NANOFLUIDS

OVERVIEW OF PRESENTATION• Introduction• Conventional methods of heat transfer• Materials for nanoparticles and fluids• Preparation of nanofluids • Modeling of thermal conductivity• Possible microscopic mechanisms• Thermal conductivity• Convection• Boiling • Advantages of nanofluids• Disadvantages of nanofluids• Application and Further research• Conclusions• References

INTRODUCTION

• Need for efficient working• Miniaturization • Proper working• Low conductivity of conventional fluids

[water, ethylene glycol, mineral oil]• Limitation of solid-liquid suspensions

INTRODUCTION(CONTI…)

• Suspension of nanometer (10-9) sized particles

• Nanofluid technology

(i) Nanoscience

(ii) Nanotechnology

(iii) Thermal engineering • Coined by Choi• Less than 100nm• Low volume fraction

Bright field image of Cu nanoparticles (<10nm)dispersed in ethylene glycol

CONVENTIONAL METHODS OF HEAT TRANSFER

Disperse micrometer

or millimeter sized

particles in heat

transfer fluids.

Major problem• Settling down• Cause wearing• Large mass

J.A. Eastman,” Mechanisms of enhanced heat transfer in nanofluids”

CONTI..

Increasing surface area

/flow velocity

Advantages • High surface to volume ratio• Thermal effectiveness

Cannot be applied

to miniaturized products,

already been maximized

NANOPARTICLES AND BASE FLUIDS

Nanoparticles

• Aluminum oxide (Al2O3)

• Titanium dioxide (TiO2)

• Copper oxide (CuO)

Base fluids • Water• Oil• Ethylene glycol

U.S. Choi and J.A. Eastman, “Enhanced heat transfer using nanofluids” U.S. Patent #6,221,275

PREPARATION OF NANOFLUIDS

Inert gas condensation (2-step).

Schematic of IGC .

1-capacitance manometer

2-LN2 filling tube

3- LN2 exhaust

4-glass vacuum chamber

5-cold plate

6-evaporation boat

7-moisture trap

8-argon gas cylinder

9-disc shutter,

10-high vacuum valve, 11-vacuum pumps, 12-power supply

Muhammad Raffi, “Synthesis and characterization of nanoparticles”

CONTI…

Advantages of IGC• Wide variety of nanopowders• Commercialized

Disadvantages• Agglomeration • Poor dispersion

• Direct evaporation• Chemical vapor deposition• Chemical precipitation

} 1-step

THERMAL CONDUCTIVITY MODELS

1) Maxwell’s model(Classical model)

Applicable to• Homogenous• Isotropic composite material

with randomly dispersed non interacting

spherical particles having • uniform size• dilute solutions

Appropriate for predicting properties such as

electrical conductivity, dielectric constant and

magnetic permeability

CONTI..

The expressions for the ratio of effective

conductivity to fluid conductivity

• (ke / kf)=1+([3ϕ (α-1)]/[(α+2)-ϕ(α-1)])

• ϕ -volume fraction or concentration of the dispersed particles

• α -ratio of thermal conductivity of the particle to that of the fluid and

CONTI..

2) Hamilton and crosser model (H&C model)

• Applicable to non-spherical

The expressions for the ratio of effective

conductivity to fluid conductivity is

• (ke / kf)=[α+(n-1)(1+ ϕ(α-1))]/ [α+(n-1)- ϕ(α-1)]

• n -shape factor to account for differences in the shape of the particles

CONTI…

3) Jang and Choi model

Four modes of energy transport • Collision of the base fluid molecules• Thermal diffusion in particles inside the base

fluids• Collision between particles• Thermal interactions of particles with the base

fluid molecules

POSSIBLE MICROSCOPIC MECHANISMS

Keblinski et al.(2002)-four possible

mechanisms:

• Brownian motion

• Molecular-level layering of liquid

• Ballistic nature of heat conduction

• Nanoparticle clustering

COMPARISON OF DIFFERENT DIFFUSION TIME SCALES

For water based

Nanofluids containing

5nm particle size at 300K

specifications are as

follows:

kf= 0.613 W/m-K

cp =4.179 kJ/kg-K

µ = 0.798 x 10-3 kg/m-s

ρ = 995.7 kg/m3

M. Reza Azizian, Hikmet S. Aybar, Tuba Okutucu, “effect of nanoconvection on thermal conductivity of nanofluids”

THERMAL CONDUCTIVITY

Measurement method:

Hot-wire transition• Time 2-8 sec• No convection• Heat applied suddenly

Platinum wire(1.06*10-7Ωm)

k={q/[4π(T2-T1)]}*ln (t2/t1)

k-conductivity

T-temperature

T-timeS.K.Das and others, ”nanofluids science and technology”

THERMAL CONDUCTIVITY OF OXIDE NANOFLUIDS

1) Measurement method: THW2) Linear relation between thermal conductivity and volume fraction3) Enhancement in water-Cuo nearly equals ethylene glycol-Al2O3 system4) Ethylene glycol-Cuo (24nm)system indicates maximum enhancement5) Water- glycol-Al2O3 (38nm)system least increment

Enhanced thermal conductivity of oxide nanofluids (Lee et al., 1999)

CONTI…

Comparison of ethylene glycol-Al2O3 system using H-C theory

Comparison of ethylene glycol-CuO system using H-C theory

Size of the dispersed particles also effects conductivity

EFFECT OF pH AND SIZE ON CONDUCTIVITY

Thermal conductivity enhancement with pH, Xie et al.(2002b)

1) At isoelectric point(IEP): repulsive force between particles are zero-agglomeration2) Hydration forces increase at pH value away from pHIEP –mobility

CONTI…

Thermal conductivity enhancement with SSA for alumina particles, Xie et al.(2002b)

Nonmonotonic behavior:1) Conductivity increases with specific surface area (SSA) up to SSA=28 m2/g and then decreases 2) As SSA increases with i.e. at reduced diameter, surface area effect is dominating, hence conductivity increases3) Beyond the maximum, size effect is dominant, phonon scattering effect causes decrease in conductivity

PARTICLE AGGLOMERATION-TIME SCALE

Karthikeyan, Philip and Raj, ”Time dependence thermal conductivity of Cuo-water nanofluid”

1)Due to van der waals forces2)Agglomeration is time dependent3)Decrease in conductivity due to lower surface to volume ratio and concentration4)Viscosity also increases with concentration

Karthikeyan, Philip and Raj ,0.1% volume fraction of CuO particles in water after a) 20 minutes b) 60 minutes c) 70 minutes

CONDUCTIVITY OF METALLIC NANOFLUIDS

Effective conductivity of CuO-ethylene glycol nanofluid, Eastman et al. (2001)

Three samples of CuO-ethylene glycol were takenfirst two stabilizer not added1) Labeled old- kept for 2 months2) Labeled new- 2 days old3) 1% Thioglycol acid added 40%increase in conductivity of

acidic nanofluid for 3%Concentration.

Thioglycol acid improvesdispersion

Metallic nanofluids greaterconductivity compared to oxidenanofluids

CONVECTION IN NANOFLUIDS

Experimental investigations• Nanofluids at low volume fraction

behave like Newtonian fluids.• Viscosity, temperature, heat capacity,

flow velocity, pressure drop etc have a bearing effect on heat transfer coefficient ‘h’ .

Animation of shadowgraphshowing travelling waves moving across the top surface, in contrast to the fixed pattern seen in classic convection. This "oscillating" convection occurs if the nanoparticles are initially evenly dispersed throughout the fluid. If not, the convection is completely shut down

http://www.google.co.in/url?q=http://focus.aps.org/story/v23/st9&sa=X&ei=n_VETcz7JseIrAf4_KQM&ved=0CCoQuAIwAg&usg=AFQjCNG_ETkh3VJSP9DVAVdNFpvlSHX32w

CONTI…

1)Alumina and Titania particles with mean diameter 13nm and 27nm respectively 2)‘h’ increased 45% at 1.34% volume fraction and 75% at 2.78% for alumina particles in water3)Greater heat transfer by Alumina-water fluid than using Titania particles4)Pressure drop decreased significantly compared to increase in ‘h’

Pak and Cho, heat transfer coefficient vs Reynlods number

CONTI…

1)Entry length effect2)In laminar flow boundary layer is thin hence ‘h’ is higher3)Enhancement increases with concentration only at entrance

Measured local heat transfer coefficient for convection inside a tube, Wen and Ding (2004)

BOILING OF NANOFLUIDS

• Boiling is the process of changing liquid into vapor at a constant temperature known as saturation temperature at a given pressure

• Critical heat flux is the flux at which a small change in flux will lead to a larger in wall superheat

Boiling classified as:• Pool boiling –heat flux• Film boiling-mass flux

CONTI…• CHF is the maximum heat flux under which a boiling

surface stays in nucleation regime of boiling• Film boiling undesirable because portions of the

surface become covered with vapor• Increase in CHF is due to increase in Surface

roughening (more nucleation sites)• Nucleating small bubbles is desirable as it helps in

agitation of fluid.• Boiling heat transfer depends on heat of

vaporization, density of vapor and liquid, and surface tension and not on ‘k’

CHF IN POOL BOILING

Under lower volume fraction of vapor flow boiling issimilar to nucleate boiling and depends mostly on heat flux flux Results show a 50% to

200% rise in CHF over pool boiling of waterUpto 0.01 g/l CHF increases exponentiallyBeyond 0.02 g/l CHF remains constant, 300% more than water

Enhancement of CHF with particle concentration, You et al(2003)

ADVANTAGES OF NANOFLUIDS• Compared with suspended particles of millimeter-or-micrometer dimensions

which were used in base fluids to enhance heat transfer of such fluids, nanofluids exhibit higher thermal conductivities.

• Many types of particles such as metallic and non-metallic, can be added into fluids to form nanofluids.

• Suspended particles of the order of millimeters or even micrometers may cause some severe problems such abrasive action of the particles causes the clogging of flow channels, erosion of pipelines etc which are not that severe in case of nanofluids.

• Micro and millimeter sized particles tend to settle rapidly. But nanoparticles can remain suspended in base fluids for a longer time.

• The much larger relative surface area of nanoparticles compared to those of conventional particles improves heat transfer capabilities

DISADVANTAGES

• Processing cost• Agglomeration at higher pH value and also at

high temperatures because of the ability of the particle to overcome thermal energy barrier leading to an increase in van der waals forces and hence resulting in decrease of conductivity

• Use of surfactants for stability which results in lowering of conductivity due to the formation of a thermal boundary layer around the particles

APPLICATION AND FURTHER RESEARCH

• Cooling application• Biomedical• Tribology• Defense

• Production of nanofluids• Key energy transport mechanisms• Thermal conductivity models• Long term stability• Green nanofluids

CONCLUSION• Nanofluids containing small amounts of

nanoparticles have substantially higher thermal conductivity than those of base fluids. The thermal conductivity enhancement of nanofluids depends on the particle volume fraction, size, type of base fluid and nanoparticles, pH value of nanofluids.

• No clarity on the dominant mechanism responsible for drastic increase in nanofluids.

• Although nanoconvection time scale can be compared with that of heat diffusivities scale, it cannot be ascertained as the dominant mechanism because in nanofluids the concentration of particles is low

CONTI…

• Increase in fluid velocity increased ‘h’ but led to a rise to a larger Darcy’s friction factor

• Increase in heat transfer with concentration at entrance

• CHF increases exponentially upto 0.01g/l

REFERENCES

• Sarit k. Das, Stephan U.S. Choi, Wenhua Yu, T. Pradeep-2007, “NANOFLUIDS: SCIENCE AND TECHNOLOGY”, A john Wiley & sons, INC., Publication.

• S Kakac, B. Kosoy, D. Li, A.Pramuanjaroenkij-2010, “Microfluidics Based Microsystems: Fundamentals and Applications”, Springer Publication.

• Prof. M. Kostic, Nanofluids, ”Advanced Flow And Heat Transfer Fluids”.

• Visinee Trisaksria, Somchai Wongwises, “Critical review of heat transfer characteristics of nanofluids”.

• Emily Pfautsch, “ Forced convection of nanofluids over a flat plate”.• Muhammad Raffi, “Synthesis and characterization of metal

nanoparticles”.

THANK YOU