Mechanisms of Enhanced Heat Transfer in Nanofluids

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1 Mechanisms of Enhanced Heat Transfer in Nanofluids J.A. Eastman, Materials Science Division, Argonne National Laboratory Fluctuations and Noise in Out of Equilibrium Systems, Sep 14-16, 2005 Supported by,, Office of Basic Energy Sciences, under contract W Eng-38

2 Motivation for improved heat transfer fluids Thermal loads are increasing in a wide variety of applications Microelectronics: smaller features and faster operating speeds Transportation: higher power engines; lighter radiators for fuel economy Lighting: brighter optical devices Utilization of solar energy for power generation Conventional approach to increasing heat dissipation is to increase heat exchanger size Produces an undesired increase in thermal management system size New approach is needed Macintosh G5 uses combination of fans and liquid cooling to enable fast processor speed

3 Nanofluids Motivation Fluids have low k compared to most solids Material Metallic Solids: Silver 429 Copper 401 Aluminum 237 Nonmetallic Solids: Diamond 3300 Silicon 148 Alumina (Al 2 O 3 ) 40 Room Temperature Thermal Conductivity (W/m-K) Metallic Liquids: 644K 72.3 Nonmetallic Liquids: Water Ethylene Glycol Engine Oil U.S. Choi and J.A. Eastman, Enhanced Heat Transfer Using Nanofluids, U.S. Patent #6,221,275 Goal is to enhance effective fluid thermal conductivity and heat transfer coefficient by suspending solid nanoparticles

4 Nanofluids motivation Heat transfer fluid in a pipe Heat transfer fluid in a pipe Nanoparticles flow Microparticles sink Surface atoms Nanoparticles Better dispersion behavior Less clogging and abrasion Much larger surface area-tovolume ratio The relative large mass of microparticles can damage the pipe s wall

5 Outline Synthesis Thermal conductivity in stationary fluids Flow convection and boiling

6 Synthesis of Nanofluids Nanofluids are produced by several techniques: Direct evaporation (1 step) Gas condensation (IGC)/dispersion (2 step) Chemical vapor condensation (1 step) Chemical precipitation (1 step) Gas condensation advantages: Wide variety of nanopowders can be produced Powder production process has already been commercialized Gas condensation disadvantages: Agglomeration Often poor dispersion properties ~30 nm diameter CuO produced by IGC

7 Direct evaporation Less agglomeration than gascondensation Restricted to low vapor pressure liquids and materials that can be vaporized at low to moderate T Small particle size, but little control over size Small sample sizes; slow production rate; scalable? Resistively Heated Crucible Cooling System Liquid ~10 nm diameter Cu in ethylene glycol

8 Chemical Vapor Condensation Synthesis N 2 Carrier Gas O 2 Inlet 3 Zone Furnace Nanoparticles Particle Collector Collection Chamber Precursor Pump 50 nm ~10 nm diameter TiO 2 in H 2 O ~10 nm diameter Fe 2 O 3 in H 2 O powder can be directly deposited into liquids (less agglomeration than IGC, but more than direct evaporation) size control is possible scale-up should be straight-forward (but hasn t been done) layered oxide nanoparticles can be produced (e.g., for biomed. applications)

9 Chemical synthesis Chemical synthesis techniques can produce small, ~monodisperse nanoparticles with no agglomeration Effect of surface molecules on thermal properties? Few existing nanofluid thermal properties studies have used this type of particle 10 nm diameter thiol-stabilized AuPd nanoparticles produced by coreduction of PdCl 2 and HAuCl 4 O.M. Wilson et al., Phys. Rev. B, 66, (2002) M. Brust et al., J. Chem. Soc. Chem. Commun., 801 (1994)

10 Thermal Conductivity Most studies of nanofluids thermal properties have focused on thermal conductivity Thermal conductivity = k Heat flux = q T heat source Temperature gradient, dt/dx x q =!k "T "x Fourier s Law Heat transfer behavior also depends on other properties (specific heat, density, viscosity)

11 Thermal conductivity measurement S. Choi Many studies have used transient hot-wire technique Thin Pt wire suspended in fluid is heated resistively k is calculated from time-temperature profile For electrically conducting fluids, wire must be coated Other techniques (e.g., 3-ω) have also been used

12 Thermal conductivity of oxide nanofluids Measured k/k o for 3 different oxides in H 2 O H. Masuda, 1993 Enhancement depended on material; at 4 vol.% loading: ~30% for Al 2 O 3 ~10% for TiO 2 ~1% for SiO 2 Adjusted ph to create stable dispersions ph =3 for Al 2 O 3 ; ph=10 for TiO 2 and SiO 2

13 Oxide nanofluids Thermal conductivity ratio (k/k 0 ) S. Lee, U.-S. Choi, S. Li, J.A. Eastman ASME Journal of Heat Transfer 121, pp (1999) water + Al 2 O 3 water + CuO ethylene glycol + Al 2 O 3 ethylene glycol + CuO Saw smaller enhancement for Al 2 O 3 -in-h 2 O than Masuda Nanoparticles produced by IGC; dispersed in H 2 O ultrasonically, but no ph adjustment Larger effect for ethylene glycol than for waterbased nanofluids Volume fraction Larger improvement for CuO than Al 2 O 3 is surprising

14 Other studies of oxide nanofluids Wang et al. observed 17% improvement in k/k o for just 0.4 vol.% CuO in H 2 O >5x effect Masuda observed! 50 nm particle size? 0.25 vol.% Surfactant used to improve dispersion But, agglomeration still observed L.-P. Zhou, B.-X. Wang, Ann. Proc. Chinese Eng. Thermophysics, 889 (2002) B.-X. Wang et al., proceedings of the 15th Symp. on Thermophysical Properties (2003) Other studies have seen enhancement intermediate to observations of Masuda and Lee et al. (e.g., H. Xie et al., J. Appl. Phys., 91, 4568 (2002))

15 Copper-containing nanofluids Do metal nanoparticles behave like oxides? J.A. Eastman et al., Appl. Phys. Lett., 78, 718 (2001) 10 nm diameter Cu nanoparticles produce much larger increase in k than 30 nm diameter oxide nanoparticles thioglycolic acid improves dispersion behavior (but adding acid alone does not affect k) Is larger k enhancement due to smaller particle size or larger particle conductivity?

16 Iron-containing nanofluids Behavior is similar to Cu (without added surfactant) Fe nanoparticles produced by chemical vapor condensation k/k o appears to be non-linear with vol. % T.-K. Hong, H-S. Yang, J. Appl. Phys., 97, , (2005) 0.55 vol.% Fe k/k o increases with sonication time

17 Au-containing nanofluids H.G. Patel et al., Appl. Phys. Lett., 83, 2931 (2003) Au-thiolate nanoparticles in toluene exhibit Even larger improvement seen for very dilute Au-citrate in H2O nanofluids k/ko vol.% ~5x larger enhancement than enhancement seen for Cu-in-ethylene glycol 5% enhancement for vol.% Au-citrate nm dia. Also see a strong T-effect Au-thiolate 3-4 nm dia.

18 Another look at Au-containing nanofluids S.A. Putnam, D.G. Cahill, et al., submitted (2005) Investigated 2 types of chemically-synthesized Au nanoparticles 4 nm diameter alkanethiolateprotected Au in ethanol 2 nm dodecanethiol functionalized Au in toluene Maximum enhancement ~0.02 vol.% ~2 orders-of-magnitude less increase than seen by Patel et al. Measured k using optical technique Importance of different fluid, loading %, measurement technique?

19 Macroscopic theory predictions k k o! 1+ 2V p 1" V p Macroscopic theory based on Maxwell s predictions for dielectric behavior of composites R. Hamilton and O. Crosser, I&EC Fundamentals, 1, 187 (1962) Predicts increase in conductivity in nanofluids is approximately independent of particle size and particle conductivity Other versions of effective media theory give similar predictions

20 Some data are in good agreement with effective media theory while other studies show anomalous behavior Comparison with macroscopic theory Al 2 O 3 P. Keblinski, J.A. Eastman, D.G. Cahill Materials Today, pp , June 2005

21 Possible mechanisms Possible microscopic mechanisms: (P. Keblinski et al., ASME Journal of Heat Transfer 45, pp (2002)) Brownian motion (but thermal motion is expected to be faster than expected particle motion) Effect of particles on liquid local ordering (effectively decreases average spacing between particles) Ballistic rather than diffusive thermal transport in the particles (but isn t expected to affect transport between particles) Copper (bulk amorphous) Al (liquid) Nanoparticle clustering (would probably lead to poor dispersion properties) P. Geysermans et al., Jn. Chem. Phys., 113, 6382, 2000.

22 Effect of temperature on k/k o S.K. Das, N. Putra, P. Theisen, W. Roetzel, ASME J. Heat Transfer, 125, no. 4, 567 (2003) H. Patel, S.K. Das et al., Appl. Phys. Lett, 83, 2931 (2003) Observed a large increase in conductivity with increasing temperature Concluded that particle motion is important

23 Another observation of T-dependence J. Wu, J.A. Eastman, (2005) 1.4 Thermal conductivity ratio (k/k o ) % TiO 2 / water 1 % TiO 2 / water 2 % TiO 2 / water 4 % TiO 2 / water Linear relationship for low particle loadings smaller T-dependence for lower loadings Saturation occurs when temperature is high enough for extensive inter-particle interaction? Temperature ( C)

24 Importance of controlling interparticle interactions? As-processed ph value of nanofluid High surface charges Low surface charges Zero surface charge Well-dispersed Weakly attracted agglomerated

25 ph-dependent thermal conductivity of Al 2 O 3 /water J. Wu, J.A. Eastman, (2005) Thermal Conductivity Ratio,!/! Increased thermal conductivity at low ph Extremely increased thermal conductivity at high ph Increased thermal conductivity near PZC (similar to enhancement seen previously for Cu-in-ethylene glycol) ph Value 0.5 vol%al 2 O 3 /water Al 2 O 3 : 48 nm

26 ph-dependent thermal conductivity of TiO 2 /water J. Wu, J.A. Eastman, (2005) 1.6 Thermal Conductivity Ratio,!/! Increased thermal conductivity at low ph Extremely increased thermal conductivity at high ph Increased thermal conductivity near PZC ph Value 0.5 vol%tio 2 /water TiO 2 : anatase, 10 nm

27 Another view of ph-dependence pp Xie et al. investigated effect of ph on k of Al 2 O 3 - containing nanofluids Concluded that k/k o increases with difference from ZPC ph Avoided region near ZPC (ph~9)

28 Carbon-nanotube nanofluids S. Choi et al., Appl. Phys. Lett., 79, 2252 (2001) Carbon nanotubes in oil show the largest enhancement in k todate (25 nm diameter x 50 µm length) Is increased enhancement due to higher conductivity of the nanotubes or to their length? 200 nm 1 µm P.M. Ajayan, RPI

29 Other views of C-nanotubes Enhancements in k observed to depend on fluid Smaller enhancement than seen by Choi et al. Wen and Ding observed ~25% 0.8 vol.% in H 2 O (J. Thermophys. Heat Trans., 18, 481, 2004); Assael et al. observed 38% 0.6 vol.% (proc. 27th Int. Therm. Cond. Conf.,153, 2005) Effects of different fluids, different preparation techniques? TCNT = treated carbon nanotubes DW = distilled water EG = ethylene glycol DE = decene

30 Outline Synthesis Thermal conductivity in stationary fluids Flow convection and boiling

31 Pool boiling heat transfer: background S.M. You, J.H. Kim, K.H. Kim, Appl. Phys. Lett., 83, 3374 (2003) Pool boiling heat transfer (BHT) defined as a process of vigorous heat transfer occurring with a phase change from liquid to vapor in a pool of initially quiescent liquid. Nucleating small bubbles is desirable rather than coalescing large ones; critical heat flux (CHF) is the maximum heat flux under which a boiling surface stays in the nucleate boiling regime Film boiling due to CHF is undesirable because portions of the surface become covered with vapor (lower k leads to increased T) Surface roughening is known to increase CHF (more nucleation sites) Increased k of nanofluids is not expected to increase BHT (depends on heat of vaporization, density of vapor and liquid, and surface tension); nanoparticles may have other effects

32 Effect of nanofluids on boiling critical heat flux 0.05 g/l Al 2 O 3 nanoparticles in deionized H 2 O Saw 200% increase in CHF; constant BHT coefficient; Vassalo et al. saw similar behavior with SiO 2 (Int. J. Heat Mass Trans., 47, 407 (2004) 30% larger bubbles; consistent with increased surface tension Existing theories would predict only 15% increase in CHF for observed bubble size increase S.M. You, J.H. Kim, K.H. Kim, Appl. Phys. Lett., 83, 3374 (2003)

33 Measurements under flow conditions heat transfer depends not only on k, but also on parameters including specific heat, viscosity, flow rate, and density For forced convection in tubes: h k 2/3 * C p 1/3 * ρ 0.8 / η Determine heat transfer coefficient from T-rise and pressure drop across test section

34 Heat transfer under forced convection S.U.-S. Choi et al., Mater. Sci. Forum, , 629 (1999) ~15% increase in heat transfer coefficient observed for 1% CuO in water Larger increase than expected based on k enhancement

35 Other heat transfer observations Cu nanoparticles in deionized H 2 O 2 vol.% Cu improved heat transfer coefficient >39% Y. Xuan, Q. Li, J. Heat Trans., 125, 151 (2003) Friction factor unchanged at this loading In contrast, Pak and Cho (Exper. Heat Trans., 11, 151, 1998) found that 3 vol.% Al 2 O 3 or TiO 2 in H 2 O decreased convective heat transfer coefficient 12% Putra et al. (Heat Mass Trans., 39, 775, 2003) saw a reduction under natural convection of Al 2 O 3 - and CuO-H 2 O nanofluids Wen and Ding (Int. J. Heat Fluid Flow, in press 2005) also saw a reduction under natural convection of TiO 2 -H 2 O nanofluids; adjusted the ph to be far from ZPC (well dispersed, but non-interacting nanofluids)

36 Conclusions Many studies have observed significantly improved heat transfer properties in nanofluids (thermal conductivity, pool boiling critical heat flux, heat transfer coefficient) Many conflicts exist between different studies Different sample preparation techniques, particle size, surface treatment, fluid and nanoparticle materials, measurement techniques may be important Degree of interaction between particles appears to be important (control with ph for H 2 O based nanofluids) Systematic studies are needed; improved synthesis techniques would help (control of size, surface properties, dispersion behavior) New theories are needed that take into account all important characteristics of nanofluids

37 Collaborators Steve Choi Ho-Soon Yang Jie Anny Wu Loren Thompson Guo-Ren Bai Pawel Keblinski (RPI) Simon Phillpot (U. FL) Supported by,, Office of Basic Energy Sciences, under contract W Eng-38