**2. Introduction to nanofluids**

Nanofluids are a new generation of HTFs with anomalous behavior, engineered by homogeneously suspending nanometer-sized materials or structures within conventional fluids. In other words, nanofluids are nanoscale colloidal suspensions containing condensed nanomaterials. Nanofluids have been found to possess enhanced thermo-physical properties such as thermal conductivity, thermal diffusivity, viscosity, and convective heat transfer coefficients compared to those of base fluids such as water (DiW), ethylene glycol (EG) or oils. They have demonstrated great potential applications in many fields such as microelectronics, transportation, industrial cooling, magnetic sealing, reducing pollution, space and defense, energy storage, air conditioning, power transmission systems, medical therapy and diagnosis, antibacterial activity nanodrug delivery, fuel cells, components and tools wear, friction reduction and nuclear systems cooling, etc. [3–8]. Among diverse techniques to cool down or maintain certain temperature in these systems, the use of fins, vanes or radiators as well as forced air/fluids through cooling channels are being used, even though these are costly. Diverse machinery and devices use inexpensive conventional HTFs to intensify heat dissipation. However, the inherent limitation of these fluids is the relatively low thermal conductivity; water for instance, is roughly three orders of magnitude less conductive than copper or aluminum (**Table 1**). What these conventional fluids lack in thermal conductivity however, is compensated by their ability to flow.

The main mechanism for heat transfer in fluids is convection; its efficacy mostly depends on the thermo-physical properties of the conventional fluids. Furthermore, if the thermal conductivity of conventional fluids were enhanced, it would be much more effective. Hence, since the solid materials possess several orders of higher thermal conductivities, compared

**Material Thermal conductivity** 

Multi wall nanotubes (MWCNTs)

O3

O4

Molybdenum disulfide (MoS<sup>2</sup>

Cobalt oxide (Co<sup>3</sup>

Silicon oxide (SiO<sup>2</sup>

Tungsten disulfide (WS<sup>2</sup>

**Table 1.** Typical thermal conductivities for diverse conventional fluids and solid materials.

Titania (TiO<sup>2</sup>

Zirconia (ZrO<sup>2</sup>

carbon (diamond)

Nonmetallic solids Alumina (Al<sup>2</sup>

Conventional fluids Water (DiW) ~0.598–0.609 [9–11]

Carbon structures Single wall nanotubes (SWCNTs) 3000–6000 [18–21]

Metallic solids Aluminum 237 [27]

**(W/m K)**

~3000 [22, 23] 900–2320 [24, 25]

Thermal Transport and Challenges on Nanofluids Performance

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Ethylene glycol (EG) ~0.251 [12–14] Engine oil ~0.145 [12, 14] Mineral oil ~0.115 [3, 15] Kerosene ~0.139 [16] R141b refrigerant ~0.089 [17]

Carbon (graphite) 119–165 [24] Graphene ~3000 [4] Graphite 130–2000 [26]

Copper 398 [27] Gold 315 [27] Silver 424 [27]

Aluminum nitride (AlN) 319–550 [29] Boron nitride (h-BN) ~300 [30, 31] Boron nitride nanotubes (BNNTs) ~600–960 [32–34]

Copper oxide (CuO) 76.5 [36]

Silicon carbide (SiC) 148–270 [27, 38]

Zinc oxide (ZnO) 13–29 [13, 39]

) 31–41 [26, 28]

) 12.8 [35]

) 1.4–12 [39–42]

) 32–124 [43, 44]

) 8.4–11.2 [13, 39, 42]

) 2.2 [45]

) 34.5 ± 4 [37]

**Reference**

217


needs of mankind [2]. Nowadays, with increasing pressure of globalized markets and companies' profit race, a dramatic search to obtain proper materials performance, optimizing components and devices designs, improving efficiencies, reducing tools wear, materials consumption and pollution, and obtaining the most possible revenue. In addition to issues regarding materials scrap, maintenance and components wear among others, a hot topic in industry is the heat dissipation. Among diverse forms of energy used, over 70% is produced in or through the form of heat [2]. Heat transfer is a crucial area of research and study in thermal engineering. Heat is transferred either to input energy into a system or to remove the energy produced in a system. Hence, reducing energy loss and intensifying heat transfer processes are becoming paramount tasks to be addressed. Therefore, thermal management plays a vital factor concerning devices, machinery or apparatuses performance; thermal transport role has been subjected to countless investigations and is under the scope of the operational useful life of these components and devices. Being this an opportunity area for successful heat management and energy efficient

Nanotechnology is a science that deals with diverse characteristics and properties of materials at a nanometric level (1 nm = 10−9 m). Recently, diverse techniques, equipment, and instrumentations have been devised, as well as various relevant and interesting characteristics and properties of these materials were sorted out. Hence, with aid of nanotechnology, with novel developments linking electronic, optical, mechanical, and magnetic properties, industrial devices have emerged, and this trend is certainly continuing in this century. Cooling of electric, electronic and mechanical devices has been a hot topic in today's fast-growing technologies. The heat required to be dissipated from systems is continually increasing due to industrial and economic trends to miniaturize designs, make better use of resources, obtain more power output, develop and use more environmentally friendly materials, and obtain more profits.

Nanofluids are a new generation of HTFs with anomalous behavior, engineered by homogeneously suspending nanometer-sized materials or structures within conventional fluids. In other words, nanofluids are nanoscale colloidal suspensions containing condensed nanomaterials. Nanofluids have been found to possess enhanced thermo-physical properties such as thermal conductivity, thermal diffusivity, viscosity, and convective heat transfer coefficients compared to those of base fluids such as water (DiW), ethylene glycol (EG) or oils. They have demonstrated great potential applications in many fields such as microelectronics, transportation, industrial cooling, magnetic sealing, reducing pollution, space and defense, energy storage, air conditioning, power transmission systems, medical therapy and diagnosis, antibacterial activity nanodrug delivery, fuel cells, components and tools wear, friction reduction and nuclear systems cooling, etc. [3–8]. Among diverse techniques to cool down or maintain certain temperature in these systems, the use of fins, vanes or radiators as well as forced air/fluids through cooling channels are being used, even though these are costly. Diverse machinery and devices use inexpensive conventional HTFs to intensify heat dissipation. However, the inherent limitation of these fluids is the relatively low thermal conductivity; water for instance, is roughly three orders of magnitude less conductive than copper or aluminum (**Table 1**). What these conventional fluids lack in thermal conductivity however, is compensated by their ability to flow.

fluid-based heat transfer systems, with aid of reinforced materials.

**2. Introduction to nanofluids**

216 Microfluidics and Nanofluidics

**Table 1.** Typical thermal conductivities for diverse conventional fluids and solid materials.

The main mechanism for heat transfer in fluids is convection; its efficacy mostly depends on the thermo-physical properties of the conventional fluids. Furthermore, if the thermal conductivity of conventional fluids were enhanced, it would be much more effective. Hence, since the solid materials possess several orders of higher thermal conductivities, compared with that of conventional fluids, an idea to introduce conducting particles to fluids was considered. Among diverse particles geometry, different particle shapes occur naturally or are engineered for specific applications, as shown in **Figure 1**.

Choi-Eastman group [68] for copper nanostructures dispersed in water as well. Nanofluids research has been exploited and novel developments have been able to fulfill industrial necessities. This research area has been increasing through time, starting at 5 publications in 2003, reaching up to more than 2100 publications by 2017, according to scientific search engine "sciencedirect.com" and keyword Nanofluids. Nanofluids are a novel class of stable heat-transfer suspensions which are engineered containing homogeneously dispersed solid nanofillers. Compared to micro- or millifluids, nanofluids tend to be more stable, since nanofillers possess unique properties, such as large surface area to volume ratio, as well as dimension-dependent physical properties, which make nanostructures better and more stably dispersed in conventional fluids. Nevertheless, some limitations of the effective incorporation of nanostructures within conventional HTFs are dispersion and solubility, because these tend to aggregate and sediment over time. In some cases, additives or surfactants are used to stabilize the nanostructures within the fluids, even though the surfactants could affect and diminish the thermal conductivity of the nanofluids, since surfactants introduce defects at the interfaces [70, 71]. Therefore, one of the main advantages of nanofluids is that they can be specially engineered to optimally fulfill specific objectives, such as enhanced thermal conductivity, a higher thermal energy storage capacity, higher heat transfer coefficients, a better temperature stabilization and less pressure drop, among others. Hence, search for new nanofillers which can get high

Thermal Transport and Challenges on Nanofluids Performance

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219

It has been demonstrated that nanofluids for heat transfer applications have provided better thermal performance than conventional fluids [3, 12, 73, 74]. Hence, the advent of nanofluidbased heat transfer systems can make compact designs with high efficient thermal, physical and electrical performance for instruments and devices. Experiments on convection heat transfer of nanofluids were conducted by several research groups [75–77], showing significant improvements in heat transfer rates of nanofluids. Meanwhile, the thermal conductivity enhancement of nanofluids show a temperature-dependent characteristic and increase of enhancement with rising temperature, which makes the nanofluids more suitable for applications at elevated temperatures [3, 6, 78–81]. Additionally, previous research has shown that nanofluids display better performance in their thermo-physical properties, such as thermal conductivity, thermal diffusivity, viscosity, friction, etc., compared to conventional fluids [3, 4, 82–87]. Hence, nano-

The manipulation of matter on the nanometer scale has become a central focus from both fundamental and technological perspectives. Unique, unpredictable, and highly intriguing physical, electrical, mechanical, optical and magnetic phenomena result from the confinement of matter into nanoscale features. Morphology control in nanostructures has become a key issue in the preparation of electronic or mechanical nanodevices and functional materials [88]. A wide variety of combinations of nanostructures and conventional fluids can be used to synthesize and prepare stable nanofluids for diverse applications. Nanofluids could be manufactured by two methods. The first step method is a process in which, simultaneously,

thermal conductivities at lower filler fractions is important [3, 72].

fluids could be used for aforesaid engineering applications.

**3. Synthesis and preparation of nanofluids**

Heat transfer using fluids is a very complicated phenomenon, and various factors such as fluid stability, composition, viscosity, surface charge, interface, and morphology of the dispersed nanostructures influence the observed results [3, 6, 46–59]. Optimization and high efficiency of components and devices have gained great importance since these factors play a crucial role in diverse fields. Solid materials such as metals, CNTs, oxide/nitride/carbide ceramics, semiconductors, and composite materials having higher thermal conductivity can be homogeneously suspended and stabilized within conventional fluids, resulting in better thermal transport performance composite fluids. Nevertheless, improvement in thermal conductivity cannot be achieved by just increasing the solid filler concentration because each system presents a threshold, in which beyond a certain limit, increasing the filler fraction will also increase the viscosity, which will adversely affect the fluid properties and performance.

Most early studies used suspensions of millimeter or micrometer-sized particles, which led to countless problems, such as a tendency to rapidly sediment, unless flow rate is increased; not only losing the improvements in thermal conductivity, but also forming sludge sediments, increasing the thermal resistance and impairing the heat transfer capacity of the conventional fluids. Furthermore, fluids of this scale size could have considerably larger pressure drops [60–64], thus making flow through small channels much more difficult since diverse parameters are critical for device performance, such as morphology and stability of nanostructures, fluids composition, viscosity, fast sedimentation, channels clogging, erosion or wear, among others, which are often very serious for systems consisting of small channels [3, 65–69].

A revolution in the field of HTFs arose with the advent of nanofluids (NFs), a term introduced by Prof. Choi's research group in the late 1990s at Argonne National Lab [68]. The first investigations were performed by Masuda et al. [69] for Al2 O3 nanoparticles within water, and by

**Figure 1.** Diverse particle shapes and geometries.

Choi-Eastman group [68] for copper nanostructures dispersed in water as well. Nanofluids research has been exploited and novel developments have been able to fulfill industrial necessities. This research area has been increasing through time, starting at 5 publications in 2003, reaching up to more than 2100 publications by 2017, according to scientific search engine "sciencedirect.com" and keyword Nanofluids. Nanofluids are a novel class of stable heat-transfer suspensions which are engineered containing homogeneously dispersed solid nanofillers. Compared to micro- or millifluids, nanofluids tend to be more stable, since nanofillers possess unique properties, such as large surface area to volume ratio, as well as dimension-dependent physical properties, which make nanostructures better and more stably dispersed in conventional fluids. Nevertheless, some limitations of the effective incorporation of nanostructures within conventional HTFs are dispersion and solubility, because these tend to aggregate and sediment over time. In some cases, additives or surfactants are used to stabilize the nanostructures within the fluids, even though the surfactants could affect and diminish the thermal conductivity of the nanofluids, since surfactants introduce defects at the interfaces [70, 71]. Therefore, one of the main advantages of nanofluids is that they can be specially engineered to optimally fulfill specific objectives, such as enhanced thermal conductivity, a higher thermal energy storage capacity, higher heat transfer coefficients, a better temperature stabilization and less pressure drop, among others. Hence, search for new nanofillers which can get high thermal conductivities at lower filler fractions is important [3, 72].

It has been demonstrated that nanofluids for heat transfer applications have provided better thermal performance than conventional fluids [3, 12, 73, 74]. Hence, the advent of nanofluidbased heat transfer systems can make compact designs with high efficient thermal, physical and electrical performance for instruments and devices. Experiments on convection heat transfer of nanofluids were conducted by several research groups [75–77], showing significant improvements in heat transfer rates of nanofluids. Meanwhile, the thermal conductivity enhancement of nanofluids show a temperature-dependent characteristic and increase of enhancement with rising temperature, which makes the nanofluids more suitable for applications at elevated temperatures [3, 6, 78–81]. Additionally, previous research has shown that nanofluids display better performance in their thermo-physical properties, such as thermal conductivity, thermal diffusivity, viscosity, friction, etc., compared to conventional fluids [3, 4, 82–87]. Hence, nanofluids could be used for aforesaid engineering applications.
