Review on Mono and Hybrid Nanofluids: Preparation, Properties, Investigation, and Applications in IC Engines and Heat Transfer

Abstract

Nano fluids are widely used today for various energy-related applications such as coolants, refrigerants, and fuel additives. New coolants and design modifications are being explored due to renewed interest in improving the working fluid properties of heat exchangers. Several studies have investigated nanofluids to enhance radiator and heat exchanger performance. A new class of coolants includes single, binary, and tertiary nanoparticle-based hybrid nano-coolants using ethylene glycol/deionized water combinations as base fluids infused with different nanoparticles. This review article focuses on the hydrothermal behavior of heat exchangers (radiators for engine applications) with mono/hybrid nanofluids. The first part of the review focuses on the preparation of hybrid nanofluids, highlighting the working fluid properties such as density, viscosity, specific heat, and thermal conductivity. The second part discusses innovative methodologies adopted for accomplishing higher heat transfer rates with relatively low-pressure drop and pump work. The third part discusses the applications of mono and hybrid nanofluids in engine radiators and fuel additives in diesel and biodiesel blends. The last part is devoted to a summary of the research and future directions using mono and hybrid nanofluids for various cooling applications.

1. Introduction

One of the dominant threats in the current energy scenario is the depletion of energy reserves. Thermal systems such as refrigerators, heat pumps, and air conditioners place enormous energy demands. Due to limited energy resources, several investigations have been conducted to improve thermal system efficiency and performance by reforming the design of system components or changing the working fluid. Many small heating devices (such as tube heat exchangers, plate heat exchangers, and mini-channel heat exchangers) with mono/hybrid nanofluid usage improve the system’s performance.

1.1. Heat Exchanger

Heat exchangers (HEs) play an essential role in thermal energy management. Regarding the energy crisis, efficient HEs are needed to develop new energy-efficient technologies for industries to reduce energy consumption. Therefore, researchers are focusing on improving the design of equipment and the thermal properties of working fluids. Energy optimization also becomes very important due to the limitations associated with conventional fuels. Energy savings can be achieved by increasing the performance of HEs. HEs available are plate-type heat exchangers (PHEs), double-pipe heat exchangers (DPHEs), heat pipes (HPs), and mini-channel/heat sinks (Figure 1). Because of compactness, high effectiveness, flexibility, ease of handling, and high thermal performance, PHEs originated to meet the requirements of the dairy industries and other engineering applications (such as heat recovery, HVAC, cooling, offshore oil, breweries, power generation, dairy, food processing, chemical, pulp and paper production, and refrigeration). In DPHEs, both fluids can move in the same or opposite directions [1]. Power plants use shell-and-tube or double-pipe HEs to generate electricity. Heaters and economizers are components of these plants [2]. DTHEs have become widespread in use due to their simple design, easy cleaning, and low cost involved [3]. Portable devices (laptops, mobile phones, etc.) are preferable, which require limited space. Cooling these small objects is a challenge. Thus, mini/micro channels have emerged [4]. HPs also play an essential role in cooling these small devices [5]. Improved heat transfer in HEs is accompanied by higher pumping power. Therefore, the benefit of enhanced heat transfer and associated pressure loss must be balanced [6].

1.2. Hybrid Nanofluids

Industries with cooling solution requirements have focused on the use of modified fluids with various additives [7] to obtain improved thermal properties. Nanofluids are colloidal mixtures of nanosized particles (10–100 nm) suspended in base fluids [8]. They possess good physical or chemical properties and thermal or rheological properties [9,10]. Hybrid nanofluids are suspensions of a mixture of dissimilar nanoparticles or nanocomposites infused in the conventional base fluid, which yield better thermal conductivity and heat transfer characteristics due to hybridization [11]. They are used in phase change materials, heat exchangers, solar energy, electronics, agriculture, chemical, manufacturing, and automobile industries [12,13,14,15,16,17,18,19,20,21,22,23,24,25]. The two-step method is used for preparing hybrid nanofluids. Different nanoparticles are prepared and mixed in the primary liquid through magnetic or mechanical stirring. The solution is sonicated and characterized to ensure stable and homogeneous mixing, providing improved heat transfer characteristics [26]. Enhanced heat transfer is due to increased surface area, collision, interactive effect, and proper mixing of nanoparticles in base fluids (causing micro turbulences). Hybrid nanofluids play four roles (used as refrigerant, lubricant, absorbent, and secondary refrigerant) in improving the thermal system performance in low-temperature applications. The effects of nanoparticle concentration and size on the performance of water-based CuO nanofluids were investigated in [27]. The synthesis, stability, and thermo-physical properties of hybrid nanofluids were studied in [28]. Zaynon and Azmi [29] presented the influence of nanoparticle type, concentration, temperature, shape, and size on the nanofluid properties. The amount of grapheme required in the base fluid to improve thermal performance was suggested in [30].

1.3. Secondary Refrigerant

Global warming has been a significant concern for environmentalists during the past couple of decades. Refrigeration industries replaced chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) with hydrofluorocarbons (HFCs) to overcome the ozone depletion problem. Supermarket refrigeration systems utilize direct expansion systems with separate evaporation units. Up to 30% loss of refrigerant charges is estimated annually [31]. This refrigerant charge leak exacerbates the global warming problem. There is a necessity for a secondary refrigerant to reduce the leak. Liquid cooling systems are used in industrial refrigeration and commercial air conditioning [32]. A secondary circuit cooling system uses primary and secondary refrigerants. The primary refrigerant (while undergoing a phase change in the evaporator) cools the secondary pumping fluid to the supermarket for cooling. Secondary refrigerants minimize the leakage of primary refrigerants with the possibility of load sharing and easy maintenance. They improve the thermophysical properties of the primary refrigerants, overcoming the additional investment cost and pump work requirement.

1.4. Objectives

The remainder of this review describes the hydrothermal characteristics of a nanofluid-driven single/hybrid plate heat exchanger (PHE), covers the creation of hybrid nanofluids, the empirical relationship with thermo-physical properties, and innovative ways to achieve high heat transfer with relatively low-pressure drops, as well as the research on using single and hybrid nanofluids for heat exchangers (HE) and internal combustion engines (ICEs), and highlights the potential for using single and hybrid nanofluids in the low-temperature sector.

2. Preparation of Mono/Hybrid Nanofluids

Nanofluids are organized according to their preparation using one- or two-step methods (Figure 2). In a one-step approach, nanoparticles are prepared and mixed directly in a base fluid using physical or chemical processes. In the two-step method, nanoparticles are obtained using physical or chemical methods and then effectively infused in an essential base liquid [33]. Several investigators have reviewed the preparation of different mono/hybrid nanofluids based on various base fluids [34,35,36,37,38,39,40,41,42,43,44]. Spherical ZnO particles were synthesized using a sol–gel annealing process at 500–600 °C in [45]. The ball milling process was used to grind aluminum nitride carbon nanocomposite (a nontoxic ceramic) for heat transfer experiments [46]. Making nanofluids through a single-step method is expensive and time-consuming. The control of particle agglomeration is the primary problem in the two-step method. Ultrasonication minimizes nanoparticle sedimentation and improves nanofluid stability [47]. Due to simplicity, 95% of researchers used a two-step method when preparing nanofluids (see

Table 1. Examples of hybrid nanofluids adopting two-step preparation.

Author(s)	Nanoparticle	Base Fluid
Jana et al. [49]	Au–CNT, Cu–CNT	Water
Han et al. [50]	Sphere–CNT	Oil
Turcu et al. [51]	Fe3O4–polypyrrole	Water
Jha and Ramaprabhu [52]	Cu–MWCNT	Water/EG
Han and Rhi [53]	Ag–Al2O3	Water
Baby and Sundara [54]	CuO–HEG	Water/EG
Paul et al. [55]	Al–Zn	EG
Suresh et al. [56]	Al2O3–Cu	Water
Botha et al. [57] *	Ag–SiO2	Oil
Ho et al. [58]	Al2O3–PCM	Water
Baby and Sundara [59]	Ag–HEG	Water/EG
Amiri et al. [60]	Ag–MWCNT	Water
Chen et al. [61]	Ag–MWCNT	Water
Aravind and Ramaprabhu [62]	Graphene–MWCNT	Water and EG
Bhosale and Borse [63]	Al2O3–CuO	Water
Balla et al. [64]	CuO–Cu	Water
Abbasi et al. [65]	ϒ-Al2O3–MWCNT	Water
Nine et al. [66]	Cu–Cu2O	Water
Munkhbayar et al. [67] *	Ag–MWCNT	Water
Sundar et al. [68]	Nanodiamond–nickel	Water/EG
Parameshwaran et al. [69]	Ag–TiO2	PCM
Batmunkh et al. [70]	Ag–TiO2	Water
Madhesh et al. [71]	Cu–TiO2	Water
Chen et al. [72]	MWCNT–Fe3O4	Water
Parekh [73]	Mn0.5Zn0.5Fe2O4	Oil
Luo et al. [74]	Al2O3–TiO2	Lubricating oil
Madhesh and Kalaiselvam [75]	Cu–TiO2	Water
Zubir et al. [76]	Graphene oxide–CNT	Water
Qadri et al. [77]	Graphene–Cu2O	Water/EG
Karimi et al. [78]	NiFe2O4	Water
Chakraborty et al. [79]	Cu–Al	Water
Megatif et al. [80]	CNT–TiO2	Water
Abbasi et al. [81]	MWCNT–TiO2	Water
Toghraie et al. [82]	ZnO–TiO2	EG
Bhanvase et al. [83]	PANI–CuO	Water
Asadi et al. [84]	CuO–TiO2	Water
Chen et al. [85]	Al2O3	Liquid paraffin
Asadi et al. [86]	MWCNT	Water
Gulzar et al. [87]	Al2O3–TiO2	Therminol-55
Alarifi et al. [88]	MWCNT–TiO2	Oil
Akram et al. [89]	CGNP	DI Water
Sharafeldin and Grof [90]	WO3	Water
Chen et al. [91]	MWCNT	Water
Ali et al. [92]	Al	Water
Mahbubul et al. [93]	Al2O3	Water
Mahyari et al. [94]	GO–SiC	Water/EG
Chen et al. [95]	Fe3O4–MWCNT	Brine water
Okonkwo et al. [96]	Al2O3–Fe	Water
Terueal et al. [97]	MoSe2	Water
Li et al. [98]	SiO2	Liquid paraffin
Geng et al. [99]	ZnO–MWCNT	Oil
Li et al. [100]	SiO2	EG

* Single-step method.

).

Jana et al. [49] infused various volume fractions of CNTs in water to obtain CNT suspensions. Au nanoparticles were suspended with CNT in varying volume fractions to obtain CNT–Au suspensions. The hybrid suspension was sonicated for 1 h using an ultrasonic cleaner to get an adequately dispersed solution. Bhosale and Borse [63] prepared a hybrid nanofluid (Al₂O₃–CuO water) by mixing 2.5 mg of CuO and Al₂O₃ in distilled water. Later, the concentration was varied to 0.25%, 0.5%, and 1.0% volume. Toghraie et al. [82] prepared ZnO–TiO₂/EG hybrid nanofluids by dispersing equal volumes of ZnO and titanium dioxide (TiO₂) nanoparticles in a given amount of pure EG as a base liquid. The stability of the prepared nanofluid was confirmed, ensuring no sedimentation. Paul et al. [55] synthesized Al–Zn nanoparticles by stirring. They prepared hybrid nanofluids through a two-step process. Al–Zn nanoparticles were added to ethylene glycol (base fluid), followed by sonication and magnetic stirring. Suresh et al. [56] obtained a hybrid powder of alumina–copper using a thermochemical method, including spray-drying, oxidation of the precursor powder, hydrogen reduction, and homogenization. They used different volume fractions (0.1%, 0.33%, 0.75%, 1.0%, and 2.0%). Baby and Sundara [54] used a hydrogen-induced exfoliation and chemical reduction process of graphite oxide (GO) to synthesize grapheme decorated with CuO (CuO/HEG). The HEG obtained was functionalized by acid treatment and coated with CuO nanoparticles. CuO/HEG was dispersed in the base liquid (water/EG) by ultrasonication. Nine et al. [66] reported an economical and beneficial process for synthesizing Cu₂O and Cu/Cu₂O nanoparticles with a mean size of less than 30 nm. A ball milling process was used to synthesize Cu/Cu₂O–water hybrid nanofluids. Madhesh et al. [71] prepared a copper–titania hybrid nanofluid by uniformly dispersing an aqueous solution of titania (5 g) and copper acetate (0.5 g) in an ultrasonic vibrator for 2 h using reducing agents at 45 °C and atmospheric pressure. A one-step method was described for a hybrid nanofluid containing silver and silica nanoparticles by Botha et al. [57]. Ho et al. [58] prepared phase change material (PCM) suspensions using interfacial poly-condensation and emulsion techniques. Nanofluid Al₂O₃–water was obtained by adding Al₂O₃ nanoparticles in water (base liquid). Chen et al. [61] prepared Ag/MWCNT nanocomposites using the silver mirror reaction. Functionalized MWCNTs were used to fabricate Ag/MWCNT nanocomposites using sodium dodecyl sulfate (SDS) as a surfactant and formaldehyde as a reducing agent.

3. Characterization and Stability of Mono/Hybrid Nanofluids

Several forces, such as van der Waals attraction, buoyancy, gravity, and electrostatic repulsion, cause destabilization and sediment formation. Van der Waals attraction and gravity decrease the stability of colloidal suspensions. Stability is a critical factor in the effectiveness of nanofluids for technological applications. All thermo-physical properties of nanofluids depend on their stability. The instability of nanofluids can reduce their effectiveness in many heat transfer applications. It is caused by the tendency of nanoparticles to form clusters in liquids. An SEM image of the Al₂O₃–MWCNT/water hybrid nanofluid is shown in Figure 3 [101].

The particle aggregation causes the separation of nanoparticles from base fluids and forms sedimentation [102]. The coagulation rate is determined from the collision frequency of particles in Brownian motion and cohesion probability [103]. Removal of agglomeration propensity yields stable nanofluids. Methods adopted for assessing the stability of nanofluids are the sedimentation method, spectral absorbance, centrifugation method, transmittance measurement, zeta potential measurement, and dynamic light scattering [104]. For long-term stable and homogenous nanofluids, the following surfactants can be added [105,106]: anionic (sodium dodecyl sulfate and sodium dodecyl benzene sulfonate), cationic (cetyl trimethyl ammonium bromide), nonionic (Span-80 and Tween-20), and polymer (polyvinyl pyrrolidone, polyvinyl alcohol, and gum Arabic). Surfactants improve the wettability of the nanoparticles and the base fluids by reducing the base fluid’s surface tension and improving the nanoparticles’ dispersibility [107].

Ultrasonic mills, baths, stirrers, and high-pressure homogenizers are used for the dispersion of nanoparticles. Baby and Sundara [59] used an economical method to synthesize hydrogen-functionalized, exfoliation-induced silver-decorated graphene (Ag/HEG) and prepared nanofluids. Ag/HEG was distributed in a mixture of deionized water/ethylene glycol using ultrasonic agitation without surfactant. The hybrid nanofluid was observed to be stable for more than 3 months. Aravind and Ramaprabhu [62] prepared MWCNT nanocomposites with graphene shells and synthesized them by chemical vapor deposition. The prepared hybrid nanofluid was stable for an extended period. Megatif et al. [80] prepared a CNT–TiO₂ hybrid nanocomposite and dispersed it in water to obtain a hybrid nanofluid. The surfactant SDBS was added to the suspension for proper dispersion. They sonicated the solution for 15 min and tested its stability. The solution was stable for 2 days.

Although most (95%) of the researchers adhered to the two-step method, nanofluids synthesized by the expensive and complex one-step method improve the stability of nanoparticle suspensions in base oils due to high sedimentation rates with short sonication times [108]. Ultrasonication lessens the sedimentation of nanoparticles, resulting in enhanced nanofluid stability. A better understanding of the mechanisms of nanofluids at the atomic level is required to address particle transport, aggregation, and stability issues with minimal experimentation.

No sophisticated equipment is required to produce nanofluids using a simple two-step method. Dispersion of nanoparticles requires sonication times of 3–10 h [109]. Amin et al. [110] critically reviewed the properties of single and hybrid nanofluids based on organic and synthetic materials. Malika and Sonavan [111] used a two-step method to prepare CuO–ZnO/water hybrid nanofluids. Ultrasonication provided nanofluid stability. FESEM/EDS, dynamic light scattering, and zeta potential measurements provide insight into nanoparticle morphology, shape, and size. The stability of Al₂O₃–CuO/(50/50) EG/W (ethylene glycol/water) hybrid nanofluids at 60 °C was confirmed by zeta potential measurements [112].

The stability of trihybrid nanofluids was tested by mixing three types of nanoparticles (i.e., Al₂O₃, TiO₂, and SiO₂ with volume concentrations of 0.05–0.3%) in a water/ethylene glycol-based fluid [113] and a recommended sonication time of 10 h at a zeta potential of 25.1 mV. To improve the stability of nanofluids, Afshari et al. [114] highlighted properties such as the acidity degree of the nanofluid, ultrasonication, nanoparticle material, base fluid type, nanoparticle concentration, surfactants, and surface modification of nanoparticles. Arora and Gupta [115] reviewed stability evaluation techniques (spectral absorbance, sedimentation, zeta-potential, and electron microscopy) and enhancement techniques (ultrasonication, surfactant addition, particle surface modifications, and pH change). Future research should focus on industrial applications to minimize pressure losses, the concentration of nanoparticles, and the long-term stability of hybrid nanofluids.

The stability characteristics of mono and hybrid nanofluids have been studied using zeta potential measurements and vibrating sample magnetometry (VSM) analysis [116]. To maintain nanofluid stability, Zainon and Azmi [29] recommend analysis by sonication, pH modification, surfactant, TEM, field emission scanning electron microscopy (FESEM), XRD, zeta potential, and UV/visible spectroscopy techniques. Bumataria et al. [117] used single and hybrid nanofluids to study heat transfer consider in heat pipe technologies. The use of dispersing agents and sonication increases the stability of nanofluids [118]. Excellent suspension stability could be obtained by adding small amounts of SDBS and PEG to DW (hybrid nanofluid 25% Al₂O₃ + 75% TiO₂) [119]. The hybrid nanofluid’s stability was high, as the zeta potential value (i.e., the electrostatic repulsive force between the nanoparticle and the base fluid) was 42.6 mV compared to the reference value of 30 mV. Said et al. [120] investigated the stability of carbon nanofibers (CNF), functionalized carbon nanofibers (F-CNF), reduced graphene oxide (rGO), and F-CNF/rGO nanofluids. Hybrid nanofluids (FCNF/rGO) showed higher stability than CNF, F-CNF, and rGO nanofluids.

Muthoka et al. [121] investigated the stability of hybrid nanofluids with two nanoparticles in PCM/DI water. The stability of surfactant-free MgO and 24 wt.% primary liquid was poor after 24 h, whereas the functionalized MWCNT solution showed no separation after 24 h. It was confirmed that the nanofluid’s low-temperature stability was increased using a surfactant. Acid treatment with CNF was used to test stability [122]. The zeta potential of 0.02 vol.% F–CNF nanofluids measured after 2 and 90 days was −42.9 and −41.8 mV, indicating improved stability compared to the −16.3 and −15.5 mV UNV zeta potentials, which were characterized by relatively unstable dispersion. Alawi et al. [123] synthesized aqueous nanofluids PEG–GnP, PEG–TGr, Al₂O₃, and SiO₂. The dispersion stability of the carbon-based nanofluid and the metal oxide nanofluid was observed for 30 days, and the high dispersibility of PEG–HNP and PEG–TGr in an aqueous medium with low sedimentation was confirmed. Compared to GnP/DW nanofluids, TiO₂/DW nanofluids showed superior stability [124]. The addition of CTAB surfactant showed excellent stability of ternary hybrid nanofluids [125]. Uysal [126] used a 500 rpm homogenizer to mix and stabilize nano-graphene in vegetable oil. Al-Waeli et al. [127] demonstrated high nanofluid stability (over 80 days) with CTAB and tannic acid + ammonia solution. The stability of Al₂O₃/water nanofluids using CTAB and SDBS surfactants was investigated for various pH values [128]. Kazemi et al. [129] visually observed the stability of SiO₂/water and G/water nanofluids. SiO₂/water nanofluids were found stable at all pH values (see

Table 2. Stability of different nanofluids with surfactants.

Author(s)	Nanoparticle	Base Fluid	Surfactant (s)
Xian et al. [130]	COOH-GnP, TiO2	DW/EG	SDC, CTAB *, SDBS
Almanassra et al. [131]	CNT	Water	GA *, PVP, SDS
Cacua et al. [132]	Al2O3	Water	CTAB, SDBS *
Kazemi et al. [129]	SiO2, graphene	Water	CMC *
Ouikhalfan et al. [133]	TiO2	DW	CTAB *, SDS
Siddiqui et al. [134]	Cu-Al2O3	DI water
Cacua et al. [128]	Al2O3	DI water	CTAB, SDBS *
Etedali et al. [135]	SiO2	DI water	CTAB *, SLS *
Giwa et al. [136]	Al2O3-Fe2O3	DW	SDS *, NaDBS *
Kazemi et al. [137]	G-SiO2	DW	CMC *
Gallego et al. [138]	Al2O3	Water	SDBS *
Shah et al. [139]	(rGO)	EG	CTAB *, SDBS, and SDS
Ilyas et al. [140]	GnP	Saline water	SDS *

* Recommended surfactant for improved stability of hybrid nanofluids.

for the stability of various nanofluids).

Brownian motion of nanoparticles, micro-convection, clustering, and pH value strongly affect the thermal properties of hybrid nanofluids [141]. Solidification and clustering of nanocomposites of different sizes in nanofluids affect their thermal properties [142,143]. The stabilization and evaporation of single and hybrid nanofluids have been studied in specific systems from a statistical point of view [144,145].

4. Thermo-Physical Properties of Mono/Hybrid Nanofluids

This section summarizes the influence of using hybrid nanofluids on effective thermal conductivity, dynamic viscosity, density, and specific heat [146,147,148]. Devices commonly used for measuring the properties (density, specific heat, thermal conductivity, and viscosity) of working fluids, including nanofluids and hybrid nanofluids, are shown in Figure 4. The density of a liquid can be measured by taking the weight of the liquid and dividing it by its volume. A digital scale can be used to measure mass. Viscosity can be measured with a Brookfield DV1 digital viscometer, and thermal conductivity and specific heat can be measured with a hot disc thermal constant analyzer.

4.1. Density and Specific Heat of Mono/Hybrid Nanofluids

Density and specific heat capacity are among the most important thermo-physical properties of hybrid nanofluids when studying the heat transfer properties of working fluids. Ho et al. [149] studied aqueous hybrid nanofluids of Al₂O₃ nanoparticles and microencapsulated particles of phase change materials. They measured various thermo-physical properties of the hybrid nanofluids. Baghbanzadeh et al. [150] studied the therm-o-physical properties of aqueous hybrid nanofluids containing different weights of silica and MWCNTs. They observed that the density and viscosity of the hybrid nanofluids increased with concentration but decreased with increasing temperature. Labib et al. [151] found that density increased more than the viscosity with the volume fraction of a hybrid suspension containing carbon nanotubes and oxides.

Some studies of hybrid nanofluids concluded that the specific heat increases with particle volume concentration and temperature, while the density enhances with concentration and drops with the temperature. The increase in specific heat is related to the formation of nanostructures at the solid–liquid interface, whereas particle aggregation harms the increase in specific heat. The hybrid nanofluid’s density ratio [152] is obtained from the mass balance and the specific heat capacity from the energy balance. The generalized form of the density and specific heat equations for hybrid nanofluids are as follows:

$${\rho }_{nf}=\sum {\varphi }_p{\rho }_p+{\rho }_{bf}(1-\sum {\varphi }_p),$$

(1)

$${\rho }_{nf}{c}_{nf}=\sum {\varphi }_p{\rho }_p{c}_p+{\rho }_{bf}{c}_{bf}(1-\sum {\varphi }_p),$$

(2)

where ρ is the density, $\varphi$ is the volume fraction, and c is the specific heat.

4.2. Viscosity of Mono/Hybrid Nanofluids

Viscosity is one of the critical thermophysical properties for studying the behavior of hybrid nanofluids because the required pressure drop and pump operation depend on it. Numerous physical parameters affect the viscosity of nanofluids. Large particles have a relatively higher viscosity than small particles [153]. Nanofluids at low volume concentrations exhibit Newtonian behavior [154]. Ho et al. [149] reported the viscosity of a 10 wt.% hybrid suspension compared to water. Baghbanzadeh et al. [150] studied the rheological properties of aqueous hybrid nanofluids containing silicon dioxide and MWCNTs in weight ratios of 80:20 and 50:50. They observed that the nanofluid’s viscosity increased as the concentration and temperature decreased. The viscosity increase was the smallest for the 50:50 wt.% ratio. Abashi et al. [65] measured the viscosity of MWNT–TiO₂/water suspensions. They observed an increase in fluid viscosity, and the enhancement was more significant when there were more MWCNT nanoparticles than TiO₂ nanoparticles in the solution. Sundar et al. [68] experimentally determined the viscosity of the nanocomposite hybrid nanofluid MWCNT–Fe₃O₄ and observed an increase in viscosity up to 1.5 times at 3 vol.% concentration and 60 °C compared to water.

Esfe et al. [155] and Dardan et al. [156] studied an oil-based hybrid nanofluid containing MWCNT nanoparticles. They observed a significant increase in viscosity. Soltani and Akbari [157] studied ethylene glycol-based hybrid nanofluids containing MgO and MWCNT nanoparticles. They observed an increase in viscosity of up to 168% when increasing the particle volume fraction to 1.0%. Esfe et al. [158] studied the viscosity of MWCNT–TiO₂ hybrid nanofluids using brine as the base fluid. They observed an 83% increase in viscosity at 10 °C. Asadi and Asadi [159] proposed a correlation for the viscosity of motor oil-based hybrid nanofluids containing MWCNTs and ZnO nanoparticles (15:85). The maximum increase in viscosity was about 45% at 55 °C and 1.0% concentration. Suresh et al. [56] tested a hybrid nanofluid of aluminum oxide and copper, showing that the viscosity increased by 115% at a concentration of 2.0 vol.%. At low concentrations, the effect was negligible. Suspensions containing silica and silver particles had lower viscosities than those containing only silica particles [57]. A 24% increase in viscosity was observed at a concentration of 0.02 vol.% [160].

Yarmand et al. [161] found a 30% increase in viscosity and 0.09% density for the nanocomposite hybrid nanofluid GNP–silver at 40 °C. The thermal conductivity and viscosity of nanodiamond–Fe₃O₄ nanofluids were noted for 20 to 60 °C temperature and concentrations up to 0.2 vol.% [162]. They suggested a correlation for the viscosity ratio at different temperatures, which was exponential and depended only on the volume concentration. An expression was proposed for the viscosity of hybrid oil-based MWCNT nanofluids at 25–60 °C [155,163,164]. A correlation for the viscosity of the hybrid nanofluids was established according to the shape function [165].

$$\frac{{\mu }_{nf}}{{\mu }_{bf}}=1+{\psi }_1\varphi +{\psi }_2{\varphi }^2,$$

(3)

where $\varphi$ is the total volume fraction of particles. μ_nf and μ_bf are the viscosities of the hybrid nanofluid and base fluid, respectively, and Ψ₁ and Ψ₂ are coefficients, whose values were given by Sheikholeslami and Shamlooei [165]. Correlations for the viscosity and viscosity ratio for different hybrid nanofluids are summarized in

Table 3. Summary of correlation for the viscosity of hybrid nanofluids.

Author(s)	Nanoparticle/Base Fluid	Working Condition	Correlation
Esfe et al. [160]	Ag–MgO (50: 50)/water	φ = 0–2%	$\frac{{\mu }_{nf}}{{\mu }_{bf}}=1+32.795\varphi -7214{\varphi }^2+714600{\varphi }^3-0.1941\times 10^8{\varphi }^4$
Asadi and Asadi [159]	MWCNT–ZnO (15:85)/engine oil	T = 5–55 °C, φ = 0.125–1.0%	${\mu }_{nf}=796.8+76.26\varphi +12.88T+0.7965\varphi T-196.9\sqrt{T}-16.53\Phi \sqrt{T}$
Afrand et al. [163]	MWCNT–SiO2 (equal portion)/SAE40	T = 25–60 °C, φ = 0–1%	$\frac{{\mu }_{nf}}{{\mu }_{bf}}=0.00337+\exp (0.07731{\varphi }^{1.452}{T}^{0.3387})$
Asadi et al. [166]	MWCNT–MgO (20:80)/SAE50	T = 25–50 °C, φ = 0–2%	$\frac{{\mu }_{nf}}{{\mu }_{bf}}=\left(328201T^{-2.053}{\varphi }^{0.09359}\right)$
Dardan et al. [156]	MWCNT–Al2O3 (25:75)/SAE40	T = 25–50 °C, φ = 0–2%	$\frac{{\mu }_{nf}}{{\mu }_{bf}}=1.123+0.3251\varphi -0.08994T+0.002552T^2-0.00002386T^3+0.9695{\left(\frac{T}{\varphi }\right)}^{0.01719}$
Soltani and Akbari [157]	MWCNT–MgO/EG	T = 30–60 °C, φ = 0.1–1.0%	$\frac{{\mu }_{nf}}{{\mu }_{bf}}=\left[0.191\varphi +0.240\left(T^{-0.342}{\varphi }^{-0.473}\right)\right]\times \exp \left(1.45T^{0.120}{\varphi }^{0.158}\right)$

. The viscosity data obtained from the test and proposed empirical relations were compared for the 0.1% volume concentration of alumina nanofluid (Figure 5). It can be observed that some results matched the test data, whereas others showed deviation. The reason can be negligence of particle size or working temperature, whereas different relationships may be valid for other working parameters. Therefore, researchers must choose an appropriate empirical ratio or develop one suitable for the nanofluid synthesized.

4.3. Thermal Conductivity of Mono/Hybrid Nanofluids

Thermal conductivity is one of the essential physical properties of fluid for improving the heat transfer performance of a working fluid because the heat transfer characteristics increase as the thermal conductivity increases. The mechanism of Brownian motion enhances the thermal conductivity of nanofluids at low temperatures [167]. Hamze et al. [168] studied the influence of thermal conductivity on FLG properties. Triton X-100 surfactant increased thermal conductivity and dynamic viscosity in [169]. Batmunkh et al. [70] showed a 0.8% increase in thermal conductivity for silver–titanium hybrid nanofluids. Hybrid silver–magnesium nanofluids showed a rise of 8.6% in thermal conductivity at a concentration of 0.02 vol.% [160]. Charab et al. [170] developed a thermal conductivity model for Al₂O₃–TiO₂ hybrid nanofluids. They found a nonlinear relationship between particle volume concentration and thermal conductivity due to stability issues in nanofluids.

Baghbanzadeh et al. [150] observed 23.3% and 8.8% increases in thermal conductivity for MWCNT nanofluids and silica nanofluids. The effect of nanoparticle shape on fluid thermal conductivity was examined using alumina–MWCNT hybrid nanofluids [171]. The thermal conductivity was found to be enhanced. However, spherical particles give better results than cylindrical particles. Sundar et al. [68] and Shahsavar et al. [172] saw increased thermal conductivity when using CNT–Fe₃O₄ hybrid nanofluids. Farbod and Ahangarpour [173] and Munkhbayar et al. [67] studied the thermal properties of water-based hybrid Ag–MWCNT nanofluids. They found that the resulting thermal conductivity was 20% and 14.5% higher than the base fluid. Soltani and Akbari [157] analyzed ethylene glycol-based hybrid nanofluids containing MgO and MWCNT nanoparticles. Thermal conductivity increased by 14.5% at 40 °C. Harandi et al. [174] experimentally studied the thermal conductivity of EG-based hybrid nanofluids containing functionalized MWCNTs and Fe₃O₄ nanoparticles. They found an augmentation in thermal conductivity of 30% at a temperature of 50 °C and a concentration of 2.3%.

Botha et al. [57] studied an oil-based hybrid nanofluid of silver and silica (a nano lubricant) and observed a 15% increase in thermal conductivity for 0.6 wt.% Ag and 0.7 wt.% silica by weight. Al₂O₃–Cu hybrid nanofluids gave better results than aluminum oxide nanofluids with an augmented thermal conductivity of 12.11% [56]. Adding 0.2 vol.% ND–Fe₃O₄ nanoparticles increased the thermal conductivity of water by 17.8% [162]. Yarmand et al. [175] studied the thermal conductivity of graphene (0.06 wt.%) activated a carbon suspension in ethylene glycol and observed 4.17% and 6.47% increases at 20 °C and 40 °C. Various researchers have proposed correlations for hybrid nanofluids with various base fluids (deionized water, motor oil, vegetable oil, glycols, mixtures of glycols, and water).

Table 4. Summary of correlation for the thermal conductivity of hybrid nanofluids.

Author(s)	Nanoparticle/Base Fluid	Working Condition	Correlation
Chougule and Sahu [176]	-	-	$\frac{k_{eff}}{k_{bf}}=1+\frac{k_{p1}{\varphi }_1{r}_{bf}}{k_{bf}{r}_{p1}\left(1-\left\{{\varphi }_1+{\varphi }_2\right\}\right)}+\frac{k_{p2}{\varphi }_2{r}_{bf}}{k_{bf}{r}_{p2}\left(1-\left\{{\varphi }_1+{\varphi }_2\right\}\right)}$
Takabi and Salehi [177]	Al2O3–Cu/water	φ = 0.1–2.0%	$\frac{k_{nf}}{k_{bf}}=\frac{\frac{{\varphi }_1{k}_1+{\varphi }_2{k}_2}{\varphi }+2(1-\varphi )k_{bf}+2({\varphi }_1{k}_1+{\varphi }_2{k}_2)}{\frac{{\varphi }_1{k}_1+{\varphi }_2{k}_2}{\varphi }+(2+\varphi )k_{bf}-({\varphi }_1{k}_1+{\varphi }_2{k}_2)}$
Esfe et al. [178]	CNTs–Al2O3/water	T = 27–57 °C, φ = 0–1%	$\frac{k_{nf}}{k_{bf}}=1.05+0.005T+0.06\varphi +0.0099\varphi T+0.00317T^2+0.026{\varphi }^2+0.0034T^2\varphi +0.00735T{\varphi }^2$
Esfe et al. [160]	Ag–MgO (equal)/water	φ = 0–2%	$\frac{k_{nf}}{k_{bf}}=\frac{0.1747\times 10^5+\varphi }{0.1747\times 10^5-0.1498\times 10^6\varphi +0.1117\times 10^7{\varphi }^2+0.1997\times 10^8{\varphi }^3}$
Esfe et al. [179]	Cu–TiO2/water–EG (60:40)	T = 30–60 °C, φ = 0.1–2.0%	$\frac{k_{nf}}{k_{bf}}=1.07+0.000589T-\frac{0.000184}{\varphi T}+4.44T\varphi \times cos\left(6.11+0.00673T+4.41\varphi T-0.0414\mathit{sin}T\right)-32.5\varphi$
Esfe et al. [180]	DWCNT–ZnO/water–EG (60:40)	T = 25–50 °C, φ = 0.025–1.0%	$\frac{k_{nf}}{k_{bf}}=0.0288\times \mathit{ln}\left(\varphi \right)+1.085\exp (0.001351T+0.13{\varphi }^2)$
Harandi et al. [174]	MWCNT–Fe3O4 (equal)/EG	T = 25–50 °C, φ = 0.1–2.3%	$\frac{k_{nf}}{k_{bf}}=1+0.0162{\varphi }^{0.7038}{T}^{0.6009}$
Afrand [181]	MgO–fMWCNT/EG	T = 25–50 °C, φ = 0.0–0.6%	$\frac{k_{nf}}{k_{bf}}=0.8341+1.1{\varphi }^{0.243}{T}^{-0.289}$
Vafaei et al. [182]	MgO–MWCNT/EG	T = 25–50 °C, φ = 0.0–0.6%	$\frac{k_{nf}}{k_{bf}}=0.9787+\mathit{exp}\left(0.3081{\varphi }^{0.3097}-0.002T\right)$
Esfe et al. [183]	SWCNT–MgO (20:80)/EG	T = 30–50 °C, φ = 0.0–2.0%	$\frac{k_{nf}}{k_{bf}}=0.90844-0.06613{\varphi }^{0.3}{T}^{0.7}+0.01266{\varphi }^{0.31}T$
Esfe et al. [184]	MWCNT–SiO2 (15:85)/EG	T = 30–50 °C, φ = 0.0–2.0%	$\frac{k_{nf}}{k_{bf}}=0.905+0.002069T\varphi +0.04375{\varphi }^{0.09265}{T}^{0.3305}-0.0063{\varphi }^3$
Esfe et al. [185]	MWCNT–SiO2 (30:70)/EG	T = 30–60 °C, φ = 0.025–0.86%	$\frac{k_{nf}}{k_{bf}}=1.01+0.007685T\varphi -0.5136{\varphi }^2{T}^{-0.1578}+11.5{\varphi }^3{T}^{-1.175}$
Esfe et al. [186]	DWCNT–SiO2/EG	T = 30–50 °C, φ = 0.03–1.71%	$\frac{k_{nf}}{k_{bf}}=0.9896-0.07122\varphi +\left(0.02705{\varphi }^{0.7685}{T}^{0.627}\right)+1.531\times 10^{-5}{T}^2$
Esfe et al. [187]	SWCNT–Al2O3 (15:85)/EG	T = 30–50 °C, φ = 0.0–2.5%	$\frac{k_{nf}}{k_{bf}}=0.963+0.008379\left[{\varphi }^{0.4439}\times T^{0.9246}\right]$
Rostamian et al. [188]	CuO–SWCNT (50:50)/water–EG (60:40)	T = 20–50 °C, φ = 0.02–0.75%	$\frac{k_{nf}}{k_{bf}}=1+0.04056\varphi T-0.003252{\left(\varphi T\right)}^2+0.0001181{\left(\varphi T\right)}^3-0.000001431{\left(\varphi T\right)}^4$
Zadkhast et al. [189]	MWCNT–CuO/water	T = 25–50 °C, φ = 0.0–0.6%	$\frac{k_{nf}}{k_{bf}}=0.907\mathit{exp}(0.36{\varphi }^{0.3111}+0.000956T)$

summarizes the correlations proposed by the researchers for the thermal conductivity of hybrid nanofluids.

Elias et al. [190] proposed the following correlation for the thermal conductivity of hybrid nanofluids according to their shape function:

$$\frac{k_{nf}}{k_{bf}}=\left(\frac{k_1+(n_1-1)k_{bf}-(n_1-1)(k_{bf}-k_1){\varphi }_1}{k_1+(n_1-1)k_{bf}+(k_{bf}-k_1){\varphi }_1}\right)\left(\frac{k_2+(n_2-1)k_{nf}-(n_2-1)(k_{nf}-k_2){\varphi }_2}{k_2+(n_2-1)k_{nf}+(k_{nf}-k_2){\varphi }_2}\right),$$

(4)

where n₁ and n₂ are shape functions whose values differ for different shapes. k_nf and k_bf are the thermal conductivities of the hybrid nanofluid and base fluid, respectively. k₁ and k₂ are the thermal conductivities of particles 1 and 2, respectively. $\varphi$₁ and $\varphi$₂ are the volume fractions of particles 1 and 2, respectively. Different shapes, such as cylinders, blades, bricks, and plates, were considered in [190], revealing that cylindrical particles performed better than other shapes.

For thermal conductivity, some empirical relationships have been presented that are not valid for all nanofluids with different concentrations of nanoparticles. This can be seen from Figure 6, where test data are compared with the results obtained from other empirical relations. Therefore, researchers must choose an appropriate empirical ratio or develop a relation suitable for the developed nanofluid. It was observed that the viscosity and thermal conductivity of the working fluid increased with the addition of nanoparticles. The researchers observed this at low concentrations of nanoparticles and assumed that the same trend would continue at higher concentrations. Experiments revealed that the Prandtl number (ratio of viscosity to thermal conductivity) increases up to a specific concentration of nanoparticles. Then, it starts to fall because the two properties do not increase proportionally [190]. This increasing–decreasing trend in the Prandtl number affects the Nusselt number, which in turn affects the performance of the heat exchanger.

Table 5. Experimental data for thermo-physical properties of different fluids.

T (°C)	Water	TiO2 (0.1 vol.%)	Al2O3 (0.1 vol.%)
	Thermal conductivity, $k$ (W/m-K)
10	0.5823	0.5919	0.5922
15	0.5896	0.5979	0.5994
20	0.5964	0.6036	0.6047
25	0.6014	0.6091	0.6109
	Density, $\rho$ (kg/m3)
10	997.8	1001.0	1000.7
15	996.8	1000.0	999.5
20	996.0	999.0	998.6
25	994.7	997.9	997.3
	Viscosity, $\mu$ (mPa·S)
10	0.9549	0.9684	0.9684
15	0.8706	0.8935	0.8786
20	0.8150	0.8275	0.8187
25	0.7493	0.7690	0.7535
	Specific Heat, $C_P$ (J/kg·K)
10	4183	4168	4170
15	4183	4169	4169
20	4183	4169	4169
25	4183	4169	4169

represents the thermo-physical properties of different fluids measured by devices mentioned in Figure 4. It also shows the variation of properties at different working temperatures (10–25 °C). It can be observed that the thermal conductivity, density, and viscosity increase with the addition of nanoparticles in the base fluid. The fluid’s thermal conductivity increases with the temperature increase, whereas density and viscosity decrease. No change is observed for the specific heat as the studied temperature range is small.

Table 6. Comparison of experimental and theoretical data for thermo-physical properties of Al₂O₃ nanofluid.

T (°C)	K_Test (W/m-K)	K_th (W/m-K)	$\mathit{\mu }\_\mathit{t}\mathit{e}\mathit{s}\mathit{t}$ (mPa·S)	$\mathit{\mu }\_\mathit{t}\mathit{h}$ (mPa·S)	${\mathit{C}}_{\mathit{P}}\_\mathit{t}\mathit{e}\mathit{s}\mathit{t}$ (J/kg·K)	${\mathit{C}}_{\mathit{P}}\_\mathit{t}\mathit{h}$ (J/kg·K)	$\mathit{\rho }\_\mathit{t}\mathit{e}\mathit{s}\mathit{t}$ (kg/m3)	$\mathit{\rho }\_\mathit{t}\mathit{h}$ (kg/m3)
10	0.5922	0.5829	0.9684	0.9559	4170	4170	1000.7	1001
15	0.5994	0.5902	0.8786	0.8715	4169	4169	999.5	1000
20	0.6047	0.597	0.8187	0.8158	4169	4169	998.6	999
25	0.6109	0.602	0.7535	0.7501	4169	4169	997.3	997.7

compares the test data and theoretical data obtained from Equations (1)–(4) for different thermo-physical properties of alumina nanofluid with 0.1 vol.% concentration. It can be observed that the specific heat and density data are the same for the experimental data and theoretical prediction from the empirical relation. Whereas there is a deviation in the case of thermal conductivity and viscosity, as indicated in the provided empirical relations, the influence of temperature and particle size is neglected. Hence, correlations containing the effect of temperature and particle size are needed for predicting accurate results.

5. Hydrothermal Characteristics of Heat Exchanger

Focused extensive research is being conducted to enhance the performance of heat exchangers under laminar and turbulent flow. Providing the corrugation and chevron angle on a flat surface augments the heat transfer behavior because it increases the surface area and includes turbulence, further increasing the heat transfer coefficient [191]. Different mono and hybrid nanofluids have been introduced to enhance the performance of thermal devices, and their hydrothermal characteristics have been studied [192,193,194,195,196,197,198,199,200,201,202]. Heat transfer increases with the heat transfer coefficient, which nanofluids enhance due to increased thermal conductivity via several mechanisms. It was observed that heat transfer and pressure drop both increase using nanofluids, but the rise in pressure drop is comparatively insignificant. This section is divided into two parts: experimental and numerical studies of mono/hybrid nanofluids for heat exchangers (plate heat exchanger, tubular heat exchanger, mini-channel heat exchanger, and heat pipe).

5.1. Experimental Studies on Heat Exchangers

Pandey and Nema [203] analyzed an alumina–water solution as the refrigerant in a corrugated plate heat exchanger. They observed enhanced heat transfer properties with the Reynolds number. Tiwari et al. [204,205] conducted experimental studies on plate heat exchangers using various nanofluids to improve their performance. Barzegarian et al. [206] studied the hydrothermal characteristics of a plate heat exchanger for domestic hot water application using TiO₂–water nanofluid. Tabari et al. [207] used TiO₂–water nanofluid in a PHE for milk pasteurization considering mass concentrations (0.25%, 0.35%, and 0.8%) and concluded that the heat transfer rate increased due to the increased thermal conductivity. The effect of plate orientation in an alumina nanofluid plate heat exchanger was studied experimentally by Prashant et al. [208]. They found that the heat transfer decreased with the Reynolds number when the plate orientation was changed from horizontal to vertical. The heat transfer coefficient was reduced by 10–15% at a tilt angle of 30°. Huang et al. [209] used a mixture of Al₂O₃–water and MWCNT–water at a ratio of 2.5:1 in a plate heat exchanger and observed an increase in HTC and a pressure drop when using the hybrid solution compared to the primary fluid. Bhattad et al. [210,211] conducted experiments with plate heat exchangers using different hybrid nanofluids as refrigerants and observed changes in the volume ratio of the particles and an influence of particle size on the heat exchanger performance. They discussed the ranking of different fluids based on the achieved thermal conductivity, viscosity, density, and specific heat properties. According to heat transfer performance, studied nanofluids were arranged in ascending order as TiO₂, CuO, Al₂O₃, MgO, AlN, SiC, and MWCNT. Similarly, according to pump work, different fluids were arranged in descending order as MWCNT, CuO, TiO₂, Al₂O₃, MgO, AlN, and SiC. Bhattad [212,213] performed experiments with hybrid nanofluids in plate heat exchangers and found that hybrid nanofluids increased the heat transfer efficiency of the heat exchanger.

Kavitha et al. [214] experimented with CuO–water nanofluid as a coolant to improve a two-tube heat exchanger. Jassim and Ahmed [215,216] experimentally evaluated the effect of Al₂O₃ nanofluids on heat exchanger performance. Mansoury et al. [217] experimentally studied the heat transfer characteristics and flow of Al₂O₃–water nanofluids in heat exchangers. Pipe heat exchangers offer higher heat transfer coefficients than plate heat exchangers. Henein et al. [218] improved the thermal performance of a heat pipe vacuum tube solar collector using MgO–MWCNT/water hybrid nanofluids as the working fluid. It was interpreted that the energy and exergy efficiency increased as the mass ratio of MWCNT nanoparticles increased. The heat transfer properties of heat exchangers were improved [219,220,221,222,223]. Cylindrical (MWCNT) and various spherical (MgO) nanoparticles have been used to study the properties of two-pipe heat exchangers [224,225]. The heat transfer rate was increased by 115% using the MWCNT nanofluid. Subramanian et al. [226] experimentally found that the heat transfer of the TiO₂–water nanofluid was higher than that of the primary fluid (water). The pump work required due to the differential pressure could be estimated from the equations mentioned in the study by Dalkilic et al. [227]. Bahmani et al. [228] studied forced convection in a tube heat exchanger using a nanofluid (aluminum oxide/water). The extreme increase rate of the average Nusselt number and increase rate of thermal efficiency were 32.70% and 30%, respectively. Later, Bahmani et al. [229] studied forced convection in a two-tube heat exchanger with nanofluids at various Reynolds numbers ranging from 100 to 1500. Kristiawan et al. [230] observed enhancement in the thermal performance of helical micro-fin tubes using titania nanofluid with different particle concentrations. They also proposed a correlation to predict the Nusselt number.

Heat pipes are designed to act as heat transfer and thermostats. They were first introduced by Akachi [231]. A survey was conducted on heat pipe stability and operating limitations [232]. The heat transfer ability of nanofluid-filled PHP depends more on factors such as the nanofluid’s thermal conductivity and viscosity. A variety of nanoparticles, including metals [233], oxide particles [234], graphene, graphene oxide [235], and diamond particles [236], have been tested to understand their effect on the heat transfer properties of a PHP. Heat flux increases with nanofluid concentration, while a higher concentration leads to a higher viscosity [237]. Qu and Wu [238] tested the thermal performance of a PHP using SiO₂/water and Al₂O₃/water nanofluids. Experimental results showed that the Al₂O₃/water nanofluid improved the heat transfer performance of the PHP. Goshayeshi et al. [239] compared the properties of CLPHP charged with nanofluid Fe₃O₄ and ɤFe₂O₃. Results showed that Fe₃O₄ exhibits higher thermal performance than ɤFe₂O₃.

The thermal resistance and critical thermal load of FP–PHP decreased with increasing nanoparticle concentration/mass fraction [240]. Li [235] investigated the change in heat transfer efficiency in a PHP of graphene/water–ethylene glycol nanosuspensions at various concentrations and packing factors. Xu et al. [241] performed experiments considering hybrid working fluids. Zufar et al. [242] investigated a PHP using various hybrid nanofluids and concluded that the thermal resistance of the PHP for SiO₂–CuO and Al₂O₃–CuO hybrid nanofluids was 50% and 34% lower than that of water, respectively. Khodami et al. [243] studied a PHP heat exchanger prototype. The results showed that silver nanofluid improved exergy efficiency and reduces exergy loss. Jahani et al. [244] studied the thermal properties of nanofluidic PHP charges. The results showed the improvement in PHP performance using silver nanofluid. Su et al. [245] showed that self-wetting nanofluids have better heat transfer properties than charged self-wetting liquids and nanofluids alone over the entire operating range.

Several experimental studies on the hydrothermal behavior of HyNfs in mini/microchannel heat sinks are available [246]. Selvakumar and Suresh [247] synthesized hybrid AlO–Cu nanoparticles and studied the properties of HyNf Al₂O₃–Cu/water in a mini-channel. Ahammed et al. [248] showed a 63.13% increase in the convective heat transfer coefficient when using HyNf Al₂O₃–graphene in a mini-channel. Nimmagadda and Venkatasubbaiah [249] experimentally studied the behavior of Al₂O₃–Ag HyNf in microchannels. Ho et al. [250] looked at nano-encapsulated phase change materials at MCHS and found up to 70% improvement in heat transfer. Kumar and Sarkar [251] determined various combinations of HyNf-based nanoparticles in MCHS and showed that the Al₂O₃–AlN combination performed best. Similar investigations on micro/mini-channel heat exchange devices using HyNfs were performed in [252,253,254,255,256,257,258,259].

5.2. Numerical Studies on the Heat Exchangers

Pantzali et al. [260] investigated the operation of a miniature plate heat exchanger whose surface was modulated using nanofluids. It has been reported that using the nanofluid of CuO/water reduces the equipment’s size and the pump’s operation. Gherasim et al. [261] studied the heat transfer and flow properties of plate-like HEX using a homogeneous model with CuO and aluminum oxide nanofluids. They found that the heat transfer rate increased at higher pressure drops using nanofluids. The performance of various nanofluids (1.0 vol.%) in a small plate heat exchanger was examined [262]. They found the need for an increase in the convective heat transfer coefficient, as well as decreases in the volumetric flow rate and pumping power. A single-phase numerical model obtained promising results with various nanofluids [263]. Stogiannis et al. [264] numerically observed decreases in coolant consumption and pumping power with SiO₂ nanofluid coolant in a PFC. In the course of numerical studies conducted by Jokar and O’Halloran [265], a clear conclusion was drawn regarding augmented thermal conductivity and a drop in heat transfer with increasing volumetric concentration. Goodarzi et al. [266] and Bhattad et al. [267] numerically studied MWCNT hybrid nanofluids on HEX plates in the turbulent regime. They noted that the heat transfer rate and pump work both increased.

Ding et al. [268] performed a numerical study of TiO₂/water nanofluids in a two-tube heat exchanger. The results showed that the nanofluid’s heat transfer capacity was higher than that of deionized water. Bhattad and Babu [269] performed thermal evaluations of shell-and-tube heat exchangers using various alumina/water hybrid nanofluids. They found a 16.5% heat transfer rate gain using MWCNT/alumina hybrid nanofluid. Jafarmadar et al. [270] investigated PHP’s entropy generation and thermal analysis using Al₂O₃, CuO, and Ag nanofluids. An optimal concentration of 0.5–1% of the nanoparticle volume was determined on the basis of mathematical modeling. Yan et al. [271] and Khetib et al. [272] performed a numerical study to determine the effect of different types of nanofluids on MCHS. Kalteh et al. [273] conducted a numerical study of forced convection heat exchange between alumina and copper nanofluids at MCHS. At a volume fraction of 0.03, the average Nusselt number of copper nanofluids increased by 29.41%.

Some of the relevant studies with significant results are listed in

Table 7. Summary of studies on HEX with mono and hybrid nanofluids.

Author(s)	Operating Variables	Nanofluid Characteristics	Findings
Pantzali et al. [274]	Nanofluid as coolant, PHE, Tci = 30, Thi = 50 °C, Ωh = 40–56 mL/s, Ωc = 10–100 mL/s	Al2O3, CNT, TiO2, CuO/water (0.5–4.0 vol.%), surfactant: CTAB	The use of nanofluids was advantageous in laminar flow.
Zamzamian et al. [275]	Hot side: nanofluid, PHE, cold side: water, Ωh: 3 lpm, Ωc: 2.5 lpm, Thi = 45–75 °C	Al2O3, CuO/EG (0.1–1.0 wt.%), surfactant: SDS, SDBS, and CTAB	HTC increased with increasing concentration and temperature by 3% to 49%.
Kabeel et al. [276]	Hot side: nanofluid, PHE, cold side: water, laminar flow, Thi = 40 °C, Ωc = 3 m3/h	Al2O3/water (1–4 vol.%)	HTC and pump work increased with $\phi$. For 4.0 vol.%, HTC increased by 13%.
Tiwari et al. [277]	Nanofluid as coolant, PHE, Ωh = 3 lpm, Ωc = 1.0–4.0 lpm, Tci = (25–50 °C), Thi = 70 °C	CeO2/water (0.5–3.0 vol.%)	HTC increased by 39% at 0.75 vol.% with almost no pressure drop.
Khairul et al. [278]	PHE, nanofluid as coolant, Ωc = 2–5 lpm, Ωh = 2 lpm,Tci = 300 K	CuO/water (0.5–1.5 vol.%)	HTC increased by 27.20%, while exergy loss was reduced by 24% at 1.5 vol.%.
Huang et al. [279]	Hot side: nanofluid, PHE, cold side: water, counter flow, Re = 58–624, Ω = 0–0.16 lps, Thi = 33 °C, Tci = 22 °C	Al2O3/water (0.56–2.84 vol.%) and MWCNT/water (0.0111–0.0555 vol.%)	HTC and the pressure drop increased with concentration.
Tabari and Heris [280]	PHE, hot side: nanofluid, cold side: milk, counter flow, Pe = 300–1100, Thi = 68–72 °C	MWCNT/water (0.25–0.55 wt.%)	HTC and heat transfer rates increased upon adding MWCNTs to the base fluid.
Abed et al. [281]	PHE, height = 2.5–5 mm, pitch = 6–12 mm, constant heat flux: 6 kW/m2, Ti = 300K, turbulent flow	Al2O3, CuO, SiO2, and ZnO/water (0–4 vol.%)	The most desirable channel parameters were a trapezoid height of 5 mm and a vertical pitch of 6 mm.
Behrangzade and Heyhat [282]	PHE, hot side: nanofluid, cold side: water, Ωc = 2–4 lpm, Ωh = 4–8 lpm, Thi = 30–55 °C	Ag/water (100 ppm)	Overall, HTC increased by 16.79% for 100 ppm nanofluid.
Sarafraz and Hormozi [283]	PHE, Reh = 700–25000, Thi = 50–70 °C	MWCNT/water nanofluid (0.5–1.5 vol.%)	HTC increased with flow rate and volume concentration.
Sun et al. [284]	PHE, Re = 1000–2800	(Cu, Fe2O3, and Al2O3)/DI water (0.1–0.5%)	Overall, HTC increased with mass fraction of particles.
Kumar et al. [285]	PHE, Tci = 20 °C, Thi = 50 °C, Ωc = 0.5–2 lpm, Ωh = 3 lpm, β = 30°/30°, 30°/60°, 60°/60°	ZnO/water (0–2 vol.%), surfactant: CTAB	The optimum increase in HTRR and HTCR, as well as reduction in exergy loss, was observed at 1.0 vol.% for β-60°/60°.
Kumar et al. [286]	PHE, Nanofluid as coolant, Tci = 20 °C, Thi = 50 °C, Ωc = Ωh = 3 lpm, b = 2.5–10.0 mm	TiO2, Al2O3, ZnO, CeO2, GNP, MWCNT nanofluids, Cu + Al2O3 hybrid nanofluid/DI water (0.5–2.0 vol.%), surfactant: CTAB	Exergy destruction was lowest and exergetic efficiency was maximum for 5 mm spacing at 0.75 vol.%.
Ahmed et al. [287]	Microchannel, Re = 50–300, laminar flow	Al2O3 and SiO2/water (0.3–0.9 vol.%)	Al2O3 had the lowest thermal resistance. SiO2 was preferred due to lower pressure drop.
Ardeh et al. [288]	Microchannel, Re = 50–400, laminar flow	Al2O3–SiO2, Al2O3–Cu/water (0–5 vol.%)	Al2O3–SiO2 hybrid nanofluid had a lower thermal resistance and better thermal performance.
Wang et al. [289]	Microchannel, Re = 340–640, laminar flow	Al2O3/water (1–4 vol.%), D = 20–40 nm	Nanoparticles with small diameters and high concentrations provided higher heat transfer performance.
Adio et al. [290]	Microchannel, Re = 100–700, laminar flow	Al2O3/water (0.5–4 vol.%)	A 43.6% enhancement in heat transfer coefficient was observed.
Adio et al. [291]	Microchannel, Re = 100–400, laminar flow	CuO/Water (0.5–4 vol.%)	A 6.5% enhancement in heat transfer was observed.
Ali et al. [292]	Microchannel, Re = 100–350, laminar flow	Al2O3/water (0–3 vol.%)	The Nusselt number at 3 vol.% and Re = 350 showed a 0.67% enhancement.
Kumar et al. [293]	Microchannel, Re = 200–600, laminar flow	Al2O3/water (0.25–0.75 vol.%)	A 40% heat transfer coefficient enhancement was obserbed at 0.75 vol.% fraction.
Elbadawy and Fayed [294]	Microchannel, Re = 200–1500, laminar flow	Al2O3/water (0.01–0.05 vol.%)	Cooling performance was enhanced.
Kahani [295]	Microchannel, Re = 100–300, laminar flow	Al2O3/water (0–1 vol.%)	The average Nusselt number increased to 1.36 at a 1 vol.% fraction.
Pourfattah et al. [296]	Microchannel, Re = 25–100, laminar flow	CuO/water (0.02–0.04 vol.%)	The heat transfer coefficient was highest at a 0.04 vol.% fraction.
Kumar et al. [297]	Microchannel, Re = 100–500, laminar flow	Al2O3/water (2–7 vol.%), D = 10–40 nm	Nu increased and thermal resistance decreased when the nanoparticles diameter was reduced.
Arjun and Rakesh [298]	Microchannel, turbulent flow	Al2O3/water (0–5 vol.%)	The heat transfer coefficient improved by about 12% to 5% particle volume fraction.
Reddy et al. [299]	Microchannel, Re = 100–700, laminar flow	CuO/water (0–4 vol.%)	The heat transfer coefficient and viscosity increased, while the specific heat capacity decreased.
Darzi et al. [300]	Straight-tube HEX	Al2O3/water	The heat transfer increased with the concentration.
Maddah et al. [301]	Straight, twisted tape	Al2O3/water	The heat transfer was enhanced by 12% to 52% compared to the tube with twisted tapes.
Sarafraz and Hormozi [302]	Straight	Silver/EG–water (0.1–1 vol.%)	The 0.1–1 vol.% fraction increased the heat transfer coefficient by 22–67%.
Jafarimoghaddam et al. [303]	Straight	Cu/oil	The heat transfer coefficient increased by 17.32%.
Shirvan et al. [304]	Straight	Al2O3/water, φ = 0.03	The Nusselt number was enhanced by 57.7% with Re = 150 and φ = 0.03.
Akyurek et al. [305]	Wire coil turbulator	Al2O3/water	The addition of a wire coil increased the Nusselt number and the heat transfer coefficient.
Albadr et al. [306]	Shell-and-tube HEX	Al2O3/water, φ = 2 v%	The overall heat transfer coefficient increased by 57%.
Godson et al. [307]	Shell-and-tube HEX	Ag/water, φ = 0.01–0.04 vol.%	There was a 12.4% rise in heat transfer coefficient.
Dharmalingam et al. [308]	Shell-and-tube HEX	Al2O3/water	There was a 17% rise in overall heat transfer coefficient.
Aghabozorg et al. [309]	Shell-and-tube HEX	Fe2O3–CNT/water (0.2 wt.%)	There were 34.02% and 37.50% increases in the heat transfer coefficient for laminar and turbulent flow.
Tan et al. [310]	Shell-and-tube HEX	MWCNT/DI water, φ = 0.2–1 wt.%	There was a 24.3% increase in the heat transfer coefficient.
Naik and Vinod [311]	Shell-and-tube HEX	Fe2O3, CuO, Al2O3/(CMC), φ = 1 wt.%	There were 26% and 29% increases in the overall heat transfer coefficient for Al2O3 and CuO nanofluids.
Said et al. [312]	Shell-and-tube HEX	CuO/water	A 7% increase in overall heat transfer and a 11.39% increase in convective heat transfer were observed.

. The literature survey in the present section showed that the Nusselt number plays a very significant role in convective studies of fluid flow problems. Hence, the quantitative data for the Nusselt number are presented in

Table 8. Quantitative data of Nusselt number for different fluids.

T (°C)	Water	TiO2 (0.1 vol.%)	Al2O3 (0.1 vol.%)
10	10.11	10.19	10.48
15	11.05	11.35	12.21
20	12.27	12.68	13.89
25	14.35	14.97	16.71

.

The survey revealed that many empirical relations are available for calculating the Nusselt number on the basis of nondimensional numbers, i.e., Reynolds and Prandtl numbers. However, the Nusselt number, by definition, depends on the thermal conductivity, geometric parameters, and heat transfer coefficient. Therefore, it is suggested to validate the results in two ways. For this purpose, conducting experiments and obtaining the required parameters are recommended. The table depicts the Nusselt number for base fluid, titania nanofluid, and alumina nanofluid at different temperatures. It can be observed that the Nusselt number increases with the addition of nanoparticles and the increase in working temperature. Hybrid nanofluid plays an extraordinary role in heat transmission in the presence of a magnetic field. The temperature transfer rate increases as the Prandtl number increases. When the surface stretching rate increases, the velocity profile decays while the temperature profile increases [313].

6. Exergy Analysis of Mono/Hybrid Nanofluids

When the conversion of heat energy into valuable work is incomplete, this indicates the presence of unavailable power. The amount of available energy is termed exergy. The exergy increases as the operating condition moves far from the ambient temperature. Hence, for every thermal system where heat and work transfer are essential, one must estimate the exergy, irreversibility, exergetic efficiency, etc. Bhattad et al. [48] performed exergy destruction analysis in their research using different nanoparticle combinations for coolant purposes. Bhattad et al. [101] analyzed exergy destruction with alumina and multiwalled carbon nanotubes. They observed that the irreversibility increased with flow rate and decreased with inlet temperature, as it was directly proportional to flow rate and inversely proportional to inlet temperature. Exergetic efficiency showed the opposite trend. Bhattad et al. [314,315] performed exergy destruction analysis with different brine solution-based fluids and nanoparticle combinations for low-temperature applications. They used working fluid as a secondary refrigerant in plate evaporators. The irreversibility and nondimensional exergy destruction (NDE) were reduced while using brine-based hybrid nanofluids as a secondary refrigerant compared with the corresponding base fluid for all low-temperature applications. Hybrid nanofluids enhanced the exergetic efficiency and irreversibility distribution ratio (IDR). The irreversibility/exergy destruction of the system can be calculated as follows:

$$I=T_e{S}_{gen},$$

(5)

where T_e is the ambient temperature in Kelvin.

S_gen is the entropy generation rate, which can be calculated as follows:

$$S_{gen}=\dot{m}\left[c_p\ln \left(\frac{T_o}{T_i}\right)+\frac{\mathsf{\Delta }p}{\rho T_{av}}\right],$$

(6)

where T_av is the average temperature of the inlet and outlet temperature.

The second law of efficiency or exergy efficiency (η_II) is the ratio of exergy gain to exergy loss, which is given by

$${\eta }_{II}=\frac{E_g}{E_l}.$$

(7)

The scaling of exergetic parameters has become essential from the design point of view for thermal systems. Hence, different parameters such as non-dimensional exergy destruction, irreversibility distribution ratio, and Bejan number are discussed in the present investigation. Non-dimensional exergy destruction (NDE) is the ratio of irreversibility to maximum heat transfer rate, which signifies the effect of design parameters on the exergy destruction for a given operating condition. A non-dimensional parameter, entropy generation number, was introduced by Mishra et al. [316] to study the influence of heat capacity on entropy generation. The sustainability of the device was evaluated in terms of the exergy ratio to understand the utilization of resources [317]. Bejan [318] introduced the irreversibility distribution ratio (IDR) concept to show the relative influence of heat transfer and pressure drop on irreversibility. The irreversibility distribution ratio was defined as the ratio of entropy generation due to heat transfer to that due to pressure drop.

Bhattad et al. [211] performed exergetic analyses of the plate heat exchanger using Al₂O₃–TiO₂ hybrid nanofluid as a coolant for sub-ambient temperature application. They observed a 4.01% reduction in the exergetic efficiency for TiO₂ nanofluid. The study showed that the exergetic performance decreased continuously with the increase in TiO₂ ratio in the hybrid solution. Bhattad et al. [212] observed that the irreversibility was enhanced with the flow rate and nanoparticle suspension for MgO–alumina hybrid nanofluid. Bhattad et al. [213] observed an augmentation of the coolant exergy rate, irreversibility rate, and non-dimensional exergy by 4.8%, 7.5%, and 3.5%, respectively. At the same time, the second law of efficiency is reduced when using nanoparticles and increasing flow rate, and it decreases with the coolant inlet temperature. The authors of [319] investigated a novel nanofluid ternary hybrid nanofluid (THdNF) obtained from a mixture of three different nanoparticles, resulting in better overall performance even at low concentrations. Bahiraei et al. [320] examined twisted turbulator inserts in the concurrent pipe and found lower entropy generation when arranged counter-currently. Bahiraei and Heshmatian [321] investigated the effect of nanofluid on the cooling devices on the entropy generation and revealed a net reduction in the temperature. In the interim, various research studies focusing on entropy and exergy investigation are available in the open literature using different nanofluids [322,323,324]. Kumar and Sahoo [325,326] investigated combined exergo-economic and environmental impact analysis of THdNF as the working fluid with various turbulator inserts used for automobile applications. They found the highest 24.7% exergy change and 6.4% exergy efficiency at the lowest Reynolds number without inserts. Rai and Sahoo [327] performed exergy analysis for a 5% water in diesel emulsion (WiDE) fuel, with 50 ppm carbon nanotube (CNT) and 50 ppm aluminum oxide (Al₂O₃) nano-additive fuels, on a diesel engine with changeable engine speed and load. The exergy-based sustainability was highest for 5% WiDE–CNT fuel at 2000 engine rpm with full engine load. The exergy destruction and entropy generation rates with the 5% WiDE–Al₂O₃ and 5% WiDE–CNT nano additive fuels had 2.07% and 4.15% higher values, respectively, compared to the diesel fuel.

7. Applications of Mono/Hybrid Nanofluids

Due to their improved thermophysical characteristics, nanofluids and hybrid nanofluids can be used in radiators (as coolant), biodiesel blends, fuel additives, refrigerators, heat pumps, and air conditioning applications as primary fluids (nano refrigerants) and secondary fluids (secondary refrigerants on the evaporator side and nano lubricants as refrigerants on the condenser side). The thermophysical characteristics, pressure drop, and heat transfer characteristics of nano lubricants and nano refrigerants in refrigeration systems were reviewed by Saidur et al. [328] and Bhattad et al. [329]. Alawi et al. [330] discovered that the thermal conductivity of nano refrigerants during pool boiling is highly influenced by the temperature at low concentrations. The properties, flow characteristics, and uses of nano refrigerants at sub-ambient temperatures were evaluated by Celen et al. [331]. The use of nano lubricants and nano refrigerants has been considered in refrigeration, air conditioning, and heat pump systems [332]. The effects of employing nano coolants on cooling devices in terms of energy consumption and heat transfer efficiency were reviewed by Alawi et al. [333]. A review was conducted on the thermophysical characteristics and effectiveness of nano refrigerants in refrigeration systems [334]. Nano refrigerants were thoroughly evaluated by Nair et al. [335], who covered their manufacture, characteristics, heat transfer abilities, and effects on the performance of refrigeration systems. Hybrid nano lubricants were utilized by Zawawi et al. [336] to enhance the performance of refrigeration systems. ZnO/water (0.5 vol.%) nanofluid was described by Fard et al. [337] as the hot fluid in a plate heat exchanger. They demonstrated that the heat transfer behavior of the nanofluid was superior to that of the basic fluid. Javadi et al. [338] looked into how utilizing Al₂O₃, TiO₂, and SiO₂ nanofluids affected the heat transfer of a plate heat exchanger. The SiO₂ nanofluid exhibited a lower pressure drop, while the Al₂O₃ nanofluid had the highest heat transfer coefficient. The thermal characteristics of the two nanofluids were contrasted in a plate heat exchanger operating at low temperatures [339]. Carbon nanotubes and aluminum oxide outperformed water in terms of increased heat transfer and reduced pump power loss.

Nanofluids can be utilized in auxiliary circuits as additional fluids (evaporators and condensers) to cool the primary working fluid or refrigerant [340]. When employing nanofluids, Liu et al. [341] noticed a rise in cooling capacity and COP, as well as a fall in compressor performance. The enhanced HTC enhanced the cooling capacity [342]. Nanofluids were explored numerically by Loaiza et al. [343] for application as secondary coolants in cooling systems. It was discovered that when the concentration of nanoparticles increased and the size of the nanoparticles decreased, the evaporator area and pressure drop of the refrigerant decreased for a specific cooling capacity. Nanofluids had a minimal effect on pump power and increased chiller efficiency, cooling capacity, and efficiency. The use of Cu–H₂O nanofluids as condenser coolant in a vapor compression heat pump was modeled by Parise and Tiecher [344]. A nanoparticle percentage of 2.0% resulted in a 5.4% increase in the efficiency factor. Askari et al. [345] conducted an experimental investigation on the effectiveness of counter flow wet coolers. Nanoporous graphene nanofluids and MWCNTs are employed. They noticed increases in cooling distance, coolant flow, and cooling tower efficiency. When Kolhapure and Patil [346] employed nanofluids as refrigerants, they found that compressor operation and condenser heat transfer decreased as efficiency increased. Condensers and cooling tower sizes were reduced.

Thermal simulations of vapor compression refrigeration systems were detailed by Jaiswal and Mishra [347] utilizing Al₂O₃-, TiO₂-, CuO-, and Cu-based nanofluids in the secondary circuit and R134a refrigerant in the primary circuit. Using aqueous nanofluids in the secondary circuit boosted system performance with the same geometrical parameters from 17% to 20%. Increased cold chain efficiency employing nanofluids as secondary refrigerants was reported by Ndoye et al. [348]. To investigate the energy characteristics of secondary circuits in refrigeration systems, Soliman et al. [349] employed a variety of nanofluids. It was noted that, when concentration increased, the pump power increased, whereas the performance coefficient decreased. Excellent cooling capacity and COP for SWCNT/water suspensions were reported by Vasconcelos et al. [350]. In air conditioning systems, nanofluids can be employed as phase change refrigeration reservoirs on the evaporator side [351]. Nanotechnology is widely used in the food, food packaging, and milk pasteurization industries [352]. Zhang et al. [353] examined the ice formation process’ nucleation phenomenon using nanofluids. With nanofluids, the nucleation mechanism could be improved. A raw milk dispenser based on nanofluidic technology was created by Longo et al. [354]. Hybrid nanofluids were employed as secondary coolants for low-temperature applications by Bhattad et al. [314,315,355].

Table 9. Summary of the application of nanofluids as coolant and secondary refrigerant.

Author(s)	Operating Variables	Nanofluid Characteristics	Findings
Liu et al. [341]	MWCNT/water,	Water chiller	Efficiency improved by 5.15%, while cooling capacity increased by 4.2%.
Loaiza et al. [343]	Al2O3, TiO2, CuO, and Cu/water	Vapor compression refrigeration system	The evaporator area and pressure drop decreased for fixed cooling capacity as particle size and concentration increased.
Zhang et al. [353]	Al2O3, SiO2/water	Ice making	With the use of nanoparticles, supercooling was reduced to a lesser extent.
Kumaresan et al. [342]	MWCNT/(EG + water)	-	Higher temperatures and velocities increased heat transmission efficiency, preferably with 0.15% MWCNTs by volume.
Parise and Tiecher [344]	Cu/water	Vapor compression heat pumps	When the volume fraction of nanoparticles reached 2%, COP increased by 5.4%.
Sarkar [356,357]	Al2O3, TiO2, CuO, SiO2, and Cu/water	CO2 refrigerant system	The shell-and-tube gas cooler’s cooling effectiveness and capacity were enhanced.
Jaiswal and Mishra [347]	Al2O3, TiO2, CuO, and Cu/water	Domestic refrigeration system	The performance of the cooling system increased by 17–20%.
Longo et al. [354]	Al2O3/(EG + water)	Milk dispenser	Energy consumption was reduced by 63–70%.
Ndoye et al. [348]	Co, CuO, Fe, SiO2, Al2O3, and TiO2/water	Cold chain refrigeration plants	The cold chain became more efficient by consuming less energy and producing fewer emissions.
Askari et al. [345]	MWCNT and graphene/water	Cooling tower	Efficiency, cooling range, and cooling tower performance were all improved with lower water use, as were the tower’s attributes.
Kolhapure and Patil [346]	Al2O3/water	Air conditioning and refrigeration system	The capacitor thermal cutoff, which lowered compressor operation and used less energy, boosted COP.
Soliman et al. [349]	Al2O3, Ag, TiO2, Co, Cu, Au, Fe, CuO, diamond, and graphite/water	Refrigeration system	When the mass fraction was 0.1%, productivity increased by 10.5%.
Vasconcelos et al. [350]	SWCNT/water	Vapor compression refrigeration system	Excellent COP and cooling capacity.

provides an overview of the uses of nanofluids as primary and secondary coolants.

Arshad et al. [358] investigated the 3D flow of an engine oil-based nanofluid under the impact of rotation and partial slip phenomenon over a stretchable surface. The study’s outcomes were related to already available studies and were in good agreement. Kumar and Sahoo [359] investigated the performance characteristics of a car radiator by using a ternary hybrid nanofluid of 0.12 vol.% fraction (Al₂O₃–CuO–TiO₂/water) and water as coolant and validated results with simulation. Ternary hybrid nanofluids (THNF) showed a 14% higher heat transfer coefficient at 8 lpm, and the mixture model predicted a 5% better result than the single-phase approach. A 12.54% enhancement in BTE was observed with a fuel-saving rate of 14.28%. Kumar and Sahoo [360] investigated the thermo-hydraulic performance of a car radiator using Al₂O₃, CuO, and TiO₂ nanoparticles disseminated in an equal fraction in the range of 0.06–0.12% THNF. Coolant was operated with a flow rate of 3–8 lpm. Results revealed a 14.2% enhancement in heat transfer with a coolant flow rate of 6 lpm using a 0.12% vol. fraction of THNF. A maximum fuel saving rate of 14.28% was observed at 50% load on the engine. Preheating of fuel through radiator waste heat recovery decreased the BSFC. Rai and Sahoo [361] investigated the engine performance parameters for 5% water-emulsified fuel, 50 ppm CNT, and 50 ppm Al₂O₃–CNT (25 ppm each) hybrid nano-additive fuels, with different speeds and loads on a DICI engine. The CNT catalyst had a higher effect on the BSFC than the Al₂O₃–CNT catalyst. The BTE with 5% WiDE, 5% WiDE–Al₂O₃–CNT, and 5% WiDE–CNT nano-additive was 1.49%, 2.86%, and 3.07% higher than with diesel. Overall, compared to all fuel blends, the optimal performance was found for 5% WiDE–CNT. Najafi [362] observed the impact of adding Ag and CNT nano-additive on engine performance and combustion parameters. The results showed that adding a nanocatalyst shortened the time needed for the engine to ignite and increased the cylinder’s peak pressure and rate of heat release. The best outcome was obtained when 120 ppm of CNT nano-additive is used. Jiaqiang et al. [363] evaluated the effects of adding water and a nanocatalyst on a CI engine’s performance and emission characteristics when using diesel/biodiesel mixes. The blend of biodiesel and diesel with 90 ppm CeO₂ nano-additive was emulsified with 2%, 4%, and 6% (v/v) ratios. The investigation results showed that water emulsification up to 4% was favorable, and the addition of nanocatalyst enhanced performance and lowered emissions levels. Kumar and Raheman [364] investigated water-emulsified biodiesel blends with nano-oxide incorporation. According to the characterization analysis, the ideal fuel stability characteristics were 1% surfactant, 10% water, 2500 rpm stirrer speed, and 69.7 ppm nano-oxide.

Wro’blewski [365,366] performed energy analysis on IC engines for hydrophobic and hydrophilic multilayer nanocoatings surrounded by soot. The multilayer coating reduced the friction coefficient and, hence, improved tribological performance. Rai and Sahoo [367] carried out energy, exergy, and sustainability analyses of a diesel engine with hybrid nanofluids incorporated with various shaped (25 ppm CNT and 25 ppm spherical Al₂O₃ nano-additives). The 5% WiDE based Al₂O₃–CNT hybrid nano fuel improved BTE by 2.86% with an exergy efficiency of 4.16%. Rao and Anand [368] investigated an analysis of the energy and emissions produced by a DICI engine running on fuel containing AlO(OH) nano-additive and water in a diesel emulsion. The results of the experimental study demonstrated that adding water as an AlO(OH) nano-additive to diesel emulsion fuel considerably enhanced the engine’s energy parameters and emission characteristics. El-Seesy et al. [369] examined the optimal Al₂O₃ nano-additive concentration in diesel and Jojoba biodiesel blends to achieve higher performance. The analysis showed that all performance indicators significantly increased at a concentration of 30 mg/L Al₂O₃. Ozcan [370] studied the impact of Al₂O₃ nano-additive (50 and 100 ppm) on the energy and performance parameters of diesel engines charged with diesel/biodiesel blends through experiments. According to experimental data, the engine’s performance was improved, entropy generation decreased, and unexplained losses decreased. Hasannuddin et al. [371] evaluated the fuel qualities, emission characteristics, and performance metrics of diesel engines running on fuel containing various nano-additives and 10% water (Al₂O₃, CuO, MgO, MnO, and ZnO). According to the study, of all the nano fuels discussed, the water-in-diesel emulsion with Al₂O₃ addition had the smallest droplet size, improved torque, and decreased BSFC, BSCO, and BSNOx. Aghbashlo et al. [372] performed an energetic performance investigation of the diesel engine using four concentrations of hybrid nano-catalyst and two types of biodiesel/diesel blends (CeO₂ and MWCNT). The results showed that all fuel mixes had the same energy efficiency and sustainability index except for the nano-catalyst.

8. Conclusions

Investigations were made on different mono and hybrid nanofluids from nanocomposites, mixing other nanoparticles comprising oxides, metals, and phase change materials of different shapes such as spherical, cylindrical, and flake in a base fluid using one-step and two-step methods. Nanofluids were characterized and thermo-physical properties were measured to develop empirical relations for CFD simulations. Experimental and theoretical investigations were conducted on the heat exchangers with mono and hybrid nanofluids, and an enhancement in performance was observed. A study on different hybrid nanofluids (SiC, Al₂O₃–AlN, Al₂O₃–MgO, Al₂O₃–CuO, and Al₂O₃–MWCNT) conducted by Bhattad et al. [210] showed the ranking of different nanofluids according to different thermophysical properties. These nanofluids can serve as a coolant, secondary refrigerant, nano refrigerant, and nano lubricant for low-temperature applications such as refrigeration systems, air conditioning, and food processing, or as fuels for ICEs. From the present literature survey, the research gaps identified are as follows:

• There are limited studies on the heat transfer properties of hybrid nanofluid heat exchangers.
• The influence of individual particle proportions of hybrid nanofluids on the performance of heat exchangers is unknown.
• There is little experimental work and validation by CFD modeling of heat exchangers using hybrid nanofluids operating for low-temperature applications.
• Hybrid nanofluids can be used in biodiesel blends and as fuel additives to enhance the performance of IC engines.
• A single correlation containing the effect of temperature and particle size is needed to predict accurate thermal conductivity and viscosity of mono/hybrid nanofluids.
• Nanofluids with more thermal conductivity show better heat transfer characteristics, and nanofluids with more viscosity provide higher pressure drop and pump work. An increment in heat transfer is desirable, while an increment in pump work is undesirable; thus, another performance indicator must be determined, such as a performance index (ratio of heat transfer rate and pump work), to obtain a better nanofluid.
• Many empirical relations are available for calculating the Nusselt number on the basis of non-dimensional numbers, i.e., Reynolds and Prandtl numbers. However, the Nusselt number, by definition, depends on the thermal conductivity, geometric parameters, and heat transfer coefficient. Hence, it is suggested to validate the results in two ways. For this purpose, conducting the experiments and obtaining the required parameters are recommended.
• It can be observed that the thermal conductivity, density, and viscosity increase with the addition of nanoparticles in the base fluid. The fluid’s thermal conductivity increases with the temperature increase, whereas density and viscosity decrease. No change can be observed for the specific heat as the studied temperature range is small.
• Hybrid nanofluids can be used as a coolant in automobile radiators and ICEs.
• Few investigations are available for ternary hybrid nanofluids to analyze irreversibility, exergy, economic, and the second law of efficiency in air heat exchangers utilizing various turbulators.
• Studies were conducted primarily with water and EG/water brine as base fluids. Using different brines as primary and secondary cooling water should be explored.

Most (95%) investigators adopted a two-step method while preparing nanofluids. However, nanofluids synthesized using the one-step process (which is expensive and involved) improved the stability of the nanoparticles in the base fluid due to the high sedimentation rate with low sonification time. Ultrasonication reduces the sedimentation issue, and adding surfactants further improves nanofluid stability. FESEM/EDS, DLS, and zeta potential measurements help understand nanoparticle morphology, shape, and size. The mechanism of nanofluids at the atomic level should be understood to address particle migration, aggregation, and stability with minimal experimentation. Future research should focus on industrial applications to minimize pressure losses, specifiy the optimal concentration of nanoparticles, and ensure the long-term stability of hybrid nanofluids. Very few studies are available on ternary hybrid nanofluid to analyze irreversibility, exergy, economic, and the second law of efficiency in air heat exchangers utilizing various turbulators. Thus, this area needs to be explored more to improve automobile performance.