**1. Introduction**

Heat exchangers (HEXs) are typical thermal systems in industrial and engineering applications. They are adopted as means of heat dissipation in a wide range of thermal processes ranging from huge scale to microscale. HEXs are involved in the power production process, chemical and food industries, electronics cooling, environmental production engineering, waste heat recovery, manufacturing industry, air conditioning, and refrigeration applications. HEXs constitute a multibilliondollar industry in the United States alone, and there are over 300 companies engaged in the manufacture of a wide array of heat exchangers [1].

The performance of heat exchanger can significantly increase by the heat transfer augmentation techniques that lead to the reduction of heat exchanger size, as

well as operating cost reduction. Enhancement techniques can be classified either as passive or active techniques. A passive technique does not need any external power input, and the additional power needed to enhance heat transfer is taken from the available power in the system such as extended surfaces, treated surfaces, and twisted tape. Active techniques require external power such as mechanical aids and surface vibration [2].

mechanism increases the tangential and radial turbulent components leading to higher mix fluid layers and reduces the thermal and hydrodynamic boundary layer thickness due to increase in turbulent fluctuation and combination occurring between core region and near wall region in fluid flow, which cause higher temper-

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

Reference [8] reported for single twisted tape insert in circular tube in which heat transfer rate increased significantly than tube without inserts, and as twist ratio decreases, the heat transfer rate and friction factor become larger. Subsequently, [9] experimented the single twisted tape insert with many different twist ratios to describe the mechanism of heat transfer enhancement and proposed cor-

A recognized contribution to the field of heat transfer enhancement is done by the thermofluid team in University of Pretoria—South Africa. They have started their investigation on heat transfer enhancement since 2010 and since then, they published a large number of experimental and numerical investigation on the enhanced heat transfer by inserts. In 2013, [10] investigated the heat transfer enhancement in laminar flow regime in circular tubes using rib roughening and twisted tape inserts. The work is carried out experimentally, and Nusselt number and the friction factor were measured. The work concluded major finding that the center-cleared twisted tapes in combination with transverse ribs perform significantly better than the individual enhancement technique acting alone for laminar flow through a circular duct up to a certain amount of center-clearance. The reported results are useful for the design of solar thermal heaters and heat exchangers. In 2017, another two papers have been published by the same team [11, 12]. Both papers have been carried out numerically. In [11], heat transfer behavior in a tube with inserted twisted tape swirl generator is investigated numerically, for different values of the twist ratio and diameter ratio and for Reynolds numbers within the range of 100–20,000. Results have shown that the tube use of twist tape enhances heat transfer generally, but, accompanied with a higher pressure drop. Improvement of the thermal-hydraulic performance can only be observed for certain configurations and Reynolds numbers. In [12], simulations were conducted for laminar, transitional, and turbulent flow regimes for four different rib angle of attack values and for a plain tube without ribs, as benchmark case. Within the investigated range, the larger thermal performance factors are observed to occur for the intermediate Reynolds numbers. Maximum values in the range of 2.0–2.5 are predicted for the Reynolds number of 2000, where a subsequent drop to values within the range of 1.0–1.5 is found to occur for Reynolds numbers around 3000–4000, which may be attributed to the transitional effects. In 2018, a paper [13] is published by the same team, which reported results of experimental investigation of heat transfer performance of corrugated tube with spring tape inserts in turbulent forced convection in Re range of 10,000–5000. Air is adopted as working fluid. Results show that Nusselt numbers can be increased considerably, depending on pitch and spring ratios. The heat transfer enhancement to the pressure drop penalty is realized to be larger than unity for all cases. Values around 2.8 occur for cases with the smallest pitch and spring ratios. Predictive

ature gradient near the wall leading to increased heat transfer rate.

Nusselt number and friction factor correlations are proposed.

**25**

**1.3 Compound nanoadditive/twisted tape enhanced heat transfer**

By combining the nanofluid enhancement with inserts, [14] found that using Al2O3 nanoparticles with distilled water and full-length twisted tape gave a superior performance with a noticeable increment in friction loses than plain tube. Reference [15], achieved 20 and 2.5% increase in overall heat transfer coefficient and friction

relations for Nusselt number and friction factor.

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

Various methods of heat transfer enhancement are used in HEXs. These techniques are expressive to be manufactured and adopted to increase the thermal system efficiency by increasing the rate of heat transfer process and/or to reduce the size of thermal systems. They can be classified into two main categories; (i) active methods: which use an external power source; (ii) passive methods: which use several techniques without a power source such as turbulence generators (as propeller, spiral fin, twisted tapes, ribs … etc.), or by using additives like the nanoadditives. However, in their review paper on the tape enhanced heat transfer [2], classified third enhancement techniques. They proposed compound methods, when two passive methods are used simultaneously. The method adopted in this paper is compound enhancement as twisted tapes and nanoadditives. As such, below are some backgrounds on the nanoadditives and twisted tapes to enhance heat transfer.

#### **1.1 Nanoenhanced heat transfer**

Nanoadditives are a type of heat transfer enhancement method through enhancement of the thermal properties of base materials. They may be synthesized with base fluids to produce nanofluids for thermal transport or synthesized with phase change materials (PCM) to produce nanocomposites for thermal energy storage as in [3].

Numerous works and review papers have been published in the field of nanoenhanced fluids, such as the paper of [4] who presented and compared the preparation, stability and thermophysical properties of nanofluids. It was concluded that nanofluids have, in general, better thermo-physical properties even at a very low particle concentration (typically 1.0% or less) than conventional heat transfer fluids. The only drawback is increment in the viscosity which leads to a higher pressure drop. Similar article by [5], published in 2018, has also reviewed the fabrication, stability, and thermophysical properties of nanofluids. After presenting the progress of studies on nanofluid thermophysical characterization, they identified some possible opportunities for future research that can bridge the gap between in-lab research and commercialization of nanofluids as this nanofluids are receiving large attention due to their potential usage. Reference [6], who reviewed the current scenario and future prospective of nanofluids, who also supported the conclusion of [5], ended with that the study of nanofluids has been materialized as a new field of scientific interest and innovative application. Reference [3] investigated nanofluid characteristics concerning thermal behavior in a plain tube, which showed significant enhancement rate in heat transfer with increase in nanoparticles diameter and volume concentration. Reference [7] experimented the thermophysical properties of Al2O3 and TiO2 nanoparticles with distilled water and proposed parabolic equations for dynamic viscosity and thermal conductivity at specific conditions.

#### **1.2 Enhanced heat transfer by twisted inserts**

Twisted tape heat transfer enhancement mechanism is attributed to producing swirl flow in form of secondary recirculation act on the axial flow. This fluid

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

mechanism increases the tangential and radial turbulent components leading to higher mix fluid layers and reduces the thermal and hydrodynamic boundary layer thickness due to increase in turbulent fluctuation and combination occurring between core region and near wall region in fluid flow, which cause higher temperature gradient near the wall leading to increased heat transfer rate.

Reference [8] reported for single twisted tape insert in circular tube in which heat transfer rate increased significantly than tube without inserts, and as twist ratio decreases, the heat transfer rate and friction factor become larger. Subsequently, [9] experimented the single twisted tape insert with many different twist ratios to describe the mechanism of heat transfer enhancement and proposed correlations for Nusselt number and friction factor.

A recognized contribution to the field of heat transfer enhancement is done by the thermofluid team in University of Pretoria—South Africa. They have started their investigation on heat transfer enhancement since 2010 and since then, they published a large number of experimental and numerical investigation on the enhanced heat transfer by inserts. In 2013, [10] investigated the heat transfer enhancement in laminar flow regime in circular tubes using rib roughening and twisted tape inserts. The work is carried out experimentally, and Nusselt number and the friction factor were measured. The work concluded major finding that the center-cleared twisted tapes in combination with transverse ribs perform significantly better than the individual enhancement technique acting alone for laminar flow through a circular duct up to a certain amount of center-clearance. The reported results are useful for the design of solar thermal heaters and heat exchangers. In 2017, another two papers have been published by the same team [11, 12]. Both papers have been carried out numerically. In [11], heat transfer behavior in a tube with inserted twisted tape swirl generator is investigated numerically, for different values of the twist ratio and diameter ratio and for Reynolds numbers within the range of 100–20,000. Results have shown that the tube use of twist tape enhances heat transfer generally, but, accompanied with a higher pressure drop. Improvement of the thermal-hydraulic performance can only be observed for certain configurations and Reynolds numbers. In [12], simulations were conducted for laminar, transitional, and turbulent flow regimes for four different rib angle of attack values and for a plain tube without ribs, as benchmark case. Within the investigated range, the larger thermal performance factors are observed to occur for the intermediate Reynolds numbers. Maximum values in the range of 2.0–2.5 are predicted for the Reynolds number of 2000, where a subsequent drop to values within the range of 1.0–1.5 is found to occur for Reynolds numbers around 3000–4000, which may be attributed to the transitional effects. In 2018, a paper [13] is published by the same team, which reported results of experimental investigation of heat transfer performance of corrugated tube with spring tape inserts in turbulent forced convection in Re range of 10,000–5000. Air is adopted as working fluid. Results show that Nusselt numbers can be increased considerably, depending on pitch and spring ratios. The heat transfer enhancement to the pressure drop penalty is realized to be larger than unity for all cases. Values around 2.8 occur for cases with the smallest pitch and spring ratios. Predictive Nusselt number and friction factor correlations are proposed.

#### **1.3 Compound nanoadditive/twisted tape enhanced heat transfer**

By combining the nanofluid enhancement with inserts, [14] found that using Al2O3 nanoparticles with distilled water and full-length twisted tape gave a superior performance with a noticeable increment in friction loses than plain tube. Reference [15], achieved 20 and 2.5% increase in overall heat transfer coefficient and friction

well as operating cost reduction. Enhancement techniques can be classified either as passive or active techniques. A passive technique does not need any external power input, and the additional power needed to enhance heat transfer is taken from the available power in the system such as extended surfaces, treated surfaces, and twisted tape. Active techniques require external power such as mechanical aids and

Various methods of heat transfer enhancement are used in HEXs. These techniques are expressive to be manufactured and adopted to increase the thermal system efficiency by increasing the rate of heat transfer process and/or to reduce the size of thermal systems. They can be classified into two main categories; (i) active methods: which use an external power source; (ii) passive methods: which use several techniques without a power source such as turbulence generators (as propeller, spiral fin, twisted tapes, ribs … etc.), or by using additives like the nanoadditives. However, in their review paper on the tape enhanced heat transfer [2], classified third enhancement techniques. They proposed compound methods, when two passive methods are used simultaneously. The method adopted in this paper is compound enhancement as twisted tapes and nanoadditives. As such, below are some backgrounds on the nanoadditives and twisted tapes to enhance

Nanoadditives are a type of heat transfer enhancement method through enhancement of the thermal properties of base materials. They may be synthesized with base fluids to produce nanofluids for thermal transport or synthesized with phase change materials (PCM) to produce nanocomposites for thermal energy

Numerous works and review papers have been published in the field of nanoenhanced fluids, such as the paper of [4] who presented and compared the preparation, stability and thermophysical properties of nanofluids. It was concluded that nanofluids have, in general, better thermo-physical properties even at a very low particle concentration (typically 1.0% or less) than conventional heat transfer fluids. The only drawback is increment in the viscosity which leads to a higher pressure drop. Similar article by [5], published in 2018, has also reviewed the fabrication, stability, and thermophysical properties of nanofluids. After presenting the progress of studies on nanofluid thermophysical characterization, they identified some possible opportunities for future research that can bridge the gap between in-lab research and commercialization of nanofluids as this nanofluids are receiving large attention due to their potential usage. Reference [6], who reviewed the current scenario and future prospective of nanofluids, who also supported the conclusion of [5], ended with that the study of nanofluids has been materialized as a new field of scientific interest and innovative application. Reference [3] investigated nanofluid characteristics concerning thermal behavior in a plain tube, which showed significant enhancement rate in heat transfer with increase in nanoparticles

diameter and volume concentration. Reference [7] experimented the

**1.2 Enhanced heat transfer by twisted inserts**

thermophysical properties of Al2O3 and TiO2 nanoparticles with distilled water and proposed parabolic equations for dynamic viscosity and thermal conductivity at

Twisted tape heat transfer enhancement mechanism is attributed to producing

swirl flow in form of secondary recirculation act on the axial flow. This fluid

surface vibration [2].

*Inverse Heat Conduction and Heat Exchangers*

heat transfer.

storage as in [3].

specific conditions.

**24**

**1.1 Nanoenhanced heat transfer**

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 efficiency of the tube was not taken into consideration.

As pointed above and mentioned by many other researchers [20–23], the field of nanoenhanced heat transfer with multiple inserts is still virgin. Further experimental data are essential to support the literature, and to enhance the understanding of hydrothermal behavior in thermal systems. So far, few studies have been carried out on multiple twisted tapes with nanofluid effect on thermo-hydraulic character-

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

The objective of this chapter is to scrutinize the effect of compound multiple twisted tape (TT) inserts with TiO2-water nanofluid on heat exchange enhancement in double pipe heat exchangers. Three cases of tubes fitted with single, triple, and quintuple plain twisted tapes have been investigated experimentally and simulated numerically. In addition, two bench-mark cases have been investigated: first case is with plain tube with pure water flow, and the second case is with plain tube with nanofluid flow. Results have been manipulated and presented in terms of

Nusselt number for heat transfer and friction factor for pressure drop.

different number of inserts and with 0.1 vol.% TiO2/water nanofluid.

accomplished at 500 mm from the tube inlet.

achieved. Types of TT are shown in **Figure 1a**.

geometrical TT identifiers are shown in **Figure 1b**.

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

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

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

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

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

The twisted tapes have the same length of the tube and has a width;

The half-pitch (*y*) is defined as the distance between two points on the edge of a

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**.

*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.

istics in thermal systems.

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

**2. Problem formulation**

**2.1 Twisted tape inserts**

diameter, *Y* = *y*/*D*p.

**27**

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 thermal performance.

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 design of microchannel heat exchangers.

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.

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

As pointed above and mentioned by many other researchers [20–23], the field of nanoenhanced heat transfer with multiple inserts is still virgin. Further experimental data are essential to support the literature, and to enhance the understanding of hydrothermal behavior in thermal systems. So far, few studies have been carried out on multiple twisted tapes with nanofluid effect on thermo-hydraulic characteristics in thermal systems.

The objective of this chapter is to scrutinize the effect of compound multiple twisted tape (TT) inserts with TiO2-water nanofluid on heat exchange enhancement in double pipe heat exchangers. Three cases of tubes fitted with single, triple, and quintuple plain twisted tapes have been investigated experimentally and simulated numerically. In addition, two bench-mark cases have been investigated: first case is with plain tube with pure water flow, and the second case is with plain tube with nanofluid flow. Results have been manipulated and presented in terms of Nusselt number for heat transfer and friction factor for pressure drop.
