

93 0 ** A ^ <* J

» ^ ^ * ■

%% ■

a ■ ■ ■ water plain tube

5000 10000 15000 20000

Reynolds number. Re

Fig. 7. Nusselt number of nanofluids at 3% volume concentration (a) TiO2 nanofluid; (b) SiO2 nanofluid

A generalized regression equation is developed for the estimation of Nusselt number of water and TiO2/water nanofluid in a tube with twisted tape insert. The equation is valid for TiO2 nanofluid up to 3.0% concentration. The equation is obtained with 321 data points with an average deviation (AD) of 4.6%, standard deviation (SD) of 5.7% and maximum deviation of 12.8% given by

Nunf = 0.27Re0

Similarly, the experimental data of SiO2 nanofluid twisted tapes is subjected to regression for estimation of Nusselt number using 323 data points. The equation is applicable for concentration up to 4.0% with AD of 4.1%, SD of 5.1% and maximum deviation of 15.3% given by

Nunf =

f = 0.073Rea702 Prnf4 (1 + D

knf nf V H

The Nusselt number estimated with Eqs. (9) and (10) are in a good agreement with the experimental data as shown in Figs. 8(a) and 8(b), respectively and thus validating the equation proposed. The index for twist ratio is same for the both equations. However, the index of Prandtl in the equation is giving a significant different for the both nanofluids. The Nusselt for SiO2 nanofluid is increasing with Prandtl number whereas TiO2 nanofluid is decreasing.

0 50 100 150 200 250 0 50 100 150 200 250

Nusselt from experiment Nusselt from experiment

Fig. 8. Validation of experimental data with proposed equation (a) TiO2 nanofluid with Eq. (9); (b) SiO2 nanofluid with Eq. (10)

The enhancement of heat transfer in percent for volume concentration of 1%, 2% and 3% with different twist ratio can be summarized in Table 2. The maximum enhancement is achieved at 3.0% concentration for SiO2 nanofluid. On the other hand, the peak enhancement for TiO2 nanofluid is observed at 1.0% concentration.

Table 2. Heat transfer enhancement of nanofluids with twisted tapes

No. Concentration, % Twist ratio TiO2 Enhancement, % SiO2 Enhancement, %

1 1.0 5 15.3 4.0

2 2.0 5 12.0 14.6

3 3.0 5 11.4 27.9

4 1.0 10 20.0 10.5

5 2.0 10 11.9 21.6

6 3.0 10 12.9 24.6

7 1.0 15 17.6 8.9

8 2.0 15 17.1 9.1

9 3.0 15 10.4 20.1

10 1.0 93 17.0 4.7

11 2.0 93 8.1 15.5

12 3.0 93 1.7 23.6

3. Conclusions

The following conclusions are made from the present experimental observations:

• The heat transfer coefficient of nanofluids with twisted tape is higher than the water flow for the same twist ratio

• SiO2/water and TiO2/water nanofluids are higher than water by 27.9% and 11.4%, respectively at 3.0% volume concentration and twist ratio of five.

• The maximum heat transfer enhancement with twisted tape for TiO2/water and SiO2/water nanofluids is found at 1.0% and 3.0% volume concentration, respectively.

• The forced convective heat transfer in the turbulent range for TiO2/water and SiO2/water nanofluids with twisted tape can be estimated from Eqs. (9) and (10), respectively.

Acknowledgements

The financial support by Universiti Malaysia Pahang (UMP) under GRS100354 and Automotive

Excellence Center (AEC) under RDU130391 are gratefully acknowledged.

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