ZnO Ethylene Glycol

8/10/2019 ZnO Ethylene Glycol

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Heat transfer performance and transport properties of ZnOethylene

glycol and ZnOethylene glycolwater nanofluid coolants

K.S. Suganthi, V. Leela Vinodhan, K.S. Rajan

Centre for Nanotechnology & Advanced Biomaterials (CeNTAB), School of Chemical & Biotechnology, SASTRA University, Thanjavur 613401, India

h i g h l i g h t s

High thermal conductivity and low-viscous ZnOethylene glycol nanofluids prepared.ZnOethylene glycolwater nanofluids prepared by hierarchical method.

Liquid layering and Brownian motion contribute to thermal conductivity enhancement.

Improvement in nanofluid cooling performance inline with thermal conductivity rise.

a r t i c l e i n f o

Article history:

Received 20 May 2014

Received in revised form 14 August 2014

Accepted 5 September 2014

Available online 26 September 2014

Keywords:

ZnOethylene glycol

ZnOethylene glycolwaterTransient heat transfer

Nanofluid

Liquid layering

Heat transfer rate ratio

a b s t r a c t

Experiments were carried out on preparation and characterization of ZnOethylene glycol (EG) and ZnO

ethylene glycolwater nanofluids and analysis of their performance as coolants. Favorable interactions

between ZnO nanoparticles and ethylene glycol molecules ensured superior transport properties of

ZnOEG nanofluids. These interactions were utilized during formulation of ZnOEGwater nanofluids

with preservation of ethylene glycol molecules over ZnO nanoparticles surface rendering them with bet-

ter transport properties. ZnOEG nanofluids containing 4 vol.% nanoparticles showed thermal conductiv-

ity enhancement of 33.4% and viscosity reduction of 39.2% at 27 C. Similarly, 2 vol.% ZnOEGwater

nanofluids showed thermal conductivity enhancement of 17.26% and viscosity reduction of 17.34% at

27 C. Disturbance of hydrogen bonding network of ethylene glycol by ZnO nanoparticles resulted in

reduced dispersion viscosity. Empirical models were developed to predict the thermal conductivity

enhancement and viscosity reduction of the nanofluids apart from elucidating mechanisms for the same.

Transient heat transfer experiments showed that ZnOEG and ZnOEGwater nanofluids had better heat

absorption characteristics compared to their respective base fluids. The enhancements in heat transfer

were proportional to thermal conductivity enhancements, which showed that superior thermal conduc-

tivity of nanofluids could be harnessed for cooling applications.

2014 Elsevier Ltd. All rights reserved.

1. Introduction

Thermal management and energy storage systems are thrust

areas of research in fields such as automobile/industrial cooling,

renewable energy utilization as evident from the recent literature

[111]. Maintenance of ideal working temperature by removing

the heat dissipated is essential for proper functioning of high speed

engines, microprocessors, etc. This has become a great challenge

due to increasing thermal loads driven by technological advance-

ments. The strategies to improve efficiency of heat transfer sys-

tems include active modes (increasing coolant velocity, use of

coolants with higher thermal conductivity) and passive modes

(use of fins, channels with expansions and constrictions, higher

heat transfer area). Thermal conductivity of liquids (0.1


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It is challenging to achieve higher thermal conductivity and

lower viscosity simultaneously in nanofluids. Introduction of solid

nanoparticles in liquids will eventually increasethermal conductiv-

ity[4547]as well as viscosity and hence higher pumping power

[43,45]. However, we have recently demonstrated the preparationof metal oxide (MOx)propylene glycol (PG) nanofluids with lower

viscosity and higher thermal conductivity than pure propylene gly-

col [24,40,48]. Favorable interactions between metal oxide nano-

particles and propylene glycol molecules led to lower viscosity of

MOxPG nanofluids. This makes the MOxPG nanofluids as excel-

lent prospects for cooling applications. These research findings

motivated us to investigate transport properties of ethylene glycol

based nanofluids, since ethylene glycol (EG) is chemically similar to

propylene glycol having hydrogen bonding networks.

The properties of ethylene glycol based nanofluids have been

widely studied[39,4956]due to their use as coolants in automo-

biles. Sandethylene glycolwater dispersions prepared using stir-

red bead milling and ultrasonication showed thermal conductivity

enhancement of above 20% at a particle concentration of1.8 vol.%[50]. Single-walled CNT inclusions were dispersed in ethylene gly-

col using bile salt as dispersant and at 0.2 vol.% of nanotube load-

ing, thermal conductivity increased up to 14.8% [53]. Thermal

conductivity of Al2O3ethylene glycol nanofluids have been stud-

ied over a wide temperature range (298411 K) using a liquid

metal transient hot wire apparatus [49]. Viscosities of CuOEG

water mixture nanofluids have been studied over a temperature

range of 35 to 50 C [52]. The dispersions showed Newtonian

flow behavior over the temperature range investigated[52].

ZnOethylene glycol nanofluids containing 5 vol.% of nanoparti-

cles, prepared by 3-h ultrasonication yielded thermal conductivity

enhancement of 26.5%[51]. These nanofluids were non-Newtonian

(shear-thinning) at higher concentrations and Newtonian at lower

concentrations [51]. Kole and Dey [39] prepared ZnOEG nanofl-uids by extended ultrasonication and the optimum ultrasonication

time was found to be 60 h. Approximately 40% enhancement in

thermal conductivity of ZnOEG nanofluids was reported for parti-

cle volume concentration of 3.75 vol.% [39].

Understanding the mechanism of interaction between nanoma-

terials and base fluid molecules could lead to nanofluid formula-

tion methods, which result in well dispersed nanofluids. In our

earlier work [44], one such formulation method was reported

which resulted in nanofluids with improved transport properties.

Surfactant-free ZnOpropylene glycol nanofluids prepared using

ultrasonication had higher thermal conductivity [57] and lower

viscosity compared to those of propylene glycol [48]. Formation

of propylene glycol molecular layers over ZnO nanoparticles in

ZnOpropylene glycol nanofluids was found to be responsible forhigher thermal conductivity, while the disturbance to hydrogen

bonding network of propylene glycol contributed to lowering of

their viscosity in comparison to that of pure propylene glycol. Thus,

a coolant with higher thermal conductivity and lower viscosity was

prepared and the underlying mechanisms were understood

[48,57]. With these findings, a new method of formulation ofZnOPGwater nanofluids was developed in which, water was

commixed with ZnOPG dispersion instead of dispersing ZnO

nanoparticles in PGwater mixture using the conventional

method. This method aided in preservation of propylene glycol

molecular layers over ZnO nanoparticles and foreclosed the inter-

action of ZnO nanoparticles and water molecules. The proposed

method allowed preparation of ZnOPGwater nanofluids without

any surfactants and with better transport properties [44]. How-

ever, viscosity reduction of ethylene glycol based nanofluids has

not been evidenced in literature thus far.

In this work, ZnOethylene glycol (EG) nanofluids were

prepared using ultrasonication without using any surfactant.

ZnOEGwater nanofluids were prepared using the method similar

to that proposed in our earlier work[44]. Transport properties ofZnOEG and ZnOEGwater nanofluids were studied as a function

of temperature and nanoparticle volume concentration. Models

have been developed to predict the viscosity and thermal conduc-

tivity of ZnOEG nanofluids and ZnOEGwater nanofluids. These

nanofluids have been tested for their cooling performance under

transient conditions.

2. Materials and methods

2.1. Materials

Zinc nitrate hexahydrate, ammonium carbonate, ethylene

glycol were procured from Merck, India. All the chemicals were

used as procured without any purification.

2.2. Synthesis and characterization of ZnO nanoparticles

ZnO nanoparticles were synthesized using chemical precipita-

tion method using Zinc nitrate hexahydrate as precursor at room

temperature [58,59]. Ammonium carbonate has been used as

reducing agent in the synthesis procedure described in our earlier

work [59]. Morphology and crystallographic patterns of the

synthesized ZnO nanoparticles were examined using scanning

electron microscopy (JSM 6701F, JEOL, Japan) and powder X-ray

diffractometry (D8 Focus, Bruker, Germany).

2.3. Formulation of nanofluids

ZnOEG nanofluid of concentration 4 vol.% was prepared by dis-persing the synthesized ZnO nanoparticles in ethylene glycol by

Nomenclature

Symbol Meaninga, b, c coefficients in Eq.(14)cp specific heat (J/kg K)k thermal conductivity (W/mK)m mass (kg)

Q amount of heat transferred (W)T temperature (C)t time (s)Uf uncertainty associated with the measurement of param-

eter fUxj uncertainty in the measurement of variableXjx function of nanoparticle concentrationy function of nanoparticle concentration and temperature

A, B functions of nanoparticle concentrationN number of factors in Eq.(1)

Greek symbols/ nanoparticle volume concentrationl viscosity (mPa s)q density (kg/m3)

Subscriptsbf base fluidnf nanofluidr relative/ratio

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ultrasonication (Vibracell, Sonics, USA). Ultrasonication (130 W,

20 kHz) was carried out until ZnOEG dispersion attained maxi-

mum thermal conductivity and minimum viscosity. Viscosity and

thermal conductivity of ZnOEG dispersions were measured at reg-

ular intervals of time during ultrasonication. ZnOEG dispersions

of different volume fractions were prepared by diluting 4 vol.%

ZnOEG nanofluids with required volume of ethylene glycol.

A hierarchical method demonstrated in our earlier work [44]has been utilized for the preparation of 2 vol.% ZnOEGwater

nanofluids by mixing water with 4 vol.% of ZnOEG in equal vol-

umes. The prepared nanofluids thus had a base fluid composition

of 50 vol.% water and 50 vol.% ethylene glycol. Different concentra-

tions of ZnOEGwater nanofluids were prepared by further dilut-

ing 2 vol.% ZnOEGwater nanofluids with 5050 vol.% ethylene

glycolwater mixture.

2.4. Characterization of nanofluids

Transport properties like viscosity and thermal conductivity

were studied for the ZnOEG and ZnOEGwater nanofluids as a

function of temperature and nanoparticle volume fraction. Thermal

conductivity of nanofluids was measured using transient-hot wire

method (Decagon devices, USA). KS-1 probe has been used for mea-

surements. Temperature of the sample was maintained using a cir-

culating water bath (TC-502, Brookfield Engineering, USA).

Viscosities of nanofluids were measured using a rotational vis-

cometer (LVDV-II + Pro, Brookfield Engineering, USA). S18 and

S64 spindles were used for ZnOEG at higher (27140 C) and

lower temperatures (1020 C) respectively. S00 spindle was used

for ZnOEGwater nanofluids over the entire temperature range

investigated. During the viscosity measurements, temperature of

the ZnOEG nanofluids and ZnOEGwater nanofluids were main-

tained using a temperature controller (Thermosel, Brookfield

Engineering, USA) and constant temperature bath (TC-502, Brook-

field Engineering, USA). Hydrodynamic particle size distribution of

ZnOEG dispersions was measured using dynamic light scattering

technique (NanoZS, Malvern instruments, UK) as a function ofultrasonication time.

Thermal conductivity, viscosity and hydrodynamic size distribu-

tion measurements were repeated at least three times to ascertain

the repeatability and reproducibility of measurements. Uncertain-

ties in viscosity, thermal conductivity and average hydrodynamic

size have been shown in the graphs as standard deviations.

Coefficients of variation during thermal conductivity and viscosity

measurements for the standards provided by the respective manu-

facturer were 0.25% and 0.77% respectively. Maximum coefficient

of variation in thermal conductivity and viscosity measurement of

nanofluids and base fluids were 0.87% and 1.76% respectively.

Total uncertainty in the measurement of a parameter was

calculated taking into account of the random error only, as the sys-

tematic error was negligible compared to random error. Uncertain-ties in relative viscosity, thermal conductivity ratio and heat

transfer ratio were calculated using the following formula [60]

and have been expressed as standard deviation.

Uf XNj1

Uxj@f

@xj

2" #0:51

2.5. Heat transfer experiments

In order to compare the performance of nanofluids as coolants

in relation to their base fluids, transient heat transfer experiments

were carried out using nanofluids and base fluids. The experimen-

tal setup (Fig. 1) comprised a stainless steel sample reservoir,electrical heating coil connected to AC power supply and a

temperature sensor connected to data logger. In order to maintain

constant heat flux boundary condition, test section was heated bysupplying constant AC power through the heating filament wound

over the test section, which was subsequently insulated. In a typ-

ical experiment, the test section was filled with a constant volume

of test fluid. A constant voltage of 10 V was supplied. Temperature

increase of test fluid was measured as a function of time for about

1020 min. The test fluids studied were: 1.5 vol.% ZnOEG, 1 vol.%

ZnOEG, 0.25 vol.% ZnOEG, and EG and 2 vol.% ZnOEGwater,

1.5 vol.% ZnOEGwater, 0.5 vol.% ZnOEGwater and EGwater.

3. Results and discussion

ZnO nanoparticles used in this study had uniform spherical

morphology with a size range of 2540 nm (Fig. 2a). Highly crystal-

line nature of the ZnO nanoparticles synthesized through chemicalprecipitation method was evident from the X-ray diffraction pat-

tern (Fig. 2b), which are in accordance with those for hexagonal

wurtzite phase (JCPDS No. 89-1397).

As evident from the scanning electron micrograph, synthesized

ZnO nanoparticles prevail in slightly agglomerated state. Nanofl-

uids with high dispersion quality are known to have superior

transport properties [55,61,62]. The advantages of well dispersed

nanofluids to be used as coolants are (i) higher surface area for heat

transfer, (ii) lower viscosity and lesser pumping power, and (iii)

good colloidal stability due to the smaller size of aggregates[46].

Ultrasonication plays a critical role in the formulation of nano-

fluids by assisting in breakage of aggregates resulting in high qual-

ity dispersions. However, there exists an optimum ultrasonication

time in the preparation of nanofluids [24,40,48,61,62]. In case ofextending ultrasonication beyond the optimum ultrasonication

time, re-aggregation may occur leading to formation of larger

aggregates resulting in instability of the nanofluids [59]. Other

than the physical and chemical properties of the dispersed phase

of nanofluids, dispersion characteristics have significant influence

on thermal conductivity, viscosity and stability of nanofluids.

Hence, it is pertinent to determine optimum ultrasonication

energy (or) time required to prepare nanofluids with superior dis-

persion characteristics, evidenced in terms of increased thermal

conductivity and reduced viscosity.

Fig. 3a shows the influence of ultrasonication time on thermal

conductivity and viscosity of 4 vol.% ZnOEG nanofluid. Thermal

conductivity of ZnOEG nanofluid (4 vol.%) gradually increased

with increasing ultrasonication time and saturated at about 30 h.Also, viscosity of the dispersion decreased with increasing

Fig. 1. Schematic representation of the experimental setup used for heat transfer

experiments.

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sonication time.Fig. 3b shows the influence of ultrasonication time

on the average hydrodynamic diameter of the ZnOEG dispersion,

from which it is evident that the average hydrodynamic diameter

decreased with ultrasonication time.

As ultrasonication time increased, larger aggregates of ZnO

nanoparticles had been broken up into smaller aggregates

(Fig. 3b). Formation of smaller aggregates resulted in increase in

the fraction of heat conducting paths and decrease in the fraction

of continuous phase with relatively higher thermal resistance. In

other words, size reduction of aggregates or dispersion of nanopar-

ticles resulted in increased surface area which resulted in increase

in thermal conductivity. As ZnOEG dispersion was ultrasonicated,

larger aggregates were broken up progressively and hence the per-

sistent increase in thermal conductivity. At about

30 h, saturationin thermal conductivity increase was observed showing that nano-

particles/clusters were well dispersed.

Aggregate sizes in solidliquid dispersions have profound influ-

ence on their viscosity. Greater the ratio of aggregate size to pri-

mary particle size, higher is the resistance to fluid flow [59,63]

and hence higher viscosity. As ultrasonic processing progressed,

viscosity decreased (Fig. 3a) probably due to the reduction in the

size of the aggregates.

3.1. Viscosity of dispersions

3.1.1. Influence of nanoparticle concentration on viscosity

3.1.1.1. ZnOEG dispersions. Viscosities of ZnOEG nanofluids were

independent of shear rate over a range of 66238 s1

(Fig. 4a) forthe concentration range investigated (0.254 vol.%). This reveals

that dispersion of ZnO nanoparticles did not alter the flow behavior

significantly and nanofluids remained to be Newtonian.

Relative viscosity (lr= lnf/lb)ZnO nanoparticle volume con-centration relationship of ZnOEG nanofluids (Fig. 4b) shows that

relative viscosity of ZnOEG dispersions (averaged over shear rates

in the range 66238 s1) decreased with increasing nanoparticle

loading. Viscosities and relative viscosities of ZnOEG nanofluids

showed a contradictive behavior with those reported for other

nanofluid systems as well as ZnOEG nanofluids in literature. Gal-

lego et al.[64]reported increasing viscosity of ZnOEG nanofluids

with increasing nanoparticle concentration. However, those nano-

fluids were prepared by ultrasonication for a time period of 16 min

only, which was much lower compared to the ultrasonication time

used in this study. Yu et al.[51]dispersed ZnO nanoparticles of size1020 nm in ethylene glycol using ultrasonic processing for 3 h.

ZnOEG dispersions thus prepared had higher viscosities (100%

increase) than base fluid and showed shear thinning characteristics

at particle concentrations P3 vol.%. Moosavi et al. [65] observed

26% enhancement in viscosity of ZnOEG nanofluids for a very

low particle concentration of 0.6 vol.%, while using ammonium cit-

rate as dispersant. From the nanofluid viscosity-ZnO nanoparticle

concentration data of the above [51,64] and the shear-thinning

nature of nanofluids in the work of Mossavi et al. [65],it appears

that those nanoparticles exhibited high degree of aggregation.

ZnOEG nanofluids prepared by Kole and Dey[39]were ultrasoni-

cated for an optimum ultrasonication time of 60 h and viscosities

of ZnOEG nanofluids (63.5 vol.%) were very close to that of base

fluid. This was attributed to the well-dispersed nature of ZnOnanoparticles in ethylene glycol. Hence the disparity observed in

Fig. 2. (a) Scanning electron micrograph of synthesized ZnO nanoparticles and (b)

X-ray diffraction pattern of ZnO nanoparticles.

Fig. 3. Influence of ultrasonication time on (a) thermal conductivity and viscosity of

4 vol.% ZnOEG nanofluids at 27 C and (b) average hydrodynamic diameter of ZnO

nanoparticles in ZnOEG nanofluids.

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the viscosities of ZnOEG nanofluids might be due to the differ-

ences in nanofluid formulation method (ultrasonication time/use

of surfactant), morphological characteristics of ZnO nanopowder

used, etc. which ultimately resulted in different aggregate size

distributions.

Intermolecular as well as intramolecular hydrogen bonding per-

sists in ethylene glycol. Metal oxide nanoparticles are known to

have hydroxyl groups on their surface when dispersed in polar liq-

uids [66]. Hydroxyl groups engaged in intramolecular and intermo-

lecular hydrogen bonding between ethylene glycol molecules are

likely to form hydrogen bonds with hydroxyl groups on nanoparti-cles surface. Hence, the hydrogen bonding network of ethylene

glycol is reorganized.

At higher nanoparticle concentration, the number of ZnO nano-

particles interacting with ethylene glycol molecules was higher

and hence, perturbations to the hydrogen bonding network of eth-

ylene glycol were enhanced leading to decrease in viscosity with

increasing nanoparticle concentration.

In our recent studies of propylene glycol based nanofluids

[24,40,48,67], rheological behavior similar to that of ZnOEG

nanofluids of the current study was observed. CuOPG,

ZnOPG, Fe2O3PG, sandPG, Mn0.43Fe2.57O4PG nanofluids had

viscosities lesser than propylene glycol and perturbations in

hydrogen bonding network was identified as the rationale

behind the unusual rheological behavior of propylene glycol

based nanofluids.

3.1.1.2. ZnOEGwater nanofluids. The influence of nanoparticle

concentration on relative viscosity of ZnOEGwater nanofluids

(Fig. 5) shows decreased viscosities at higher particle concentra-

tion. It may be recalled that the ZnOEGwater nanofluids were

prepared by addition of water to ZnOethylene glycol dispersion,

which had lower viscosity than ethylene glycol due to disturbance

in hydrogen bonding network. By adding water to ZnOEG disper-

sion, viscosity reduction has been maintained by preserving ethyl-ene glycol molecular layers over ZnO nanoparticles and thus

avoiding the direct contact of ZnO nanoparticles and water mole-

cules. It is known that the direct contact of surfactant-free ZnO

nanoparticles with water molecules promote their aggregation

[44,59,68] and increase dispersion viscosity. Hence, through pre-

vention of direct contact between surfactant-free ZnO nanoparti-

cles and water and preservation of ZnO nanoparticlesethylene

glycol interactions, 17.34% reduction in viscosity has been obtained

for 2 vol.% ZnOEGwater nanofluid. The percentage reduction in

viscosity of 2 vol.% ZnOEGwater nanofluids (17%) is comparable

to that of 2 vol.% ZnOEG nanofluid (20%).

3.1.2. Influence of temperature on viscosity

3.1.2.1. ZnOEG nanofluids. As coolants, nanofluids will be sub-

jected to temperature variations. Hence, it becomes pertinent to

investigate rheological behavior of nanofluids under thermal loads.

Influence of temperature on viscosity of ZnOEG nanofluids has

been studied over a wide temperature range of 10140 C

(Fig. 6a and b). Viscosity of ZnOEG nanofluids decreased in an

asymptotic manner with increasing temperature similar to that

of ethylene glycol, the base fluid (Fig. 6a). Viscosity variation of

nanofluids and the base fluid with temperature could be fitted into

power law as follows

l ATB 2

Value of B signifies the temperature dependency of viscosity of

the fluids. B value decreased with increasing concentration with

pure ethylene glycol having the highest B value. This shows that

the addition of nanoparticles had reduced the temperature depen-

dency of ZnOEG nanofluids.

The decrease in viscosity of liquids with increasing tempera-

tures is due to decrease in the magnitude of intermolecular attrac-

tive forces such as hydrogen bonds [69,70]. With reduction in

intermolecular forces of attraction at higher temperatures, their

influence on viscosity is also reduced. Hence at higher tempera-

tures, the perturbation of hydrogen bonds does not proportionately

lead to reduction in nanofluid viscosity, while the contribution of

Fig. 4. (a) Influence of shear rate on viscosity of ZnOEG nanofluids of different

concentrations and ethylene glycol and (b) influence of nanoparticle volume

concentration on relative viscosity of ZnOEG nanofluids.

Fig. 5. Influence of nanoparticle volume concentration on relative viscosity of ZnOEGwater nanofluids.



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nanoparticles to increased viscous dissipation is temperature-inde-

pendent. Hence, viscosity of ZnOEG nanofluid shows lower tem-

perature dependence than pure ethylene glycol, with lowest

temperature dependence observed at the highest nanoparticle con-

centration (4 vol.%) investigated.

In order to further evaluate the effect of addition of nanoparti-

cles on nanofluid viscosity, the relative viscosity of nanofluids was

calculated as follows:

lr lnanofluid lbase fluid

AnfT

Bnf

AbfTBbf

3

Fig. 7 shows the relationship between relative viscosity andtemperature at different nanoparticle concentrations. FromFig. 7,

it can be observed that relative viscosity increases with increasing

temperature, which implies that decrease in viscosity of nanofluids

with temperature is lower than that of base fluid. Highest reduc-

tion in viscosity of ZnOEG nanofluids has been observed at the

lowest temperature investigated. For instance, reduction in viscos-

ity of 4 and 2 vol.% ZnOEG nanofluids at 10 C are 66% and 45%

respectively. At lower temperatures, movement of liquid mole-

cules is dominated by intermolecular forces, which have higher

influence over liquid viscosities at lower temperatures. At higher

temperatures, intermolecular forces between molecules begin to

diminish and the movement of liquid molecules is controlled by

their translational energy. Since the addition of ZnO nanoparticles

brings out reduction of dispersion viscosity by disturbing hydrogenbonding network, viscosity reduction is more pronounced at lower

temperatures at which liquid viscosities depend on intermolecular

forces than that at higher temperatures at which influence of inter-molecular forces on liquid viscosity is negligible.

Another striking observation from viscosity measurements is

that relative viscosities of ZnOEG nanofluids were lower than

unity up to temperature of 110 C. This shows that pumping power

required to circulate ZnOEG nanofluids will be lower than that

required for pure ethylene glycol up to 110 C. As long as the tem-

perature is below 110 C, relative viscosity of 4 vol.% nanofluid is

lower than that of 2 vol.% whereas at 140 C, the relative viscosity

of 4 vol.% is higher than that of 2 vol.%.

To get better understanding, variation of relative viscosity with

nanoparticle concentration at different temperatures has been

plotted in Fig. 8. The slope of the curve gradually changes from

negative to positive as the temperature is increased from 10 to

140 C.

3.1.2.2. Viscosity model. The abundance of suspensions and disper-

sions in practical applications puts forward the demand for devel-

opment of models to predict viscosity. Einstein [71] derived an

expression to determine viscosity of dispersions of spherical,

non-interacting particles in a dilute suspension. Mooney [72]pro-

posed a model for prediction of relative viscosity of concentrated

suspensions (/> 0.05) in which empirical constants were used. A

Fig. 6. Influence of temperature on viscosity of ZnOEG nanofluids and ethylene

glycol (a) 27140 C and (b) 1020 C.

Fig. 7. Influence of temperature on relative viscosity of ZnOEG nanofluids at

different concentrations.

Fig. 8. Influence of nanoparticle volume concentration on relative viscosities ofZnOEG nanofluids at different temperatures.

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semi-empirical equation for shear viscosity of suspensions was

proposed by Krieger and Dougherty[73]for a wide range of parti-

cle concentrations. Taking particle aggregation into account, Chen

et al. [74] proposed a modified form of KriegerDougherty equa-

tion that predicted viscosity enhancement of dispersions with

aggregation well. All these models were used to predict the

enhancement in viscosity of dispersions due to particle addition.

Since viscosity reduction has been observed for ZnOEG nanofl-uids, these models cannot be directly used for viscosity prediction.

A new empirical model is developed to predict the viscosity of

ZnOEG nanofluids over a concentration and temperature range

of 04 vol.% and 27140 C respectively. This model takes into

account increase in viscosity due to particle addition, decrease in

viscosity due to disturbance in intermolecular forces due to nano-

particle addition and the effect of temperature as follows:

lr x y 4

where x is increase in relative viscosity due to particle addition and

y is decrease in relative viscosity due to disturbance in intermolec-

ular forces. While x is a function of nanoparticle volume fraction,

y is a function of nanoparticle volume fraction and temperature.

To determine the expression for x that describes increase ofnanofluid viscosity with nanoparticle concentration, relative vis-

cosity of nanofluid at 140 C was correlated with nanoparticle con-

centration, as the influence of intermolecular forces of attraction

on viscosity at this temperature was minimal. Relative viscosity

(lr)-nanoparticle volume concentration (/) relationship at 140 Cwas found to be lr= 1 + 5.9765/. This expression can be used topredict the increase in viscosity due to particle addition.

Therefore, Eq.(4) becomes

lr 1 5:9765/ fT;/ 5

Taking into account of the fact that the relative viscosity-nano-

particle concentration relationship is linear at all temperatures

(Fig. 8), the y in Eq.(4)is expected to vary linearly with nanopar-

ticle concentration and non-linearly with temperature.Using the relative viscosity data over a temperature range of

27140 C, the following empirical correlation was obtained using

Minitab 16 with a regression coefficient of 0.9135.

lr 1 5:9765/ 518:979/T0:9014 6

The developed empirical model predicts the relative viscosity of

ZnOEG dispersions well, as evident from Fig. 9 that shows the

comparison between predicted and experimental relative

viscosities.

3.1.2.3. ZnOEGwater nanofluids. Fig. 10(a) shows the influence of

temperature on viscosity of ZnOEGwater nanofluids and EG

water (base fluid) over a temperature range of 1055 C. Viscosity

of ZnOEGwater nanofluids decreased with increasing tempera-ture exponentially and could be fitted to power law as ZnOEG

nanofluids (Eq.(2)).

B values (from Eq.(2)) of EGwater, 1 vol.% ZnOEGwater and

2 vol.% ZnOEGwater were found to be 0.7756, 0.7298 and 0.6937

respectively. Similar to ZnOEG nanofluids, B values decreased

with increasing nanoparticle concentration. Value of B in Eq. (2)

is an indication of dependency of viscosity of the fluids on temper-

ature. Viscosity of ethylene glycolwater mixture, the base fluid

showed higher temperature dependency compared to nanofluids,

which might be due to the higher magnitude of intermolecular

forces that existed in the base fluid.

Relative viscosities of ZnOEGwater nanofluids have been cal-

culated using Eq. (3) and have been plotted against temperature

(Fig. 10b). Relative viscosities were less than one over the entiretemperature range investigated (1055 C). Relative viscosity of

2 vol.% nanofluid was lower than that of 1 vol.% nanofluids at all

temperatures.

The relative viscosity may be related to temperature and

nanoparticle concentration using the form Eq. (5) with expres-

sion for f(T, /) specifically derived for ZnOEGwater system

as follows:

fT;/ 5T0:1802/0:5455 7

Fig. 9. Comparison between experimental and predicted relative viscosities of

ZnOEG nanofluids.

Fig. 10. (a) Influence of temperature on viscosity of ZnOEG nanofluids and

ethylene glycol and (b) influence of temperature on relative viscosity of ZnOEG

nanofluids at different concentrations.



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Eq.(5)for ZnOEGwater nanofluid becomes

lr 1 5:9765/ 5T0:1802

/0:5455 8

The developed empirical model predicts the relative viscosity of

ZnOEGwater dispersions well, as evident from Fig. 11that shows

the comparison between predicted and experimental relative

viscosities.

3.2. Thermal conductivity of dispersions

3.2.1. ZnOEG nanofluids

Fig. 12(a) shows the variation of thermal conductivity of ZnO

EG nanofluids with nanoparticle volume concentration at 27 C.

Linear variation of thermal conductivity with nanoparticle concen-

tration depicts the well dispersed nature of nanoparticles or

nanoclusters in ZnOEG nanofluids. About 33.4% increase in ther-

mal conductivity has been observed for 4 vol.% ZnOEG nanofluids.

Thermal conductivity ratios of ZnOEG nanofluids prepared in this

study had relatively higher thermal conductivity ratios than that of

other researchers [51,64,65] except that of Kole and Dey [39]

(Fig. 12b). The differences in thermal conductivity of ZnOEG

nanofluids can be attributed to the difference in morphology of

the nanomaterial used and ultrasonic processing. ZnOEG nanofl-

uids prepared by Kole and Dey[39]being sonicated for prolonged

time (>60 h) had smaller sizes of aggregates (


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Liquid layering of propylene glycol molecules over ZnO nano-

particles was found to influence thermal conductivity enhance-

ment in ZnOPG nanofluids and its unique temperature

dependence [57]. As propylene glycol and ethylene glycol have

similarity in chemical nature, the course of thermal conductivity

change with change in temperature can be explained by the liquid

layering of ethylene glycol molecules on ZnO nanoparticles

surface.Fig. 12(c) shows the influence of temperature on thermal con-

ductivity of ZnOEG nanofluids and ethylene glycol, the base fluid.

Highest thermal conductivity enhancement was achieved at the

lowest temperature investigated, 10 C(Fig. 12c). The trend of var-

iation of thermal conductivity of ZnOEG nanofluids with temper-

ature could be divided into two sections, (i) a temperature range

(1030 C), within which nanofluid thermal conductivity

decreased with temperature and (ii) a temperature range (30

60 C), within which nanofluid thermal conductivity was tempera-

ture-independent.

Ordered arrangement of liquid molecules over solid surfaces

have higher thermal conductivity compared to bulk liquid, since

the ordered liquid molecular structure mimics the atomic arrange-

ment in crystalline solids with high thermal conductivity. The

thickness of such ordered layers greatly influences thermal con-

ductivity enhancement of nanofluids and have been shown to

increase with decreasing temperature[57]. The thickness of ethyl-

ene glycol molecular layer increased with decreasing temperature

and the maximum thermal conductivity enhancement was

observed at the lowest temperature (10 C). It can also be observed

from Fig. 12c that the magnitude of the slope of the curve (10

30 C) increases with increasing concentration. Thermal conductiv-

ity increase at 10 C for 4 vol.%, 2 vol.% and 1 vol.% are 55%, 36% and

14% respectively. The amount of liquid layers contributing to ther-

mal conductivity enhancement will be higher at higher concentra-

tions owing to the presence of larger number of nanoparticles.

In our earlier work [57], a simplified form of Yu and Chois

model given below, was used to predict thermal conductivity

enhancement of ZnOPG nanofluids at lower temperatures.

krknfkbf

1 3/1 b3

11

In the above equation, / is the volume fraction, b is the ratio of

thickness of liquid layer to the radius of nanoparticle and knfand kbfare the thermal conductivity of nanofluid and thermal conductivity

of base fluid respectively. Since temperature dependency of ther-

mal conductivity of ZnOEG nanofluids followed the same trend

as ZnOPG nanofluids, simplified Yu and Chois model [57] was

used to predict thermal conductivity enhancement of ZnOEG

nanofluids.

By fitting the experimental thermal conductivity data of ZnO

EG nanofluids in Eq.(11), b values at 10, 20, 27, 30 C were calcu-lated to be 0.6598, 0.6175, 0.407, and 0.33233 respectively.

Decreasing b values with increasing temperature showed that

the thickness of liquid layers surrounding nanoparticles decreased

with increasing temperature. A comparison of experimental and

calculated thermal conductivity ratios (using Eq. (11)) is made in

Fig. 13, from which it is evident that Eq. (11)with temperature-

dependent b values predict the thermal conductivity ratio well

for the temperature range of 1030 C.

Other than liquid layering, Brownian motion of nanoparticles

and particle clustering are two important mechanisms responsible

for thermal conductivity enhancement in nanofluids[75]. Increase

in thermal conductivity with increasing temperature is attributed

to the Brownian motion of particles due to their higher energy at

higher temperatures and micro-convection induced by Brownianmotion of particles[18,7678]. No significant difference in thermal

conductivity was observed over the temperature range of

3050 C. Since viscosity of the dispersion is relatively low in thistemperature range, an increase in thermal conductivity due to

Brownian motion of particles is expected. However, no such

increase in thermal conductivity was observed. Hence, we postu-

late that the increase in thermal conductivity due to Brownian

motion of particles might have been offset by the decrease in ther-

mal conductivity due to decrease in thickness of liquid layers in

accordance with decreasing b at higher temperatures.

3.2.2. ZnOEGwater nanofluids

Fig. 14(a) shows the influence of nanoparticle concentration on

thermal conductivity ratio of ZnOEGwater nanofluids. The ther-

mal conductivity was found to increase with nanoparticle concen-tration in a linear fashion, which indicates absence of significant

particleparticle interactions. The solvation layers of ethylene gly-

col on ZnO nanoparticles surface has been preserved in ZnOEG

water nanofluids also, as evidenced by negligible particleparticle

interaction and colloidal stability. If water molecules are in contact

with surfactant-free ZnO nanoparticles, agglomeration of nanopar-

ticles, instability of the dispersion and deterioration of transport

properties become inevitable. However, they have been eliminated

by the use of simple hierarchical method of preparation of ZnO

EGwater nanofluids.

Thermal conductivity ratio-nanoparticle volume concentration

relationship of ZnOEGwater nanofluids at 27 C is as follows:

kr 1 8:195/ 12

From Eqs.(9) and (12), it is evident that the thermal conductiv-

ity enhancements of ZnOEG and ZnOEGwater nanofluids are

comparable. This might be yet another testimony to the preserva-

tion of layers of ethylene glycol over ZnO nanoparticles.

The influence of temperature on thermal conductivity of

ZnOEGwater nanofluids (2 vol.%) is shown inFig. 14b. Tempera-

ture-independent behavior of ZnOEGwater nanofluids can be

attributed to the combined influence of liquid layering and Brown-

ian motion. As temperature increases, thickness of liquid layers

decreases and hence thermal conductivity decreases. This is offset

by Brownian motion due to lower dispersion viscosity. Hence,

these two phenomena counterbalanced each other resulting in

temperature-independent thermal conductivity of ZnOEGwaternanofluids.

Fig. 13. Comparison of experimental and predicted thermal conductivity ratios of

ZnOEG nanofluids.



10/12

3.3. Transient heat transfer experiments

3.3.1. Data reduction

The heat transfer performance of nanofluids was analyzed

through evaluation of amount of heat transferred (Q) over a fixed

length of time as follows:

Q mcpdT

dt Vqcp

dT

dt 13

In Eq.(13),Vis the volume of the test fluid. The product of den-

sity and specific heat (qCp) was calculated as follows[79,80]:

qcpnanofluid /qcpnanoparticle 1 /qcpbase fluid 14

The test fluid temperaturetime data were fitted using a poly-

nomial equation for the entire duration of the experiment as

follows:

T at2 bt c 15

The coefficients a and b and the intercept c for each of the

test fluids are shown in Table 1. Starting from a constant initial

temperature, amount of heat absorbed (Q) was calculated for a per-

iod of 10 min using Eqs. (13) and (15).

3.3.2. Heat transfer rate ratio (Qratio)

To compare the cooling capability of nanofluids with respect tothat of base fluids, the ratio of heat removed by nanofluid ( Qnf) to

the heat removed by base fluid (Qbf) has been calculated as Qratioas follows:

QratioQnfQbf

16

For both ZnOEG and ZnOEGwater nanofluids, Qratio were

found to be greater than one at all concentrations (Fig. 15). The bet-

ter thermal performance of both ZnOEG and ZnOEGwater

nanofluids might be attributed to enhanced thermal conductivity

of the nanofluids. The better performance of nanofluids in compar-

ison with base fluid indicates that they function as better coolants

in comparison to the base fluids at all nanoparticle concentrations

investigated. The Qratio was found to increase linearly with nano-particle concentration for both the nanofluids. It may be recalled

that the thermal conductivity of both these nanofluids increase lin-

early with nanoparticle concentration as discussed in Section 3.2.

Hence, it is evident that there is a direct correlation between ther-

mal conductivity enhancement of nanofluids and enhancement in

heat transfer rate with nanofluid coolants.

3.4. Stability of nanofluids

ZnOEG and ZnOEGwater nanofluids were colloidally stable

over a period of two months and no settling of particles was

Fig. 14. (a) Influence of nanoparticle volume concentration on thermal conductivity

of ZnOEGwater nanofluids at 27 C and (b) influence of temperature on thermalconductivity of 2 vol.% ZnOEGwater nanofluids.

Table 1

Coefficients a and b and the intercept c for each of the test fluid.

Nanoparticle volume concentration (%) ZnOEG ZnOEGwater

a b c a b c

0 8.383E06 0.0292 30.981 6.78E06 0.0286 31.803

0.25 8.594E06 0.0302 30.411

0.5 7.53E06 0.0313 30.373

1 9.829E06 0.0332 29.489

1.5 9.361E06 0.0339 31.096 6.84E06 0.0321 28.981

2 1.23E05 0.0354 32.685

Fig. 15. Influence of nanoparticle volume concentration on Qratio of ZnOEG and

ZnOEGwater nanofluids.

http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-


11/12

observed during characterization of transport properties and heat

transfer experiments. Taking the average hydrodynamic size as

the particle diameter, particle settling velocity and Brownian

velocity can be used to assess the stability of ZnOEG and ZnO

EGwater nanofluids [50]. Brownian velocities of particles in

ZnOEG and ZnOEGwater were 1.64 105 m/s and

7.109 105 m/s respectively. Particle settling velocities of ZnO

EG and ZnOEGwater were 1.607

10

9

m/s and7.048 109 m/s respectively. Brownian velocities of the particles

are higher than that of particle settling velocities which implies

that settling of particles is less probable ensuring the colloidal

stability.

4. Conclusions

Spherical ZnO nanoparticles with a size range of 2540 nm

were synthesized by room temperature chemical precipitation

and dispersed in ethylene glycol by probe ultrasonication without

any surfactant. ZnOEGwater nanofluids were prepared by hier-

archical formulation strategy, which preserved ethylene glycol

molecular layers over ZnO nanoparticles surface. The influence

of nanoparticle concentration and temperature on thermal conduc-tivity and viscosity of ZnOEG and ZnOEGwater nanofluids

revealed increased thermal conductivity and reduced viscosity.

ZnOEG and ZnOEGwater nanofluids showed thermal conduc-

tivity of 33.4% and 17.26% enhancements in thermal conductivity

and 39.2% and 17.34% reduction in viscosity at particle volume

concentrations of 4 and 2 vol.% respectively. Thickness of ethylene

glycol layers of ZnO nanoparticles increased at lower temperatures

resulting in higher thermal conductivity under such conditions.

Liquid layering of molecules contributed to thermal conductivity

enhancements in ZnOEG nanofluids, while liquid layering as well

as Brownian motion to temperature independent thermal conduc-

tivity of ZnOEGwater nanofluids. Empirical models were devel-

oped to predict the transport properties of the nanofluids. Our

results on transient heat transfer experiments using ZnOEG and

ZnOEGwater nanofluids demonstrate their superior cooling

capability in comparison with the respective base fluids due to

their increased thermal conductivities.

Acknowledgements

This work is supported by (i) INSPIRE fellowships (Reg. Nos.:

IF110312, IF130529) of Department of Science and Technology

(DST), India. (ii) PG teaching Grant No.: SR/NM/PG-16/2007 of

Nano Mission Council, Department of Science & Technology

(DST), India (iii) Grant No.: SR/FT/ET-061/2008, DST, India and

(iv) Research & Modernization Project #1, SASTRA University,

India. The authors thank SASTRA University for the infrastructural

support extended during the work.

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