1. Introduction

Nanoparticles, which were named as ultra-fine particles during the 1970s and 1980s, have size usually less than 100 nm. When a bulk material is considered then its physical properties remain nearly constant, but in case of nanoparticles it is not true. Nanoparticles are being used in many consumer goods such as paints, cosmetics and textiles. Nanoparticles are mixed with base fluid such as water, ethylene glycol and oil, to improve its properties. This mixture of nanoparticles and base fluid which is known as nanofluid can be used in different heat transfer applications.

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In the following chapter effects of using titanium oxide nanofluids in heat transfer applications have been presented. Titanium oxide nanofluids have better thermal properties as compared to simple liquids. Due to their better heat transfer characteristics they can be used as an alternate for simple liquids in many heat transferring systems such as radiator of cars, heat exchangers and heat sinks. The disadvantage associated with these nanoparticles is their potential toxicity.

## 2. Thermal performance enhancement by using TiO2 nanoparticles

Liquids have poor thermal properties, which is a barrier in development of energy efficient systems. Hence, some kind of technique should be adopted to overcome this problem. Addition of nanoparticles to base fluid showed better thermo-physical properties as compared to simple fluid. The reasons attributed to this enhancement in heat transfer performance could include Brownian motion and reduction in thermal boundary layer. This behaviour of nanofluids attracts many researchers to do research work in this field.

Hamid et al. [1] experimentally found performance factor <sup>η</sup> <sup>¼</sup> <sup>h</sup>nf <sup>h</sup>bf ¼ h' and friction factor of TiO2 nanofluids. Reynolds number, temperature, concentration and thermal properties have significant effect on the performance factor. At 30�C and volume concentration less than 1.2%, the performance factor was less than base fluid. However, at 50 and 70�C for all concentrations, nanofluid showed better improvement as compared to base fluid. Enhancement in Reynolds number, temperature and concentration enhanced the performance factor, while pressure drop increased as Reynolds number and concentration increased. We can convert weight concentration to volume concentration by using Eq. (1)

$$\mathcal{O} = \frac{\omega \rho\_{\rm bf}}{[(1 - 0.01\omega)\rho\_{\rm p} + 0.01\omega \rho\_{\rm bf}]} \tag{1}$$

where ω is weight concentration, ρbf is density of base fluid and ρ<sup>p</sup> is density of nanoparticles. To find volume concentration if mass is given, Eq. (2) can be used

$$\mathcal{O} = \frac{m\_{\text{p}}/\rho\_{\text{p}}}{m\_{\text{p}}/\rho\_{\text{p}} + m\_{\text{bf}}/\rho\_{\text{bf}}} \times 100\tag{2}$$

Density of nanofluid ρnf can be calculated from Eq. (3)

$$
\rho\_{\rm nf} = \mathcal{Q}\rho\_{\rm p} + (1 - \mathcal{Q})\rho\_{\rm bf} \tag{3}
$$

Vakili et al. [2] measured enhancement in convective heat transfer coefficient of TiO2 nanofluid flowing through a vertical pipe. According to experimental findings, thermal conductivity has nonlinear dependence on concentration. Increment in values of Reynolds number, nanoparticle concentration and heat flux also improved convective heat transfer coefficient. TiO2 nanofluid with water/ethylene glycol as base fluid showed more enhancement in convective heat transfer coefficient as compared to TiO2 nanofluid with distilled water as base fluid. Azmi et al. [3] used TiO2 and Al2O3 nanofluids in his experimentation to find and compare heat transfer coefficient and friction factor. At 30�C, Al2O3 nanofluid showed higher enhancement in viscosity and its viscosity varies with temperature whereas TiO2 nanofluid has viscosity independent of temperature. Lower heat transfer coefficient than water and ethylene glycol is obtained for TiO2 nanofluid for all concentrations (0–1%) at 30�C. Similar trend in heat transfer coefficient for both nanofluid is achieved at higher temperature. Friction factor augmentation for both nanofluids with volume concentration is not considerable.

In the following chapter effects of using titanium oxide nanofluids in heat transfer applications have been presented. Titanium oxide nanofluids have better thermal properties as compared to simple liquids. Due to their better heat transfer characteristics they can be used as an alternate for simple liquids in many heat transferring systems such as radiator of cars, heat exchangers and heat sinks. The disadvantage associated with these nanoparticles is their potential toxicity.

Liquids have poor thermal properties, which is a barrier in development of energy efficient systems. Hence, some kind of technique should be adopted to overcome this problem. Addition of nanoparticles to base fluid showed better thermo-physical properties as compared to simple fluid. The reasons attributed to this enhancement in heat transfer performance could include Brownian motion and reduction in thermal boundary layer. This behaviour of

TiO2 nanofluids. Reynolds number, temperature, concentration and thermal properties have significant effect on the performance factor. At 30�C and volume concentration less than 1.2%, the performance factor was less than base fluid. However, at 50 and 70�C for all concentrations, nanofluid showed better improvement as compared to base fluid. Enhancement in Reynolds number, temperature and concentration enhanced the performance factor, while pressure drop increased as Reynolds number and concentration increased. We can convert

where ω is weight concentration, ρbf is density of base fluid and ρ<sup>p</sup> is density of nanoparticles.

mp=ρ<sup>p</sup> þ mbf=ρbf

Vakili et al. [2] measured enhancement in convective heat transfer coefficient of TiO2 nanofluid flowing through a vertical pipe. According to experimental findings, thermal conductivity has nonlinear dependence on concentration. Increment in values of Reynolds number, nanoparticle concentration and heat flux also improved convective heat transfer coefficient. TiO2 nanofluid with water/ethylene glycol as base fluid showed more enhancement in convective heat transfer coefficient as compared to TiO2 nanofluid with distilled water as base fluid. Azmi et al. [3] used

<sup>Ø</sup> <sup>¼</sup> ωρbf

<sup>Ø</sup> <sup>¼</sup> <sup>m</sup>p=ρ<sup>p</sup>

<sup>h</sup>bf ¼ h' and friction factor of

½ð<sup>1</sup> – <sup>0</sup>:01ωÞρ<sup>p</sup> <sup>þ</sup> <sup>0</sup>:01ωρbf� (1)

ρnf ¼ Øρ<sup>p</sup> þ ð1 � ØÞρbf (3)

� 100 (2)

2. Thermal performance enhancement by using TiO2 nanoparticles

nanofluids attracts many researchers to do research work in this field.

Hamid et al. [1] experimentally found performance factor <sup>η</sup> <sup>¼</sup> <sup>h</sup>nf

182 Application of Titanium Dioxide

weight concentration to volume concentration by using Eq. (1)

To find volume concentration if mass is given, Eq. (2) can be used

Density of nanofluid ρnf can be calculated from Eq. (3)

Wang et al. [4] added TiO2 nanoparticles in paraffin wax (a phase changing material) to improve its thermal properties. Thermal properties varied with the concentration of nanoparticles. A drop in phase change temperature is observed when loading of nanoparticle was less than 1 wt% while a drop in latent heat capacity is observed when nanoparticle loading was greater than 2 wt %. Thermal conductivity of composite decreased as the temperature is increased and it is lower in liquid state than in solid state. Azmi et al. [5] experimentally investigated the effects of working fluid temperature and concentration on thermal conductivity, viscosity and heat transfer coefficient. These thermo-physical properties were greatly influenced by temperature and concentration. Thermal conductivity has direct relation with temperature at low concentration. Range of variation in viscosity is 4.6–33.3% depending on temperature and concentration.

Sajadi et al. [6] study the turbulent heat transfer behaviour of TiO2/water base nanofluid. The basic aim was to study the effects of volume concentration on heat transfer coefficient and on pressure drop. Dispersion of nanoparticle is improved by mixing an ultrasonic cleaner. Increasing concentration of nanoparticle has no significant effect on heat transfer but pressure drop increased. When the Reynolds number is increased then the ratio of heat transfer coefficient for nanofluid to base fluid is decreased while the Nusselt number is increased for both base and nanofluids. Wei et al. [7] did experimentation to find thermal conductivity and stability of TiO2/diathermic oil nanofluid. Effect of temperature and concentration on thermal conductivity had been examined. Thermal conductivity was having a linear correlation with concentration. Thermal conductivity of nanofluid increased with an increase in temperature. Zeta potential values of different samples indicated good stability of nanofluids. To calculate thermal conductivity, classical models are available such as Hamilton-Crosser (H-C) [8] is presented in Eq. (4)

$$\frac{k\_{\rm nf}}{k\_{\rm f}} = \frac{k\_{\rm p} + (n-1)k\_{\rm f} + (n-1)\mathcal{O}(k\_{\rm p} - k\_{\rm f})}{k\_{\rm p} + (n-1)k\_{\rm f} - \mathcal{O}(k\_{\rm p} - k\_{\rm f})} \tag{4}$$

where knf is thermal conductivity of nanofluid, k<sup>f</sup> is thermal conductivity of base fluid, k<sup>p</sup> is thermal conductivity of nanoparticles, <sup>Ø</sup> is volume fraction and <sup>n</sup> <sup>¼</sup> <sup>3</sup> <sup>ψ</sup>. Ψ is the sphericity.

Yu et al. [9] also gave a model to find thermal conductivity of nanofluids and it is presented in Eq. (5) as follows:

$$\frac{k\_{\rm rf}}{k\_{\rm f}} = \frac{k\_{\rm p} + 2(k\_{\rm p} - k\_{\rm f})(1 + \beta)^{3}\mathcal{O} + 2k\_{\rm f}}{k\_{\rm p} - 2(k\_{\rm p} - k\_{\rm f})(1 + \beta)^{3}\mathcal{O} + 2k\_{\rm f}}\tag{5}$$

where β is the ratio of the nano-layer thickness to the original particle radius.


Timofeeva [10] gave a model, which is given in Eq. (6)

$$\frac{k\_{\rm eff}}{k\_{\rm f}} = 1 + 3\mathcal{Q} \tag{6}$$

Basic information such as nanoparticle size, concentration of nanoparticles in base fluid and results, related to these research works can be obtained from Table 1.
