Nanofluids for Improved Efficiency in Cooling Systems

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Nanofluids for Improved Efficiency in Cooling

Systemsrk spony Efficiency and Renewable E n

Abstract

The present scenario of high thermal loading coupled with high flux

levels demands exploration of new heat transfer augmentation

mechanisms. In this context, ‘Nanofluids’ may emerge as alternative heat

transfer fluids. The term ‘Nanofluids’ is used to indicate a special class of

heat transfer fluids that contain stabilized nanoparticles (≤50 nm) of

metallic/non metallic substances uniformly and stably suspended in an

engineering fluid. This paper deals with the property characterization, performance and

potential applications of nanofluids .

Keywords Nano size, Properties, Potential benefits, Applications

INTRODUCTION

Heat transfer technology stands at the cross roads of miniaturization on one hand and

astronomical increase in heat flux on the other. The usual enhancement techniques for heat

transfer can hardly meet the challenge of ever increasing demand of heat removal in

processes involving electronic chips, laser applications or similar high energy devices. The

factors which limit the usual techniques are many folded. One major limitation is the poor

thermal characteristics of usual heat transfer fluids. Metals in solid form have orders-of-

magnitude higher thermal conductivities than those of fluids. This inherent inadequacy of

these fluids makes the heat removal mechanism less effective even with the best utilization of

their flow properties. For example, the thermal conductivity of copper at room temperature is

about 700 times greater than that of water and about 3000 times greater than that of engine

oil, as shown in Fig. 1.

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0

500

1000

1500

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1 2 3 4 5 6 7 8

Th

erm

al c

on

du

cti

vit

y W

/mK

1. Engine oil (0.143)2. Ethylene glycol (0.253)3. Water (0.613)4. Alumina 5. Aluminium 6. cooper7. Silver 8.Carbon Nanotubes

Figure 1: THERMAL CONDUCTIVITY OF TYPICAL MATERIALS[1]

Therefore, the thermal conductivities of fluids that contain suspended solid metallic

particles could be expected to be significantly higher than those of conventional heat transfer

fluids. The idea of increasing thermal conductivity of fluids with conducting particles

suspended on them is not new. Ahuja [2] and Liu et al. [3] carried out the studies on

practical implication of hydrodynamics and heat transfer of slurries. The major problems

with such suspensions are the rapid settling of these particles, the abrasive action of the

particles and clogging in small flow passages Thus even though the slurries have higher

conductivities, they are hardly useable as heat transfer fluids. These problems can be

overcome by using nano sized particles. Nanofluids are new class of heat transfer fluids and

are engineered by suspending nanometer-sized particles like copper oxide, carbon nanotubes

etc. in conventional heat transfer fluids such as water, ethylene glycol, or engine oil. The

average size of particles used in nanofluids is below 50 nm. Modern nanotechnology provides

great opportunities to process and produce materials with average crystallite sizes below 50

nm.








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Nanofluids have single-phase heat transfer coefficients than their

base fluids. In particular, the heat transfer coefficient increases appear to

go beyond the mere thermal conductivity effect, and cannot be predicted

by traditional pure-fluid correlations such as Dittus-Boelter’s.

THERMO PHYSICAL PROPERTIES OF NANOFLUIDS

There have been models to evaluate the thermal conductivities of fluids suspended

with nano/micro sized particles. With the Hamilton and Crosser model applied to copper

nanoparticles in water, the effective thermal conductivity of the copper/water system was

estimated[1]. The effects of particle volume fraction and shape on the thermal conductivity

ratio for a copper-water system are plotted in Fig. 2

FIGURE 3: EFFECT OF PARTICLE VOLUME FRACTION AND SPHERICITY ON THERMAL CONDUCTIVITY RATIO FOR COPPER/WATER SYSTEM.[1]








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The results clearly show that the thermal conductivity of the fluid/particle system

depends on both particle volume fraction and the shape. Assuming that the sphericity

(defined as the ratio of the surface area of a sphere with a volume equal to that of the particle

to the surface area of the particle) of copper nanoparticles is 0.3, the thermal conductivity of

water can be enhanced by a factor of 1.5 at the low nanoparticle volume fraction of 5%. This

finding demonstrates theoretically the feasibility of nanofluids,i.e, metallic nanoparticles are

capable of significantly increasing the thermal conductivity of conventional heat transfer

fluids.

.

The thermal conductivities behavior of nanofluids with low particle concentrations(1-5

vol.%) was also studied experimentally[1]. Thermal conductivities of four oxide nanofluids

were measured. In particular, water and ethylene-glycol- based nanofluids, containing copper

oxide and aluminum oxide nanoparticles, were tested. The experimental results show that

these nanofluids have substantially higher thermal conductivities than the same liquids

without nanoparticles. For example, a 20% improvement in the thermal conductivity of

ethylene glycol was seen when 4% copper oxide was dispersed in this fluid (see Fig. 4).

Measurements show that less than 1.% copper nanoparticles in ethylene glycol improve the

effective thermal conductivity 40%.








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Figure 4: THERMAL CONDUCTIVITY OF WATER AND ETHYLENE GLYCOL IMPROVES WITH INCREASING VOLUMEFRACTION OF COPPER OXIDE OR ALUMINUM OXIDE NANOPARTICLES DISPERSED IN BASE FLUIDS.[1]








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FIGURE 5: THERMAL CONDUCTIVITY OF ENGINE OIL WITH CARBON NANOTUBES

Fig. 5 shows the increase in thermal conductivity with the addition of carbon

nanotubes to engine oil. It is observed that at a volume fraction of 1% the thermal

conductivity increases by 250%








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Figure 6: TEMPERATURE-DEPENDENT THERMAL CONDUCTIVITIES OF NANOFLUIDS .[4]

Fig.6 shows the dependence of thermal conductivity of water alumina system on

temperature. For 4% volume fraction the enhancement goes from 9.4% to 24.3% with

temperature rising from 21°C to 51°C. This temperature dependant property implies that

nanofluids are “smart” fluids sensing their thermal environment and adjusting their thermal

conductivity accordingly.

POTENTIAL BENEFITS OF NANOFLUIDS AS A COOLANT

There is now great industrial interest in nanofluids. Some of the specific potential

benefits of nanofluids are described below.

Improved Heat Transfer and Stability: Because heat transfer takes place at the surface of

the particle, it is desirable to use a particle with a large surface area. Nanoparticles provide

extremely high surface areas for heat transfer and therefore have great potential for use in

heat transfer. The much larger relative surface areas of nanophase powders, when compared

with those of conventional micrometer-sized powders, should markedly improve the heat

transfer capabilities and stability of the suspensions.








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Reduced Pumping Power: In heat exchangers that use conventional fluids, the heat transfer

coefficient can be increased only by significantly increasing the velocity of the fluid in the

heat transfer equipment. However, the requited pumping power increases significantly with

increasing velocity. For a nanofluid flowing in the same heat transfer equipment at a fixed

velocity, enhancement of heat transfer due to increased thermal conductivity can be

estimated. For example, to improve the heat transfer of a conventional fluid by a factor of 2,

pumping power must be increased by a factor of about 10. However, if a nanoparticle-based

fluid with a thermal conductivity =3 times that of a conventional fluid were used in the same

heat transfer equipment, the rate of heat transfer would be doubled .Therefore, the potential

savings in pumping power is significant with nanofluids.

Minimal Clogging: Nanophase metals are believed to be ideally suited for applications in

which fluids flow through small passages, because the metallic nanoparticles are small

enough that they are expected to behave like molecules of liquid. This will open up the

possibility of using nanoparticles even in microchanels for many envisioned high-heat-load

applications.

Miniaturized Systems: Nanofluid technology will support the current industrial trend toward

component and system miniaturization by enabling the design of smaller and lighter heat

exchanger systems. Miniaturized systems will reduce heat transfer fluid inventory.

Cost and Energy Savings: Successful employment of nanofluids will result in significant

energy and cost savings because heat exchange systems can be made smaller and lighter,

existing system for such fluids but also to develop a method for direct evaporation of

nanoparticles into high-vapor-pressure fluids such as water.

OTHER POTENTIAL APPLICATIONS

The application of nanofluids is not limited to cooling. Nanofluids have got wide

variety of applications in various fields such as biomedical engineering, nanomedicine etc...

For example in cancer therapy ferrofluids (a type of nanofluid) will be forced in to tumor

interior. Then the patient is subjected to a rapidly shifting magnetic field. Magnetic particles








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in the Ferro fluid quickly changes their orientation, causing friction and heat which will

destroy the tumors.

CONCLUSION

Downscaling, or miniaturization, has been the major trend in modern science and

technology. Stable suspensions of carbon nanotubes, oxide and metallic nanoparticles in

conventional heat transfer fluids can be achieved by maintaining the particle size below a

threshold level. Studies of nanofluids reveals high thermal conductivities and heat transfer

coefficients compared to those of conventional fluids. Nanofluids are one of the most discussed

and emerging research topic.

REFERECES

[1] Choi, U.S,(1998) , “ Nanofluid technology current status and future”, Second

Korean- American Scientists and Engineers Association Research Technologies

October 22- 24, Vienna, pp 2- 21

[2] Ahuja, A. S., (1975), ‘‘Augmentation of Heat Transport in LaminarFlow

of Polystyrene Suspension: Experiments and Results,’’ J. Appl. Phys., 46(8) 8,

pp 3408-3416

[3] Liu, K. V., Choi, U. S., and Kasza, K. E., (1988), ‘‘Measurement of Pressure

Drop and Heat Transfer in Turbulent Pipe Flows of Particulate Slurries,’’








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Argonne National Laboratory Report, ANL-88-15.

[4]S.K. Das, N. Putra, P. Theisen, W. Roetzel (2003) Temperature Dependence

of Thermal Conductivity Enhancement for Nanofluids, ASME J. Heat Transfer,

125, no. 4, pp 567-574






Nanofluids for Improved Efficiency in Cooling Systems

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