5. Application of TiO2 nanofluids in heat sinks

Heat sink (which is a form of heat exchanger) is used to absorb excessive heat from a system to maintain its temperature at an optimum value and to avoid from overheating. These are made of conductive metals. Air or liquid is used to remove the heat from heat sink. Design of heat sink is such as to maximize surface area contact with air or cooling liquid. Due to limitations, we cannot increase area beyond limits rather we can use cooling liquid with higher thermal conductivity. This enhancement in thermal conductivity will result in higher heat transfer and can be achieved by addition of nanoparticles to base fluid.

Ali et al. [33] experimentally compared performance of staggered and inline pin fin heat sinks under laminar flow of TiO2 (rutile) and TiO2 (anatase). TiO2 (rutile) with staggered pin fin heat sinks showed best performance. The arrangement of staggered pin fin allows more liquid to interact with pin fins, which makes its performance better than inline pin fin. By using TiO2 (rutile) with staggered pin fin heat sinks minimum temperature of base obtained is 29.4C. The lowest thermal resistance is obtained for TiO2 (rutile) with staggered pin fin heat sinks at Reynolds number of 587 which was 0.012C/W. Schematic diagram of nanofluid flow is shown in Figure 3.

Mohammed et al. [34] used six different nanofluids in the experimentation to find enhancement in the heat transfer coefficient, wall shear stresses, friction factor and pressure drop in triangular micro-channel heat sink. Order of achieved enhancement in heat transfer coefficient is Diamond> SiO2 > CuO > TiO2 > Ag > Al2O3. CuO/water and TiO2/water showed same performance in terms of heat transfer coefficient. Order of pressure drop occurred along the length of the channel in experiment is SiO2 > Diamond > Al2O3 > TiO2 > CuO > pure water > Ag.

Figure 3. Schematic diagram of nanofluid flow through heat sink. CR is coolant reservoir, DAS is data acquisition system, F is flow meter, HS is heat sink, HT is heater, P is pump, R is radiator, T is thermocouple and V is valve in flow diagram.

Ag also has lowest wall shear stress. Diamond/water nanofluid has lowest thermal resistance among six nanofluids.

Naphon and Nakharintr [35] performed experiments on mini- rectangular fin heat sinks with different widths by using TiO2/de-ionized water nanofluid to check heat transfer enhancement. Average outlet temperature and plate temperature decreased as Reynolds number is increased. Average heat transfer rate is increased with mass flow rate of nanofluids. Nusselt number has direct relation with Reynolds number. Increase in Reynolds number decreased the thermal resistance while slight increase in pressure drop is observed. Average heat transfer rate of heat sink with largest width is higher than the sinks with smaller width.

To calculate thermal resistance in Ref. [35], Eq. (23) can be used as

heat transfer rate and entropy generation increased with increase in volume concentration of nanoparticles. While Prandtl number decreased as concentration of nanoparticles in base fluid is increased, pressure drop is lowest for SiO2. Overall heat transfer coefficient is highest for Al2O3. Prandtl number is highest for lowest concentration of nanoparticles. Entropy generation is lowest for SiO2 as 25, while for TiO2 and Al2O3 this value increases to 40 and 38.7,

Ashrafi et al. [31] used nanofluid as coolant in heat exchangers of swimming pool. In shell and tube heat exchanger, nanofluid flows through tubes while cold water in shell. Results show that when weight concentration of nanoparticle and Peclet number is increased, the convective heat transfer coefficient is also increased. Kumar et al. [32] used shell and tube heat exchanger to check heat transfer characteristics of TiO2/water, CuO/water, TiO2/ethylene glycol and CuO/ethylene glycol nanofluids with different concentrations. Hot water flows through shell and nanofluid flows through tubes. CuO/water nanofluids showed highest enhancement among all nanofluids used in the experimentation. Convective heat transfer coefficient is improved by increasing Reynolds

Heat sink (which is a form of heat exchanger) is used to absorb excessive heat from a system to maintain its temperature at an optimum value and to avoid from overheating. These are made of conductive metals. Air or liquid is used to remove the heat from heat sink. Design of heat sink is such as to maximize surface area contact with air or cooling liquid. Due to limitations, we cannot increase area beyond limits rather we can use cooling liquid with higher thermal conductivity. This enhancement in thermal conductivity will result in higher heat transfer and

Ali et al. [33] experimentally compared performance of staggered and inline pin fin heat sinks under laminar flow of TiO2 (rutile) and TiO2 (anatase). TiO2 (rutile) with staggered pin fin heat sinks showed best performance. The arrangement of staggered pin fin allows more liquid to interact with pin fins, which makes its performance better than inline pin fin. By using TiO2 (rutile) with staggered pin fin heat sinks minimum temperature of base obtained is 29.4C. The lowest thermal resistance is obtained for TiO2 (rutile) with staggered pin fin heat sinks at Reynolds number of 587 which was 0.012C/W. Schematic diagram of nanofluid flow is shown

Mohammed et al. [34] used six different nanofluids in the experimentation to find enhancement in the heat transfer coefficient, wall shear stresses, friction factor and pressure drop in triangular micro-channel heat sink. Order of achieved enhancement in heat transfer coefficient is Diamond> SiO2 > CuO > TiO2 > Ag > Al2O3. CuO/water and TiO2/water showed same performance in terms of heat transfer coefficient. Order of pressure drop occurred along the length of the channel in experiment is SiO2 > Diamond > Al2O3 > TiO2 > CuO > pure water > Ag.

number, volume concentration, volume flow rate and temperature.

5. Application of TiO2 nanofluids in heat sinks

can be achieved by addition of nanoparticles to base fluid.

Different enhancements achieved by researchers are given in Table 3.

respectively.

196 Application of Titanium Dioxide

in Figure 3.

$$R\_{\rm th} = \frac{1}{\overline{h} \times A\_{\rm s}} = \frac{1}{\frac{Q\_{\rm avg}}{A T\_{\rm LMTD}}} \tag{23}$$

Parameters used in above equation include Rth as thermal resistance, h as average heat transfer coefficient and A<sup>s</sup> as total heat transfer surface area of heat sink.

Sohel et al. [36] used three different nanofluids to check thermo-physical properties and heat transfer performance of nanofluids in a circular copper micro-channel. CuO/water nanofluid showed best thermo-physical properties and heat transfer performance among the three nanofluids. Reduction in friction factor for CuO/water nanofluid is 9.38%, for Al2O3/water is 1.13% and for TiO2/water is 1.79%. Reduction in thermal resistance for CuO/water nanofluid is 11.62%, Al2O3/water is 6.37% and TiO2/water is 5.84%. Khaleduzzaman et al. [37] performed experimental work to find out the effect of nanoparticles volumetric concentration on flow rates, heat transfer coefficient and thermal resistance for water block heat sink. The interface temperature reduced as the volume flow rate increased. When the volume fraction and flow rate increased, thermal resistance decreased. Augmentation in heat transfer coefficient occurred when volume fraction and flow rate increased. Ijam et al. [38] performed cooling of copper mini-channel heat sink using two different types of nanofluid. Effects of nanofluid



Researcher

Ali and

TiO2

5–30 nm

Nearly

–

Laminar

 4.31% for TiO2

–

Staggered and inline

A maximum of 37.78% in Nusselt number is obtained for

TiO2 nanofluid with staggered

pin fin heat sinks.

Highest and lowest heat

transfer coefficient is obtained for diamond/

water and Al O2

nanofluid, Highest pressure drop is

obtained for SiO2/water nanofluid while Ag/water

nanofluids

pressure drop and wall

shear stresses. An increase of about 42.3% in average heat

transfer rate is obtained

for heat sink with

w = 2 mm. Maximum

as compared to pure

water in mass flow rate,

Reynolds number, heat

transfer coefficient,

thermal heat flux at 4%

concentration

water nanofluid, we have

12.76, 1.84, 7.74 9.97 and

6.20%, respectively.

 for TiO2/

conductivity

 and

improvement

 have lowest

respectively.

3/water

(rutile)/H O2

enhancement

198 Application of Titanium Dioxide

pin fin heat sinks

(anatase) and

3.99% TiO2

(rutile) by

volume

(anatase)-water

5–30 nm

between

260 and

600

TiO2

(rutile)-water

Mohammed

Al O2

3-water

–

100–1000 –

Laminar

 2% by volume

 10–20 mL/min

 Triangular micro

channels 20 micro channels with 10 mm

length

Ag-water

CuO-water

Diamondwater SiO2-water

TiO2-water

Naphon and

TiO2-water

 21 nm 80–200

 20, 22 and

Laminar

 0.4% by

4.5–8 g/s

 Three minirectangular

 fin heat

sinks with three different height of fins

1, 1.5 and 2 mm

volume

24C

Nakharintr

Sohel et al. [36] Al O2

3-water

–

Maximum

–

Laminar

 0.5–4% by

Nanofluid

Circular microchannel heat sinks with diameter of each

is 0.4 mm

volume

inlet velocity

= 1.5 m/s

up to 1000

TiO2-water

CuO-water

 [35]

et al. [34]

Arshad [33]

and base fluid

Nanoparticle

Size

 Reynolds

Temperature

Flow

Nanoparticle

Flow rates

 Design

Results

concentration

number

range

type

range

 4. TiO2 nanofluids used for heat sink

volume fraction and inlet velocity on thermal conductivity, heat transfer coefficient, pumping power and pressure drop had been investigated. Fluid at low velocity absorbs more heat than the fluid at higher velocity. Mass flow rate and particle volume fraction has direct relation with heat transfer coefficient. By increasing mass flow rate and inlet velocity of fluid, pressure drop also increases. When volume fraction is increased then thermal resistance is decreased. Improvement in heat flux with volume fraction of 0.8% and nanofluid inlet velocity of 0.1 m/s for Al2O3 is 17.3% while for TiO2 is 16.53%.

Xia et al. [39] compared heat transfer performance of fan-shaped micro-channel heat sink with rectangular micro-channel heat sink by using TiO2/water and Al2O3/water nanofluids. Pressure drop and convection of heat transfer is higher in fan-shaped micro-channel heat sink. Enhancement in heat transfer is greater for Al2O3/water nanofluids when compared with TiO2/ water nanofluids, while thermal conductivity behaviour of TiO2/water nanofluid is better.

To calculate friction resistance coefficient in Ref. [39] Eq. (24) is used

$$f = \frac{2\Delta P D\_h}{\rho L u\_m^2} \tag{24}$$

where ΔP is pressure drop between inlet and outlet, D<sup>h</sup> is hydrodynamic diameter, ρ is density, L is length of micro-channel and u<sup>m</sup> is mean velocity. Different enhancements obtained by researchers are given in Table 4.

## 6. Application of TiO2 nanofluid in nucleate pool boiling

Nucleate pool boiling is a boiling type which takes place when temperature of surface is about 5�C greater than the saturation temperature of liquid. This boiling region is the most desirable as we can obtain high heat transfer rates with a small value of ΔTexcess. Usually two methods are used to increase heat transfer rate in this region. One way is to increase the nucleation sites by doing surface treatment and other way is use of nanofluids.

Ali et al. [40] experimentally found boiling heat transfer coefficient enhancement by using TiO2 (Rutile)/water nanofluid. Two different concentrations of 12 and 15% by weight are used in experimentation. Experimental setup accuracy is checked by using Pioro [41] correlation, which is given by Eq. (25) as follows:

$$\frac{hl}{k} = 0.075 \text{C}\_{\text{sf}} \left( \frac{q}{h\_{\text{fg}} \rho\_{\text{g}}^{0.5} [\sigma g (\rho - \rho\_{\text{g}})]^{0.25}} \right)^{0.66} Pr'' \tag{25}$$

In above equation Csf and n are constants, which are dependent on fluid and heating surface.

By increasing wall super heat a decrease in heat flux enhancement is observed. Average heat flux and boiling heat transfer coefficient enhancement obtained at 15% concentration is 2.22 and 1.38 while for 12% concentration is 1.89 and 1.24, respectively.

Trisaksri et al. [42] experimentally investigated nucleate pool boiling heat transfer at different concentration and pressure of a refrigerant-based nanofluid on cylindrical copper tube. TiO2- R141 nanofluid with three different concentrations had been used. When concentration of nanoparticle is increased, a decline in boiling heat transfer for R141 is observed. Effect of pressure is dominant at low concentrations. Rohsenow [43] correlation used in experimentation to predict nucleate boiling heat transfer is given by Eq. (26) as follows:

$$\frac{\mathbb{C}\_{p,l}(T\_{\rm s} - T\_{\rm sat})}{h\_{\rm fg}Pr\_{\rm l}^{m}} = \mathbb{C}\_{\rm sf} \left(\frac{q}{\mu\_{\rm l}h\_{\rm fg}} \sqrt{\frac{\sigma}{g(\rho\_{\rm l} - \rho\_{\rm v})}}\right)^{0.33} \tag{26}$$

Suriyawong and Wongwises [44] performed experimentation on two different circular plates made of copper and aluminium with different roughness (0.2–4 µm) to check nucleate boiling heat transfer characteristics. When concentration was greater than 0.0001% by volume, a decrease in heat transfer coefficient had been observed. The reason for this deterioration is sedimentation of nanoparticles on heating surface and decrease in nucleation sites. Rough surfaces provide more heat transfer coefficient as compared to smooth surfaces because more nucleation sites are presented on such surfaces. Aluminium plate showed high heat transfer coefficient than copper plate.

Das et al. [45] checked the effects of surface modification on nucleate boiling heat transfer. In experimentation, Cu surface is coated with crystalline TiO2 nanostructure. Increase in surface roughness, surface wet ability or surface coating thickness provide enhancement in boiling heat transfer coefficient.

## 7. Conclusion

(24)

Pr<sup>n</sup> (25)

volume fraction and inlet velocity on thermal conductivity, heat transfer coefficient, pumping power and pressure drop had been investigated. Fluid at low velocity absorbs more heat than the fluid at higher velocity. Mass flow rate and particle volume fraction has direct relation with heat transfer coefficient. By increasing mass flow rate and inlet velocity of fluid, pressure drop also increases. When volume fraction is increased then thermal resistance is decreased. Improvement in heat flux with volume fraction of 0.8% and nanofluid inlet velocity of 0.1 m/s

Xia et al. [39] compared heat transfer performance of fan-shaped micro-channel heat sink with rectangular micro-channel heat sink by using TiO2/water and Al2O3/water nanofluids. Pressure drop and convection of heat transfer is higher in fan-shaped micro-channel heat sink. Enhancement in heat transfer is greater for Al2O3/water nanofluids when compared with TiO2/ water nanofluids, while thermal conductivity behaviour of TiO2/water nanofluid is better.

> <sup>f</sup> <sup>¼</sup> <sup>2</sup>ΔPDh ρLu<sup>2</sup> m

where ΔP is pressure drop between inlet and outlet, D<sup>h</sup> is hydrodynamic diameter, ρ is density, L is length of micro-channel and u<sup>m</sup> is mean velocity. Different enhancements obtained by

Nucleate pool boiling is a boiling type which takes place when temperature of surface is about 5�C greater than the saturation temperature of liquid. This boiling region is the most desirable as we can obtain high heat transfer rates with a small value of ΔTexcess. Usually two methods are used to increase heat transfer rate in this region. One way is to increase the nucleation sites

Ali et al. [40] experimentally found boiling heat transfer coefficient enhancement by using TiO2 (Rutile)/water nanofluid. Two different concentrations of 12 and 15% by weight are used in experimentation. Experimental setup accuracy is checked by using Pioro [41] correlation,

hfgρ<sup>0</sup>:<sup>5</sup>

In above equation Csf and n are constants, which are dependent on fluid and heating surface. By increasing wall super heat a decrease in heat flux enhancement is observed. Average heat flux and boiling heat transfer coefficient enhancement obtained at 15% concentration is 2.22

q

<sup>g</sup> <sup>½</sup>σgð<sup>ρ</sup> � <sup>ρ</sup>gÞ�<sup>0</sup>:<sup>25</sup>

!<sup>0</sup>:<sup>66</sup>

To calculate friction resistance coefficient in Ref. [39] Eq. (24) is used

6. Application of TiO2 nanofluid in nucleate pool boiling

by doing surface treatment and other way is use of nanofluids.

<sup>k</sup> <sup>¼</sup> <sup>0</sup>:075Csf

and 1.38 while for 12% concentration is 1.89 and 1.24, respectively.

for Al2O3 is 17.3% while for TiO2 is 16.53%.

200 Application of Titanium Dioxide

researchers are given in Table 4.

which is given by Eq. (25) as follows:

hl

This chapter gives an overview of titanium oxide nanofluids application in different heat transfer systems. Because of high thermal conductivity of these fluids as compared to simple water or other fluids, heat transfer systems using titanium oxide nanofluids performed more efficiently. Pressure drop due to the presence of nanoparticles was not significant. Therefore, no extra pumping power was required for circulation of nanofluids.

### Nomenclature


