Thermophysical Properties of NanofluidThermophysical Properties of Nanofluidss(Density, Specific Heat and Thermal Conductivity)(Density, Specific Heat and Thermal Conductivity)

M.Reza AzizianM.Reza AzizianSupervisor: Prof. Dr. Hikmet Ş. AybarSupervisor: Prof. Dr. Hikmet Ş. Aybar

CoCo-S-Supervisor: Asstupervisor: Asst.. PProf. Dr. Tuba Okutucurof. Dr. Tuba Okutucu

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

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

IntroductionIntroduction

- HHeat eat transfer in cooling processestransfer in cooling processes can be found in many industrial areacan be found in many industrial areass.. - The conventional methods The conventional methods to to increase cooling rates:increase cooling rates: 1- Extended surfaces such as fins1- Extended surfaces such as fins 2- Increasing flow rates2- Increasing flow rates

- These conventional methods have their own limitations:These conventional methods have their own limitations: 1- 1- FFins: undesirable increase in the ins: undesirable increase in the size of size of thermal management systemthermal management system 2- Increasing flow rates: increase2- Increasing flow rates: increasess pumping power pumping power

- There is aThere is ann immediate need for new and innovative concepts to achieve immediate need for new and innovative concepts to achieve ultra high performance cooling.ultra 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 transfer fluids are The thermal conductivity characteristics of ordinary heat transfer fluids are not adequate to meet our requirements.not adequate to meet our requirements.

- The thermal conductivity characteristics of ordinary heat transfer fluids The thermal conductivity characteristics of ordinary heat transfer fluids are are about two orders of magnitude less efficient in conducting heat compareabout two orders of magnitude less efficient in conducting heat comparedd to metals.to metals.

- Because a solid metal has a larger thermal conductivity than a base fluid, Because a solid metal has a larger thermal conductivity than a base fluid, suspending metallic solid fine particlesuspending metallic solid fine particless into the base fluid is expected to into the base fluid is 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 has been well to improve the physical properties of liquids is hardly new, it has been well known for 100 years, since the idea can be traced back to James Clerk known for 100 years, since the idea can be traced back to James Clerk Maxwell’s theoretical work (Maxwell, 1873).Maxwell’s theoretical work (Maxwell, 1873).

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

NanofluidsNanofluids

- NanofluidNanofluidss ha haveve a potential to reduce such problems. a potential to reduce such problems.

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

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

Au, Cu), Carbon nanotubes Au, Cu), Carbon nanotubes

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

- Two-step technique: The two step method starts with producing Two-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 and inert-gas condensation processing)evaporation and inert-gas condensation processing), and proceeds to , and proceeds to disperse them into a base fluid; most of the nanofluids are producedisperse them into a base fluid; most of the nanofluids are producedd by two by two step method.step method.

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

- Very recently an interesting schemeVery recently an interesting scheme:: thethe synthesis of metal nanoparticles in synthesis of metal nanoparticles in deionized water, using multi-beam laser ablation in liquids, is reported by deionized water, using multi-beam laser ablation in liquids, is reported by Argonne National Laboratory, where the nanoparticle size and distribution Argonne National Laboratory, where the nanoparticle size and distribution will be controlwill be controlledled by the laser parameters. by the laser parameters.

- ZrO- ZrO22 in water that produce with two - Cu nanoparticles produced by in water that produce with two - Cu nanoparticles produced by

Step method direct evaporation into ethyleneStep method direct evaporation into ethylene

glycolglycol

Density and Specific heatDensity and Specific heat

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

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

- For this propose we define a nanofluid as a mixture consisting of a For this propose we define a nanofluid as a mixture consisting of a continuouscontinuous base fluid component called “matrix” and a discontinuous solid base fluid component called “matrix” and a discontinuous solid component called “particles”, the subscript “m” representcomponent called “particles”, the subscript “m” representss the base fluid the base fluid matrix and the subscript “p” representmatrix and the subscript “p” representss particles in base fluid. particles in base fluid.

(1 )m p m m p pe m p

e m p m p

m m V Vmf f

V V V V V

( ) ( ) ( ) ( )

( ) ( )m p p m p p p m m p p p

p e e e eee m p m p m m p p

Q Q mC T mC T C V C VQC

m T m m T m m T V V

(1 )( ) ( )p m p pf C f C (1 )( ) ( )

(1 )p m p p

pem p

f C f CC

f f

Thermal Conductivity (Experimental)Thermal Conductivity (Experimental)

- Before suggesting a theoretical model for thermal conductivity letBefore suggesting a theoretical model for thermal conductivity let’’s first s first look at the parameters that affect the thermal conductivity of nanofluidlook at the parameters that affect the thermal conductivity of nanofluidss from experiments.from experiments.

- According to the report of Argonne National Laboratory, eight parameters According to the report of Argonne National Laboratory, eight parameters aaffect the thermal conductivity of nanofluids, they got these results from ffect the thermal conductivity of nanofluids, they got these results from about 124 researchers experimentsabout 124 researchers experiments.. TThese effects are:hese effects are:

1- Particle volume concentration1- Particle volume concentration 2- Particle materials2- Particle materials 3- Particle size 3- Particle size 4- Particle shape4- Particle shape 5- Base fluid material5- Base fluid material 6- Temperature6- Temperature 7- Additive7- Additive 8- Acidity8- Acidity

- Effect of Particle Volume Concentration- Effect of Particle Volume Concentration

From the experimental results the general From the experimental results the general trend is clear: thermal conductivity trend is clear: thermal conductivity enhancement increases with increaseenhancement increases with increase in in particle volume concentration.particle volume concentration.

(Al(Al22OO33 in water) in water)

- Effect of Particle MaterialEffect of Particle Material

The thermal conductivity ratio is seen to The thermal conductivity ratio is seen to increase faster for metalincrease faster for metalss than oxide than oxide particles.particles.

(Particles in ethylene glycol)(Particles in ethylene glycol)

- Effect of Particle SizeEffect of Particle Size

The trend in this part is not obvious.The trend in this part is not obvious.

MMost of the researchers report thatost of the researchers report that larger particle diameters produce largelarger particle diameters produce largerr enhancement in thermal conductivity enhancement in thermal conductivity but in some cases the experiments show but in some cases the experiments show indicate the oppositeindicate the opposite..

A consistent trend appears where larger A consistent trend appears where larger particle diameters produce a large particle diameters produce a large enhancement in thermal conductivityenhancement in thermal conductivity::

AlAl22OO33 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.

e.g. e.g. SiC in waterSiC in water

- Effect of Base Fluid Material- Effect of Base Fluid Material

The 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 Temperature

AAll experimentll experiments s show increased thermal show increased thermal conductivity enhancement with increased conductivity enhancement with increased temperature.temperature.

(Al(Al22OO33 in water) in water)

- Effect of AdditivesEffect of Additives

Experiments 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 conduction through Maxwell was one of the first to analytically investigate conduction through suspended particles.suspended particles.

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

- When we go to the nanoscale we have to consider some effects that When we go to the nanoscale we have to consider some effects that dodo not not exist in large scalesexist in large scales. T. To improve the predictions we consider:o improve the predictions we consider:

1- Effect of nanoparticle-matrix interfacial layer1- Effect of nanoparticle-matrix interfacial layer

2- Effect of nanoparticle Brownian motion2- Effect of nanoparticle Brownian motion

3- Effect of nanoparticle cluster/aggregate 3- 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, solid-like liquid layer, and nanoparticles, solid-like liquid layer, and a bulk liquid. a bulk liquid.

- TThe solid-like nanolayer acts as a he solid-like nanolayer 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 interfacial layer assumption, for copper-particle-in-ethylene-glycol copper-particle-in-ethylene-glycol suspension.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 nanolayer occurs when considering the nanolayer 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 addingcould be potentially better than adding larger-size nano-particles.larger-size nano-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 Brownian motion of nanoparticles will be another potential factor in calculating be another potential factor in calculating the thermal conductivity of nanofluids.the thermal conductivity of nanofluids.

- To develop such a theory for the To develop such a theory for the thermal conductivity of nanofluids thermal conductivity of nanofluids it isit is assumed that energy transport in assumed that energy transport in nanofluids involvnanofluids involvee four modes: four modes:

1- Collision between base fluid 1- Collision between base fluid molecules.molecules.

2- Thermal diffusion in nanoparticles. 2- Thermal diffusion in nanoparticles.

3- Collision between nanoparticles. 3- Collision between nanoparticles.

4- Thermal interaction of dynamic or 4- Thermal interaction of dynamic or dancing nanoparticles with base fluid dancing nanoparticles with base fluid molecules.molecules.

- The S.P. Jang and U.S. Choi model is The S.P. Jang and U.S. Choi model is able to predict a particle-size and able to predict a particle-size and temperature-dependent conductivity of temperature-dependent conductivity of nanofluids, while no existing theories nanofluids, while no existing theories explain or predict these effects.explain or predict these effects.

- The S.P. Jang and U.S. Choi model The S.P. Jang and U.S. Choi model predicts the experimentpredicts the experimentalal data for data for temperature dependency with excellent temperature dependency with excellent agreementagreement.. In contrast, conventional In contrast, conventional theories with motionless nanoparticles fail theories with motionless nanoparticles fail to predict this behaviour (horizontal dashed to predict this behaviour (horizontal dashed line).line).

- The Maxwell model predicts decreasing The Maxwell model predicts decreasing nanofluid conductivity with decreasing nanofluid conductivity with decreasing particle size, but according to S.P. Jang and particle size, but according to S.P. Jang and U.S. Choi model when the particle size U.S. Choi model when the particle size decreasedecreasess the random motion is larger and the random motion is larger and the convectionthe convection like effect becomelike effect becomess dominant.dominant.

Effect of Particle ClusteringEffect of Particle Clustering-Sometimes nanofluids are in form of Sometimes nanofluids are in form of clusterclusterss when the concentration is high when the concentration is high or when the time is increaseor when the time is increasedd..

- 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 takeenergy interaction takess place at the place 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 CuO nanofluid of 0.1 vol% Mesh like structure observed, in water based CuO nanofluid of 0.1 vol% after sonication for (a) 20min, (b) 60min and (c) 70 min.after sonication for (a) 20min, (b) 60min and (c) 70 min.

- It can be seen It can be seen that that the structure formation begthe structure formation beginin only after 60 min from the only after 60 min from the sonication(vibration of liquid with ultrasonic bath).sonication(vibration of liquid with ultrasonic bath).

ConclusionConclusion

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

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

- In our project we have a plan to apply the formula for thermal In our project we have a plan to apply the formula for thermal conductivity of gas mixtureconductivity of gas mixturess to nanofluid to nanofluids.s.