**2. Problem formulation**

factor, respectively, by using nanofluid with single twisted tape insert in double pipe heat exchanger than that without insert. Reference [16] presented an experimental analysis of the turbulent flow in tube fitted with (single, dual, triple, and quadruple) twisted tapes and nanofluid under turbulent flow conditions. The results shown that Nusselt number and friction factors increased as the number of tapes and volumetric concentration increased. Also, the increment in heat transfer rate by increase in nanofluid volumetric concentration only was higher than that of increase in twisted tape number only. It must be mentioned that the volumetric

Reference [17] has carried out experimental and numerical investigations on similar tube flow and similar nanofluid with plain tube, tube fitted with dual plain tape inserts, tube fitted with dual twisted tapes inserts and tube fitted with dual helical screw twisted tape inserts. He concluded that, a maximum enhancement of 82.2% is achieved in the Nusselt number by using tube fitted with dual helical screw twisted tapes inserts and TiO2/water nanofluid flow than that observed with the plain tube and distilled water flow. And the maximum friction factor observed for the same model of the tube fitted with dual helical screw twisted tape inserts and nanofluid are up to 17.34% than that of the plain tube. Reference [18] carried out a wide range of experimental study on the convective heat transfer enhancement using combined techniques. One of these techniques is the use of twisted tape along the whole tube length of a micro-fin tube that effectively combined the features of extended surfaces, turbulators, and artificial roughness. Nanofluids are used for improving the thermo-physical properties of the fluid. Ag-water nanofluid in a micro-fin tube with nonuniform twisted tapes insert is examined under turbulent flow. The effects of the twist ratios of nonuniform twistedtapes of 3.0 > 2.8 > 2.6, 3.0 > 2.6 > 2.2, and 3.0 > 2.4 > 1.8, in counter and co-current flow arrangements and nanofluid concentrations of 0.007, 0.016, and 0.03% vol. are investigated. They claimed that heat transfer, friction loss, and thermal performance factors are increasing as the twist ratios are decreasing for nonuniform twisted tapes and increasing nanofluid concentrations. The optimum condition are achieved in using the micro-fin tube with a nonuniform twisted-tape in a counter-current-arrangement with twist ratios in a series of 3.0> 2.4> 1.8 with Ag-water nanofluid at a concentration of 0.3% vol. The enhancements are up to 112.5% for the heat transfer rate and 1.62 for

On other combined enhancement attempt in microchannel heat exchangers, [19] carried out experimental investigation on heat transfer for pulsating flow of GOP-water nanofluid. The effects of mass fraction of graphene oxide (GOPs) and flow pulsating frequency on heat transfer and pressure drop in a microchannel with arrayed pin-fins have been investigated. Five different mass fractions of graphene oxide nanofluids were prepared and used as working fluids. Experiments were performed under the condition that the pulsating frequency was from 1 to 5 Hz, the mass fraction was from 0.02 to 0.2%, and the average Reynolds numbers were 272, 407, and 544. The results show that the heat transfer is enhanced significantly when the frequency is in the range of 2–5 Hz. For the frequency of 1 Hz, the pulsating flow shows a negative effect on temperature uniformity. With the increase of mass fraction, the heat transfer performance is improved, while no significant change is found in pressure drop. The pulsating flow leads to a significant enhancement of pressure drop for frequency at 2 Hz. The combination of pulsating and nanofluid can obtain higher heat transfer efficiency under limited size of microchannel heat sink and low inlet Reynolds numbers. The results provided good guide for the

Since conventional fluids, such as water, have a relatively poor heat transfer characteristic, the nanoenhancing technique opens the door to gain more benefits from these conventional fluids especially in heat transfer intensification field.

efficiency of the tube was not taken into consideration.

*Inverse Heat Conduction and Heat Exchangers*

thermal performance.

**26**

design of microchannel heat exchangers.

The basic geometry adopted in this investigation is a straight tube with 1000 mm length, *L* and 50 mm internal diameter, *D*. Pure tube, with pure water flow and no inserts, was considered as the benchmark case to compare the thermal enhancement and the pressure drop. The other cases were investigated with a different number of inserts and with 0.1 vol.% TiO2/water nanofluid.

It is worth mentioning that this research meant to investigate the flow characteristics only in compound thermal and hydrodynamic in fully developed region where the entrance effect becomes insignificant beyond a pipe length of 8 times the diameter for turbulent flow [8, 14]. The fully developed region was calculated to be accomplished at 500 mm from the tube inlet.

#### **2.1 Twisted tape inserts**

Twisted tape inserts (TT) are heat transfer enhancement devices which are dividing the flow within the tube resulting in higher velocity near the tube surface. They, also, creating spiral flow creates swirl or secondary flow in the main flow which increases local velocities and promotes mixing. They are widely used over decades to generate the swirl flow in the thermal fluid resulting in increased heat transfer coefficient, with a penalty of increased pressure drop across the flow passage. Thus, reduction in the thermal system, like the heat exchangers, can be achieved. Types of TT are shown in **Figure 1a**.

Main parameters that are commonly adopted to characterize the TT are the empty tube Reynolds number (Re), half-pitch (*y*), and twist ratio (*Y*). The main geometrical TT identifiers are shown in **Figure 1b**.

The half-pitch (*y*) is defined as the distance between two points on the edge of a TT, which lies down on the same plane as the TT completes 180° of revolution.

The twist ratio, *Y*, is defined as the ratio of the half-pitch to the internal tube diameter, *Y* = *y*/*D*p.

Tube fitted with single twisted tape (STT), tube fitted with triple twisted tapes (TTT), and tube fitted with quintuple twisted tapes (QTT) are considered in the present experimental and numerical investigations. The schematics of cross sectional view of these models and twisted tapes and the geometries are illustrated in **Figure 2**.

The twisted tapes have the same length of the tube and has a width; *L* = 1000 mm, width, *w* (mm), thickness, *δ* (mm) and pitch of 180° twist, *y* (mm) as explained in **Table 1**. The swirl direction corresponding to tape arrangement was co-swirl flow, and all tapes were aligned to be twisted in the same direction.

**Figure 1.**

*Twisted tape inserts; (a) different types of twisted tape inserts (courtesy of visual capitalist [24], with permission); (b) topologies of twisted tape insert.*

and twisted tape inserts. In the present study, 0.1 vol.% TiO2 nanoparticles of size (<50 nm) mixed with distilled water was sonicated continuously by ultrasonic vibrator generating pulses of 240 W at 40 + 4 kHz to break down any possible nanoparticle agglomeration. Nanofluid was found to be stable during tests period of tests and no intermediate mixing process considered necessary. TiO2 particles and

Volume concentration φ desired in this study was 0.1% for TiO2/water. The thermophysical parameters are calculated for nanofluid; **Table 3** contains the popular and valid models, which were used to evaluate property values and shows

Experimental setup was designed and fabricated to investigate the thermohydraulic characteristics of the double pipe heat exchanger, as shown in **Figure 3**. The test section comprised of copper tube of 1000 mm length with inside and

water have the considered properties illustrated in **Table 2**.

**3. Experimental implementation**

values.

**29**

**3.1 Apparatus**

**Model No. of tapes**

*Geometries and details of TT inserts.*

**Density ρ (kg/m3 )**

*Properties of titanium oxide particles and water.*

*Note:* φ *is the percentage of volume fraction of nanoparticles.*

**Table 1.**

**Table 2.**

Thermal conductivity*, k*

Dynamic viscosity, *μ*

**Table 3.**

**Tape width (mm)**

*DOI: http://dx.doi.org/10.5772/intechopen.93359*

**Tape pitch (***y***) (mm)**

*Applications of Compound Nanotechnology and Twisted Inserts for Enhanced Heat Transfer*

TiO2 4230 692 8.4 — Water 997 <sup>4179</sup> 0.613 <sup>855</sup> � <sup>10</sup>�<sup>6</sup>

**Specific heat (***C***p) (J/kg**�**K)**

**Property Formula Relation**

Density, *ρ ρnf* ¼ *φρnp* þ ð Þ 1 � *φ ρbf* (1) Pak and Cho

Specific heat, *C*<sup>p</sup> ð Þ *CP nf* ¼ *φ*ð Þ *CP np* þ ð Þ 1 � *φ* ð Þ *CP bf* (2) Maddah et al.

*Correlations of 0.1% volume of TiO2-in-water nanofluid properties and predicted values.*

STT 1 48 100 1000 1 Aluminum TTT 3 21 100 1000 1 Aluminum QTT 5 16 100 1000 1 Aluminum

**Tape length (mm)**

**Thermal conductivity,** *k* **(W/m**�**K)**

**no.**

*knf* ¼ ½ �Þ 1 þ 3*φ kbf* (3) Said et al. [7] 0.6148 W/m�K

*<sup>μ</sup>nf* <sup>¼</sup> <sup>1</sup> <sup>þ</sup> <sup>2</sup>*:*<sup>5</sup> *<sup>φ</sup>* <sup>þ</sup> <sup>6</sup>*:*<sup>2</sup> *<sup>φ</sup>*<sup>2</sup> <sup>½</sup> �Þ*μbf* (4) Batchelor [26] 857.143 � <sup>10</sup>�<sup>6</sup>

**Recommended by**

[25]

[15]

**Tape thickness (***δ***) (mm)**

**Tape material**

**Dynamic viscosity,** *μ* **(kg/m**�**s)**

**Value**

1000.2 kg/m<sup>3</sup>

4175.5 J/kg�K

kg/m�s

#### **Figure 2.**

*Schematic views for tubes fitted with; (a) single twisted tape (STT), (b) triple twisted tapes (TTT), and (c) quintuple twisted tapes (QTT).*

#### **2.2 Working fluids**

Water was used in the primary tests to investigate the effect of inserts only on the thermo-hydraulic performance of tubes. Latterly, nanofluid of TiO2 predispersed in water was used to obtain the enhancement by both nanotechnology


*Applications of Compound Nanotechnology and Twisted Inserts for Enhanced Heat Transfer DOI: http://dx.doi.org/10.5772/intechopen.93359*

**Table 1.**

*Geometries and details of TT inserts.*


#### **Table 2.**

*Properties of titanium oxide particles and water.*


#### **Table 3.**

*Correlations of 0.1% volume of TiO2-in-water nanofluid properties and predicted values.*

and twisted tape inserts. In the present study, 0.1 vol.% TiO2 nanoparticles of size (<50 nm) mixed with distilled water was sonicated continuously by ultrasonic vibrator generating pulses of 240 W at 40 + 4 kHz to break down any possible nanoparticle agglomeration. Nanofluid was found to be stable during tests period of tests and no intermediate mixing process considered necessary. TiO2 particles and water have the considered properties illustrated in **Table 2**.

Volume concentration φ desired in this study was 0.1% for TiO2/water. The thermophysical parameters are calculated for nanofluid; **Table 3** contains the popular and valid models, which were used to evaluate property values and shows values.

#### **3. Experimental implementation**

#### **3.1 Apparatus**

Experimental setup was designed and fabricated to investigate the thermohydraulic characteristics of the double pipe heat exchanger, as shown in **Figure 3**. The test section comprised of copper tube of 1000 mm length with inside and

**2.2 Working fluids**

*(c) quintuple twisted tapes (QTT).*

**Figure 2.**

**28**

**Figure 1.**

*permission); (b) topologies of twisted tape insert.*

*Inverse Heat Conduction and Heat Exchangers*

Water was used in the primary tests to investigate the effect of inserts only on

the thermo-hydraulic performance of tubes. Latterly, nanofluid of TiO2 predispersed in water was used to obtain the enhancement by both nanotechnology

*Schematic views for tubes fitted with; (a) single twisted tape (STT), (b) triple twisted tapes (TTT), and*

*Twisted tape inserts; (a) different types of twisted tape inserts (courtesy of visual capitalist [24], with*

where *v* is the mean velocity (=measured flow rate/pipe inlet cross section area), *ν* is the kinematic viscosity of the working fluid, and *Dp* is the pipe or tube diameter. Prandtl number is considered 7.56 for water. For nanofluid, Pr is evaluated using

*Applications of Compound Nanotechnology and Twisted Inserts for Enhanced Heat Transfer*

Pr <sup>¼</sup> *Cpnf* � *<sup>μ</sup>nf knf*

The friction factor was predicted using the known Darcy-Weisbach equation

where Δ*p* is the measured pressure drop over the 1.0 m pipe length. The measured values were vindicated with results gained from well-established correlations,

*nf φ*<sup>0</sup>*:*<sup>074</sup> (9) Duangth-

*D <sup>π</sup>*�4*<sup>δ</sup> D* h i<sup>0</sup>*:*<sup>2</sup> *<sup>π</sup>*

*Adopted correlations from the literature to predict Nusselt number used to verify the experimental results.*

�0*:*<sup>25</sup> (12) Blasius, in

�0*:*<sup>375</sup> (14) Duangtho-

*w π*

*Adopted correlations from the literature to predict friction factor used to verify the experimental results.*

� �<sup>2</sup> � ��0*:*<sup>74</sup> " # (15) Bergles and

**Correlation Eq. No. Suggested**

*<sup>f</sup>* <sup>¼</sup> <sup>0</sup>*:*79 ln *Re Dh* � <sup>1</sup>*:*<sup>64</sup> � ��<sup>2</sup> (13) Petukhov,

*<sup>π</sup>*�4*<sup>δ</sup> D*

2 *L Dh*

� � (11)

**by**

ongsuk and Wongwises [21]

Manglik [9]

**by**

[27]

in [27]

ngsuk and Wongwises [21]

Manglik [9]

(8) Gnielinski, in [27]

h i<sup>0</sup>*:*<sup>8</sup> (10) Bergles and

**Case of HEX**

Tube with water flow

Tube with nanofluid flow

Tube with twisted tape(s) and water flow

> **Case of HEX**

Tube with water flow

Tube with water flow

Tube with nanofluid flow

Tube with triple twisted tapes and water flow

*<sup>f</sup>* <sup>¼</sup> *<sup>Δ</sup><sup>p</sup>* 0*:*5 *ρ Vin*

**Correlation Eq. No. Suggested**

(Eq. 11), for both, water and nanofluid flow, as follows:

*y w* h i<sup>1</sup>*:*<sup>1</sup> *Tb*

*Ts* h i<sup>0</sup>*:*<sup>45</sup> *<sup>π</sup>*þ2�2*<sup>δ</sup>* (7)

Eq. (7), as follows:

*DOI: http://dx.doi.org/10.5772/intechopen.93359*

illustrated in **Table 5**.

*Nu* <sup>¼</sup> <sup>0</sup>*:*<sup>074</sup> *Re* <sup>0</sup>*:*<sup>707</sup>

*<sup>f</sup>* ð Þ<sup>8</sup> *Re Dh* ð Þ �<sup>1000</sup> *Pr* <sup>1</sup>þ12*:*<sup>7</sup> *<sup>f</sup>* ð Þ<sup>8</sup> 1 <sup>2</sup> *Pr*<sup>2</sup> <sup>3</sup>�1 � �

*nf Pr*<sup>0</sup>*:*<sup>385</sup>

*Nu* <sup>¼</sup> <sup>0</sup>*:*<sup>024</sup> *Re* <sup>0</sup>*:*<sup>8</sup>*pr*<sup>0</sup>*:*<sup>4</sup> � <sup>1</sup> <sup>þ</sup> <sup>0</sup>*:*<sup>769</sup>

*Nu* ¼

**Table 4.**

**Table 5.**

**31**

*f* ¼ 0*:*318 *Re Dh*

*<sup>f</sup>* <sup>¼</sup> <sup>0</sup>*:*961*φ*<sup>0</sup>*:*<sup>052</sup> *Re Dh*

*<sup>f</sup>* <sup>¼</sup> <sup>0</sup>*:*<sup>079</sup> *Re* �0*:*<sup>25</sup> *<sup>π</sup>*þ2�2*<sup>δ</sup>*

*D <sup>π</sup>*�4*<sup>δ</sup> D* h i<sup>1</sup>*:*<sup>25</sup> *<sup>π</sup>*

*<sup>π</sup>*�4*<sup>δ</sup> D* h i<sup>1</sup>*:*<sup>75</sup> � � <sup>1</sup> <sup>þ</sup> <sup>2</sup>*:*06 1 <sup>þ</sup> <sup>2</sup>*<sup>y</sup>*

**Figure 3.** *Schematic diagram of the experimental setup.*

outside diameters of 50 and 53 mm, respectively. An electric heater with 5 kW capacity was wounded on the outer surface of the copper tube and insulated by glass-wool insulation. Thermocouples, referred to as (T1–6) in **Figure 3**, were installed to measure wall and fluid temperature at inlet, fully developed, and outlet regions. All thermocouples were calibrated before being fixed, and all were connected to the data logger to record temperature readings. A differential pressure manometer was used to measure pressure drop along the test section. The working fluid was heated inside the test section and allowed to cool by passing through a cooler (evaporative cooling system). By recirculation, the working fluid returns to the storage tank in the flow loop and then is pumped again to the test section. Flow meter was placed at the entrance of the test section to measure the working fluid flow rate. Throttle valves were incorporated to allow controlling the working fluid flow rate and for maintenance and emergency, if any. To ensure steady state condition for each run, period of 30 minutes was permitted prior to starting the measurement and data acquisition.

#### **3.2 Verification of experimental measurements**

Heat transfer in convective flow is commonly evaluated in terms of Nusselt number, Nu, as in Eq. (5).

$$\text{Nu} = \frac{h.D\_p}{k} = \frac{\left[\frac{\equiv}{q} \;/(T\_s - T\_b)\right]D\_p}{k} \tag{5}$$

Nusselt number values were validated with the experimental results of previous researches shown in **Table 4**. Values of Reynolds numbers, Re, were evaluated as per Eq. (6), as follows:

$$\text{Re} = \frac{\text{VD}\_p}{v} \tag{6}$$

*Applications of Compound Nanotechnology and Twisted Inserts for Enhanced Heat Transfer DOI: http://dx.doi.org/10.5772/intechopen.93359*

where *v* is the mean velocity (=measured flow rate/pipe inlet cross section area), *ν* is the kinematic viscosity of the working fluid, and *Dp* is the pipe or tube diameter.

Prandtl number is considered 7.56 for water. For nanofluid, Pr is evaluated using Eq. (7), as follows:

$$\text{Pr} = \frac{\text{Cp}\_{\text{nf}} \times \mu\_{\text{nf}}}{k\_{\text{nf}}} \tag{7}$$

The friction factor was predicted using the known Darcy-Weisbach equation (Eq. 11), for both, water and nanofluid flow, as follows:

$$f = \frac{\Delta p}{0.5 \,\rho \,\,\,\text{V}\_{in}^{-2} \left(\frac{L}{D\_h}\right)}\tag{11}$$

where Δ*p* is the measured pressure drop over the 1.0 m pipe length. The measured values were vindicated with results gained from well-established correlations, illustrated in **Table 5**.


#### **Table 4.**

outside diameters of 50 and 53 mm, respectively. An electric heater with 5 kW capacity was wounded on the outer surface of the copper tube and insulated by glass-wool insulation. Thermocouples, referred to as (T1–6) in **Figure 3**, were installed to measure wall and fluid temperature at inlet, fully developed, and outlet

regions. All thermocouples were calibrated before being fixed, and all were

measurement and data acquisition.

*Schematic diagram of the experimental setup.*

*Inverse Heat Conduction and Heat Exchangers*

number, Nu, as in Eq. (5).

**Figure 3.**

per Eq. (6), as follows:

**30**

**3.2 Verification of experimental measurements**

Nu <sup>¼</sup> *<sup>h</sup>:Dp*

*k* ¼

connected to the data logger to record temperature readings. A differential pressure manometer was used to measure pressure drop along the test section. The working fluid was heated inside the test section and allowed to cool by passing through a cooler (evaporative cooling system). By recirculation, the working fluid returns to the storage tank in the flow loop and then is pumped again to the test section. Flow meter was placed at the entrance of the test section to measure the working fluid flow rate. Throttle valves were incorporated to allow controlling the working fluid flow rate and for maintenance and emergency, if any. To ensure steady state condition for each run, period of 30 minutes was permitted prior to starting the

Heat transfer in convective flow is commonly evaluated in terms of Nusselt

*q*

Nusselt number values were validated with the experimental results of previous researches shown in **Table 4**. Values of Reynolds numbers, Re, were evaluated as

> Re <sup>¼</sup> *VDp v*

<sup>¼</sup> *<sup>=</sup>*ð Þ *Ts* � *Tb* h i

*Dp*

*<sup>k</sup>* (5)

(6)

*Adopted correlations from the literature to predict Nusselt number used to verify the experimental results.*


#### **Table 5.**

*Adopted correlations from the literature to predict friction factor used to verify the experimental results.*
