Annals of Nuclear Energy 69 (2014) 7–13

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Annals of Nuclear Energy

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

Experimental investigation of TiO2/Water nanofluid effectson heat transfer characteristics of a vertical annuluswith non-uniform heat flux in non-radiation environment

http://dx.doi.org/10.1016/j.anucene.2014.01.0330306-4549/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Address: Department of Engineering, University ofShahid Beheshti, Evin, GC, P.O.Box: 1983963113, Tehran, Iran. Tel.: +9809386714575.

E-mail address: yasser.abbassi@gmail.com (Y. Abbassi).

Yasser Abbassi a,⇑, Mansour Talebi b, Amir Saeed Shirani a, Jamshid Khorsandi b

a Department of Engineering, University of Shahid Beheshti, Iranb Nuclear Science & Technology Research Institute, Iran

a r t i c l e i n f o a b s t r a c t

Article history:Received 25 May 2013Received in revised form 3 January 2014Accepted 10 January 2014Available online 18 February 2014

Keywords:ExperimentTiO2/Water nanofluidVertical annuliCosine heat fluxNon-radiation environment

In this paper, an experimental study carried out to investigate the heat transfer performance of a 10 nmTiO2/Water nanofluid (deionized water) in a vertical annulus with non-uniform heat flux at its inner tube.The experimental apparatus is a vertical annulus which is designed to simulate flow over nuclear fuelrods in non-radiation environment. Electrically produced heat flux has cosine shape. The effects of nano-particles volume concentration (0.25%, 0.5%, 1% and 1.5%) and different flow rates on wall temperatureprofile, maximum wall temperature, local and averaged heat transfer coefficient and local and averagedNusselt number is studied by this experiment. Experiments were conducted in different Reynolds num-ber and low nanoparticles concentrations. It is observed that by increasing Reynolds number or nanopar-ticles volume fractions, inner wall temperature (cladding temperature) decreases and its profile shapeflattens. The heat transfer coefficient for nanofluid is found to be higher than that for pure water andit increases with increasing volume concentrations. Results also indicate that at very low volume concen-trations (less than 0.005) nanofluids has no major impact on heat transfer parameters. Effect of pressureand entrance temperature on heat transfer parameters is also considered. It is understood that heat trans-fer parameters are independent of pressure and entrance temperature in our experiment ranges.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Ordinary heat transfer fluids such as water, oil and EG arewidely used to prevent the overheating or to enhance heat transferrate of different systems used in microelectronics, industry, trans-portation, electronic, nuclear engineering and etc. But, the poorheat transfer properties of these coolants compared with those ofmost solids are the primary obstacle of high compactness andthe efficiency of the heat transfer systems. Therefore, manyresearchers have centralized their works on improvement of highperformance heat transfer fluids in last decades. In the early re-searches, suspension and dispersion of millimeter or micrometer-sized particles were employed. However, heat transfer fluidscontaining suspended particles of micro/millimeter sizes had ma-jor disadvantages like erosions of the components by abrasivereactions, clogging in small passages, sedimentation of particles

and increased pressure drop. Nanofluid is a new class of fluid forimproving both thermal conductivity and suspension stability inthe various industrial fields. It is consisting of uniformly dispersedand suspended nanometer-sized particles which were first pio-neered by Choi and Eastman (1996) in 1995. Compared with sus-pensions of micro/millimeter sized particles, the heat transferrate and stability of nanoparticles suspension is improved. Nanofl-uids have developed as a new class of heat transfer fluids and havegrown tremendously in the past few years. Researchers are beingchallenged to figure out many unanticipated thermophysical fea-tures of these fluids, to propose new models to explain their behav-ior. A plenary summary of the previous studies on the properties ofnanofluids is rendered recently by Duangthongsuk and Wongwises(2009).

Sajadi and Kazemi (2011) investigated turbulent heat transfercharacteristics of TiO2/Water nanofluid in a circular pipe for max-imum nanoparticles volume concentration of 0.25%. The resultsindicated that addition of small amounts of nanoparticles to thebase fluid considerably augmented heat transfer, while Nusseltnumbers are approximately the same for all nanoparticles volumeconcentration. Recently Abbasian Arani and Amani (2012, 2013)

Nomenclature

Cp specific heat (J/kg K)d diameter of particles (nm)dh hydraulic diameter (m)h heat transfer coefficient (W/m2 K)k thermal conductivity (W/m K)l test section length (m)Nu Nusselt numberP power (W)Pr Prandtl numberq00 heat flux (W/m2)_q generated heat (W)rin inner tube radius (mm)Re Reynolds numberT temperature

Greek symbolsa thermal diffusivity (m2/s)j Boltzmann constant (J/K)k mean free path (m)l viscosity (kg/s m)q density (kg/m3)/ nanoparticles concentration

Indicesbf basefluidnf nanofluidnp nanoparticles

8 Y. Abbassi et al. / Annals of Nuclear Energy 69 (2014) 7–13

presented an experimental study on heat transfer and pressuredrop of TiO2–Water in turbulent flow regime for 30 nm particlesize diameter. They carried out their experiment investigation forReynolds number range between 8000 and 51,000 and 0.002–0.02 volume concentrations. They concluded that by using thenanofluid at high Reynolds number (greater than 30,000) morepower compared to low Reynolds number needed to compensatethe pressure drop of nanofluid, while increments in the Nusseltnumber for all Reynolds numbers are approximately equal.

In addition to mentioned benefits of nanofluid, there are lots ofphenomena which make nanofluid a proper fluid for some specificapplications. These phenomena include critical heat flux enhance-ment, accelerated quenching, wettability enhancement and etc.(Ciloglu and Bolukbasi, 2011; Barber et al., 2011). These character-istics make nanofluid a potential coolant for nuclear reactors sys-tems such as core, ECCS and IVR coolant (Kim et al., 2007;Buongiorno, 2008). El-wakil shows (El-Wakil, 1971) that an annu-lar with cosine heat flux (for a constant heat transfer coefficient)has a wall temperature profile as Fig. 1. It is shown that wall tem-perature profile has a maximum above midpoint of rod.

This paper aims to consider effects of nanofluid coolant on heattransfer around simulated nuclear fuel rods (in non-radiation envi-ronment) in various situations. Hence vertical annulus with a co-sine heat flux (an electrical heater produces cosine heat flux) in apressurized loop is selected. The nanofluid used in this study isTiO2/Water with average particle sizes of 10 nm. The particle con-centrations used in the experiments were of 0.25 vol.%, 0.5 vol.%,1.0 vol.% and 1.5 vol.%. We focus on titanium dioxide as a nanopar-ticle that was not studied extensively in literature such asaluminum and copper. Also titanium dioxide has important char-acteristics as safe material for human and excellent chemical andphysical stability (Murshed et al., 2005; Anoop et al., 2009). Inaddition, in order to investigate flow rate effect on mentionedcharacteristics, experiments also conducted for Reynolds numberin range of 900–4500.

Fig. 1. Heat flux, clad temperature and coolant bulk temperature of typical verticalannuli (constant heat transfer coefficient).

2. Materials and methods

2.1. Experimental apparatus

The schematic of the experimental apparatus is shown in Fig. 2.This set up has two closed-loop systems. The nanofluid feedingsystem contains a collection tank, a pump and pressurizer as reser-voir tank. Primary loop contains pressurizer, heat exchanger,pump, preheater and test section. Pressurizer is attached to a100 bars nitrogen capsule and system pressure can be increased

up to 25 bars. Pressurizer (as it names induces) prepare and stabi-lize primary loop desired pressure. A water heat exchanger re-moves heat from the loop and delivers it to secondary loop. Avariable round pump circulates fluid in pipes. The most importantpart of system is test section which is vertical annuli. Inner andouter diameters of annuli are 33 mm and 55 mm respectivelyand its length is 1.44 m. both tubes are made of stainless steel.Heat generation mechanism is located in inner. Length of heatingsurface is 1 m. In this study, temperature of inner tube outer wallis important and is measured by 22 k-type thermocouples (11thermocouples in right side and 11 thermocouples in left side)which are installed with distance about 10 cm from each otheron test section inner tube wall. The secondary cycle containsequipment to remove generated heat in test section. All of the ther-mocouples and sensors have a precision of 0.1 C and were cali-brated before they are attached to the test section. The rotametercalibrated in different temperate of hot water.

2.2. Preparation of nanofluids

In the present study, deionized water was used as liquid med-ium. The desired volume concentrations used in this study are0.0025 (0.25%), 0.005 (0.5%), 0.01 (1.0%) and 0.015 (1.5%). TiO2

Nanoparticles with average diameters of 10 nm which are pro-vided by Us-Nano Inc. are suspended in deionized water. Purity

Fig. 2. Schematic diagram of experimental apparatus.

Y. Abbassi et al. / Annals of Nuclear Energy 69 (2014) 7–13 9

of nanoparticles is with +99%. An ultrasonic vibrator with magneticstirrer was used for approximately 4 h in order to break downagglomeration of the nanoparticles.

2.3. Data analysis

In order to obtain heat transfer parameters, nanofluid proper-ties such as density, specific heat, viscosity and thermal conductiv-ity should be measured or calculated by theoretical models.

– Density and specific heat

The effective density of the nanofluid is given by Wang andMujumdar (2007):

qnf ¼ ð1� /Þqbf þ ð/Þqnp ð1Þ

The heat capacitance is defined as (Wang and Mujumdar, 2007):

Cp;nf ¼ð1� /Þqbf Cp;bf þ ð/ÞqnpCp;np

qnf ¼ ð1� /Þqbf þ ð/Þqnpð2Þ

– Dynamic viscosity of nanofluidIn this study Corcione (2011) empirical correlation proposed for

nanofluids dynamic is used.

lnf

lbf¼ 1

1� 34:87ðdnp=dbf Þ�0:3/1:08ð3Þ

where dbf is the equivalent diameter of base fluid molecule, givenby:

dbf ¼ 0:16M

Npqbf 0

!1=3

ð4Þ

In which M is the molecular weight of base fluid, N is the Avo-gadro number, and qbf0 is the mass density of base fluid calculatedat temperature T = 293 K.

– Thermal conductivity

Corcione correlation (Corcione, 2011) was used for determina-tion of nanofluid effective thermal conductivity versus nanofluidtemperature, particles mean diameter, volume fraction of nano-fluid, particles Reynolds number and thermal conductivity of nano-particles and base fluid as follows:

knf

kbf¼ 1þ 4:4Re0:4

np Pr0:66bf

TTfr

� �10 knp

kbf

� �0:03

/0:66 ð5Þ

where Tfr is freezing point of basefluid (about 273.16 K). Reynolds isnanoparticle Reynolds number, defined as:

Renp ¼qbf uBdnp

lbf¼

2qbf jBT

pl2bf dnp

ð6Þ

where jB is Boltzmann’s constant (1.38066 � 10�23 J/K). This corre-lation is applicable for nanoparticles diameter between 10 nm and150 nm, volume concentration between 0.2% and 9% and nanofluidtemperature between 294 K and 324 K.

Water and nanoparticle properties is calculated according Abb-asian Arani and Amani (2013) correlations.

lnlbf

0:001792

� �¼ �1:24� 6:44

237:15T

� �þ 7:68

273:15T

� �2

ð7Þ

kbf ¼ �1:549404þ 0:01553952� T � 3:65967� 10�5T2

þ 2:9401� 10�8T3 ð8Þ

10 Y. Abbassi et al. / Annals of Nuclear Energy 69 (2014) 7–13

qbf ¼ �764:475639þ 19:25155� T � 0:07714568� T2

þ 1:364893� 10�4T3 � 9:339158� 10�8T4 ð9Þ

Cp;bf ¼ 198531:690492� 2894:853934� T þ 17:2363068

� T2 � 0:05126994� T3 þ 7:616133� 10�5T4

� 4:517821� 10�8T5 ð10Þ

All of the four above correlations are valid over 273 < T (K) < 373.These correlations are independent of pressure.

The thermal conductivity of TiO2 calculated by following corre-lation over 273 < T (K) < 350. Arani et al. (2012) obtained thiscorrelation by curve fitting on the presented data by Powell et al.(1966).

knp ¼ 100� ð0:1813� 4:768Þ � 10�4T þ 5:089� 10�7T2 ð11Þ

They also calculated a specific correlation for specific heat ofTiO2 by curve fitting on data of Smith et al. (2009). The molecularweight of TiO2 is 79.8988 g/mol.

Cp;np ¼ 58:4528þ 3:02195� T þ 3:02923� 10�3T2 ð12Þ

The density of Rutile TiO2 is 4250 kg/m3 (Palik, 1985).

2.4. Heat transfer analysis

In this study it is assumed that heat generation is completelycosine and has a maximum in center of test section.

q00ðyÞ ¼ P4rinl

cospyl

� �ð13Þ

q00ðyÞ ¼ P2

1þ sinpyl

� �� �ð14Þ

In this equation P stand for total power of test section heater, rin isinner tube radius and l = 1 m is length of heating area.

In order to calculate local and average heat transfer coefficienttemperature of wall and fluid bulk is needed. Wall temperatureis measured through experiment. In order to obtain better resultsaverage of measured temperature in left and right side of wall isselected as wall temperature.

Twall ¼Tright þ Tleft

2ð15Þ

and bulk temperature is calculated by following expression (Incrop-era and DeWitt, 1996).

Tbulk ¼ Tin þ_q

_mCpð16Þ

Hence, local heat transfer coefficient is obtained by Incroperaand DeWitt (1996):

hðyÞ ¼ q00

Twall � Tbulkð17Þ

Averaged heat transfer coefficient is derived by Incropera andDeWitt (1996):

�h ¼ 1l

Xn

i¼1

hðiÞ � DyðiÞ ð18Þ

where i is number of region around thermocouple by height ofDy = 10 cm.

Local and Nusselt number is calculated by below equations(Incropera and DeWitt, 1996):

Nu ¼ hDh

kð19Þ

where Dh = 0.044 m is hydraulic diameter and thermal conductivityis calculated by presented equations in previous section.

2.5. Uncertainty

The uncertainty of averaged heat transfer coefficient is definedas:

dð�hÞ�h

@ðPÞP

� �2

þ @ðDTÞDT

� �2

þ @ðDyÞDy

� �2" #

ð20Þ

The values of uncertainties calculated in different Reynoldsnumbers and nanoparticles volume fractions. The maximumuncertainties of averaged heat transfer coefficient for differentReynolds number is 4.9%. The maximum uncertainty of Reynoldsnumber is 1.8%.

3. Result and discussion

In this section measured data and calculated parameters wouldbe presented. In order to observe general trend of experimentaldata, cubic fitting of data is provided too. Parameters reported hearare: wall temperature profile, maximum wall temperature (PCT),local and averaged heat transfer coefficient and local and averagedNusselt number.

3.1. Wall temperature profile and maximum temperature

Experimental data directly present wall temperature. Depen-dence of wall temperature to Reynolds number and nanoparticlevolume fraction is illustrated in Figs. 3 and 4. Tough there is no ma-jor effect on wall temperature in low nanoparticle concentrations,in higher volume fractions nanoparticles effect wall temperature.By increasing nanoparticle concentrations wall temperature de-creases. The reason behind this behavior is higher thermal conduc-tivity of nanofluid in comparison to pure water. IncreasingReynolds number decreases wall temperature. Almost all of the

Fig. 3. Dependence of wall temperature to volume concentrations (Re = 3500).

Fig. 4. Dependence of wall temperature to Reynolds number (u = 1%).

Fig. 6. Local heat transfer coefficient.

Y. Abbassi et al. / Annals of Nuclear Energy 69 (2014) 7–13 11

profiles reach the peak in y/L = 0.6. Measured wall maximum tem-perature is illustrated in Fig. 5.

3.2. Heat transfer coefficients and Nusselt number

Calculated local and averaged heat transfer coefficients in thisexperiment are illustrated in Figs. 6 and 7. It is observed that using

Fig. 5. Measured maximum wall temperature.

low nanoparticles concentration has no major effect on local heattransfer coefficient.

Variation of Nusselt number with different particle concentra-tions at Re = 4200 is illustrated in Fig. 8. It is evident that TiO2

nanoparticles suspended in water enhance the Nusselt number, ex-cept for 0.25% and 0.5% volume fractions. Same results are obtainedby Rayatzadeh et al. (2013) for different geometry.

3.3. Effect of system pressure and entrance temperature dependency

In order to investigate dependency of experiment to pressure ofsystem, different pressures were applied to primary loop and wall

Fig. 7. Averaged heat transfer coefficient as a function of Reynolds and volumeconcentration.

Fig. 8. Local Nusselt number.

Fig. 9. Wall temperature dependency to pressure.

Fig. 10. Dependency of wall temperature to the entrance temperature for differentvolume concentrations.

12 Y. Abbassi et al. / Annals of Nuclear Energy 69 (2014) 7–13

temperatures were measured. Result is illustrated in Fig. 9. Resultsshow that in primary loop pressure range from 1 bar to 20 bar walltemperature and hence other parameters are independent ofpressure.

To understand how wall temperature profile behave when en-trance temperature differ, couple of experiments were conducted.Results of dependency of wall temperature profile to entrance tem-perature are shown in Fig. 10. It is understood that entrance tem-perature has no major effect on wall temperature profile.

3.4. Validation of results

El-wakil shows (El-Wakil, 1971) that an annular with cosineheat flux for a constant heat flux has a wall temperature flux sim-ilar to our results (Fig. 1). In order to validate result we refer tosome of the experimental data on TiO2 experiments. Rayatzadehet al. (2013) observe Nusselt number enhancement. It is evidentfrom our experiment that TiO2 nanoparticles suspended in waterenhance the Nusselt number, except for 0.25% and 0.5% volumefractions. Same results are obtained by Rayatzadeh et al. (2013)for different geometry. There is a similar experiment with Al2O3

nanoparticles which verify our results (Rashid and Talebi, 2012)which shows heat transfer and Nusselt number enhancement be-cause of addition of Al2O3 nanoparticles.

4. Conclusions

Nanofluid is a novel suspension with a wide scope of applica-tions. This study focuses on the effect of TiO2/Water nanofluid onheat transfer in a vertical annulus with non-uniform heat flux,and conducts related experimental research, leading to the follow-ing conclusions.

� For low volume fraction of TiO2 nanoparticles in base fluid (lessthan 1%), there are no major changes in wall temperature, max-imum wall temperature, heat transfer coefficient and Nusseltnumber. This enables engineer to benefit from other nanofluidcharacteristics (increased wettability and critical heat flux dueto low concentrations) without changing the design of system.� For higher volume fractions of TiO2 nanoparticle, wall tempera-

ture decreases and heat transfer and Nusselt number increase.� For a typical case, it is observed that increasing volume concen-

tration of nanoparticles from 1% to 1.5% in has same effect ofincreasing Reynolds number from 2100 to 4200.� Addition of TiO2 nanoparticles to 1.5% volume fraction, doubled

heat transfer coefficient rather than base fluid.

Y. Abbassi et al. / Annals of Nuclear Energy 69 (2014) 7–13 13

� The dispersion also flattens temperature profile and makes thetemperature gradient between fluid and wall steeper which isresulted in Nusselt number enhancement.

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