# On the Role of Nanofluids in Thermal-hydraulic Performance of Heat Exchangers—A Review

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## Abstract

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## 1. Introduction

_{2}and NO

_{x}emissions compared to other power generating systems [3,4]. Furthermore, the advances in Brayton cycle configurations have resulted in the widening of the range of possible applications, so today gas turbines can be seen employed in power plants, aviation, and marine propulsion. Gas turbines operate on the thermodynamic basis of the Brayton cycle, where it can be divided into two main categories, namely the open and the closed cycles. The major difference between the two is that in the open-cycle the working fluid (air) needs to be replaced with each complete cycle, whereas in the closed cycle, the heat transfer fluid (e.g., air or other gaseous fluid) is continuously reused. Thus, the closed cycle is seen as more cost-effective than the open system due to it requiring less fuel consumption while providing higher thermal transport efficiency [3,5]. Nevertheless, for propulsion applications (e.g., aircraft), open-cycle gas turbines are preferable because of their small construction scale, ease of load control, and higher turbine inlet temperature [6,7]. One of the key components that intensively influences the closed-cycle performance is the heat exchanger (HE), as it transports the required heat from a thermal source (e.g., solar, nuclear, or fossil) to the gas turbine cycle. In advanced closed-cycle system, reheaters, recuperators, and intercoolers, which are forms of HEs, are occasionally used to enhance thermal efficiency. Despite the achievements that started in 1935, when the closed-cycle gas turbine was first patented by Keller and Ackeret [5], researchers have come to a point where limited improvement in the cycle performance can be accomplished via modifying the design set-up, using different forms of heat exchanging devices, or adding turbulators to promote heat exchange [8,9]. Hence, they have concluded that to surpass the current limitations, an advanced type or category of heat transfer fluid needs to be employed [10]. This is when nanofluids, which were discovered in 1993 by Masuda et al. [11] and named by Choi and Eastman in 1995 [12], were seen as one of the primary solutions for overcoming the aforementioned cycle improvement limitations. The primary advantage of using nanofluids as working fluids, is that they exhibit significantly enhanced heat transfer characteristics when compared to their conventional counterparts [13,14].

- Nanoparticles:
- Material type;
- Size and shape;
- Attraction/extraction characteristics with the hosting basefluid molecules;
- Volumetric concentration;
- Density;
- Specific heat capacity; and
- Thermal conductivity.

- Basefluids:
- Type;
- Temperature;
- pH value;
- Molecular attraction/extraction behaviour towards the dispersed particles;
- Density;
- Specific heat capacity;
- Viscosity; and
- Thermal conductivity.

- Preparation route:
- Single-step method; or
- Two-step approach.

- Chemical or physical dispersion/s (if added).
- Nanofluid long- and short–term dispersion, kinetic, and chemical stabilities.

## 2. Heat Exchangers

#### 2.1. Plate Heat Exchangers

- -
- All-welded PHEs: The gaskets have been entirely eliminated, and as a result, the reliability of the HE has been enhanced by replacing a fully welded plate exchanger instead of the gaskets. This leads to eliminating the limitation associated with the operating pressure and temperature. The main constraint of this model is that no mechanical methods can be used for cleaning purposes, and therefore cleaning can only be achieved via chemical routes.
- -
- Wide-gap PHEs: Having the free-flow channel for highly viscous fluids and other products that contain coarse particles leads to eliminating the clogging problem that is usually encountered in HEs of the shell and tube type.
- -
- Free-flow PHEs: This unique design provides a wide flow path for fluids of high viscosity and fouling tendency and is also suitable for fluids that contain fibrous materials. This special design (free-flow design) can be considered as an improved design in comparison to the wide-gap PHEs. The main aspect of the design is that there is no contact point that can restrict the fluid flow in the flow path of the free-flow plates.

#### 2.2. Plate-Fin Heat Exchangers

- Fluid leakage possibilities do not exist or rarely occur, and as such there is no risk associated with the fluid mixing or contamination.
- PFHEs are designed for low-pressure applications (i.e., less than 1000 kPa).
- This class of HEs are mainly used for gas-to-gas applications and, in particular cases, in gas-to-liquid systems (e.g., the WR-21 marine propulsion gas turbine cycle).
- PFHEs offer high area density (i.e., up to approximately 6000 m
^{2}/m^{3}). - Various fins geometries (rectangular, tubular, offset strip, and wavy fin) can be utilized between the plates for various applications.
- PFHEs are designed for operating temperatures up to approximately 800 °C. The type of fin-to-plate bonding and the materials define the maximum operating temperatures.

- PFHEs provide superior thermal performance compared to their counterparts by using extended surfaces.
- PFHEs can operate effectively with temperature differences as low as 1 °C for single phase streams; while between multiphase streams, the temperature difference can be as low as 3 °C.
- For cryogenic applications, brazed aluminum PFHEs are the optimum choice due to the high surface compactness, the capability of handling multiple streams, and the highly desirable low-temperature properties at which they are able to operate.
- In cryogenic applications, a thermal effectiveness of the order of 95% or higher can be attained.
- PFHEs have large heat transfer surface per unit volume, and low weight per unit heat transfer.
- Exchanging heat between many process streams is possible in PFHEs.
- PFHEs can be used in different temperatures (from 0 to 800 °C) and pressures (up to 140 bar) by selecting the proper materials. However, they rarely get exposed simultaneously to a high temperature and pressure operating environment [58].

## 3. Nanofluids: Fundamentals and Characteristics

#### 3.1. Fabrication Approaches

#### 3.2. Dispersion Stability

#### 3.3. Nanofluids Thermophysical Properties

- Effective thermal conductivity:
- Cylindrical cell method;
- Steady-state parallel-plate method;
- Temperature oscillation approach;
- 3-ω method;
- Thermal comparator method;
- Thermal constants analyzer approach;
- Flash lamp method; and
- Transient hot-wire method.

- Effective viscosity:
- Capillary tube viscometer;
- Rotating viscometer;
- Capillary viscometer;
- Pressure differences over capillaries device; and
- Torsional oscillating cup.

## 4. Application of Nanofluids in Heat Exchangers

#### 4.1. Plate Heat Exchangers

#### 4.1.1. CNT-Based Nanofluids

_{nf}/h

_{w}) took place at the Pe number of 1000 and solid concentration of 0.55 wt. % by 33%. Figure 9 presents the variations of the relative h with respect to the Pe number in different solid concentrations.

_{2}O

_{3}-water nanofluid on the convective heat transfer and pressure drop in a PHE were investigated by Huang et al. [88]. They performed the experiments over a different range of Re numbers, solid concentrations, and flow velocities. They reported that, at a constant Re number, the nanofluids showed better heat transfer performance compared to that of the pure water. However, at a constant flow velocity, the heat transfer performance of the nanofluids deteriorated. They reported that the heat transfer enhancement of MWCNT-water nanofluid is higher than that of the Al

_{2}O

_{3}-water nanofluid. This would be because of the higher thermal conductivity of the MWCNT-water nanofluid. They also found that increasing the solid concentration of nanoparticles leads to decreasing the convective heat transfer coefficient, as can be seen in Figure 11. Moreover, they declared that increasing the solid concentration results in increasing the pressure drop and friction factor. Based on the experimental data, they also proposed a new correlation to predict the heat transfer and friction factor of the nanofluids.

_{2}, Al

_{2}O

_{3}, ZnO, CeO

_{2}, graphene nanoplate, and MWCNT-water) under the effects of variable spacing between the plates was studied. The spaces varied from 2.5 to 10 mm. The thermophysical properties of all the nanofluids investigated, including thermal conductivity, viscosity, density, and specific heat, were experimentally measured at different solid concentrations of 0.5, 0.75, 1.00, and 1.25 vol. %. As can be seen in Figure 12, all the measured thermophysical properties of the MWCNT-water nanofluid exhibited significantly higher values than those of the other nanofluids. Their results indicated that, at the spacing of 5 mm, the MWCNT-water nanofluid showed the highest heat transfer coefficient compared to the other nanofluids studied (see Figure 13a). The maximum heat transfer of the MWCNT-water nanofluid was 53% higher than that of the pure water. Moreover, the results showed that the minimum values of the pressure drop corresponded to the use of the MWCNT-water nanofluid (see Figure 13b).

#### 4.1.2. Hybrid Nanofluids Containing CNT Nanoparticles

_{2}O

_{3}-water nanofluid, with the ratio of 1:2.5, on the heat transfer performance of a chevron corrugated PHE has been experimentally studied by Huang et al. [185]. They added a small amount of MWCNT to the nanofluid in order to increase the thermal conductivity of the mixture. They conducted the experiment with pure water, Al

_{2}O

_{3}-water, and the hybrid MWCNT/Al

_{2}O

_{3}-water nanofluid, and compared the results of the convective heat transfer coefficient and the pressure drop. They found that, at the same flow velocity, the thermal performance of the hybrid nanofluid was marginally superior to that of the pure water and Al

_{2}O

_{3}-water nanofluid (Figure 14a). They also noted that the pressure drop of the hybrid nanofluid is slightly lower than that of the Al

_{2}O

_{3}-water nanofluid (Figure 14b), whilst being higher than that of pure water. Based on the experimental data, they proposed a new correlation for predicting the Nu number. They concluded that the studied hybrid nanofluid would be a promising substitute for the water and Al

_{2}O

_{3}-water nanofluid in heat transfer applications.

_{2}O

_{3}/MWCNT-water, TiO

_{2}/MWCNT-water, ZnO/MWCNT-water, and CeO

_{2}/MWCNT-water), with the ratio of 80:20, in different solid concentrations (0.25 to 2 vol. %) and at the temperature of 35 °C in a PHE. They began by measuring the thermophysical properties of the hybrid nanofluids, including thermal conductivity, dynamic viscosity, density, and specific heat. They reported that the maximum enhancement in thermal conductivity belonged to the CeO

_{2}/MWCNT-water hybrid nanofluid by 26.59%. Based on the Mouromtseff number [187], they found that the solid concentration of 0.75 vol. % is the optimum in which the maximum heat transfer would be achieved. Then, they investigated the effects of using the hybrid nanofluids on the heat transfer performance of a PHE and reported that, as expected, the CeO

_{2}/MWCNT-water nanofluid possessed the best performance compared to the other hybrid nanofluids studied. They also reported that the CeO

_{2}/MWCNT-water nanofluid showed the lowest exergy loss and total entropy generation.

_{2}O

_{3}-MWCNT with different ratios (5:0, 4:1, 3:2, 2:3, 1:4, and 0:5), at the solid concentration of 0.01 vol. %, on the thermal performance of a counterflow corrugated PHE, have been experimentally studied by Bhattad et al. [188]. They performed the experiments over a range of the fluid inlet temperatures (10 to 25 °C) and flow rates (2.0 to 0.4 L/m), and studied the heat transfer coefficient, Nu number, pressure loss, and performance index. They found that the Al

_{2}O

_{3}-MWCNT-water hybrid nanofluid possessed a better thermal performance compared to Al

_{2}O

_{3}-water nanofluid. Moreover, variations of the Nu number versus Re number (Figure 15) revealed that the MWCNT-water nanofluid possessed the highest Nu number in all the Re numbers studied, while the Al

_{2}O

_{3}-water nanofluid showed the lowest Nu number. They also found than the maximum enhancement in heat transfer coefficient took place at the ratio of 0:5 (Al

_{2}O

_{3}:MWCNT) by 15.2%, while the minimum enhancement in heat transfer coefficient took place at the ratio of 5:0. They also reported that adding nanomaterials has a negligible effect on the pumping power and pressure loss.

_{2}O

_{3}-SiC, Al

_{2}O

_{3}-AIN, Al

_{2}O

_{3}-MgO, Al

_{2}O

_{3}-CuO, and Al

_{2}O

_{3}-MWCNT), with the ratio of 4:1, on the hydrothermal performance of a counterflow corrugated PEH have been investigated over a range of inlet fluid temperatures and flow rates. They reported that, amongst all the hybrid nanofluids studied, the Al

_{2}O

_{3}/MWCNT-water nanofluid possessed the highest heat transfer performance. The maximum enhancement was reported as 31.2%. However, using this hybrid nanofluid leads to a negligible increase in pumping power (0.08%). They concluded that the Al

_{2}O

_{3}/MWCNT-water hybrid nanofluid would be a good heat transfer fluid for enhancing the thermal performance of the PHE.

#### 4.1.3. Other Types of Nanofluids

_{2}-water [190], ZnO-water [191], silver-water [192], graphene-water/EG [193], Al

_{2}O

_{3}-water [194], and so forth, on the thermal-hydraulic performance of different PHEs. The studies were performed on various solid concentrations, flow rates, and inlet fluid temperatures. Moreover, the effects of using nanofluids instead of conventional working fluids on the convective heat transfer coefficient, pressure drop and pumping power, Nu number, and the overall heat transfer coefficient have also been studied. The following conclusions have been reported by almost all the researchers; however, there are some differences between the reported values:

- It is reported by all the researchers that the Nu number and the convective heat transfer coefficient are enhanced by adding nanoparticles to the working fluids. Moreover, increasing the solid concentration of nanoparticles and the Re number leads to enhancing the Nu number and the convective heat transfer coefficient.
- Adding nanoparticles to the working fluids leads to increasing the dynamic viscosity of the resultant fluids (nanofluids) which, in turn, leads to increasing the pressure loss and pumping power. However, in some literature, it is reported that the increase in the pressure drop is negligible [190,195].
- Replacing the conventional working fluids with nanofluids will bring certain advantages from the heat transfer performance point of view. However, they impose some extra cost in terms of energy consumption; increasing the pressure drop leads to increasing the pumping power and energy consumption.

_{2}O

_{3}-water nanofluid.

#### 4.2. Plate-Fin Heat Exchangers

_{2}-, TiO

_{2}-, ZnO-, Fe

_{2}O

_{3}-, Al

_{2}O

_{3}-, and CuO-water) combined with a wavy channel on the thermal-hydraulic performance of a plate-fin HE have been studied by Aliabadi et al. [206]. The effects of using a parallel and corrugated wavy channel, different solid concentrations (0.1, 0.2, 0.3, and 0.4 wt. %), and the type of basefluid, including deionized water (DIW) and a mixture of DIW with EG in different mixing ratios of 100:0, 75:25, and 50:50, on the thermal-hydraulic performance have been experimentally studied. Two different definitions for the performance evaluation criteria have been employed for the thermal-hydraulic performance as follows:

_{2}-DIW nanofluid showed the best heat transfer performance. Moreover, it was found that using a mixture of DIW and EG with SiO

_{2}nanoparticle showed better thermal-hydraulic performance compared to the SiO

_{2}-DIW nanofluid. The ratio of 75% DIW and 25% EG was reported as the optimum ratio for having the best thermal-hydraulic performance (Figure 19).

## 5. Discussion and Future Direction

## 6. Concluding Remarks

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## Nomenclature

γ | Alpha phase |

ANL | Argonne National Laboratory |

CNTs | Carbon nanotubes |

${C}_{p}$ | Specific heat capacity (kJ/kg·K) |

DIW | Deionized water |

DW | Double-walled |

EG | Ethylene glycol |

$\eta $ | thermal-hydraulic performance |

f | Friction factor |

${\mathrm{f}}_{\mathrm{V}}$ | Volumetric concentration |

h | Convective heat transfer coefficient (W/m^{2}·K) |

HE | Heat exchanger |

h_{nf}/h_{w} | Relative convective heat transfer coefficient |

ṁ | Volume flow rate (lpm) |

MW | Multi-walled |

Nu | Nusselt number |

P | Pressure (Pa) |

Pe | Peclet number |

PFHEs | Plate-fin heat exchangers |

PHEs | Plate heat exchangers |

Re | Reynolds number |

SANSS | Submerged arc nanoparticles synthesis system |

SEM | Scanning electron microscope |

SW | Single-walled |

T | Temperature |

TC | Thermal conductivity (W/m·K) |

TEM | Transmission electron microscope |

u | Flow velocity |

$\mathrm{V}$ | Volume (m^{3}) |

VEROS | Vacuum evaporation onto a running oil substrate |

vol. % | Volume percentage |

wt. % | Wight percentage |

X | spacing distance (mm) |

Greek letters | |

$\mathsf{\rho}$ | Density (kg/m^{3}) |

Δ | Difference |

Subscripts | |

bf | Basefluid |

h | Hot |

i | Used nanofluid |

nf | Nanofluid |

np | Nanoparticles |

ref | Reference liquid |

w | Water |

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**Figure 1.**Data obtained from the Scopus database for the words ‘Heat exchanger’ and ‘Nanofluid’ from 2009 to 2019, where (

**a**) shows the number of publications per year, and (

**b**) demonstrates the type and percentage of these publications.

**Figure 2.**A schematic view of different flow patterns and pass arrangements in plate heat exchangers (PHEs), where (

**a**) series flow, (

**b**) single-pass looped, (

**c**) multi-pass with equal pass, and (

**d**) multi-pass with unequal pass [49].

**Figure 3.**A schematic view of different elements of a plate-fin heat exchanger (PFHE) [49].

**Figure 4.**A schematic view of the two types of flow arrangement in a PFHE, where (

**a**) Crossflow, and (

**b**) Counterflow arrangement [49].

**Figure 5.**Eastman et al. one-step method for nanofluids fabrication. Reproduced with permission from [146]. Cambridge University Press, 2011.

**Figure 6.**Illustration of the two-step nanofluid fabrication method using an ultrasonicator device [158].

**Figure 8.**Thermal conductivity of selected nanomaterials [179].

**Figure 9.**Variations of relative convective heat transfer coefficient versus Peclet number in different solid concentrations. Reproduced with permission from [180]. Taylor & Francis, 2015.

**Figure 10.**(

**a**) Variations of Nu number of the nanofluid and water versus Re number, and (

**b**) Variations of the Nu number versus Re number in different inlet fluid temperature. Reproduced with permission from [181]. Elsevier, 2016.

**Figure 11.**Variations of the convective heat transfer coefficient versus flow velocity (u) in different solid concentrations for: (

**a**) Al

_{2}O

_{3}-water nanofluid, and (

**b**) MWCNT-water nanofluid. Reproduced with permission from [88]. Elsevier, 2015.

**Figure 12.**Variation of the relative thermophysical properties of the studied nanofluids. Reproduced with permission from [183]. Elsevier, 2016.

**Figure 13.**Variations of (

**a**) the convective heat transfer coefficient, and (

**b**) pressure drop versus the spacing value at the solid concentration of 0.75 vol. %. Reproduced with permission from [183]. Elsevier, 2016.

**Figure 14.**Variation of the (

**a**) convective heat transfer coefficient, and (

**b**) pressure drop versus fluid velocity for the three different fluids. Reproduced with permission from [185]. Elsevier, 2016.

**Figure 15.**Variations of Nu number versus Re number in different ratios of the Al

_{2}O

_{3}and MWCNT nanoparticles. Reproduced with permission from [188]. Elsevier, 2019.

**Figure 16.**A schematic view of the: (

**a**) studied plate-fin heat exchanger equipped with vortex-generator, and (

**b**) the computation domain. Reproduced with permission from [204]. Elsevier, 2014.

**Figure 17.**A schematic view of the different studied plate-fin channel; (

**a**) plain channel, (

**b**) perforated channel, (

**c**) offset strip channel, (

**d**) louvered channel, (

**e**) wavy channel, (

**f**) vortex-generator channel, and (

**g**) pin channel. Reproduced with permission from [205]. Elsevier, 2014.

**Figure 18.**The variations of the thermal-hydraulic performance of different studied channels versus solid concentration in different flow rates: (

**a**) 2 lpm, (

**b**) 3.5 lpm, and (

**c**) 5 lpm. Reproduced with permission from [205]. Elsevier, 2014.

**Figure 19.**Variations of the thermal-hydraulic performance versus volumetric flow rate for different ratios of DIW-EG mixture as the basefluid for the SiO

_{2}-based nanofluid [206].

**Figure 20.**The studied geometry of the plain and vortex-generator channel. Reproduced with permission from [207]. Springer Nature, 2015.

**Figure 21.**Variations of the thermal-hydraulic performance versus Re number for: (

**a**) different nanofluids flow inside plain channel, and (

**b**) different heat transfer enhancement methods. Reproduced with permission from [207]. Springer Nature, 2015.

Origin | Nanoparticles | Basefluids | Source |
---|---|---|---|

Metals | Cu * | Water, EG, oil, acetone, and water & EG mixture. | [68,74,75,76,77,78,79] |

Ag * | Water, and toluene. | [80,81] | |

Au * | Water, and toluene. | [81,82,83] | |

Al * | Water, oil, EG, kerosene. | [15,84,85,86,87] | |

Oxides | Al_{2}O_{3} * | Water, EG, oil, and water & glycerine mixture. | [82,88,89,90,91,92,93,94,95,96] |

CuO * | Water, oil, and R-134a *. | [92,93,97,98,99,100,101] | |

ZnO * | Water, EG, and oil. | [102,103,104,105,106,107,108,109] | |

TiO_{2} * | Water, EG, oil, water & EG mixture, and bioglycol & water mixture. | [110,111,112,113,114,115,116] | |

SiO_{2} * | Water, EG, glycerol, oil, and glycerol & EG mixture. | [80,117,118,119,120,121] | |

Carbon-based | MWCNTs * | Water, EG, water & EG mixture, and fullerenes oil. | [88,122,123,124,125,126,127,128] |

DWCNTs * | Water, and EG. | [129,130] | |

SWCNTs * | Water, water & EG mixture. | [131,132] | |

Nanodiamond | Water, EG, propylene glycol, midel oil, silicone oil, mineral oil, transformer oil, and engine oil. | [133] | |

Graphene | Water, water & EG mixture. | [134,135,136] | |

Graphite | Water, texatherm oil. | [137,138,139] |

_{2}O

_{3}, CuO, ZnO, TiO

_{2}, SiO

_{2}, MWCNTs, DWCNTs, SWCNTs, and R-134a are referred to copper, silver, gold, aluminium, aluminium oxide (also known as alumina), copper oxide (also known as cupric oxide), zinc oxide, titanium oxide, silicon dioxide (also known as silica), multi-walled CNTs, double-walled CNTs, single-walled CNTs, and 1,1,1,2-Tetrafluoroethane, respectively.

**Table 2.**A summary of the recently published literature on the effects of using nanofluids on heat transfer performance and pressure drop of PHEs.

Reference | Nanofluid | Considered Conditions and Objectives | Type of HE | Findings |
---|---|---|---|---|

Tiwari et al. [196] | CeO_{2}-water* | - -
- Solid concentrations: 0.5 to 3 vol. %
- -
- Flow rates: 1 to 4 lpm
- -
- The effects of nanofluid on heat transfer and pressure drop.
| Chevron corrugated PHE | They found that the optimum solid concentration (0.75 vol. %) in which the heat transfer reached its maximum enhancement by 39%. They reported that increasing the flow rate of the nanofluid and the hot water leads to enhancing the heat transfer coefficient. Moreover, the increase in the pressure drop at the optimum solid concentration is negligible while the heat transfer has been significantly improved. |

Barzegarian et al. [190] | TiO_{2}-water | - -
- Solid concentrations: 0.3 to 1.5 wt. %
- -
- Flow regime: turbulent
- -
- Re numbers: 159 to 529
- -
- Effects of solid concentration and Re number on heat transfer and pressure drop.
| Brazed PHE | Their results revealed that increasing the Re number and solid concentration results in enhancing the convective heat transfer coefficient, and the maximum enhancement took place at the highest solid concentration by 23.7%. They also reported that the increase in pressure drop by increasing the solid concentration is negligible. |

Kumar et al. [191] | ZnO-water | - -
- Solid concentrations: 0.5 to 2.0 vol. %
- -
- The effects of nanofluid on heat transfer performance and finding the optimum solid concentration.
| Chevron-type PHE | They reported that the solid concentration of 1.0 vol. % is the optimum solid concentration where the maximum heat transfer rate is achieved. |

Unverdi and Islamoglu [197] | Al_{2}O_{3}-water | - -
- Solid concentrations: 0.25 to 1 vol. %
- -
- Flow rates: 90 to 300 kg/h
- -
- Re number: 600 to 1900
- -
- The effects of nanofluid on heat transfer and pressure drop.
| Chevron-type PHE | They reported that increasing the solid concentration and flow rate results in enhancing the Nu number by the maximum of 42.4%. They also reported that the maximum increase in the heat transfer and pressure drop took place at the highest solid concentration and Re number by 6.4% and 8.4%, respectively. |

Pourhoseini et al. [192] | Ag-water | - -
- Nanofluid concentrations: 0 to 10 mg/L
- -
- Flow rate: 2 to 8 lpm
- -
- Nanofluid inlet temperature: 36, 46, and 56 °C
- -
- The effects of flow rate and solid concentration on heat transfer performance.
| CR14-45 COMER PHE | They found that the effect of flow rate on heat transfer performance is more significant than the effect of solid concentration. |

Wang et al. [193] | Graphene nanoplatelets-EG/water (50:50) | - -
- Solid concentrations: 0.01 to 1.0 wt. %
- -
- Re number: 10 to 400
- -
- The effects of using nanofluid on heat transfer and pressure drop.
| Miniature PHE | They reported the maximum enhancement of 4% in heat transfer as the solid concentration increased. Moreover, they reported that the increase in Re number leads to enhancing the heat transfer performance in all the studied solid concentrations. The same trend as was observed for the pressure drop; increasing the solid concentration and Re number leads to increasing the pressure drop. |

Mansoury et al. [194] | Al_{2}O_{3}-water | - -
- Solid concentrations: 0.2 to 1 vol. %
- -
- Flow regime: turbulent
- -
- The effects of nanofluid on heat transfer and pressure drop in different HEs.
| Different HEs; a Double-pipe, a Shell and tube, and a PHE | They reported that the maximum heat transfer of 60% is achieved in the double-pipe HE, while the minimum enhancement took place in the PHE by 11%. Moreover, the minimum increase in pressure drop has been experienced in the PHE. |

Elias et al. [198] | Al_{2}O_{3}-water | - -
- Solid concentrations: 0 to 0.5 vol. %
- -
- Temperatures: 25 to 55 °C
- -
- Re numbers: 180 to 350
- -
- The effects of using nanofluid on heat transfer performance and pressure drop.
| Chevron-type PHE | The results revealed the maximum enhancement of 7.8% in the heat transfer coefficient at the solid concentration of 0.5 vol. %. Moreover, increasing the solid concentration leads to increasing the pressure drop. |

Tayyab at al. [199] | CuO-water | - -
- Solid concentrations: 0.2 to 0.6 vol. %
- -
- Flow rates: 1 to 9 lpm
- -
- Different surface roughness
- -
- The effects of nanofluid on heat transfer performance in different HEs.
| Different HEs: Shell and tube, concentric, spiral, and PHE | The results revealed that the heat transfer performance of the nanofluid in the PHE is better than the other studied HEs. The maximum enhancement in heat transfer for the PHE is 26% while for the other HEs, 21% is reported. |

Attalla and Maghrabie [200] | Al_{2}O_{3}-water | - -
- Solid concentrations: 1.2 to 2.6 vol. %
- -
- Re numbers: 500 to 5000
- -
- The effects of using nanofluid on the Nu number, friction factor, and heat transfer enhancement.
| PHE | The results revealed that the heat transfer performance and the pressure drop has been increased as the solid concentration and surface roughness increased. Moreover, it is found that the influence of the surface roughness is more noticeable than the solid concentration. |

Talari et al. [195] | Al_{2}O_{3}-water | - -
- Solid concentrations: 0 to 5 vol. %
- -
- Finding the optimum solid concentration for heat transfer intensification.
| Corrugated PHE | They declared that since the heat transfer enhancement of the nanofluid showed a monotonic increase, it is not possible to find an optimum solid concentration. |

Sözen et al. [201] | Kaolin-water | - -
- Solid concentration: 2 wt. %
- -
- Temperatures: 40, 45, and 50 °C
- -
- The effect of using nanofluid on heat transfer performance.
| Spiral PHE | It is revealed that using nanofluid instead of the based fluid leads to having 17.6% enhancement in heat transfer rate. Moreover, increasing the Re number leads to decreasing the effectiveness of the PHE. |

Meisam et al. [202] | Al_{2}O_{3}-waterTiO _{2}-waterSiO _{2}-water | - -
- Solid concentrations: 0.05, 0.1, and 0.2 wt. %
- -
- Temperatures: 30 to 50 °C
- -
- Re numbers: 35.9 to 160.6
- -
- Flow rates: 0.4 to 2 L/m
- -
- The effects of using different nanofluids on heat transfer performance have been studied.
| PHE | The results revealed that adding nanoparticles to the basefluid leads to considerable enhancement in heat transfer performance. The maximum enhancement in the heat transfer achieved by using SiO_{2}-water nanofluid at the highest solid concentration and Re number of 37 by 2.82%, while the minimum enhancement has been experienced by using Al_{2}O_{3}-water nanofluid at the solid concentration of 0.1 wt. % and Re number 158 by 1.64%. |

Soman et al. [203] | γ-Al_{2}O_{3}-water* | - -
- Solid concentrations: 0.1, 0.2, and 0.3 wt. %
- -
- Mas flow rate: 0.016-0.082 kg/s
- -
- Re number: 200 to 1100
| Dimpled PHE | It is revealed that increasing the mass flow rate leads to increasing the heat transfer rate in the PHE. Moreover, increasing the mass flow rate has a direct effect on the heat transfer performance. A new correlation for predicting the Nu number has also been proposed. |

_{2}, and γ-Al

_{2}O

_{3}are referred to cerium dioxide, and the alpha phase of alumina, respectively

**Table 3.**A summary of the available literature on the effects of nanofluids, vortex-generators, winglets, and perforations on the thermal-hydraulic performance of plate-fin HEs.

Reference | Nanofluid | Considered Conditions and Objectives | Type of HE | Findings |
---|---|---|---|---|

Aliabadi et al. [208] | Al_{2}O_{3}-water | - -
- Considering the effects of perforations, winglets, and nanofluids on heat transfer performance.
- -
- Solid concentration: 0.1 and 0.3 wt. %
- -
- Waviness aspect ratio: 0.33 to 0.51
- -
- Perforation diameter: 5 mm
- -
- Winglets heights and width: 5 mm
- -
- Re number: 3900 to 11,400
| Wavy plate-fin HE | It is revealed that the Nu number for the wavy channel is higher than that of the plain channel. Moreover, the performance factor values show that applying these three techniques leads to improving the thermal-hydraulic performance of the HE. |

Aliabadi et al. [209] | Al_{2}O_{3}-water | - -
- Solid concentrations: 0.1–0.4 wt. %
- -
- Re number: 4500–11,500
- -
- Waviness aspect ratio: 0.33 to 0.51
- -
- Winglets height: 2 to 6 mm
| Wavy plate-fin HE | It is reported that using the nanofluid instead of the basefluid leads to increasing the thermal performance and pressure drop by 11.3% and 6.2%, respectively. Moreover, increasing the waviness aspect ratio and winglets height results in increasing the heat transfer and pressure drop. |

Aliabadi and Salami [210] | Al_{2}O_{3}-water | - -
- Solid concentration: 0 to 4 wt. %
- -
- Re number: 6000 to 22,000
- -
- The effects of nanofluids, channel height, channel length, stip length, strip pitch, and strip thickness.
| Offset-strip | It is reported that the most effective factor on the thermal-hydraulic performance is the channel height. Moreover, using nanofluid results in having better thermal performance compared to the basefluid. |

Aliabadi and Mortazavi [211] | Al_{2}O_{3}-water | - -
- Solid concentration: 0.1 to 0.4 wt. %
- -
- Re number: 4000 to 10,000
- -
- The effects of nanofluid, waviness aspect ratio and the arrangement of the winglets.
| Chevron plate-fin HE combined with holes and winglets | It is found that the HE equipped with holes and winglets showed enhanced Nu number by a maximum of 1.6%. Moreover, it is reported that employing the nanofluid as the working fluid also leads to enhancing the Nu number. The optimum solid concentration has been reported as 0.3%. |

Aliabadi et al. [212] | Al_{2}O_{3}-water | - -
- Solid concentrations:0.1 and 0.3 wt. %
- -
- Re number: 100–900
| Plate and plate pin fin HE | It is reported that the plate-pin fin showed better heat transfer performance and lower pressure drop. Moreover, using nanofluid leads to enhancing the heat transfer coefficient and the best performance achieved at the solid concentration of 0.3 wt. %. |

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**MDPI and ACS Style**

Almurtaji, S.; Ali, N.; Teixeira, J.A.; Addali, A.
On the Role of Nanofluids in Thermal-hydraulic Performance of Heat Exchangers—A Review. *Nanomaterials* **2020**, *10*, 734.
https://doi.org/10.3390/nano10040734

**AMA Style**

Almurtaji S, Ali N, Teixeira JA, Addali A.
On the Role of Nanofluids in Thermal-hydraulic Performance of Heat Exchangers—A Review. *Nanomaterials*. 2020; 10(4):734.
https://doi.org/10.3390/nano10040734

**Chicago/Turabian Style**

Almurtaji, Salah, Naser Ali, Joao A. Teixeira, and Abdulmajid Addali.
2020. "On the Role of Nanofluids in Thermal-hydraulic Performance of Heat Exchangers—A Review" *Nanomaterials* 10, no. 4: 734.
https://doi.org/10.3390/nano10040734