**4.1. Thermal performance of nanofluids**

Heat transfer is classified into various mechanisms, such as thermal conduction, thermal convection and thermal radiation. In diverse fields, thermal transport is a critical parameter to obtain efficient performance of machinery and devices. Heat convection occurs when bulk flow of a fluid (liquid or gas) carries heat along with the flow of matter in the fluid, this process could be "forced," where fluid motion is generated by an external source such as a pump, fan or other mechanical means, or "natural," by density differences in the fluid occurring due to temperature gradients. Radiation heat transfer is the transfer of energy by means of photons in electromagnetic waves in much the same way as electromagnetic light waves transfer light. On the other hand, heat conduction is the direct microscopic exchange of kinetic energy of particles through the boundary between two systems. When an object is at a different temperature from another body or its surroundings, heat flows so that the body and the surroundings reach the same temperature, at which point they are in thermal equilibrium. The thermal conductivity (*k*) of liquids can be successfully measured if the time taken to measure *k* is very small so that the convection current does not develop [202].

Diverse techniques have been proposed to measure nanofluids thermal conductivity over the past years. The most common techniques to measure the effective thermal conductivity of nanofluids are the transient hot-wire method [3, 147, 202–207], steady-state method [109, 208–210], cylindrical cell method [211], temperature oscillation method [183, 212, 213], and 3-ω method [40, 214–216] to name some. Eastman et al. reported a 40% enhancement with only 0.40 vol.% of copper oxide (CuO) particles [217], while Choi et al. reported a remarkable 160% increase in thermal conductivity of MWCNTs/engine oil nanofluid at 1.0 vol.% filler fraction of nanotubes [218]. Ilyas et al. [58] study a commercial brand oil (THO) with incorporation of MWCNTs where significant thermal conductivity improvement of ~22 and ~30% was achieved at 35 and 60°C, respectively, at 1.0 wt.%. Hwang et al. [41] investigated the thermal conductivity of DiW- and EG-based nanofluids reinforced with MWCNTs, CuO and SiO<sup>2</sup> . It was observed that thermal conductivity was improved almost linearly as filler fraction increased. For DiW-based systems, the addition of SiO<sup>2</sup> , CuO and MWCNTs at 1.0 vol.% filler fraction showed an increase of 3, 5 and ~12%, respectively. Also, CuO/EG nanofluid at 1.0 vol.% showed an increase of ~9%. Wen and Ding et al. [192] also investigated the effects of MWCNTs within DiW, with addition of 0.25 wt.% gum Arabic (GA) dispersant with respect to DiW. For MWCNTs at 0.50 wt.% and 1.0 wt.%, an increase in thermal conductivity enhancement was achieved up to ~30% and ~38% at 25°C, and ~35% and ~80% at 30°C, respectively. It was found that these improvements were slightly higher than the results reported by Liu et al. [14], Assael et al. [219], Wen et al. [220], and Xie et al. [221], but lower than the results showed by Choi et al. [218]. There are diverse factors that cause these discrepancies among the different groups; as mentioned by Sing [55], these discrepancies should rely on the dependency of thermal conductivity is on diverse important factors such as the structure and properties of the CNTs, their aspect ratio, clustering, addition of dispersants, temperature and the experimental errors involved as well.

create a low thermal resistance path which can enhance the thermal conductivity, according to Saterlie et al. [197]. The ordered structure could have higher thermal conductivity than that of the conventional, therefore an enhancement of the effective thermal conductivity. Various researchers have suggested that there is liquid layering on the nanoparticles, which helps to enhance the heat transfer properties of nanofluids [197–201]. Yu et al. [200] proved the formation of layers by the liquid molecules close to a solid surface, even though the thickness and thermal conductivity of the nanolayers are not well known yet. According to Elis Josna Mary et al. [164], a temperature-dependent, linear variation in thermal conductivity increase with

Diverse studies on nanofluids have been carried out by many researchers. This section deals with literature review on nanofluids; nanofluids preparation and characterization, thermophysical properties, as well as nanofluids applications, which lays foundation and basis for

Heat transfer is classified into various mechanisms, such as thermal conduction, thermal convection and thermal radiation. In diverse fields, thermal transport is a critical parameter to obtain efficient performance of machinery and devices. Heat convection occurs when bulk flow of a fluid (liquid or gas) carries heat along with the flow of matter in the fluid, this process could be "forced," where fluid motion is generated by an external source such as a pump, fan or other mechanical means, or "natural," by density differences in the fluid occurring due to temperature gradients. Radiation heat transfer is the transfer of energy by means of photons in electromagnetic waves in much the same way as electromagnetic light waves transfer light. On the other hand, heat conduction is the direct microscopic exchange of kinetic energy of particles through the boundary between two systems. When an object is at a different temperature from another body or its surroundings, heat flows so that the body and the surroundings reach the same temperature, at which point they are in thermal equilibrium. The thermal conductivity (*k*) of liquids can be successfully measured if the time taken to mea-

Diverse techniques have been proposed to measure nanofluids thermal conductivity over the past years. The most common techniques to measure the effective thermal conductivity of nanofluids are the transient hot-wire method [3, 147, 202–207], steady-state method [109, 208–210], cylindrical cell method [211], temperature oscillation method [183, 212, 213], and 3-ω method [40, 214–216] to name some. Eastman et al. reported a 40% enhancement with only 0.40 vol.% of copper oxide (CuO) particles [217], while Choi et al. reported a remarkable 160% increase in thermal conductivity of MWCNTs/engine oil nanofluid at 1.0 vol.% filler fraction of nanotubes [218]. Ilyas et al. [58] study a commercial brand oil (THO) with incorporation of MWCNTs where significant thermal conductivity improvement of ~22 and ~30% was achieved at 35 and 60°C, respectively, at 1.0 wt.%. Hwang et al. [41] investigated the thermal conductivity of

filler fraction was observed, which could be attributed to liquid layering.

sure *k* is very small so that the convection current does not develop [202].

**4. Nanofluids application fields**

**4.1. Thermal performance of nanofluids**

further investigations.

230 Microfluidics and Nanofluidics

Nanodiamonds (NDs) dispersed in EG and mineral oil (with addition of OA) were studied by Branson et al. [222]. It was observed that addition of 0.88 vol.% of NDs enhanced the thermal conductivity by ~12%. In MO, for instance, an enhancement of ~6% and ~11% is achieved at NDs loading of 1.0 vol.% and 1.9 vol.%, respectively. According to Branson et al., the differences on enhancement efficiencies are attributable to divergence in thermal boundary resistance at nanoparticle/surfactant interfaces [222]. Research by Khairul et al. [173] on the effects of Al<sup>2</sup> O3 and CuO nanoparticles filler fraction and use of SDBS surfactant on viscosity and thermal conductivity of water-based nanofluids was performed. It was observed that thermal conductivity of the nanofluids increased nonlinearly with increasing nanoparticles filler fraction, with a maximum increase of 10 and 14% for Al<sup>2</sup> O3 and CuO, respectively, at 0.15 wt.%, similar to what Kong et al. found for the maximal enhancement for Al2 O3 nanofluids [223]. On the field of oxide nanostructures, Yiamsawasd et al. [224] reported a maximum thermal conductivity enhancement of 20% for TiO<sup>2</sup> /water nanofluid. Elis Josna Mary et al. [164] investigated CeO2 /EG nanofluid and observed a temperature effect on thermal conductivity rise of 17% and ~11% at 10 and 30°C, respectively. Serebryakova et al. [225] investigated the effects of dispersing Al<sup>2</sup> O3 within EG/water mixtures. It was observed that thermal conductivity performance was improved by 5% at 1.5 vol.%. Mariano et al. [226] estimated thermal conductivity behavior and rheological properties of Co3 O4 /EG nanofluids and obtained thermal conductivity enhancement of 27% at 5.0 wt.%. Aluminum nitrides (AlN) can also find many applications in the heat exchange process. Thermal conductivity performance of AlN/ethanol nanofluid was investigated by Hu et al. [227]. Results showed a 20% increase in the thermal conductivity of ethanol with 4.0 vol.% at room temperature. Furthermore, a strong temperature dependence of the thermal conductivity was observed in this research. Yu et al. [228] thermal conductivity of AlN dispersed in two different conventional fluids, such as EG and propylene glycol (PG), was investigated. It was found a 39 and 40% thermal conductivity improvement for EG and PG, respectively, having the same particle size and nanoparticles filler fraction.

A great improvement on 2D-nanostructure-based nanofluids was obtained by Taha-Tijerina et al. [3], where exfoliated h-BN and graphene were homogeneously dispersed within mineral oil with superb thermal conductivity increase up to ~80% at very low filler fractions (<0.10 wt.%) with no significant increase in viscosity. Continuing with 2D structures, several research studies have developed graphene-based nanofluids with high nanoparticle stability and significant enhancements [229–235]. Shaikh et al. studied the effect of exfoliated graphite dispersed within PAO oil at various concentrations, ranging from 0.10 vol.% up to 1.0 vol.%. It was observed that addition of 2D-structures improved the thermal conductivity from 18% up to ~130%, respectively [230]. Hadadian et al. [236] prepared highly stable graphene oxide (GO)-based nanofluid. Thermal transport of EG increased by 30% with 0.07 GO mass fraction. Other EG-based nanofluids synthesized by Yu et al. [237, 238] have shown better enhancements of 61 and 86% with GO [237] and graphene nanosheets [238], respectively, at 5.0 vol.% loading.

Diverse theories explain the mechanisms that could affect the behavior of nanofluids; the most accepted being Brownian motion [40, 104, 239, 240], percolation theory [55, 104, 198, 241, 242], micro convection cell model [104, 198, 239–242], and liquid layering theory [55, 104, 193, 198, 242, 243]. **Table 2** shows the influence of oil-based nanofluids on thermal conductivity.

> Similarly, **Table 3** shows the results from diverse investigations on other water-based nanofluids; and various materials and sizes used as reinforced nanoparticles. **Table 4** shows the

**Filler Nanoparticles morphology Filler fraction TC enhancement Ref.**

O3 Spherical ~60 nm diam. 5.0 vol.% ~20% [141]

O3 Spherical ~27–56 nm diam. 1.6 vol.% ~10% [247]

O3 Brick ~20 × 40 × 40 nm 7.0 vol.% 16% [140]

O3 Platelet ~15 nm diam., 5 nm thick 7.0 vol.% 28% [140]

O3 Blade ~8 × 15 nm, 5 nm thick 7.0 vol.% 23% [140] Au Spherical ~10–20 nm diam. 0.00026 vol.% ~8% [11] Ag Spherical ~60–80 nm diam. 0.001 vol.% ~5% [11]

Cu3 Spherical ~60–100 nm diam. 1.0 vol.% ~48% [197]

4.0 vol.% 4.0 vol.%1

3.0 vol.%

2.0 vol.% 3.0 vol.%

5.0 vol.%

**Filler fraction TC enhancement Ref.**

Thermal Transport and Challenges on Nanofluids Performance

http://dx.doi.org/10.5772/intechopen.72505

~10% ~17% ~45%

0.5 vol.% ~8.5% [92]

1.0 vol.% 160% [218]

1.0 vol.% ~175% [19]

~11% ~10% ~25%

~9% ~14%

11% 16% 21%

~12% ~60% [18]

233

[246]

[123]

[47]

[12]

0.25 vol.% 0.5 vol.% 1.0 vol.%

influence of various nanofluids in thermal management properties as well.

**morphology**

Diameter: 10–50 nm

Diameter: 10–30 nm

Diameter: 25 nm

Diameter: 20–300 nm

**Filler Type of oil Nanoparticles** 

MWCNT Engine oil (15 W-40) Rods ~length: 0.3–10 μm

MWCNT Mineral oil Rods ~length: 10–50 μm

MWCNT Synthetic PAO oil Rods ~length: 50 μm

MWCNT Poly-α-olefin (PAO) Rods ~length: 1–100 μm

**Table 2.** Influence of oil-based nanofluids in thermal management.

Al2

Al2

Al2

Al2

Al2

Al2

*Notes*: if not specified, measurements were conducted at room temperature.

O3 Spherical ~131 nm diam. 1.0 vol.%1

CeO2 Spherical 74 nm diam. 2.0 vol.%

CeO2 Spherical 30 nm diam. 1.0 vol.%

CuO Spherical ~36 nm diam. 1.0 vol.%



**Table 2.** Influence of oil-based nanofluids in thermal management.

**Filler Type of oil Nanoparticles** 

CeO2 Transformer oil @ 50C

Al2

232 Microfluidics and Nanofluidics

Al2

**morphology**

GO [237] and graphene nanosheets [238], respectively, at 5.0 vol.% loading.

AlN Mineral oil Spherical ~50 nm diam. 0.05 vol.% ~8% [244]

with no significant increase in viscosity. Continuing with 2D structures, several research studies have developed graphene-based nanofluids with high nanoparticle stability and significant enhancements [229–235]. Shaikh et al. studied the effect of exfoliated graphite dispersed within PAO oil at various concentrations, ranging from 0.10 vol.% up to 1.0 vol.%. It was observed that addition of 2D-structures improved the thermal conductivity from 18% up to ~130%, respectively [230]. Hadadian et al. [236] prepared highly stable graphene oxide (GO)-based nanofluid. Thermal transport of EG increased by 30% with 0.07 GO mass fraction. Other EG-based nanofluids synthesized by Yu et al. [237, 238] have shown better enhancements of 61 and 86% with

Diverse theories explain the mechanisms that could affect the behavior of nanofluids; the most accepted being Brownian motion [40, 104, 239, 240], percolation theory [55, 104, 198, 241, 242], micro convection cell model [104, 198, 239–242], and liquid layering theory [55, 104, 193, 198, 242, 243]. **Table 2** shows the influence of oil-based nanofluids on thermal conductivity.

~8–10 atomic layer thick

~0.8–1.2 nm thick

~5 atomic layers thick

~5 atomic layers thick

O3 Engine oil Spherical ~80 nm diam. 0.5 vol.%

O3 Engine oil Spherical ~28 nm diam. 5.0 vol.%

Al Engine oil Spherical ~80 nm diam. 1.0 vol.%

CuO Mineral oil Spherical ~100 nm diam. 2.5 vol.%

Diamond Mineral oil Spherical ~<10 nm diam. 1.0 vol.%

Graphene Mineral oil (50°C) 2D-sheets ~500 by 500 nm

Graphene Heat-transfer oil 2D sheets ~0.5–2.0 μm

h-BN Mineral oil (50°C) 2D sheets ~500 by 500 nm

h-BN Synthetic fluid 2D sheets ~500 by 500 nm

**Filler fraction TC enhancement Ref.**

~9% ~12%

~26% ~30%

~20% ~37%

~12% ~23%

~5% ~11%

~10% ~80%

8% 17% 25%

~9% ~10% ~80%

0.10 wt.% 8% [6]

[194]

[209]

[194]

[245]

[222]

[3]

[147]

[3]

1.0 vol.%

7.5 vol.%

3.0 vol.%

5.0 vol.%

1.9 vol.%

0.01 wt.% 0.10 wt.%

0.05 wt.% 0.10 wt.% 0.20 wt.%

0.01 wt.% 0.05 wt.% 0.10 wt.%

Spherical ~3–7 nm diam. 0.7 vol.% ~15% [203]

Similarly, **Table 3** shows the results from diverse investigations on other water-based nanofluids; and various materials and sizes used as reinforced nanoparticles. **Table 4** shows the influence of various nanofluids in thermal management properties as well.



**5. Summary**

**Filler Conventional fluid Nanoparticles** 

O3 Ethylene glycol Spherical ~10 nm

Au Toluene Spherical ~10–20 nm

CuO Ethylene glycol Spherical ~35 nm

CuO Ethylene glycol Spherical ~23 nm

O<sup>4</sup> Kerosene Spherical ~15 nm

MWCNT Ethylene glycol Rods length: 30 μm

MWCNT Ethylene glycol Rods length: μm

SiO2 Ethanol Spherical ~23 nm

SiO2 Ethylene glycol Spherical ~23 nm

TiO<sup>2</sup> Ethylene glycol/water (20/80%)

Ethylene glycol @

Graphene Ethylene glycol @ 20°C

50°C

h-BN Stamping lubricant @ 50°C

h-BN Metal cutting fluid @ 50°C

AlN Ethylene glycol Spherical ~50 nm

Al2

Fe<sup>3</sup>

**morphology**

diam.

diam.

diam.

diam.

diam.

diam.

2D sheets ~500 × 600 nm

2D sheets ~500 × 500 nm

2D sheets ~500 × 500 nm

Diam.: 15 nm

Diam.: ~20–30 nm

Spherical ~21 nm

range

diam.

diam.

diam.

*Notes*: if not specified, measurements were conducted at room temperature.

**Table 4.** Influence of diverse nanofluids in thermal management.

~5 atomic layer thick

~5 atomic layer thick

**Filler fraction TC** 

5.0 vol.% 10.0 vol.%

0.50 vol.% 1.0 vol.%

0.01 wt.% 0.10 wt.%

0.01 wt.% 0.10 wt.%

0.05 vol.% 1.0 vol.%

0.50 vol.% 1.0 vol.%

0.14 vol.% 0.14 vol.% 6.5%

**enhancement**

http://dx.doi.org/10.5772/intechopen.72505

5.0 vol.% ~18% [217]

Thermal Transport and Challenges on Nanofluids Performance

0.011 vol.% ~9% [11]

4.0 vol.% ~22% [190]

~15.0 vol.% ~55% [209]

~15% ~34%

36%

~25% ~30%

~14% ~18%

~7% ~13%

~8% ~13%

1.0 vol.% ~5% [40]

1.0 vol.% ~4% [40]

4.0 vol.% ~15% [224]

~20% ~40% **Ref.**

235

[228]

[169]

[184]

[6]

[6]

[67]

[14]

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

*Notes*: if not specified, measurements were conducted at room temperature.

2 At 60°C.

3 With addition of CTAB.

**Table 3.** Influence of water-based nanofluids in thermal management.

<sup>1</sup> At 50°C.


**Table 4.** Influence of diverse nanofluids in thermal management.

## **5. Summary**

**Filler Nanoparticles morphology Filler fraction TC enhancement Ref.**

Graphene Sheets, 1 μm lateral 0.40 vol.% ~9% [232]

SiC Spherical ~26 nm 4.2 vol.% 16% [141] SiC Cylindrical ~600 nm 4.0 vol.% 23% [141] SiC Spherical ~37–110 nm diam. 0.1 vol.% ~12% [155] SiO2 Spherical ~12 nm diam. 1.0 vol.% ~3% [92]

1.0 vol.%1 4.0 vol.% 4.0 vol.%1

5.0 vol.% 7.5 vol.%

0.30 vol.%

0.01 wt.% 0.10 wt.%

5.0 vol.%

4.9 wt.%

0.01 wt.% 0.10 wt.%<sup>2</sup> 0.10 wt.%<sup>2</sup>

3.0 vol.%

3.0 vol.%

~14% ~29% ~15% ~36%

~24% ~55% ~78%

~7% ~12%

~9% ~19%

0.40 vol.% ~11% [232]

~18% ~30%

~1% ~5%

~38% ~126% ~288%

1.0 vol.% ~7% [221]

10% 21%

9% 18% [78]

[245]

[248]

[120]

[135]

[118]

[249]

[181]

[181]

CuO Spherical ~24 nm diam. 1.0 vol.%

CuO Spherical ~100 nm diam. 2.5 vol.%

CuO Spherical ~25 nm diam. 0.10 vol.%

G sheets, 1 μm lateral; MWCNTs ~19 nm diam.

TiO<sup>2</sup> Spherical ~15 nm diam. 1.0 vol.%

TiO<sup>2</sup> Spherical ~95 nm diam. 1.0 wt.%

Diam.: 20 nm

Diam.: 15 nm

ZnO Semi-rectangular (90–210 nm) 1.0 vol.%

ZnO Spherical, 20–40 nm diam. 1.0 vol.%

**Table 3.** Influence of water-based nanofluids in thermal management.

*Notes*: if not specified, measurements were conducted at room temperature.

CNTs Rods ~length: 35 μm

MWCNTs Rods ~length: 30 μm

GO Sheets, range of 200 nm to 1000 nm

Graphene + MWCNTs

234 Microfluidics and Nanofluidics

1 At 50°C. 2 At 60°C. 3

With addition of CTAB.

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, environmentally friendly materials, and obtain more profits. The present work offers a general overview of the recent research and development on preparation and characterization of nanofluids for thermal management applications, with emphasis on experimental data, variables and features. Nowadays, many technologies search for the highest efficiency mainly for energy savings, particularly on cooling or heat dissipation challenges within devices and machinery components. Many interesting properties of nanofluids have been reported in the past decades. Several efforts have been made trying to homogeneously disperse nanostructures within conventional HTFs to improve their properties. Nanofillers size has positive effects on conventional HTFs performance, i.e., compared to larger dispersed solid particles making flow through microchannels much easier, also since diverse parameters are critical for devices performance, such as morphology and stability of dispersed nanostructures, fluids composition, viscosity, fast sedimentation, channels clogging, erosion, wear, among others, which are often very serious for systems consisting of small channels. It is noted that nature of enhancement in thermal transport with nanoparticles concentration and temperature increment differs from fluid to fluid, which is comprehensible due to many factors such as fluids composition, viscosity, nature of fluids (morphology as well as interaction between fluid and nanofillers), etc.

**Author details**

**References**

José Jaime Taha-Tijerina

Address all correspondence to: jose.taha@udem.edu

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Thermal Transport and Challenges on Nanofluids Performance

http://dx.doi.org/10.5772/intechopen.72505

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It is found that factors such as temperature and filler fraction are more sensitive in determining the effective thermal conductivity in low viscosity fluids. The lower the filler concentration, the higher the stability (but lower thermal conductivity improvement), which means that a medium must be found between the two to prevent nanoparticles sedimentation/agglomeration, the free phonon/electron movement is affected by these defects, and hence a surfactant-free stable suspension can provide much better thermal conductivity. Nanofluids stability is a key factor to evaluate the quality of the nanofluids, and is considerably valued in the industrial applications. Additives or surfactants could be used to promote nanoparticles stabilization, but with some main drawbacks such as decrease of thermal conductivity, since surfactants could introduce defects at the solution/particle interfaces. Some nanofluids are currently expensive, partly due to the difficulty in manufacturing either the nanostructures to be afterward dispersed within conventional fluids or the nanofluids by themselves. Optimum layer thickness and filler fraction are important parameters in research of thermal transport, electrical and physical behavior and general aspects of both fundamental and applied characteristics. Mass production of nanostructures could further reduce the costs, and also using low filler fractions is another way to make nanofluids more affordable. Although nanofluids have displayed paramount and exciting potential applications, some vital hinders also exist before regular commercialization and industrialization of nanofluids.
