Thermo physical Properties of NanofluidThermo physical Properties of Nanofluid(Density, Specific Heat and Thermal Conductivity)(Density, Specific Heat and Thermal Conductivity)

M.RezaM.Reza AzizianAzizianSupervisor: Prof. Dr. Hikmet Supervisor: Prof. Dr. Hikmet ŞŞ. Aybar. Aybar

CoSupervisorCoSupervisor: Asst : Asst profprof. Dr. Tuba . Dr. Tuba OkutucuOkutucu

Mechanical Engineering DepartmentMechanical Engineering DepartmentMay 21, 2008May 21, 2008

http://http://me.emu.edu.tr/mreza/current_research.htmme.emu.edu.tr/mreza/current_research.htm

IntroductionIntroduction

-- HHeat eat transfer in cooling processestransfer in cooling processes can be found in many industrial area.can be found in many industrial area.

-- The conventional methods for increase cooling rates:The conventional methods for increase cooling rates:11-- Extended surfaces such as finsExtended surfaces such as fins22-- Increasing flow ratesIncreasing flow rates

-- These conventional methods have their own limitations:These conventional methods have their own limitations:11-- fins: undesirable increase in the thermal management systemfins: undesirable increase in the thermal management system’’s sizes size22-- Increasing flow rates: increases pumping powerIncreasing flow rates: increases pumping power

-- There is a immediate need for new and innovative concepts to achThere is a immediate need for new and innovative concepts to achieve ultra ieve ultra high performance cooling.high performance cooling.

-- Nanofluids are promising to meet and enhance the challenges.Nanofluids are promising to meet and enhance the challenges.

-- The thermal conductivity characteristics of ordinary heat transfThe thermal conductivity characteristics of ordinary heat transfer fluids are er fluids are not adequate to meet our requirements.not adequate to meet our requirements.

-- The thermal conductivity characteristics of ordinary heat transfThe thermal conductivity characteristics of ordinary heat transfer fluids er fluids about two orders of magnitude less efficient in conducting heat about two orders of magnitude less efficient in conducting heat compare to compare to metals.metals.

-- Because a solid metal has a larger thermal conductivity than a bBecause a solid metal has a larger thermal conductivity than a base fluid, ase fluid, suspending metallic solid fine particle in to the base fluid is suspending metallic solid fine particle in to the base fluid is expected to expected to enhance the thermal conductivity of that fluid. enhance the thermal conductivity of that fluid.

-- Scattering solid particles (micrometer or even mm dimension)Scattering solid particles (micrometer or even mm dimension) into liquids into liquids to improve the physical properties of liquids is hardly new, it to improve the physical properties of liquids is hardly new, it has been well has been well known for 100 years, since the idea can be traced back to James known for 100 years, since the idea can be traced back to James Clerk Clerk MaxwellMaxwell’’s theoretical work (Maxwell, 1873).s theoretical work (Maxwell, 1873).

-- These micrometer or mm dimension solids in base fluid cause probThese micrometer or mm dimension solids in base fluid cause problems lems such as sedimentation, clogging, abrasion, and increase in presssuch as sedimentation, clogging, abrasion, and increase in pressure drop.ure drop.

NanofluidsNanofluids

-- Nanofluid has a potential to reduce such problems.Nanofluid has a potential to reduce such problems.

-- Nanofluids, a name conceived by Nanofluids, a name conceived by Dr.ChoiDr.Choi, in Argonne National , in Argonne National Laboratory, to describe a fluid consisting of solid nanoparticleLaboratory, to describe a fluid consisting of solid nanoparticles with size s with size less than 100nm suspended on it with solid volume fractions typiless than 100nm suspended on it with solid volume fractions typically less cally less than 4%. than 4%.

-- Base fluid: water, organic liquidBase fluid: water, organic liquid-- Nanoparticle size: 1Nanoparticle size: 1--100 nm100 nm-- Nanoparticle materials: oxides (e.g., AlNanoparticle materials: oxides (e.g., Al22OO33, ZrO, ZrO22, SiO, SiO22, , CuOCuO), metals (e.g., ), metals (e.g.,

Au, Cu), Carbon Au, Cu), Carbon nanotubesnanotubes

-- Nanofluid can be produced by two techniques: twoNanofluid can be produced by two techniques: two--step technique and the step technique and the singlesingle--step technique. step technique.

-- TwoTwo--step technique: The two step method starts with producing step technique: The two step method starts with producing nanoparticle by one of the physical or chemical processes (e.g.,nanoparticle by one of the physical or chemical processes (e.g., evaporation evaporation and inertand inert--gas condensation processing)gas condensation processing), and proceeds to disperse them into , and proceeds to disperse them into a base fluid; most of the nanofluids are produce by two step meta base fluid; most of the nanofluids are produce by two step method.hod.

-- SingleSingle--step technique: The single step simultaneously makes and dispersstep technique: The single step simultaneously makes and disperses es the nanoparticles directly into a base fluid; best for metallic the nanoparticles directly into a base fluid; best for metallic nanofluids.nanofluids.

-- Very recently an interesting scheme to synthesis of metal nanopaVery recently an interesting scheme to synthesis of metal nanoparticles in rticles in deionizeddeionized water, using multiwater, using multi--beam laser ablation in liquids, is reported by beam laser ablation in liquids, is reported by Argonne National Laboratory, where the nanoparticle size and disArgonne National Laboratory, where the nanoparticle size and distribution tribution will be control by the laser parameters.will be control by the laser parameters.

-- ZrOZrO22 in water that produce with two in water that produce with two -- Cu nanoparticles produced by Cu nanoparticles produced by Step method dirStep method direct evaporation into ethyleneect evaporation into ethylene

glycolglycol

Density and Specific heatDensity and Specific heat

-- Calculation of the effective density and effective specific heatCalculation of the effective density and effective specific heat of nanofluid of nanofluid is straightforward.is straightforward.

-- They can be estimate base on the physical principle of the mixtuThey can be estimate base on the physical principle of the mixture rule re rule these results are in very good agreement with some experimental these results are in very good agreement with some experimental data.data.

-- For this propose we define a nanofluid as a mixture consisting oFor this propose we define a nanofluid as a mixture consisting of a f a continues base fluid component called continues base fluid component called ““matrixmatrix”” and a discontinuous solid and a discontinuous solid component called component called ““particlesparticles””, the subscript , the subscript ““mm”” represent the base fluid represent the base fluid matrix and the subscript matrix and the subscript ““pp”” represent particles in base fluid.represent particles in base fluid.

(1 )m p m m p pe m p

e m p m p

m m V Vm f fV V V V V

ρ ρρ ρ ρ

+ +⎛ ⎞= = = = − +⎜ ⎟ + +⎝ ⎠

( ) ( ) ( ) ( ) ( )( ) ( )

m p p m p p p m m p p pp e e e ee

e m p m p m m p p

Q Q mC T mC T C V C VQCm T m m T m m T V V

ρ ρρ ρ ρ ρ ρ

ρ ρ+ Δ + Δ +⎛ ⎞= = = =⎜ ⎟Δ + Δ + Δ +⎝ ⎠

(1 )( ) ( )p m p pf C f Cρ ρ= − +(1 )( ) ( )

(1 )p m p p

pem p

f C f CC

f fρ ρ

ρ ρ− +

=− +

Thermal Conductivity (Experimental)Thermal Conductivity (Experimental)-- Before suggestion a theoretical model for thermal conductivity lBefore suggestion a theoretical model for thermal conductivity lets first ets first

look at the parameters that affect the thermal conductivity of nlook at the parameters that affect the thermal conductivity of nanofluid anofluid from experiments.from experiments.

-- According to the report of Argonne National Laboratory, eight paAccording to the report of Argonne National Laboratory, eight parameters rameters effect the thermal conductivity of nanofluids, they got these reeffect the thermal conductivity of nanofluids, they got these results from sults from about 124 researchers experiments, these effects are:about 124 researchers experiments, these effects are:

11-- Particle volume concentrationParticle volume concentration22-- Particle materialsParticle materials33-- Particle size Particle size 44-- Particle shapeParticle shape55-- Base fluid materialBase fluid material66-- TemperatureTemperature77-- AdditiveAdditive88-- AcidityAcidity

-- Effect of Particle Volume ConcentrationEffect of Particle Volume ConcentrationFrom the experimental results the general From the experimental results the general trend is clear: thermal conductivity trend is clear: thermal conductivity enhancement increases with increase enhancement increases with increase particle volume concentration.particle volume concentration.(Al(Al22OO33 in water)in water)

-- Effect of Particle MaterialEffect of Particle MaterialThe thermal conductivity ratio is seen to The thermal conductivity ratio is seen to increase faster for metal than oxide increase faster for metal than oxide particles.particles.(Particles in ethylene glycol)(Particles in ethylene glycol)

-- Effect of Particle SizeEffect of Particle SizeThe trend in this part is not obvious.The trend in this part is not obvious.most of the researchers report that the most of the researchers report that the larger particle diameters produce a large larger particle diameters produce a large enhancement in thermal conductivity but enhancement in thermal conductivity but in some cases the experiments show the in some cases the experiments show the different thing.different thing.A consistent trend appears where in the A consistent trend appears where in the larger particle diameters produce a large larger particle diameters produce a large enhancement in thermal conductivity.enhancement in thermal conductivity.(Al(Al22OO33 in water)in water)

-- Effect of Particle Shape Effect of Particle Shape All of the results indicate that elongated All of the results indicate that elongated particles are superior to spherical for particles are superior to spherical for thermal conductivity enhancement. thermal conductivity enhancement. ((SiCSiC in water)in water)

-- Effect of Base Fluid MaterialEffect of Base Fluid MaterialThe results show increased thermal The results show increased thermal conductivity enhancement for poorer (lower conductivity enhancement for poorer (lower thermal conductivity) heat transfer fluid. thermal conductivity) heat transfer fluid. The results show the least enhancement for The results show the least enhancement for water, which is the best heat transfer fluid water, which is the best heat transfer fluid with the highest thermal conductivity of the with the highest thermal conductivity of the fluids compared.fluids compared.(Al(Al22OO33 in fluids)in fluids)

-- Effect of TemperatureEffect of TemperatureThe trend of all experimental shows The trend of all experimental shows increased thermal conductivity enhancement increased thermal conductivity enhancement with increased temperature.with increased temperature.(Al(Al22OO33 in water)in water)

-- Effect of AdditivesEffect of AdditivesExperiments have used fluid additives in an Experiments have used fluid additives in an attempt to keep nanoparticles in suspension attempt to keep nanoparticles in suspension and to prevent them from agglomerating.and to prevent them from agglomerating.The thermal conductivity enhancement The thermal conductivity enhancement improved by using the additive.improved by using the additive.(Cu in ethylene glycol)(Cu in ethylene glycol)

-- Effect of Acidity (PH)Effect of Acidity (PH)Limited studies have been published on the Limited studies have been published on the effect of fluid acidity on the thermal effect of fluid acidity on the thermal conductivity enhancement of nanofluids.conductivity enhancement of nanofluids.But the general trend is that acidity But the general trend is that acidity increases the thermal conductivity increases the thermal conductivity enhancement.enhancement.(Al(Al22OO33 in water)in water)

Thermal Conductivity (Theoretical Modeling)Thermal Conductivity (Theoretical Modeling)

-- Maxwell was one of the first to analytically investigate conductMaxwell was one of the first to analytically investigate conduction through ion through suspended particles.suspended particles.

-- This equation and other equations for thermal conductivity e.g.,This equation and other equations for thermal conductivity e.g., Hamilton Hamilton and Crosser, and Crosser, RayleighRayleigh, and etc predict thermal conductivity reasonably , and etc predict thermal conductivity reasonably well for dilute mixtures of relatively large particles in fluidswell for dilute mixtures of relatively large particles in fluids..

-- When we go to the When we go to the nanoscalenanoscale we have to consider some effects that there we have to consider some effects that there are not exist in large scales, to improve the predictions we conare not exist in large scales, to improve the predictions we consider:sider:11-- Effect of nanoparticleEffect of nanoparticle--matrix interfacial layermatrix interfacial layer22-- Effect of nanoparticle Brownian motionEffect of nanoparticle Brownian motion33-- Effect of nanoparticle cluster/aggregate Effect of nanoparticle cluster/aggregate

32 ( )

p meff m m

m p p m

K KK K f K

K K f K K−

= ++ − −

Effect of Nanoparticle Interfacial layerEffect of Nanoparticle Interfacial layer-- Liquid molecules close to a solid Liquid molecules close to a solid surface are known to form layered surface are known to form layered structures.structures.

-- According to the work of Yu, et al. According to the work of Yu, et al. 2000 the layered molecules are in an 2000 the layered molecules are in an intermediate physical state between a intermediate physical state between a solid and bulk liquid.solid and bulk liquid.

-- With these solid like liquid layers, the With these solid like liquid layers, the nanofluid structure consists of solid nanofluid structure consists of solid nanoparticles, solidnanoparticles, solid--like liquid layer, like liquid layer, and a bulk liquid. and a bulk liquid.

-- that the solidthat the solid--like like nanolayernanolayer acts as a acts as a thermal bridge between a solid thermal bridge between a solid nanoparticle and a bulk liquid and so is nanoparticle and a bulk liquid and so is key to enhancing thermal conductivity. key to enhancing thermal conductivity.

-- In this figure you can see the thermal In this figure you can see the thermal conductivity enhancement by modified conductivity enhancement by modified Maxwell equation according to the Maxwell equation according to the interfacial layer assumption, for copperinterfacial layer assumption, for copper--particleparticle--inin--ethyleneethylene--glycol suspension.glycol suspension.

-- A three to eight fold increase in A three to eight fold increase in thermal conductivity of nanofluids thermal conductivity of nanofluids compared to the enhancement without compared to the enhancement without considering the considering the nanolayernanolayer occurs when occurs when nanoparticles are smaller than the nanoparticles are smaller than the critical radius 5nm.critical radius 5nm.

-- This finding suggests that adding This finding suggests that adding smaller (<10 nm diameter) particles smaller (<10 nm diameter) particles could be potentially better than adding could be potentially better than adding more largermore larger--size size nanonano--particles.particles.

--

Effect of Brownian MotionEffect of Brownian Motion-- Because of the size of nanoparticles the Because of the size of nanoparticles the Brownian motion of nanoparticles will be Brownian motion of nanoparticles will be another potential factor in calculating the another potential factor in calculating the thermal conductivity of nanofluids.thermal conductivity of nanofluids.

-- To develop such a theory for the To develop such a theory for the thermal conductivity of nanofluids they thermal conductivity of nanofluids they assumed that energy transport in assumed that energy transport in nanofluids involving four modes:nanofluids involving four modes:11-- Collision between base fluid Collision between base fluid molecules.molecules.22-- Thermal diffusion in nanoparticles. Thermal diffusion in nanoparticles. 33-- Collision between nanoparticles. Collision between nanoparticles. 44-- Thermal interaction of dynamic or Thermal interaction of dynamic or dancing nanoparticles with base fluid dancing nanoparticles with base fluid molecules.molecules.

-- The model is able to predict a particleThe model is able to predict a particle--size size and temperatureand temperature--dependent conductivity of dependent conductivity of nanofluids, while no existing theories nanofluids, while no existing theories explain or predict these effects.explain or predict these effects.

-- The present model predicts the The present model predicts the experiment data for temperature experiment data for temperature dependency with excellent agreement, dependency with excellent agreement, In In contrast, conventional theories with contrast, conventional theories with motionless nanoparticles fail to predict this motionless nanoparticles fail to predict this behaviour (horizontal dashed line).behaviour (horizontal dashed line).

-- The Maxwell model predicts decreasing The Maxwell model predicts decreasing nanofluid conductivity with decreasing nanofluid conductivity with decreasing particle size, but according to this model particle size, but according to this model when the particle size is decrease the when the particle size is decrease the random motion is larger and the random motion is larger and the convectionlikeconvectionlike effect become dominant.effect become dominant.

Effect of Particle ClusteringEffect of Particle Clustering--Some times nanofluids are in form of Some times nanofluids are in form of cluster when the concentration is high cluster when the concentration is high or when the time is increase.or when the time is increase.

-- It is accepted that heat transfer is a It is accepted that heat transfer is a surface phenomenon and the thermal surface phenomenon and the thermal energy interaction take places at the energy interaction take places at the surface of nanoparticles.surface of nanoparticles.

-- When the particles get agglomerated, When the particles get agglomerated, the effective surface area to volume the effective surface area to volume ratio decreases, thus reducing the ratio decreases, thus reducing the effective area of thermal interaction of effective area of thermal interaction of particles causing a decrease in the particles causing a decrease in the thermal conductivity of the fluid.thermal conductivity of the fluid.

-- Mesh like structure observed, in water based Mesh like structure observed, in water based CuOCuO nanofluid of 0.1 nanofluid of 0.1 volvol% % after after sonicationsonication for (a) 20min, (b) 60min and (c) 70 min.for (a) 20min, (b) 60min and (c) 70 min.

-- It can be seen the structure formation being only after 60 min fIt can be seen the structure formation being only after 60 min from the rom the sonicationsonication..

ConclusionConclusion

-- A review of experimental studies clearly shows a relatively largA review of experimental studies clearly shows a relatively large chaos and e chaos and randomness in the published data.randomness in the published data.

-- Our review on theoretical models indicate that a clear understanOur review on theoretical models indicate that a clear understanding of the ding of the main main mechanism(smechanism(s) involved in thermal transport phenomena in nanofluids ) involved in thermal transport phenomena in nanofluids is not established yet.is not established yet.