NANOFLUIDS

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

Need for efficient working Miniaturization Proper working Low conductivity of conventional fluids [water, ethylene glycol, mineral oil] Limitation of solid-liquid suspensions

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 (Al 2 O 3 ) Titanium dioxide (TiO 2 ) 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-LN 2 filling tube 3- LN 2 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 (k e / k f )=1+([3 ϕ (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 (k e / k f )=[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: k f = W/m-K c p =4.179 kJ/kg-K µ = x kg/m-s ρ = kg/m 3 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 -7 Ωm) k={q/[4π(T 2 -T 1 )]}*ln (t 2 /t 1 ) k-conductivity T-temperature T-time S.K.Das and others, ”nanofluids science and technology”

THERMAL CONDUCTIVITY OF OXIDE NANOFLUIDS 1) Measurement method: THW 2) Linear relation between thermal conductivity and volume fraction 3) Enhancement in water-Cuo nearly equals ethylene glycol- Al 2 O 3 system 4) Ethylene glycol-Cuo (24nm)system indicates maximum enhancement 5) Water- glycol-Al 2 O 3 (38nm)system least increment Enhanced thermal conductivity of oxide nanofluids (Lee et al., 1999)

CONTI… Comparison of ethylene glycol- Al 2 O 3 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- agglomeration 2) Hydration forces increase at pH value away from pH IEP – 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 m 2 /g and then decreases 2) As SSA increases with i.e. at reduced diameter, surface area effect is dominating, hence conductivity increases 3) 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 forces 2)Agglomeration is time dependent 3)Decrease in conductivity due to lower surface to volume ratio and concentration 4)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 taken first two stabilizer not added 1)Labeled old- kept for 2 months 2)Labeled new- 2 days old 3)1% Thioglycol acid added  40%increase in conductivity of acidic nanofluid for 3% Concentration.  Thioglycol acid improves dispersion  Metallic nanofluids greater conductivity compared to oxide nanofluids

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 shadowgraph showing 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 IrAf4_KQM&ved=0CCoQuAIwAg&usg=AFQjCNG_ETkh3VJSP9DVAVdNFpvlSHX32 w

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 water 3)Greater heat transfer by Alumina-water fluid than using Titania particles 4)Pressure drop decreased significantly compared to increase in ‘h’ Pak and Cho, heat transfer coefficient vs Reynlods number

CONTI… 1)Entry length effect 2)In laminar flow boundary layer is thin hence ‘h’ is higher 3)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 is similar to nucleate boiling and depends mostly on heat flux flux Results show a 50% to 200% rise in CHF over pool boiling of water Upto 0.01 g/l CHF increases exponentially Beyond 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”.

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