4. Application of TiO2 nanofluids in heat exchangers

The basic function of heat exchanger is to transfer heat energy from one fluid (which is at high temperature) to other fluid (which is at low temperature). Two fluids in heat exchanger did not come into direct contact or mix with each other. To achieve high heat transfer rate in heat exchanger is our main concern. Use of nanofluid is one of simple way to attain this purpose.

Duangthongsuk et al. [19] used a double tube counter flow heat exchanger to check heat transfer and flow characteristics of TiO2 nanofluid. Nanofluid flows in inner copper tube while hot water flows in annular. Heat transfer coefficient increased as mass flow rate of hot water and Reynolds number increased while temperature of nanofluid is decreased. Hot water temperature has no significant effect on heat transfer coefficient. When compared with base fluid (water) pressure drop and friction factor are nearly the same. To calculate Nusselt number, different equations are available in literature such as Gnielinski equation [20] is defined as in Eq. (13)

$$Nu = \frac{\left(\frac{f}{8}\right)(Re - 1000)Pr}{1 + 12.7\left(\frac{f}{8}\right)^{0.5}\left(Pr^2 - 1\right)}\tag{13}$$

where Nu is Nusselt number, Re is Reynolds number, Pr is Prandtl number and f is friction factor in above equation.

To predict Nusselt number Pak and Cho [21] correlation is given in Eq. (14) as follows:

$$Nu\_{\rm nf} = 0.21 Re\_{\rm nf}^{0.8} Pr\_{\rm nf}^{0.5} \tag{14}$$

Another correlation in Eq. (15) to predict Nusselt number is given by Xuan and Li [22]

$$Nu\_{\rm nf} = 0.0059 \left( 1 + 7.6286 \mathcal{O}^{0.6886} Pr\_{\rm d}^{0.001} \right) Re\_{\rm nf}^{0.9238} Pr\_{\rm nf}^{0.4} \tag{15}$$

where Pe is Peclet number of nanofluid in above relation.

Singh et al. [23] did experimental studies on double pipe heat exchanger by using CuO/TiO2 nanofluids with different flow rates and volume concentrations. Application of CuO/TiO2 nanofluids enhanced heat transfer rate as concentration and flow rate is increased. CuO nanofluid showed better results than TiO2 nanofluids because of high thermo-physical properties. Reddy et al. [24] did experimentation to check heat transfer coefficient and friction factor in double pipe heat exchanger with and without helical coil inserts by using TiO2 nanofluid. Nanofluid flows in the inner tube while hot fluid flows in the outer tube. Enhancement in heat transfer coefficient and friction factor (in terms of pressure drop) is measured. New correlations for Nusselt number and friction factor developed are given in Eqs. (16) and (17)

$$Nu\_{\rm Reg} = 0.007523 Re^{0.8} Pr^{0.5} (1 + \mathcal{Q})^{7.6} (1 + P/d)^{0.037} \tag{16}$$

$$f\_{\rm Reg} = 0.3250 Re^{-0.2377} (1 + \mathcal{O})^{2.723} (1 + P/d)^{0.041} \tag{17}$$

Khedkar et al. [25] study TiO2/water nanofluid heat transfer characteristics in concentric heat exchanger. Nanofluid with the highest concentration has the highest overall heat transfer coefficient. Flow diagram of apparatus used in experimentation [25] is shown in Figure 2.

Duangthongsuk et al. [26] found that enhancement in heat transfer coefficient and pressure drop is related to nanoparticle concentration. If nanoparticle concentration is increased beyond the

Figure 2. Schematic representation of nanofluid flow through concentric tube heat exchanger.

limit then a decrease in the heat transfer coefficient is observed. This is attributed to increase in viscosity. In this experiment, value of heat transfer coefficient increases as volume concentration is increased up to 1% and after that decrease in heat transfer coefficient is observed. Proposed correlations to predict Nusselt number and friction factor are mentioned in Eqs. (18) and (19), respectively.

where Nu is Nusselt number, Re is Reynolds number, Pr is Prandtl number and f is friction

To predict Nusselt number Pak and Cho [21] correlation is given in Eq. (14) as follows:

Another correlation in Eq. (15) to predict Nusselt number is given by Xuan and Li [22]

Nunf <sup>¼</sup> <sup>0</sup>:0059 1 <sup>þ</sup> <sup>7</sup>:6286Ø<sup>0</sup>:<sup>6886</sup>Pe<sup>0</sup>:<sup>001</sup>

Nusselt number and friction factor developed are given in Eqs. (16) and (17)

<sup>f</sup> Reg <sup>¼</sup> <sup>0</sup>:3250Re�0:<sup>2377</sup>ð<sup>1</sup> <sup>þ</sup> <sup>Ø</sup><sup>Þ</sup>

Figure 2. Schematic representation of nanofluid flow through concentric tube heat exchanger.

NuReg <sup>¼</sup> <sup>0</sup>:007523Re<sup>0</sup>:<sup>8</sup>

where Pe is Peclet number of nanofluid in above relation.

Nunf <sup>¼</sup> <sup>0</sup>:21Re<sup>0</sup>:<sup>8</sup>

Singh et al. [23] did experimental studies on double pipe heat exchanger by using CuO/TiO2 nanofluids with different flow rates and volume concentrations. Application of CuO/TiO2 nanofluids enhanced heat transfer rate as concentration and flow rate is increased. CuO nanofluid showed better results than TiO2 nanofluids because of high thermo-physical properties. Reddy et al. [24] did experimentation to check heat transfer coefficient and friction factor in double pipe heat exchanger with and without helical coil inserts by using TiO2 nanofluid. Nanofluid flows in the inner tube while hot fluid flows in the outer tube. Enhancement in heat transfer coefficient and friction factor (in terms of pressure drop) is measured. New correlations for

Pr<sup>0</sup>:<sup>5</sup>

Khedkar et al. [25] study TiO2/water nanofluid heat transfer characteristics in concentric heat exchanger. Nanofluid with the highest concentration has the highest overall heat transfer coefficient. Flow diagram of apparatus used in experimentation [25] is shown in Figure 2.

Duangthongsuk et al. [26] found that enhancement in heat transfer coefficient and pressure drop is related to nanoparticle concentration. If nanoparticle concentration is increased beyond the

ð1 þ ØÞ

7:6

<sup>2</sup>:<sup>723</sup>ð<sup>1</sup> <sup>þ</sup> <sup>P</sup>=d<sup>Þ</sup>

ð1 þ P=dÞ

nf Pr<sup>0</sup>:<sup>5</sup>

d Re<sup>0</sup>:<sup>9238</sup>

nf (14)

nf (15)

<sup>0</sup>:<sup>037</sup> (16)

<sup>0</sup>:<sup>041</sup> (17)

nf Pr<sup>0</sup>:<sup>4</sup>

factor in above equation.

190 Application of Titanium Dioxide

$$Nu = 0.07 Re^{0.707} Pr^{0.385} \mathcal{O}^{0.074} \tag{18}$$

$$f = 0.961 \mathcal{O}^{0.052} \text{Re}^{-0.375} \tag{19}$$

Barzegarian et al. [27] used brazed plate heat exchanger to check enhancement in overall heat transfer coefficient and pressure drop by using TiO2 nanofluid. For a specified Reynolds number, increment in weight concentration of nanoparticle in nanofluid increased the overall heat transfer coefficient and pressure drop also. Increase in Reynolds number also enhanced the overall heat transfer coefficient and pressure drop. Up to 20% enhancement in pressure drop has been observed during experimentation. Tiwari et al. [28] used four different types of nanofluids (including CeO2, Al2O3, TiO2 and SiO2) with different concentrations and volume flow rates. Better heat transfer behaviour of TiO2 and CeO2 nanofluid is observed at low concentrations while that of Al2O3 and SiO2 at high concentrations. CeO2 > Al2O3 > TiO2 > SiO2 is the order of nanofluids for which maximum heat transfer coefficient is obtained.

Overall heat transfer coefficient is calculated in Ref. [29] as by using Eqs. (20)–(22)

$$
\mathcal{U} = \frac{\mathcal{Q}\_{\text{avg}}}{AF\Delta T\_{\text{LMTD}}} \tag{20}
$$

$$A = \text{N}\_{\text{t}}HW\tag{21}$$

$$\Delta T\_{\rm LMTD} = \frac{(T\_{h,o} - T\_{c,i}) - (T\_{h,i} - T\_{c,o})}{L \text{tr} \frac{(T\_{h,o} - T\_{c,i})}{(T\_{h,i} - T\_{c,o})}} \tag{22}$$

where U is over all heat transfer coefficient, A is total surface area, F is temperature correction factor, Nt is total number of plates, and H and W are height and width of plates.

Taghizadeh-Tabari et al. [29] performed experimentation on plate heat exchanger of milk pasteurization industry by using TiO2/water nanofluid. Peclet number is used in experiment to compare performance of nanofluid with different concentrations. Nusselt number and pressure drop increased as the Peclet number or concentration or both are increased. Experimental results showed dramatic increase in heat transfer coefficient while theoretical calculated results did not. Reasons behind this could include increase of nanoparticle Brownian motions, particle migration and reduction of boundary layer thickness. The performance index η ¼ ðhnf =hbfÞ=ðΔPnf =ΔPbfÞ is greater than 1 for all type of nanofluid concentrations used in the experimentation. Benefit of using nanofluid in milk pasteurization industry is to reduce energy consumption. Javadi et al. [30] did study work by using three different nanofluids in plate heat exchanger to compare thermo-physical and heat transfer characteristics with base fluid. The results confirmed that overall heat transfer coefficient, thermal conductivity, pressure drop,



Researcher

and base

fluid

Duangthongsuk

TiO2-water

 21 nm

 4000–18,000

 Nanofluid

Turbulent

 0.2% by

Hot water flow

Double tube with

Enhancement

convective heat transfer coefficient

is about 6–11% as compared to base

fluid.

 in

192 Application of Titanium Dioxide

counter flow

For inner tube,

outer diameter

is 9.53 mm and inner diameter is

8.13 mm. For outer tube,

inner diameter is 27.8 mm and

outer diameter

is 33.9 mm.

> Singh et al. [23] TiO2-water

Reddy et al. [24] TiO2

21 nm

 4000–15,000 –

 –

0.004–0.02%

–

> by volume

CuO-water

30–50 nm

10–20 nm

3500–13,500

 Nanofluid

Laminar

0.1–0.3% by

Nanofluid flow

Double pipe

At volumetric concentration

0.3% and flow rate

of 4 LPM, a

maximum

enhancement

coefficient of heat

transfer is 5% for

CuO nanofluids.

 in

 of

For inner tube,

outer diameter is 18 mm and inner diameter is 15 mm.

For outer tube,

inner diameter is 32 mm and outer diameter is 36 mm.

Double tube with

Enhancement

Nusselt number and friction factor

for 0.02% concentration

15,000 Reynolds number without

helical coils are 10.73 and 8.73%,

respectively.

However, by using

helical coils this

enhances up to

17.71 and 16.58%,

respectively.

 and

 in

> and without

helical coil inserts.

For inner tube,

outer diameter is 9.53 mm and inner

diameter is

8.13 mm. For outer tube, inner diameter is 27.8 mm and

outer diameter

is 33.9 mm.

and

volume

rate range is 1–4

LPM

turbulent

temperature

range 30 3C

volume

rate is 3 LPM and

4.5 LPM

temperature

range 15–25C

Hot water temperature

range is 35–50C

and

Wongwises

 [19]

Nanoparticle

Size

 Reynolds

Temperature

Flow

Nanoparticle

Flow rates

 Design

Results

concentration

type

number

range

range



Researcher

and base

fluid

Tiwari et al. [28] CeO2-water

Al O2

3-water

45 nm

30 nm

–

25–30C

–

0.5, 0.75, 1.0,

1–4 LPM

Plate heat

For Al O2

water and SiO2/

water nanofluids,

the maximum

enhancement

heat transfer coefficient at

optimum volume

concentration

about 35.9, 26.3,

24.1 and 13.9%,

respectively.

Maximum ratio of Nusselt number of

nanofluid to distilled water obtained is about

1.17.

Nunf/Nuwater=

Maximum

pressure drop is

about 8% as compared to

distilled water.

 1.17

 are

 in

3/water, TiO2/

CeO2/water,

194 Application of Titanium Dioxide

exchanger

10 plates and

heat exchanger

are of 0.3 m2.

1.25, 1.5, 2.0

and 3% by

volume

TiO2-water

10 nm

SiO2-water

Taghizadeh-Tabari et al. [29]

TiO2-water

 10–15 nm Nearly (70–220)

–

Laminar

0.25, 0.35 and

–

Plate heat exchanger.

11 plates with

the length of

0.2 m each and width of 0.11 m Spacing between

plates is 0.0025 m.

and

0.8% by

turbulent

weight

Critical

Reynolds

number

is 100

> Javadi et al. [30] SiO2-liquid

nitrogen

–

 –

Inlet and outlet

–

0.2–2% by

Mass flow rate of

Plate fin heat

Al O2 3 has the

highest overall

heat transfer coefficient which is

308.69 W/m2k in

2%

concentration.

exchanger.

volume

SiO2 at 0.2%

concentration

0.278 kg/s and at

2% is 2.079 kg/s. Mass flow rate of TiO2 and Al O2 3 at

0.2% is 0.456 and 0.439 kg/s while for 2%

concentration

 is

concentration

 is

temperatures

310 and 124.26 K for hot fluid while

99.719 and 301.54

K for cold fluid,

respectively

 are

TiO2-liquid

nitrogen

Al O2

3-liquid

nitrogen

10 nm

Nanoparticle

Size

 Reynolds

Temperature

Flow

Nanoparticle

Flow rates

 Design

Results

concentration

type

number

range

range

Table 3. Application of TiO2 nanofluids in different types of heat exchangers. 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, respectively.

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 number, volume concentration, volume flow rate and temperature.

Different enhancements achieved by researchers are given in Table 3.
