Nanofluids for Heat Transfer Enhancement-A Review. E.K. Goharshadi*, H. Ahmadzadeh, S. Samiee and M. Hadadian

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1 Review article Phys. Chem. Res., Vol. 1, No. 1, 1-33, June Nanoluids or Heat Transer Enhancement-A Review E.K. Goharshadi*, H. Ahmadzadeh, S. Samiee and M. Hadadian Deartment o Chemistry, Ferdowsi University o Mashhad, Mashhad 91779, an (Received 12 December 2012, Acceted 28 March 2013) PHYSICAL CHEMISTRY RESEARCH Published by the anian Chemical Society ino@hyschemres.org A nanoluid is a dilute liquid susension o articles with at least one critical dimension smaller than ~100 nm. Researches so ar suggest that nanoluids oer excellent heat transer enhancement over conventional base luids. The enhancement deends on several actors such as article shae, article size distribution, volume raction o nanoarticles, temerature, H, and thermal conductivities o nanoarticles and base luids. This aer resents an udated review on nanoluids with the emhasis on heat transer enhancement including ormulation, hysical roerties, biological and non-biological alications, stability, ossible mechanisms or the enhancement o heat conduction, and numerical modelling o nanoluids. Based on the research indings, a number o challenges are emhasized in order to understand the underlying hysics or uture industrial take-u o the nanoluids technology. Further comutational studies are also required in order to understand all o the actors aecting on the enhancement o thermal conductivity o nanaoluids. Keywords: Heat transer enhancement, Nanoluids, Thermal conductivity, Viscosity INTRODUCTION Scientists have been quite active in the ast ew decades in the search o novel aroaches to increase heat dissiation o various cooling devices. These include many electronics devices such as microrocessors where continually increasing ower densities require more innovative techniques o heat dissiation. Heat transer through a luid medium is imortant in several engineering alications including heat exchangers, rerigerators, automobiles, and ower lants. The ability o a luid medium to transer heat across a small temerature dierence enhances the eiciency o energy conversion and imroves the design and erormance o automobile engines, heat transer devices, and micro-electro-mechanical systems (MEMS). In recent years, modern technologies have ermitted the manuacturing o a new class o luids, called nanoluids. A nanoluid is the romising heat and mass transer medium in which nanoarticles are disersed. It is known that the thermal conductivity o the nanoluids is considerably higher than that *Corresonding author. gohari@erdowsi.um.ac.ir o the corresonding base luids [1,2]. The enhancement deends on several actors such as article shae, article size, volume raction o articles, and thermal roerties o solid and liquid. In site o the otential and eatures o nanoluids, these rather secial luids are still in their early develoment stages. The exerimental data on hysical roerties o nanoluids esecially thermal conductivity are very scattered. Even though data rovide insight into nanoluid roerties and heat transer beneits, a considerable amount o research remains to be done on this subject and the develoment o the ield aces several challenges. The resent review rovides a comrehensive outline o the attractive research rogress made in the area o nanoluids. It also summarizes the exerimental, theoretical, and comutational develoments o the ield. PRE-NANOFLUID STUDIES OF HEAT TRANSFER FLUIDS Heat transer luids such as water, mineral oil, and ethylene glycol lay a vital role in many industrial rocesses

2 Goharshadi et al./phys. Chem. Res., Vol. 1, No. 1, 1-33, June including ower generation, chemical rocesses, heating or cooling rocesses, and microelectronics. Heat transer technology stands at cross roads today between ever increasing demand o cooling ultra-high heat lux equiments on one hand and unrecedented ace o miniaturization on the other. In the resent days, the dierent ranges o laser alications, suerconducting magnets, high ower X-ray, and above all suer-ast comuting chis erorming trillions o oerations er second are becoming quite common. These devices are not only to oerate in their resective alications with high recision but also to do so occuying minimum sace [3]. Today s raid IT develoment requires PC erormance caable o ast data rocessing. To meet this requirement, high-erormance devices built in PCs have been develoed. Esecially, cometitive release o aster CPU roducts and shit towards more comact and thinner devices is noticeable. This leads to higher heat generation because o CPU temerature rise and causes the short lie, malunction, and ailure o CPUs. CPU cooling has been taken seriously. Pentium-IV CPU and Athlon XP, released by Intel and AMD resectively, has high heat dissiation, requiring excellent cooling erormance. For examle, 2 GHz Pentium-IV rocessor made in 0.18 µm manuacturing rocess has thermal design ower (TDP) o 75.8 W, requiring cooling erormance o 0.47 C/W (at 40 C). 2 GHz Pentium-IV rocessor made in 0.13 µm manuacturing rocess has a little lower TDP 52 W and requires cooling erormance o 0.53 C/W (at 40 C) [4]. This uts a challenge not only to the core device design but also to their thermal management. While air based cooling systems are more common and reliable, they ail miserably with increasing heat lux. Thereore, in almost all o the high heat lux alications liquid cooling is reerred. The cooling liquids usually used are water/chilled water, common rerigerants, and liquid nitrogen, or similar cryogens deending on the seciic alication. Usual rerigerants are hazardous to the environment and cryogens are costly not only due to their energy intensive roduction rocess but also due to whole range o costly equiments which use them. While water is a convenient and sae medium, its relatively oor heat transer characteristic is a major disadvantage [3]. Solid articles generally ossess ar greater thermal conductivity than conventional heat transer luids as seen in Table 1. The thermal conductivity o coer, or examle, is 700 times higher than that o water and 3000 times that o engine oil. Mixing solid articles in a liquid can, thereore, enhance the cooling otential o the liquid by increasing the thermal conductivity o the susended luid. Working on various mixtures using millimetre or micrometer size articles gets back to over a hundred years ago [5-7]. While these luids do rovide the aorementioned cooling beneits, their imlementation is comlicated by their causing severe roblems. In ractical alications, the abrasive action o the articles causes the clogging o low channels, erosion o ielines, and their momentum lead to an increase in ressure dro in ractical alications. Furthermore, they oten suer rom instability and rheological roblems. In articular, the articles tend to settle raidly. Thus, although these luids give better thermal conductivities, their use is not ractical. Recently, the advent and develoment o nanotechnology oers the oortunity to enhance the alication o heat transer luids by introducing nanoluids. A nanoluid is a class o solid-liquid comosite materials consisting o solid nanoarticles disersed in a heat transer luid. The concet o nanoluids was irst materialized by Choi [1] ater erorming a series o research at Argonne National Laboratory in USA. The irst exeriments were done by Masuda et al. [8] to show the extraordinary values o thermal conductivity o nanoluids. However, subsequent research [2,9,10] showed that the nanoluids exhibit higher thermal conductivity even or low concentration o susended nanoarticles. For instance, exeriments showed an increase in thermal conductivity by disersion o less than 1% volume raction o Cu nanoarticles or carbon nanotubes (CNTs) in ethylene glycol or oil by 40% and 150%, resectively [11]. POTENTIAL AND FEATURES OF NANOFLUIDS The ollowing eatures or dierent nanoluids have been observed consistently by dierent researchers at various organizations: 1. The most imortant eature observed in nanoluids is an abnormal rise in thermal conductivity, ar beyond exectations, and much higher than any theoretical rediction. Comaring to ure liquids, the thermal conductivity o 2

3 Nanoluids or Heat Transer Enhancement-A Review/Phys. Chem. Res., Vol. 1, No. 1, 1-33, June Table 1. Thermal Conductivity Values and Measuring Temerature o Thermal Conductivity or Some Solids and Liquids Material Metallic solids Thermal conductivity (W m -1 K -1 ) Measuring temerature (K) Aluminium (Al) Coer (Cu) Gold (Au) on (Fe) Silver (Ag) Non-metallic solids Alumina (Al 2 O 3 ) 40 CNT 3000 Coer oxide (CuO) Diamond 3300 Fullerene 0.40 Silicon (Si) 148 Liquids Ethylene glycol 0.20 Engine oil 0.14 Glycerol Water nanoluids deends strongly on to temerature increase. The large surace area o nanoarticles allows or more heat transer. 2. Abnormal viscosity increase relative to the base luid. 3. Stability. Because the nanoarticles are small, they weigh less, and the sedimentation rates are smaller. Nanoluids have been reorted to be stable over months using a stabilizing agent [1,2,12]. 4. Microchannel cooling without clogging. Nanoluids are not only a better medium or heat transer in general but they are also ideal or microchannel alications where high heat loads are needed. The combination o microchannels and nanoluids will rovide highly conducting luids and a large heat transer area. This cannot be attained with meso- or micro-articles because they clog microchannels. Nanoarticles, which are only a ew hundreds or thousands o atoms long, are orders o magnitude smaller than the microchannels. 5. Reduced chances o erosion. Nanoarticles are very small, and the momentum they can imart to a solid wall is much smaller. This reduces the chances o comonent erosion such as heat exchangers, ielines, and ums. 6. Reduction in uming ower. To increase the heat transer o conventional luids by a actor o two, uming ower must usually be increased by a actor o ten. It can be shown that i one can multily the conductivity by a actor o three, the heat transer in the same aaratus doubles [1]. The required 3

4 Goharshadi et al./phys. Chem. Res., Vol. 1, No. 1, 1-33, June increase in the uming ower will be very moderate unless there is a shar increase in luid viscosity. Thus, a very large savings in uming ower can be achieved i a large thermal conductivity increase can be brought about with a small volume raction o articles. 7. Reduced riction coeicient. Nanoluids could eectively decrease riction. 8. Cost and energy saving. Successul emloyment o nanoluids will result in signiicant energy and cost savings because heat exchange systems can be made smaller and lighter. 9. Possible sectrum o alications o nanoluids include more eicient low and lubrication, cooling and heating in new and critical alications like electronics, nuclear, biomedical instrumentation and equiments, transortation and industrial cooling, heat management in various critical alications, as well as environmental control and cleanu, and bio-medical alications. TYPES OF NANOFLUIDS The range o otentially useul combinations o nanoarticle and base luids is enormous. Nanoluids can be classiied broadly by the tye o articles into our grous: ceramic, ure metallic, alloy, and some allotroes o carbon or carbon-based nanoluids. Dierent combinations o the above articles and luids give dierent nanoluids. Table 2 shows some exerimental studies on dierent kinds o nanoluids. Ceramic Nanoluids The irst materials tried or nanoluids were ceramic articles, rimarily because they were easy to roduce and chemically stable in solution. The ceramics are classiied into three distinct categories: oxides such as alumina and zirconia, non-oxides such as carbides, nitrides, and silicides, and comosites such as combinations o oxides and non-oxides. Each one o these classes can develo unique material roerties. Among dierent kinds o ceramics, much interest has been shown on oxide nanoluids. The irst ublished reort by Masuda et al. [8] reorted 30% increases in the thermal conductivity o water with the addition o 4.3 vol. % Al 2 O 3 nanoarticles. Pure Metallic Nanoluids Although ewer studies o nanoluids containing metal nanoarticles have been carried out than those o containing oxide nanoarticles, the results have been encouraging. Usually, a much higher eective thermal conductivity is exhibited or a nanoluid consisting o a metal than that o containing the same volume raction o disersed oxide o that metal [42]. Alloy Nanoluids Alloying o metals with dierent metals is a way o develoing new materials with better technological useulness as comared to their arent metals [70]. Studies on alloy nanoarticles revealed that their hysical roerties dier rom what have been observed in bulk samles. There are ew reorts or alloy nanoluids in the literature [70,72]. Alloy nanouilds may be reared by mechanical alloying or by the inert gas condensation rocess. Carbon-Based Naoluids The large intrinsic thermal conductivity o some carbonbased nanostructures, combined with their low densities as comared to metals, make them attractive candidates or using in nanoluids. Examles o carbon-based nanoluids are ullerenes, carbon nanotubes (single-walled nanotubes (SWNTs), multi-walled nanotubes (MWNTs), and ultradisersed diamond) in dierent luids. Comared with metal or metal oxide materials, CNTs have higher thermal conductivity. For examle, thermal conductivity values or SWNT, double-walled carbon nanotube, and MWNT are 6000 W m -1 K -1, 3986 W m -1 K -1, and 3000 W m -1 K -1, resectively [85]. One o the irst studies involving CNT nanoluids was carried out by Choi et al. [74]. They measured the eective thermal conductivity o 1.0 vol.% MWNTs disersed in synthetic oly(a-olein) oil and reorted 160% increase in thermal conductivity. There are some reasons or this anomalous henomenon. First, the thermal conductivity o CNT is similar to that o grahite and aroaches or even exceeds 4

5 Nanoluids or Heat Transer Enhancement-A Review/Phys. Chem. Res., Vol. 1, No. 1, 1-33, June Table 2. Some Exerimental Studies on Dierent Kind o Nanoluids Nanoarticle Re. Nanoarticle Re. Ceramic nanoluids Metallic nanoluids SiC [13] Ag [14-20] Al 2 O 3 [8,21-37] Au [15,33,38-40] CeO 2 [41] Cu [26,27,42-48] CuO [26,27,29,31-33,41,49-57] Fe [36,58-60] Fe 2 O 3 [61] Ni [62] Fe 3 O 4 [63,64] Alloy nanoluids SiO 2 TiO 2 Ag-Cu [70] ZnO Ag-Al [72] ZrO 2 Al-Cu [72] WO 3 Carbon-based nanoluids CNT [40,53,57,74-82] Diamond [14,47,74,83] Fullerene [57] Grahite [79] Grahene [84] that o natural diamond, the best room-temerature thermal conductor. Second, nanotubes have high asect ratios. PREPARATION OF NANOFLUIDS There are two rimary methods to reare nanoluids: A two-ste rocess in which nanoarticles or nanotubes are irst roduced as a dry owder. The resulting nanoarticles are then disersed into a luid in a second ste. Single-ste nanoluid rocessing methods have also been develoed. Two-Ste Methods Several studies, including the earliest investigations [2] o nanoluids, used a two-ste rocess in which nanoarticles are irst roduced as a dry owder. This method is more extensively used to roduce nanoluids because nanoowders are commercially available nowadays. A variety o hysical, chemical, and laser-based methods are available or the roduction o the nanoarticles to be used or nanoluids [86-92]. One-Ste Methods The nanoarticles may agglomerate during the drying, storage, and transortation rocess, leading to diiculties in the ollowing disersion stage o two-ste method. Consequently, the stability and thermal conductivity o nanoluid are not ideal. In addition, the roduction cost is high. 5

6 Goharshadi et al./phys. Chem. Res., Vol. 1, No. 1, 1-33, June To reduce the agglomeration o the nanoarticles, one-ste methods have been develoed. There are some ways or rearing nanoluids using this method including direct evaoration condensation [27,93,94], chemical vaour condensation [95], and single-ste chemical synthesis. STABILITY OF NANOFLUIDS The roduction o a nanoluid aces some major challenges such as agglomeration o articles in solution due to very strong van der Waals interactions and the raid settling o articles in luids. The secial requirements or rearation o a nanoluid are durability and stability o susension with low agglomeration o articles, and no chemical change o the luid [96]. Stability o a nonoluid is strongly aected by the characteristics o the susended articles and base luids such as the article morhology and the chemical structure o the articles and base luid [57]. In order to make a stable susension, one should reduce the density dierence between the articles and the luid, increase the viscosity o the luid, and make the articles very small to revent agglomerating [16]. Methods o Imroving the Stability o a Nanoluid To obtain stable nanoluids, several methods such as electrical, hysical, or chemical [82] are used. General common methods are: (1) Controlling the surace charge o the nanoarticles by controlling the H. The stability o a nanoluid directly links to its electrokinetic roerties. Through a high surace charge density, strong reulsive orces can stabilize a well-disersed susension [97]. As the H o the solution dearts rom the isoelectric oint (IEP) o articles, the colloidal articles get more stable [98,99]. The IEP is the concentration o otential controlling ions at which the zeta otential is zero. Thus, at the IEP, the surace charge is zero. (2) Modiying the surace by addition o some suractants. This is one o the general methods to avoid sedimentation o nanoarticles. Suractants can modiy the articles-susending medium interace and revent aggregation over long time eriods. The reason is that the hydrohobic suraces o nanoarticles/nanotubes are modiied to become hydrohilic and vice versa. Selection o suitable suractants and disersants deends mainly uon the roerties o the solutions and articles. Suractant molecules adsorbed on the nanoarticle s surace can decrease the surace energy and thus revent the agglomeration o articles. Poular suractants that have been used in literature can be listed as sodium dodecylsulate [100], sodium dodecylbenzenesulonate [101], cetyltrimethylammoniumbromide [102], dodecyl trimethylammonium bromide, sodium octanoate [103], and olyvinylyrrolidone [104]. Adding suractants restricts the alication o nanoluids at high temeratures [105] when the bonding between suractant and nanoarticles is damaged and hence, the nanoluid loses its stability and sedimentation o nanoarticles occurs [106]. (3) Using ultrasonic vibration. Ultrasonic bath, rocessor, and homogenizer are owerul tools or breaking down the agglomerations in comarison with other methods like magnetic and high shear stirrer as exerienced by researchers [97]. Each o the above techniques or combination o them such as simultaneous use o ultrasonic agitation and addition o suractants are sometimes used to minimize article aggregation and to imrove disersion behavior. The best way to roduce a stable susension may be a single-ste method where instead o nanoarticles, nanoluids are roduced directly, thus reducing the chance o agglomeration [82]. Stability Evaluation The Derjaguin-Landau-Verwey-Overbeek (DLVO) theory [107,108] or colloidal interactions dictates that a colloidal system will remain stable i and only i the columbic reulsion, arising rom the net charge on the suraces o the articles in a colloid, is greater than the van der Waals orces. When the reverse is true, the colloidal articles will cluster together and orm locculates and aggregates. Although the stability o nanoluids is very imortant or their alications, there are limited studies on estimating the stability o a susension. There are some ways or evaluating the stability o a nanoluid: (1) Measuring the zeta otential which is the overall charge that a article acquires in a seciic medium and is a good 6

7 Nanoluids or Heat Transer Enhancement-A Review/Phys. Chem. Res., Vol. 1, No. 1, 1-33, June indicator or the colloidal stability o a nanoluid [109]. The higher the absolute zeta otential, the stronger the columbic reulsion between the articles, and thereore, the lower imact o the van der Waals orces on the colloid. Zeta otential measurement is one o the most critical tests to validate the quality o the nanoluids stability via a study o its electrohoretic behavior [110]. (2) Measuring article size distribution by transmission electron microscoy (TEM) or light scattering methods. The nanoluid becomes more stable when nanoarticles have narrow article size distribution. (3) UV-Vis sectrohotometric measurements. UV-Vis analysis is an eicient way to evaluate the stability o nanoluids. I nanomaterials disersed in luids have characteristic absortion bands in the wavelength range o nm, it is an easy and reliable method to evaluate the stability o nanoluids using UV-Vis sectral analysis. The variation o suernatant article concentration o nanoluids with sedimentation time can be obtained by the measurement o absortion o nanoluids because there is a linear relation between the suernatant nanoarticle concentration and the absorbance o susended articles. The outstanding advantage o UV-Vis sectral analysis comared to other methods is that it can resent the quantitative concentration o nanoluids [111]. The irst work to quantitatively characterize colloidal stability o the disersions o CNT by UV-Vis scanning sectrohometric measurements was reorted by Jiang et al. [100]. However, this method is unsuitable or high concentration o nanoluids because these disersions are too dark to dierentiate the sediment visibly [97]. (4) Cryogenic electron microscoy (Cryo-TEM, Cryoscanning electron microscoy) is another eicient method to distinguish the shae, size, distribution, and aggregation o nanoarticles in a luid i the microstructure o nanoluids is not changed during cryoation [112]. PHYSICAL PROPERTIES OF NANOFLUIDS U to now, the thermal conductivity, viscosity, density, seciic heat, and surace tension o the nanoluids have been investigated. Thermal Conductivity Among all the hysical roerties o nanoluids, the thermal conductivity is the most comlex and or many alications the most imortant one [113]. By susending some o the nanoarticles in heating or cooling luids, the heat transer erormance o the luid can be imroved signiicantly. The main reasons o such enhancement may be listed as ollows [12]: 1. The susended nanoarticles increase the surace area and the heat caacity o the luid. 2. The susended nanoarticles increase the eective (or aarent) thermal conductivity o the luid. 3. The interaction and collision between articles and luid are intensiied. 4. The mixing luctuation and turbulence o the luid are intensiied. 5. The disersion o nanoarticles lattens the transverse temerature gradient o the luid. Some exerimental studies on thermal conductivity o nanoluids are summarized in Table 3. Imortant arameters. A nanoluid is a mixture o liquid and nanoarticles, and several actors inluence on its thermal conductivity. From the exerimental results o many researchers, it is known that the thermal conductivity o nanoluids deends on many arameters including: 1. Thermal conductivity o base luid Moosavi et al. [71] measured some hysicochemical roerties including thermal conductivity, viscosity, and surace tension o ZnO nanoarticles in ethylene glycol and glycerol as base luids. They ound that the enhanced thermal conductivity ratio decreases with increasing thermal conductivity o the base luid. 2. Thermal conductivity o nanoarticles The thermal conductivity o a nanoluid containing a metal is greater than that o oxide o that metal at the same conditions [132]. 3. Volume raction The thermal conductivity o a nanouid is strongly deendent on the nanoarticle volume raction [12]. Yeganeh et al. [83] measured thermal conductivity enhancements o nanodiamond articles susended in deionized water with dierent volume ractions in the range rom 0.8% to 3%. They observed the highest enhancement in the thermal conductivity (7.2%) or a volume raction o 3%. Abareshi et al. [64] reared magnetic nanoluids by disersing the Fe 3 O 4 nanoarticles in water in the resence o 7

8 Goharshadi et al./phys. Chem. Res., Vol. 1, No. 1, 1-33, June Table 3. Summary o Some Exerimental Studies on Thermal Conductivity o Nanoluids Nanoarticle Base luid Size o nanoarticles Particle concentration Findings Re. CNT Oil D=25 nm, L=50 μm Φ = 0.3-1% Thermal conductivity was anomalously greater than theoretical redictions and is nonlinear with nanotube loadings. [74] Nitric acid treated CNT Water/ethyleneglycol/ decene D=15 nm, L=30 μm Φ = % Thermal conductivity enhanced with increasing the volume raction. The enhanced thermal conductivity ratios are reduced with the increasing thermal conductivity o the base luid. [76] Fe Ethylene glycol 10 nm Φ = 0.55% 18% increase In thermal conductivity was observed. [59] TiO 2 (rodshaed) TiO 2 (sherical shaed) Water D=10 nm, L=40 nm 15 nm Φ = 0.5-5% Maximum enhancement in thermal conductivity was 33%. Maximum enhancement in thermal conductivity was 30%. [65] Au Toluene 1.65 nm Φ = 0.003% Al 2 O 3 TiO 2 Water Water 20 nm 40 nm Φ w = 10-40% Φ = % Thermal conductivity increased with an increase in the article concentration and article thermal conductivity. [114] CuO Water 33 nm Φ = % Al 2 O 3 Water 30 nm Φ = % Thermal conductivities o the dilute Al 2 O 3 water nanoluids increase nearly linearly with the concentration. [115] Al 2 Cu Ag 2 Al Water/ethylene glycol nm nm Φ = 1-2% Φ = 1-2% The higher the volume ercent o nanoarticles, the greater was the eective thermal conductivity and the smaller the disersoid size, the greater is the enhancement in the thermal conductivity. [116] 8

9 Nanoluids or Heat Transer Enhancement-A Review/Phys. Chem. Res., Vol. 1, No. 1, 1-33, June Table 3. Continued. Nanoarticle Base luid Size o nanoarticles Al 2 O 3 Water nm Particle concentration Φ w = % Findings For weight raction o 0.15 wt%, thermal conductivity was enhanced by u to 10.1%. Re. [117] Acid treated CNT Silicone oil D= nm, L=20 μm Φ = 0.002, , 0.01% Thermal characteristics o nanoluids might be maniulated by means o controlling the morhology o CNT. [118] TiO 2 Water 21 nm Φ = 0.2-2% Thermal conductivity o nanoluids increased as the article concentrations increased and are higher than the values o the base liquids. [119] Al 2 O 3 Water 43 nm Φ = % Thermal conductivity o nanoluids increased with the nanoarticle volume concentration. [120] Al 2 O 3 Water 20, 50, 100 nm Φ w = 0.5-2% Shrinkage o article size enhanced the thermal conductivity ratio o nanoluid. [121] Diamond Water 10 nm Φ = 0.8-3% CuO Gear oil 40 nm Φ = % SiC Water 100 nm Φ = % The highest observed enhancement in the thermal conductivity was 7.2% or a volume raction o 3% An enhancement in thermal conductivity o 10.4% with 2.5% volume raction o CuO nanoarticle loading was observed Thermal conductivity o SiC/DIW nanoluids increases with an increase o volume raction. [83] [122] [123] γ-al 2 O 3 TiO 2 CuO Carboxymethyl cellulose aqueous solution 25 nm 10 nm nm Φ = 0.1-4% Thermal conductivity o nanoluids is higher than the one o the base luid and the increase in the thermal conductivity varies exonentially with the nanoarticle concentration. [124] 9

10 Goharshadi et al./phys. Chem. Res., Vol. 1, No. 1, 1-33, June Table 3. Continued. Nanoarticle Base luid Size o nanoarticles Particle concentration Findings Re. AlN Ethylene glycol/roylene glycol 50 nm Φ = % CuO Water 50 nm Φ = 0.025%, 0.05% 0.1% Fe 3 O 4 Water 10 nm Φ = 5% TiO 2 Water/ethylene glycolwater mixture 21 nm Al 2 O Φ = 0-8% At a volume raction o 0.1, the thermal conductivity enhancement ratios are 38.71% and 40.2%, resectively, or ethylene glycol and roylene glycol as the base luids. An enhancement in thermal conductivity over the base luid was witnessed or the tested temerature and volume raction A erroluid with 5.0% volume raction o nanoarticles enhanced the thermal conductivity more than 200% at maximum value. Thermal conductivity o nanoluids increased with resect to the base luid and increased with increasing concentration and temerature. [125] [126] [127] [128] Polyaniline nanoibers Water D=80 nm, L=2 μm Φ = 0.08, 0.16, 0.24% Maximum thermal conductivity enhancement ratio was 140% with [129] 0.24 vol% o nanoibers loading. CNT Water D=1-20 nm, L=10 μm Ф w = 0.25% As the number o nanotube wall increased, thermal conductivity decreased. [130] Al 2 O 3 /SiO 2 Methanol nm Φ = %. Thermal conductivity increases with an increase o the nanoarticle volume raction, and the enhancement is observed to be 10.74% and 14.29% over the baseluid at the volume raction o [131] 0.5vol% or Al 2 O 3 and SiO 2 nanoarticles. D: Diameter o nanostructures; L: length o nanotubes; Φ: volume concentrations; Ф w : mass ractions o articles T: temerature. 10

11 Nanoluids or Heat Transer Enhancement-A Review/Phys. Chem. Res., Vol. 1, No. 1, 1-33, June tetramethyl ammonium hydroxide as a disersant. They ound that the thermal conductivity ratio o the nanoluids increases with increase in volume raction. The highest enhancement o thermal conductivity was 11.5% in the nanoluid o 3 vol% o nanoarticles at 40 ºC. 4. Size o nanoarticles Nanoluids containing smaller articles show greater enhancement o thermal conductivity than that o larger articles. The stochastic motion o nanoarticles could be a robable exlanation o the thermal conductivity enhancement. This is because smaller articles are more easily to mobilize and cause a higher level o stochastic motion. Teng et al. [121] examined the eect o article size on the thermal conductivity ratio o alumina/water nanoluids. The results o their work indicated that shrinkage o article size enhances the thermal conductivity ratio o nanoluids. 5. Shae o the nanoarticles There are mainly two article shaes investigated in nanoluid research; sherical and cylindrical articles. Nanoluids with sherical shae nanoarticles exhibit a smaller increase in thermal conductivity comared with the nanoluids having cylindrical (nano-rod or tube) nanoarticles [65] because cylindrical articles usually have a large lengthto diameter ratio [133]. Murshed et al. [65] reared nanoluids by disersing TiO 2 nanoarticles in rod-shaes o 10 nm 40 nm (diameter by length) and in sherical shaes o 15 nm in deionized water and comared the thermal conductivity o resulting nanoluids. They showed that article shae could aect the enhancement o thermal conductivity so that the increase in thermal conductivity or nanoluid with rod-shae nanoarticles is larger than those o with sherical shaed articles. 6. The eect o H The number o studies regarding the H o nanoluids is limited, comared to the other arameters. Karthik et al. [126] studied the inluence o H range including the isoelectric oint on the thermal conductivity o CuO-deionized water nanoluids. They observed that thermal conductivity ratio with H increases and reaches to a maximum close to the isoelectric oint and decreases as H urther increases. 7. Asect ratio Nanoarticles with a high asect ratio such as CNTs or nanorods greatly increase the thermal conductivity o the nanoluids. 8. Temerature A otentially imortant develoment in the ield o nanoluids is the strong temerature eect on thermal conductivity. Das et al. [21] systematically discussed the relationshi between thermal conductivity and temerature or nanoluids, noting signiicant increases o thermal conductivity with temerature. Lee et al. [2] measured the thermal conductivity o oxide nanoluids over the temerature range o C. The results revealed an almost threeold increase in conductivity enhancement or coer oxide and alumina nanoluids. These indings have revolutionized the alication o nanoluids because they indicate a much larger thermal conductivity at the elevated temeratures and even more attractive as cooling luid or devices with high energy density where the cooling luid is likely to work at a temerature higher than the room temerature. These results also oen u the ossibility that nanoluids could be emloyed as smart luids sensing hot sots and roviding more raid cooling in those regions. 9. Eect o clustering The clustering eect is always resent in nanoluids and is an eective arameter in thermal conductivity. Hong et al. [134] investigated this eect or Fe (10 nm)/ethylene glycol nanoluids. The thermal conductivity was determined as a unction o ultrasonic vibration time between 0 min and 70 min. It was observed that thermal conductivity ratio increases with increasing vibration time. For longer vibration times, the increase in conductivity ratio was smaller than that o the shorter vibration times. Furthermore, the variation o thermal conductivity o nanoluid with time, ater alying the vibration, was investigated and it was ound that thermal conductivity decreases as time rogresses. Variation o average size o clusters was also determined as a unction o time, ater alying the vibration, and the results showed that cluster size increases with time. The inal conclusion was that the size o the clusters ormed by the nanoarticles has a major inluence on the thermal conductivity. In addition, the variation o thermal conductivity ratio o the Fe/ethylene glycol nanoluid with article volume raction is nonlinear. This behavior is due to the act that nanoarticles in the 11

12 Goharshadi et al./phys. Chem. Res., Vol. 1, No. 1, 1-33, June nanoluids with high volume ractions orm clusters at a higher rate. Measurement o Thermal Conductivity o Nanoluids To measure the thermal conductivity o nanoluids, transient hot wire (THW) [135], transient lane source (TPS) [129], temerature oscillation (TO) [136], steady-state arallel late techniques [137], and otical methods [113] have been reorted. Transient hot wire method. The THW technique was introduced in 1974 [135]. It is the most oular dynamic method. In this technique, a cylindrical luid volume is heated electrically using a current-carrying metallic wire stretched along the axis o the luid volume. The dierential temerature rise o the wire is calculated based on the changes in the electrical resistance o the wire at dierent times and then lotting it against the natural logarithm o the time. This lot is exected to have a linear region, rom the sloe o which the thermal conductivity o the luid can be calculated. The advantages o the THW method are [138]: 1. Caability o the exerimentally eliminating convective error 2. Fast measurement time comared with other techniques 3. Obtaining reliable data Because in general nanoluids are electrically conductive, it is diicult to aly the ordinary THW technique directly. A modiied hot-wire cell and electrical system was roosed by Nagasaka and Nagashima [139] by coating the hot wire with an eoxy adhesive which has excellent electrical insulation and heat conduction. Transient lane source. TPS method is the modiied version o THW technique or heat transer measurements. TPS unit works using temerature coeicient o nickel sensor resistance. The bath temerature value can match with temerature o samle near the sensor. This hels in measuring recise thermal conductivity values at exact temeratures. Many materials have dierent thermal conductivity values at dierent temeratures so recise measurement o the thermal conductivity at certain temeratures minimizes the uncertainty. The TPS element behaves both as the temerature sensor and the heat source. The TPS method uses the Fourier law o heat conduction as a undamental rincile or measuring the thermal conductivity. The thermal conductivity o the nanoluid is determined by measuring the resistance o the robe. Advantages o using this method are (1) ast measurements, (2) measurements in wide ranges o thermal conductivities (rom 0.02 to 200 W m -1 K -1, with 2% uncertainty), (3) no need to samle rearation and (4) lexible samle sizes. The exerimental setu (as shown in Fig. 6 reerence [140]) comrises o thermal constants analyzer, a vessel, a constant temerature bath, and a thermometer. The robe o the thermal constant analyzer is immersed vertically in the vessel containing the nanoluid. The vessel is laced in the constant temerature bath and the thermometer is immersed in the vessel to measure the temerature o the nanoluid. The thermal conductivity o the nanoluid is determined by measuring the resistance o the robe [140]. Temerature oscillation technique. TO was introduced by Santucci and co-workers [136]. It consists o illing a cylindrical volume with the luid. The thermal conductivity is measured by alying an oscillating temerature boundary condition at the two ends o the cylinder. By measuring the amlitude and hase o the temerature oscillation, the luid thermal conductivity could be calculated [141]. The simlicity o the temerature oscillation technique makes it more aealing. Steady-state arallel late method. In steady-state method, a luid layer is subjected to a stationary temerature gradient while the heat low is measured as a unction o this gradient. Two steady-state geometries have ound wide accetance: concentric cylinders and arallel lates [137]. In the irst method, the luid is enclosed between two concentric cylinders in horizontal or vertical orientation, and heat is generated in the inner cylinder. In the second method, the luid is enclosed between two arallel lates and heat is generated in the uer late. The ossibility o convection is also resent in this steady state method but the arallel-late method oers the advantage that the heat is develoed rom above and thus convection develos less easily. Otical methods. Otical methods have been roosed as non-invasive techniques or thermal conductivity measurements to imrove accuracy. Indeed, because the hot wire is a combination o heater and thermometer, intererence is unavoidable. In otical techniques, detector and heater are always searated rom each other roviding otentially more accurate data. Additionally, measurements are comleted 12

13 Nanoluids or Heat Transer Enhancement-A Review/Phys. Chem. Res., Vol. 1, No. 1, 1-33, June within several microseconds, i.e., much shorter than reorted THW measurement times o 2 to 8 s, so that natural convection eects are avoided [113]. Models or Predicting the Eective Thermal Conductivity o Nanoluids There are no theoretical ormulas currently available to redict the thermal conductivity o nanoluids satisactorily. A variety o theoretical models have been develoed to redict the eective conductivity o nanoluids. Classical Models. Pls transer this line to line 1072 theoretical models have been derived to redict the thermal conductivity o susensions. For examle, or sherical articles, the models o Maxwell [5], Jerey [142], and Davis [143] and or nonsherical articles, the model o Hamilton and Crossover [144] have been widely used. We briely discuss some o the most widely used models. Maxwell model. The Maxwell model [5] was roosed or solid-liquid mixtures with relatively large articles. According to Maxwell model the eective thermal conductivity o susensions deends on the thermal conductivity o sherical articles, base liquid, and the volume raction o the solid articles: e 2 2( P ) 2 ( ) P (1) where is the thermal conductivity ratio, k /k, and β is deined as: 1 (3) 2 Equation (2) is accurate u to the order o 2. The higherorder terms reresent air interactions o randomly disersed sheres. The ratio k e /k is called thermal conductivity enhancement. Davis model. As two revious models, Davis model [143] is alied to sherical susensions and has the orm: e 3( 1) [ ( ) O( )] [ 2 ( 1) ] () is a unction o, ( ) 2. 5or 10and ( ) 0. 5 or. Jeery model is accurate u to the order o 2. Hamilton and Crosser model. Hamilton-Crosser model [117] is an imortant model or exlaining thermal conductivity enhancement in article shae deendent cases. Hamilton and Crosser roosed the ollowing model to redict the eective thermal conductivity or liquid-solid mixtures or non-sherical articles: (4) where k and k are the thermal conductivities o the base luid and nanoarticles, resectively, and is the volume raction. Maxwell s model redicts that the eective thermal conductivity o susensions containing sherical articles increases with the volume raction o the solid articles. When the article concentration is suiciently high, the Maxwell model ails to rovide a good match with the exerimental results. Jerey model. The Maxwell equation takes into account only the article volume concentration and the thermal conductivities o article and liquid. Jerey model [142] includes the eects o article-article interactions as well: (3...) e 2 2 (2) e P ( n 1) ( n 1)( P) ( n 1) ( ) P where n is the emirical shae actor given by n = 3/ψ, and ψ is the article shericity, deined as the ratio o the surace area o a shere with volume equal to that o the article, to the surace area o the article. Comarison o Eqs. (1) and (5) reveals that Maxwell s model is a secial case o the Hamilton and Crosser model or shericity equal to one. The revious models do not consider the eect o article sizes. However, the thermal conductivities redicted by the Hamilton-Crosser model are much lower than the exerimentally measured conductivities. The classical models originated rom continuum ormulations. They tyically involve only the article size/shae and volume raction and assume diusive heat P (5) 13

14 Goharshadi et al./phys. Chem. Res., Vol. 1, No. 1, 1-33, June transer in both luid and solid hases. Although they can give good redictions or micrometer or larger-size multihase systems, the classical models usually underestimate the enhancement o thermal conductivity increase o nanoluids as a unction o volume raction [113]. Recent Models Recently, many theoretical studies have been made and several mechanisms have been roosed in order to exlain the anomalous thermal conductivity enhancement obtained with nanoluids. Based on the eective medium aroximation and the ractal theory or the descrition o nanoarticle cluster and its radial distribution, a method or redicting the eective thermal conductivity o nanoluid was established by Wang et al. [145]. They took the size eect and the surace adsortion o nanoarticles into considerations. It can be exressed as: cl ( r) n( r) (1 ) 3 dr e ( ) 2 0 cl r (6) n( r) (1 ) 3 dr ( r) 2 0 cl where cl (r) is the eective thermal conductivity o clusters and n(r) is the radius distribution unction. To lower the dierence between the exerimental data and the redicted values by the revious models, it is necessary to develo a new model using the eective volume raction. From the viewoint o the mechanism o heat transer in nanoluids, the enhancement o thermal conductivity may be due to the eects o liquid layer on the articles and the eects o Brownian motion o nanoarticles [14]. The interace liquid has a strong interaction with articles that makes the interacial liquid layer a more ordered structure. The interacial-layer liquid has a higher thermal conductivity than that o the bulk hase liquid. Since the interace between solid and liquid is regarded as a very thin nanolayer and has semisolid material roerties, the eective volume o nanoarticles can be estimated using this nanolayer. The eect o interace on articles volume is not imortant in the susension with micrometer articles but it is very signiicant in the nanoluid. Yu and Choi [146] suggested that the eective volume raction o articles is e ( r h) (1 ) 3 and the eective thermal conductivity is e P ( n 1) e ( n 1)( P) [ ] ( n 1) ( ) P e where t is the thickness o nanolayer and β is the ratio o the nanolayer thickness, t, to the article diameter, d, (β = t/d ). The thermal conductivity o nanoluid can be estimated rom this eective volume raction using the revious semiemirical models. They suggested that the thickness o the liquid layer on nanoarticles is about 3 nm. However, since the thickness o liquid layer varies with the surace structure and shae o articles, it must be estimated more recisely. Xuan et al. [147] studied the thermal conductivity o nanoluids by considering Brownian motion and clustering o nanoarticles. He roosed an equation to redict the thermal conductivity o nanoluids: k k e k k 2k 2 k 2 ( k k ) c, ( k k ) 2k P k BT 3r r cl is the aarent radius o the nanoarticle clusters, which should be determined by exeriment. µ is the dynamic viscosity o the base luid. The irst term on the right-hand side o Eq. (9) is the Maxwell model or thermal conductivity o susensions o solid articles in luids. The second term on the right-hand side o Eq. (9) adds the eect o the random motion o the nanoarticles into account. Bhattacharya et al. [148] used Brownian dynamics simulation to determine the eective thermal conductivity o nanoluids, by considering the Brownian motion o the nanoarticles. Eective thermal conductivity o the nanoluid was deined as: k k (1 ) k (10) e ' where k' is not simly the bulk thermal conductivity o the nanoarticles but also includes the eect o the Brownian motion o the nanoarticles on the thermal conductivity. Jang and Choi [149] modeled the thermal conductivity o cl (7) (8) (9) 14

15 Nanoluids or Heat Transer Enhancement-A Review/Phys. Chem. Res., Vol. 1, No. 1, 1-33, June nanoluids by considering the eect o Brownian motion o nanoarticles. The roosed model is a unction o not only thermal conductivities o the base luid and nanoarticles but it also deends on the temerature and size o the nanoarticles. Energy transort in nanoluids was considered to consist o our modes; heat conduction in the base luid, heat conduction in nanoarticles, collisions between nanoarticles (due to Brownian motion), and microconvection caused by the random motion o the nanoarticles. Among these, the collisions between nanoarticles were ound to be negligible when comared to other modes. As a result o the consideration o the three remaining modes, the ollowing exression was resented: k e d 2 k (1 ) k 3c1 k Re d Pr (11) d where c l is a roortionality constant, d the diameter o the luid molecules, Pr Prandtl number o base luid, and k is deined so that it also includes the eect o the Kaitza resistance. Koo and Kleinstreuer [150] considered the eect o thermal conductivity enhancement due to both Brownian motion and static contribution on the eective thermal conductivity o nanoluids. This model takes into account the eects o the article dynamics. For the calculation o thermal conductivity o static art (K static ), Maxwell's model is used (Eq. (1)). For contribution o Brownian motion o articles, K Brownian was considered together with the eect o luid articles moving with nanoarticles around them. As a result, the ollowing exression was roosed: k Brownian c. k BT d (12) where ρ and ρ are the density o nanoarticles and base luid, resectively. c, is seciic heat caacity o the base luid. In the analysis, the interactions between nanoarticles and luid volumes moving around them were not considered and an additional term, γ was introduced to take that eect into account. Koo and Kleinstreuer indicated that this term becomes more eective with increasing volume raction. Another arameter,, was introduced to the model in order to increase the temerature deendency o the model. Both and γ were determined by utilizing available exerimental data. It is diicult to determine theoretical exressions or and γ due to the comlexities involved and this can be considered as a drawback o the model. Xue and Xu [151] resented another theoretical study or the eective thermal conductivity o nanoluids. In their derivation, nanoarticles were assumed to have a liquid layer around them with a given seciic thermal conductivity. The resulting imlicit exression or thermal conductivity o nanoluids is: ke k ( k e k )(2k k ) ( k k )(2k ke ) ( 1 ) 0 2k k (2k k )(2k k ) 2 ( k k )( k k ) e e where subscrit reers to nanolayer. α is deined as: e (13) 3 ( ) (14) d d t By considering the eect o the interacial layer at the solid article/liquid interace, Leong et al. [152] develoed a model or determining the eective thermal conductivity o nanoluids. k e ( k k ) k 3 3 [2 1] ( k 3 ( k 1 2 2k ) ( k k 2k 3 3 ) [ ( k 3 3 ) [ 1] 2 2 k ) k ] (15) where β 1 = t/2d. This model accounts or the eects o article size, interacial layer thickness, volume raction, and thermal conductivity. I there is no interacial layer at the article/liquid interace i.e., k lr = k and β 1 = β = 1, Eq. (15) reduces to the Maxwell model (Eq. (1)). Another study regarding the eect o nanolayers was made by Sitrasert et al. [153]. They modiied the model roosed by Leong et al. [152] by taking the eect o temerature on the thermal conductivity and thickness o nanolayer into account. Sitrasert et al. rovided the ollowing relation or the determination o nanolayer thickness: t (16) ( T 273) d 15

16 Goharshadi et al./phys. Chem. Res., Vol. 1, No. 1, 1-33, June Ater the determination o nanolayer thickness, thermal conductivity o the nanolayer should be ound according to the exression: t k C k (17) r where C is 30 and 110 or Al 2 O 3 and CuO nanoarticles, resectively. Yang and Du [154] roosed a thermal conductivity model which includes the eects o the interacial layer ormed by the suractant and liquid molecules ugrading Leong et al. model [152]. Based on the analysis o disersion tye, the thickness o the interacial layer is deined by the length o the suractant molecule or nanoluid under monolayer adsortion disersion and double lengths o the suractant molecule or nanoluid under electric double layer adsortion disersion. The model or cylindrical coordinates is as ollows (nanoluids containing nanotubes): k e ( k k ) k 2 2 [2 2 ( k 1 1 2k 1] ( k ) ( k k k 2 2 ) [ ( k 2 2 ) [ 1 1 1] k ) k ] (18) Concerning theories/correlations which try to exlain thermal conductivity enhancement or all nanoluids, not a single model can redict a wide range o exerimental data [113]. Potential Mechanisms o the Enhancement o Heat Conduction in Nanoluids Dierent authors roosed dierent mechanisms o heat transer in nanoluids [145,146,149, ]. The roosed mechanisms or the anomalous enhancement discussed in the literature are: 1. Motion o the naonoarticles 2. Liquid layering at liquid/article interace 3. Nature o heat transort in nanoarticles 4. Eects o nanoarticle clustering 5. Surace charge state 6. Couled transort Because o the comlexity and contradiction in nanoluids, the research community has not reached a solid consensus on the mechanisms. Motion o the nanoarticles. The energy exchange in direct nanoarticle-nanoarticle contact arising rom article collisions in the nanoluid could result in an enhancement o the thermal conductivity. Such collisions arise rom the motion o the nanoarticles. Furthermore, even without collisions the Brownian motion o articles might enhance thermal conductivity. The movement o nanoarticles due to Brownian motion is too slow to transort signiicant amounts o heat through a nanoluid. Although, Brownian motion cannot directly result in an enhancement o the thermal transort roerties, it could have an imortant indirect role in roducing article clustering that could signiicantly enhance thermal conductivity [160]. Liquid layering at liquid/article interace. Liquid molecules are known to orm ordered layered structures at solid suraces and these interacial layers have dierent thermohysical roerties rom the bulk liquid and solid articles. Because o the ordered structure o the nanolayer, it is exected to have higher thermal conductivity than the bulk liquid [161]. Although, the resence o an interacial layer may lay a role in heat transort, it is not likely to be solely resonsible or the enhancement o thermal conductivity [162]. Nature o heat transort in nanoarticles. Macroscoic theories assume that heat is transorted by diusion. In crystalline solids, heat is carried by honons, that is, by the roagation o lattice vibrations. Such honons are created at random, roagate in random directions and are scattered by each other or by deects. When the size o the nanoarticles in a nanoluid becomes less than the honon mean-ree ath, honons no longer diuse across the nanoarticle but move ballistically without any scattering. Without going into the details o ballistic heat transort, it is diicult to envision how ballistic honon transort could be more eective than a veryast diusion honon transort [160]. Eects o nanoarticle clustering. I articles cluster into ercolating networks, they would create aths o lower thermal resistance and thereby have a major eect on the eective thermal conductivity. However, clustering to the extent that solid agglomerates san large distances is unlikely; moreover any such large clusters would most likely settle out o the luid. A urther dramatic increase o thermal conductivity can take lace i the articles do not need to be in hysical contact 16