Hybrid nanofluids preparation, thermal properties

Renewable and Sustainable Energy Reviews 68 (2017) 185–198

Contents lists available at ScienceDirect

Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

Hybrid nanofluids preparation, thermal properties, heat transfer and friction factor – A review

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L. Syam Sundara, , K.V. Sharmab, Manoj K. Singha,⁎⁎, A.C.M. Sousaa a b

TEMA-Department of Mechanical Engineering, University of Aveiro, 3810-193 Aveiro, Portugal Department of Mechanical Engineering, J.N.T. University, Hyderabad, India

A R T I C L E I N F O

A BS T RAC T

Keywords: Hybrid nanofluids Thermal properties Heat transfer Friction factor Enhancement

In the past decade, research on nanofluids has been increased rapidly and reports reveal that nanofluids are beneficial heat transfer fluids for engineering applications. The heat transfer enhancement of nanofluids is primarily dependent on thermal conductivity of nanoparticles, particle volume concentrations and mass flow rates. Under constant particle volume concentrations and flow rates, the heat transfer enhancement only depends on the thermal conductivity of the nanoparticles. The thermal conductivity of nanoparticles may be altered or changed by preparing hybrid (composite) nanoparticles. Hybrid nanoparticles are defined as nanoparticles composed by two or more different materials of nanometer size. The fluids prepared with hybrid nanoparticles are known as hybrid nanofluids. The motivation for the preparation of hybrid nanofluids is to obtain further heat transfer enhancement with augmented thermal conductivity of these nanofluids. This review covers the synthesis of hybrid nanoparticles, preparation of hybrid nanofluids, thermal properties, heat transfer, friction factor and the available Nusselt number and friction factor correlations. The review also demonstrates that hybrid nanofluids are more effective heat transfer fluids than single nanoparticles based nanofluids or conventional fluids. Notwithstanding, full understanding of the mechanisms associated with heat transfer enhancement of hybrid nanofluids is still lacking and, consequently it is required a considerable research effort in this area.

1. Introduction The single phase heat transfer fluids such as water, engine oil, ethylene glycol, propylene glycol and transformer oil are mainly used in process industries, chemical and thermal power plants. The heat transfer performance of single phase heat transfer fluids, in general, is very poor due to the low values of their thermal conductivity. The heat transfer intensification is very important to achieve significant energy and cost savings. Therefore, one possible route is increase the thermal conductivity of the working fluids. As it is well-known, solid materials possess higher thermal conductivity when compared to single phase fluids. The addition of solid particles to the single phase fluids technique was first proposed by Maxwell [1], who observed enhanced thermal conductivity values. However, the simple dispersion of solid particles in single phase fluids leads to their sedimentation and consequent clogging of the flow passages; moreover, the particles cause erosion on the flow passage walls, while increasing the pressure drop across the installations. Later on, Masuda et al. [2] dispersed micrometer size solid particles in single phase fluids and observed thermal ⁎

conductivity enhancement, but also faced particle sedimentation in the base fluid, which reduces the enhancement in thermal conductivity. In 1995, Choi [3] prepared nanofluids (fluids containing nanometer size solid particles) and observed marked enhancement of thermal conductivity. The dispersion of nanometer size particles in single phase fluids presents higher specific surface area than conventional colloidal suspensions and is more stable than conventional slurries. The commonly used nanoparticles are metals – e.g., Cu, Au, Ag, and Ni; metal oxides – e.g., Al2O3, CuO, Fe2O3, Fe3O4, SiO2, TiO2, and ZrO2; metal carbides (SiC), metal nitrides – e.g., AlN, and SiN; carbon materials – e.g., carbon nanotubes, graphite, and diamond. The preparation of nanofluids using different kind of nanoparticles along with their convective heat transfer performance has been reported by many researchers. Pak and Cho [4] conducted heat transfer and friction factor experiments for Al2O3/water and TiO2/water nanofluids in the Reynolds number range from 104 to 105 and the particle concentration ranging from 0% to 3% and observed heat transfer enhancement compared to the base fluid (water); they also propose newly-developed Nusselt number correlation. Later on, Xuan and Li [5] used Cu/water

Corresponding author. Corresponding author. E-mail addresses: [email protected] (L.S. Sundar), [email protected] (M.K. Singh).

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http://dx.doi.org/10.1016/j.rser.2016.09.108 Received 9 March 2016; Received in revised form 28 August 2016; Accepted 25 September 2016 Available online 12 October 2016 1364-0321/ © 2016 Elsevier Ltd. All rights reserved.


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enhancement of 13.56% for 0.1% vol at a Reynolds number of 1730, while Madhesh et al. [36], with Cu-TiO2 hybrid nanofluids, obtained heat transfer enhancement of 52% for 2.0% vol Sundar et al. [37] prepared nanodiamond-nickel (ND-Ni) nanocomposite (hybrid) nanofluids and determined experimentally the thermal conductivity and viscosity. Sundar et al. [38] also prepared MWCNT-Fe3O4 hybrid nanofluids and found heat transfer enhancement of 31.10% with a pumping penalty of 18% for 0.3% vol at a Reynolds number of 22000. These studies clearly indicate that hybrid nanofluids yield higher heat transfer enhancement than single nanoparticles-based nanofluids. However, to fully understand the hybrid nanofluids mechanisms enhancing heat transfer, further experiments and analyses will be required. The present review deals with hybrid nanofluids, which have the potential of making an important contribution to heat exchange equipment cost reduction by increasing its effectiveness and consequently making it smaller and lighter. In addition, an increased effectiveness may lead to substantial worldwide energy savings. Therefore, it will be presented an overview of synthesis and characterization of different hybrid nanoparticles, preparation of hybrid nanofluids and also the state of ongoing research work enabling the use of hybrid nanofluids and current challenges. It will be also reviewed and discussed their thermo-physical properties, heat transfer, friction factor and the available Nusselt number and friction factor correlations.

and Cu/transformer oil nanofluids and observed heat transfer enhancements as compared to the base fluids. In another study, Xuan and Li [6] observed heat transfer enhancement of 60% for 2.0% volume concentration of Cu/water nanofluid flowing in a tube at a Reynolds number of 25000 and they report separated Nusselt number correlations for laminar and turbulent flow, respectively. Wen and Ding [7] conducted heat transfer experiments for Al2O3/water nanofluid in a tube under laminar flow and they observed heat transfer enhancement of 47% at 1.6% volume fraction as compared to the base fluid (water). Heris et al. [8] also used Al2O3/water nanofluids in a tube under laminar flow and observed heat transfer enhancement using constant wall temperature boundary conditions. Williams et al. [9] reported convective heat transfer enhancement with alumina/water and zirconia/water nanofluids flow in a horizontal tube under turbulent flow. Duangthongsuk and Wongwises [10] found heat transfer enhancement of 20% and 32% for 1.0% vol of TiO2/water nanofluid flowing in a tube at Reynolds numbers of 3000-18000, respectively. Ghozatloo et al. [11] obtained heat transfer enhancement of 35.6% at a temperature of 38 °C for 0.1 wt% of graphene/water nanofluids flow in a tube under laminar flow. Sundar et al. [12] found heat transfer enhancement of 30.96% with a pumping penalty of 10.01% for 0.6% vol of Fe3O4/water nanofluid flow in a tube at a Reynolds number of 22000. Sundar et al. [13] observed heat transfer enhancement of 39.18% with a pumping penalty of 19.12% for 0.6% vol of Ni/water nanofluid flow in a tube at a Reynolds number of 22000. Other researchers have also observed heat transfer enhancement using different kinds of nanofluids. The examples are – Amrollahi et al. [14], Wang et al. [15], Ding et al. [16] used CNT nanofluids, Sajadi and Kazemi [17] used TiO2 nanofluids, Ghazvini et al. [18] used diamond/engine oil nanofluids, Xuan and Li [19] used Cu/water nanofluids, Ferrouillat et al. [20] used SiO2/water nanofluids, Guo et al. [21] obtained significant heat transfer rates by using Fe2O3/water nanofluids. There is a considerable body of experimental data about heat transfer enhancement with different nanofluids, which is well summarized in review articles published during the past decade [22–28]. The researchers have consistently observed higher heat transfer rates with different kinds of nanofluids (among others, Al2O3, Cu, CuO, Fe3O4, Fe2O3, CNT, nickel, nanodiamond, TiO2, and SiO2) flow in a tube under laminar or turbulent flow conditions. The heat transfer enhancement of nanofluids depends on particle concentrations, thermal conductivity of nanoparticles and mass flow rates. The thermal conductivity of nanoparticles may be altered or changed by synthesizing the hybrid (nanocomposite) nanoparticles and it is expected that fluids prepared with hybrid nanoparticles may cause further heat transfer enhancements. The hybrid nanoparticles may be defined as two or more different materials in the nanometer size; hybrid nanoparticles represent an area of nanotechnology, which is experiencing a marked growth due to its potential impact in material science and engineering. For the preparation of hybrid nanofluids there are different available methods, which enable the synthesis of hybrid nanoparticles; the use of the most common methods is succinctly reviewed in what follows. Jia et al. [29] used the hydrothermal method, Zhang et al. [30] used the solvothermal method and Shi et al. [31] used the polyols method for the synthesis of CNT/Fe3O4 hybrid nanoparticles. Guo et al. [32] used sonication and sol-gel chemistry technique for the synthesis of silica (Si) coated carbon nanotube (CNTs) coaxial nanocables. Li et al. [33] prepared CNT/SiO2 and CNT//SiO2/Ag hybrid nanoparticles using plasma treatment. Baby and Ramaprabhu [34] used the simple chemical reduction technique for the preparation of silicon dioxide (SiO2) coated on magnetite (Fe3O4) particle doped multi-walled carbon nanotubes (MWCNTs) (Fe3O4@SiO2/MWCNTs) hybrid nanoparticles. The available literature is relatively scarce in what concerns the preparation of hybrid nanofluids and the determination of their thermal properties, heat transfer and friction factor. Suresh et al. [35] prepared Al2O3-Cu hybrid nanofluids and obtained heat transfer

2. Types of hybrid (nanocomposite) materials Depending on the metal matrix, the hybrid (composite) materials can be divided into three types. (a) Metal matrix nanocomposites – the examples are, among others, Al2O3/Cu, Al2O3/Ni, MgO/Fe, Al/CNT, Mg/CNT, Al2O3/Fe-Cr, and ND/Ni, (b) Ceramic matrix nanocomposites – the examples are, among others, Al2O3/SiO2, Al2O3/TiO2, SiO2/ Ni, CNT/Fe3O4, Al2O3/SiC, and Al2O3/CNT, (c) Polymer matrix nanocomposites – the examples are, among others, polymer/layered double hydroxides, polymer/CNT, thermoplastic/thermoset polymer/ layered silicates, and polyester/TiO2. The materials used for metal matrix nanocomposites (MMNC) are, for example, Ag (silver), Al (alumina), Au (gold), Cu (copper), Fe (iron), nanodiamond (ND), Ni (nickel), Mg (magnesium), and Sn (tin). The materials used for ceramic matrix nanocomposites (CMNC) are, among others, Al2O3 (aluminum oxide), CuO (copper oxide), Fe2O3 (hematite), Fe3O4 (magnetite), NiO (nickel oxide), SiC (silicon carbide), SiO2 (silicon oxide), TiO2 (titanium oxide), and ZnO (zinc oxide). The materials used for polymer matrix nanocomposites (PMNC) are, in general, vinyl-polymer, ethylene vinyl-alcohol, poly-vinyl chloride, polyethylene, and poly-propylene. The materials used for carbon-based nanocomposites are typically: single walled carbon nanotubes (SWCNTs), multi walled carbon nanotubes (MWCNT), graphite (G), and graphene oxide (GO). The commonly used synthesis techniques for nanocomposites are chemical, mechanical (ball-milling), and vapor deposition (physical, chemical). For all the cases, the hybrid (nanocomposites) particles size should be less than 100 nm. 3. Synthesis of hybrid (nanocomposite) nanoparticles There are several techniques available for the synthesis of hybrid nanoparticles. Suresh et al. [35,39] used the thermo-chemical synthesis technique for the preparation of Al2O3-Cu hybrid nanoparticles using Cu(NO3)2·3H2O (copper nitrate) and Al(NO3)3·9H2O (aluminum nitrate) reagent grade chemicals. Sundar et al. [38] prepared MWCNT/ Fe3O4 hybrid nanoparticles using the in-situ and chemical co-precipitation method. Batmunkh et al. [40] employed the ball milling technique for the synthesis of Ag-TiO2 nanoparticles. Baby and Ramaprabhu [41–43] used catalytic chemical vapor deposition 186


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4.2. Ethylene glycol based hybrid nanofluids

(CCVD) technique for the synthesis of multi-walled carbon nanotubes (MWCNT) and hydrogen exfoliated graphene from graphite oxide (GO) hybrid nanoparticles. Aravind and Ramaprabhu [44] synthesized graphene wrapped multi walled carbon nanotubes (G/MWCNT) using chemical vapor deposition technique employing GO and MmNi3 as precursors. Han et al. [45] used precursor solution of Fe(NO3)3·9H2O (iron nitrate) and Al(NO3)3·9H2O (aluminum nitrate) for the preparation of hybrid sphere/CNT nanoparticles, they first prepared spherical nanoparticles through spray pyrolysis and then grow CNTs using the catalytic process. Baghbanzadeh et al. [46] used wet chemical method for the synthesis of silicon/MWCNT hybrid nanoparticles. Paul et al. [47] prepared hybrid Al-Zn nanoparticles using the mechanical alloying method. Abbasi et al. [48] prepared hybrid γ-Al2O3/MWCNT using the solvothermal process in ethanol. Nine et al. [49] have used the wet ball milling process to synthesize the Cu/Cu2O hybrid nanoparticles. Sundar et al. [37,50,51] used in-situ and chemical co-precipitation method for the synthesis of nanodiamond (ND)-nickel (Ni), ND-Fe3O4 and ND-Co3O4 hybrid nanoparticles. The techniques used by various researchers are summarized in Table 1. The schematic diagram of the in-situ and chemical co-precipitation method used by Sundar et al. [37,50] for the synthesis of ND-Fe3O4 and ND-Ni nanoparticles are shown in Fig. 1 and Fig. 2, respectively...

Only a few studies are available related to the preparation of ethylene glycol based hybrid nanofluids and their thermal properties. They are: Paul et al. [47], who dealt with Al-Zn nanocomposite and prepared ethylene glycol-based hybrid nanofluids. Sundar et al. [37,50,51], Baby and Ramaprabhu [41–43], Baby and Ramaprabhu [55], and Aravind and Ramaprabhu [57] also prepared ethylene glycolbased hybrid nanofluids. 4.3. Oil based hybrid nanofluids The hybrid nanofluids can also be prepared using oil as base fluid, this is the case of Botha et al. [59] using synthesized silver-silica nanoparticles in oil, and Han et al. [63] using poly-alpha-olefin (PAO) based alumina/iron/carbon nanotube nanofluids. 5. Thermal properties of hybrid nanofluids Before reporting on the heat transfer capability of hybrid nanofluids, as a matter of completeness it is important to review the density, specific heat, thermal conductivity and viscosity at different particle concentrations and temperatures.

4. Preparation of hybrid nanofluids

5.1. Density of nanofluids

The base fluids such as water, ethylene glycol, engine oil, and ethylene/water mixtures are commonly used fluids for the preparation of hybrid nanofluids. The size of the hybrid nanoparticles is very important and it should be less than 100 nm for achieving the stable hybrid nanofluids. Sundar et al. [37,50] synthesized ND-Ni and NDFe3O4 hybrid nanoparticles using the in-situ and chemical co-precipitation method and then prepared water based nanofluids. They also analyzed the particle size distribution of ND-Ni and ND-Fe3O4 using Zetasizer nanoZS and the results are shown in Fig. 3 and Fig. 4. The results indicate the particle size of ND-Ni is 22 nm and the particle size of ND-Fe3O4 is 21.2 nm, respectively. The in-situ and chemical coprecipitation method is a very effective method to obtain the small size hybrid nanoparticles...

Ho et al. [60] prepared water-based hybrid nanofluids using Al2O3 and microencapsulated phase change material (n-eicosane) nanoparticles (MEPCM) and estimated experimentally their density, specific heat, thermal conductivity and dynamic viscosity. They noted that the observed experimental density values were in good agreement with the mixture density values. The mixture density formula for hybrid nanofluids is given as follows: Table 1 Various methods to synthesis the nanocomposite materials.

4.1. Water based hybrid nanofluids Several authors used water as the base fluid for the preparation of hybrid nanofluids and verified that their thermal properties determined experimentally have higher values than those of the base fluid (water). Specific examples are given below. Nine et al. [52] prepared aluminamultiwalled carbon nanotubes (Al2O3-MWNTs)/water hybrid nanofluids, and in another study, Nine et al. [49] also studied cuprous oxide (Cu2O) and copper/cuprous oxide (Cu/Cu2O)/water hybrid nanofluids. Suresh et al. [35,39] investigated Al2O3-Cu/water nanofluids, Baghbanzadeh et al. [46] silicon-MWCNT/water nanofluids, and Munkhbayar et al. [53] silver-MWCNT/water nanofluids. Jana et al. [54] reported on CNT-gold/water and CNT-Cu/water nanofluids, Abbasi et al. [52] on Al2O3-MWNT/water nanofluids and Baby and Ramaprabhu [41–43] on Ag-HEG-MWNT/water. Baby and Ramaprabhu [55] also prepared CuO/f-HEG/water nanofluids. Chen et al. [56] prepared MWCNT-Fe2O3/water nanofluids, and Batmunkh et al. [40] Ag-TiO2/water nanofluids. Sundar et al. [37,38,50,51] investigated MWCNT/Fe3O4/water, ND-Ni/water, ND-Fe3O4/water and ND-Co3O4/water nanofluids. Madhesh et al. [36] studied CuTiO2/water hybrid nanofluids, and Aravind and Ramaprabhu [44] graphene/MWCNT/water nanofluids. Aravind and Ramaprabhu [57] used a graphene wrapped MWCNT composite for the preparation of water based nanofluids. Chen et al. [58] prepared Ag/MWCNT/water nanofluids.

Material

Method

Ref.

CNT-Fe3O4 CNT-Fe3O4 CNT-Fe3O4 Silica-MWCNT CNT/SiO2 & CNT/ SiO3/Ag Fe3O4@SiO2/ MWCNTs Al2O3-Cu Cu-TiO2 ND-Ni

Sol-gel chemistry Solvothermal system Polyols method Sol-gel chemistry Plasma treatment

Jia et al. [29] Zhang et al. [30] Shi et al. [31] Guo et al. [32] Li et al. [33]

Chemical reduction technique

Baby and Ramaprabhu [34] Suresh et al. [35,61] Madhesh et al. [36] Sundar et al. [37]

MWCNT-Fe3O4 Ag-TiO2 MWCNT-HEGO Graphene/wrapped MWCNT Hybrid sphere/CNT Nanoparticles Silicon/MWCNT Al-Zn γ-Al2O3/MWNT Cu/Cu2O ND-Fe3O4 ND-Co3O4 MWCNT-Fe2O3 Ag/MWNT

187

Thermochemical Mechanical milling In-situ and chemical coprecipitation In-situ and chemical coprecipitation Ball milling Catalytic chemical vapor deposition Chemical vapor deposition Spherical particles by spray pyrolysis and CNT by catalytic process Wet chemical Mechanical alloying Solvothermal Wet ball milling In-situ and chemical coprecipitation In-situ and chemical coprecipitation Ball milling Ball milling

Sundar et al. [38] Batmunkh et al. [40] Baby and Ramaprabhu [41–43] Aravind and Ramaprabhu [44] Han et al. [45]

Baghbanzadeh et al. [46] Paul et al. [47] Abbasi et al. [48] Nine et al. [49] Sundar et al. [50] Sundar et al. [51] Chen et al. [56] Chen et al. [58]


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Fig. 1. ND-soot, acid treated-ND and ND-Fe3O4 nanoparticles (a) schematic diagram of synthesis procedure (b) TEM results (ND-soot; left-side), (acid treated-ND; middle), (NDFe3O4 nanoparticles; right-side) (Sundar et al. [50]).

⎞ ⎛ ρhnf = ϕnp1ρnp1 + ϕnp2ρnp2 + ⎜1−ϕnp1 − ϕnp2⎟ρbf ⎠ ⎝

Al2O3 and microencapsulated phase change material (n-eicosane) nanoparticles (MEPCM) and estimated experimentally the specific heat and observed experimental values are in good agreement with those given by specific heat mixture formula. The specific heat mixture formula for hybrid nanofluids is given as follows:

(1)

where np1 and np2 are Al2O3 and MEPCM nanoparticles, ρ is the density (kg/m3) and the suffixes hnf and bf are for hybrid nanofluid and base fluid, respectively. Sundar et al. [38,50] used the mixture formula for the estimation of density of MWCNT/Fe3O4, ND-Fe3O4 nanoparticles (Eq. (2)). They substituted Eq. (2) values in Eq. (3) for the estimation of quantity of nanoparticles required for the known percentage of volume concentrations. They used Eq. (4) itself for the estimation of density of hybrid nanofluids.

⎞ ⎛ ⎜ρCNT + Fe O ⎟ = 3 4⎠ ⎝ p

⎞ ⎛ ρhnf , Cp, hnf = ϕnp1ρnp1Cp, np1 + ϕnp2ρnp2 Cp, np2 + ⎜1−ϕnp1 − ϕnp2⎟ρbf C p, bf ⎠ ⎝

where np1 and np2 are Al2O3 and MEPCM nanoparticles, respectively, ρ is the density (kg/m3), Cp is the specific heat (J/kg K). The suffixes hnf and bf are related to hybrid nanofluids and base fluids, respectively. Sundar et al. [38,50] used the mixture formula for the estimation of the specific heat of MWCNT/Fe3O4, ND-Fe3O4 nanoparticles (Eq. (6)). The Eq. (7) represents the specific heat of the hybrid nanofluid. The relations (Eqs. (6) and (7)) are as follows:

⎞ ⎛ (ρCNT ) × wCNT + ⎜ρFe O ⎟ × wFe3O4 ⎝ 3 4⎠ wCNT + wFe3O4

(2)

⎡ WCNT + Fe3O4 ⎤ ⎢ρ ⎥ ⎣ CNT + Fe3O4 ⎦ Volume concentration, ϕ×100= ⎡ WCNT + Fe3O4 ⎤ ⎡ Wwater ⎤ ⎢ρ ⎥+⎢ ⎥ ⎣ CNT + Fe3O4 ⎦ ⎣ ρwater ⎦

(3)

ρhnf = (1−ϕ)ρbf + ϕ × ρp

(4)

(5)

(Cp, CNT + Fe3O4 ) = p

(Cp, CNT ) × wCNT + (Cp, Fe3O4 ) × wFe3O4 wCNT + wFe3O4

Cp, hnf = (1−ϕ)Cp, bf + ϕ × C p, p

(6) (7)

where ϕ is the percentage of volume concentration, Cp and W is the specific heat (J/kg K) and weight (g), respectively. The suffixes are related to the carbon nanotubes (CNT), Fe3O4, hybrid (CNT-Fe3O4) nanoparticles, p is for the particle, hnf is for the hybrid nanofluid and bf is for the base fluid.

where ϕ is the percentage of volume concentration, W and ρ is the weight (g) and density (kg/m3), respectively. The suffixes are for water, Fe3O4, carbon nanotubes (CNT), hybrid nanoparticles (CNT+Fe3O4). The suffixes p is for particle, bf is for the base fluid and hnf is for the hybrid nanofluid.

5.3. Thermal conductivity of nanofluids 5.3.1. Carbon nanotubes based hybrid nanofluids The thermal conductivity is one of the most important properties in what concerns the use of hybrid nanofluids in heat exchange equip-

5.2. Specific heat of nanofluids Ho et al. [60] prepared water-based hybrid nanofluids containing 188


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Fig. 2. Schematic representation of in-situ growth of ND-Ni nanocomposite (a) As received detonated nanodiamond soot, (b) c-ND powder (c) ND-Ni hybrid nanocomposite (Sundar et al. [37]).

thermal conductivity enhancement of nanofluids containing Ag/ MWNT nanoparticles is higher than that of MWNTs nanoparticles. Baby and Ramaprabhu [41] prepared Ag/HEG-water and Ag/HEGEG hybrid nanofluids and observed thermal conductivity enhancement of 25% at 0.05% vol at a temperature of 25 °C. Baby and Ramaprabhu [42] functionalized-MWNT/functionalized-HEG-water nanofluids and observed thermal conductivity enhancement of 20% at 0.05% vol Sundar et al. [38] prepared MWCNT/Fe3O4 water hybrid nanofluids and observed thermal conductivity enhancement of 13.88% and 28.46% at temperatures of 20 °C and 60 °C, respectively, for 0.3% vol of nanofluid; the data is presented in Fig. 5.. Baby and Ramaprabhu [43] prepared MWCNT/HEG@Silver EG nanofluids and observed thermal conductivity enhancement of 8% for 0.04% vol at a temperature of 25 °C; the data is reported in Fig. 6. Aravind and Ramaprabhu [44] synthesized graphene wrapped MWCNT hybrid particles and prepared water- and EG-based nanofluids and estimated thermal conductivity at different volume fractions

ment, as it is well-established the heat transfer coefficient depends on the thermal conductivity of the fluid. Therefore, research work related to thermal conductivity of CNTs based hybrid nanofluids is briefly reviewed in what follows. Baghbanzadeh et al. [46] prepared Si/ MWCNT/water hybrid nanofluids and measured the thermal conductivity in the particle concentrations of 0.1%, 0.5% and 1.0% wt in the temperatures range from 27 °C to 40 °C. Nine et al. [52] prepared 95% of Al2O3/5% of MWCNT and 90% of Al2O3/10% of MWCNT water based hybrid nanofluids and estimated thermal conductivity at different particle concentrations. Abbasi et al. [48] prepared Al2O3/ MWCNT-water hybrid nanofluids and obtained a thermal conductivity enhancement of 14.75% for 0.01% vol Munkhbayar et al. [53] found the thermal conductivity enhancement of 14.5% for Ag/MWNT/water hybrid nanofluids. Chen et al. [56] measured thermal conductivity of MWCNT/Fe2O3 hybrid nanofluid and observed enhancement of 28% compared to base fluid. Chen et al. [58] prepared MWCNT/water and 1.0% vol of Ag-MWNT/water hybrid nanofluids and observed that the 189


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Fig. 3. Stability behavior of nanofluids (002 mg/ml) (a) sonication after 0.5 h (b) sonication after 72 h (ND-soot; left-side), (acid treated-ND;middle), (ND-Fe3O4; right-side) (c) 0.05%, 0.1% and 0.2% volume concentrations of water-based magnetic ND-Fe3O4 nanofluids (d) ND-Fe3O4 nanoparticles showing magnetic behavior under magnetic field (e) ND-Fe3O4 nanoparticle size distribution while dispersed in distilled water measured from dynamic light scattering method (~21.24 nm) (Sundar et al. [50]).

Fig. 8b reports the thermal conductivity ratio (knf / kbf ) of 60:40% EG/Wbased ND-Fe3O4 nanofluids along with 60:40% EG/W-based Fe3O4 nanofluids. They report that at ϕ =0.2% and 60 °C, ND-Fe3O4 nanofluid has 1.03-times higher thermal conductivity than that of Fe3O4 nanofluid. The thermal conductivity of increases with increasing particle concentration and temperature; however, the order of magnitude depends on the type of nanoparticle. In addition, as expected, it was observed that the thermal conductivity of hybrid nanofluids is also dependent on the type of base fluid..

and the data is shown in Fig. 7... 5.3.2. Oxides based hybrid nanofluids The available literature on research work dealing with the thermal conductivity of oxides-based hybrid nanofluids is succinctly reviewed in what follow. Paul et al. [47] prepared Al-Zn/ethylene glycol hybrid nanofluids and observed a thermal conductivity enhancement of 16% for 0.1% vol Nine et al. [46] also prepared Cu-Cu2O/water based hybrid nanofluids and observed an increase of thermal conductivity enhancement of 2% when comparing nanofluids containing Cu2O nanoparticles to those with individual Cu. Batmunkh et al. [40] prepared Ag-TiO2/ water nanofluids and measured thermal conductivity using the transient-hot wire method in the temperatures range from 15 °C to 40 °C. Ho et al. [60] prepared hybrid water-based suspension of Al2O3 and microencapsulated phase change material (n-eicosane) nanoparticles (MEPCM) and observed a thermal conductivity enhancement of 9.8% at a particle loadings of 10% wt Baby and Ramaprabhu [55] prepared copper oxide doped graphene (CuO/HEG) and observed a thermal conductivity enhancement of 28% for CuO/HEG dispersed in water at 25 °C for a volume fraction of 0.05%. Sundar et al. [50] used waterbased ND-Fe3O4 nanofluids and the thermal conductivity ratio (knf / kbf ) is shown in Fig. 8a in comparison with Fe3O4/water nanofluids. They observed at ϕ =0.2% and 60 °C, ND-Fe3O4 nanofluid showing 1.02times higher thermal conductivity compared to Fe3O4/water nanofluid.

5.4. Viscosity of nanofluids 5.4.1. Carbon nanotubes based hybrid nanofluids The viability of using hybrid nanofluids is directly related to the pressure drop, and consequently associated pumping costs. To this purpose, the viscosity is a very important property; therefore, the available literature on viscosity of hybrid nanofluids is discussed in what follows. Baghbanzadeh et al. [46] prepared water-based nanofluids by considering 80% wt of silica+20% wt of MWCNT and 50% wt of silica+50% wt of MWCNT and studied their rheological properties. They observed that viscosity of the nanofluids increased with the concentration, while they were reduced by increasing the temperature and also observed at high concentrations, the least increase in the viscosity of distilled water by adding the 50 wt% of silica+50 wt% of 190


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Fig. 4. Average particle size distribution of (a) as received detonated ND soot (b) c-ND, and (c) ND-Ni nanocomposite dispersed in water (Sundar et al. [37]).

Fig. 6. Thermal conductivity ratio of different volume fractions of Ag/MWCNT-HEG dispersed EG at different temperatures (Baby and Ramaprabhu [43]).

Fig. 5. Thermal conductivity of MWCNT-Fe3O4/water hybrid nanofluids with at different volume concentrations and temperatures (Sundar et al. [38]).

⎛ ⎞ experimental results for the viscosity and viscosity ratio ⎜μnf / μbf ⎟ are ⎝ ⎠ reported in Fig. 10a and they are compared against the data for two single nanoparticles nanofluids, namely: Fe3O4/water and Al2O3/ propylene glycol nanofluids. They observed that for particle concentration of ϕ =0.2% and temperature of 60 °C, ND-Fe3O4/water nanofluid presents 1.57-times and 1.27-times higher viscosity compared to Fe3O4/water and Al2O3/propylene glycol, respectively, for 3.0% volume concentration. Fig. 10b reports on 60:40% EG/W-based ND-Fe3O4

MWCNT nanomaterials (8.2% increase at 1.0 wt%). Sundar et al. [38] determined experimentally the viscosity of MWCNT-Fe3O4 nanocomposite nanofluids and observed increases of 1.27-times and 1.5-times for 0.3% vol at temperatures of 20 °C to 60 °C, respectively, as compared to water; the data are reported in Fig. 9.. 5.4.2. Oxides based hybrid nanofluids Sundar et al. [50] used water based ND-Fe3O4 nanofluids and their 191


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Fig. 7. Thermal conductivity of different volume fractions of graphene carbon (GC) in (a) DI water and (b) EG base fluids (Aravind and Ramaprabhu [44]).

nanofluids along with 60:40% EG/W based Fe3O4 and 60:40% EG/Wbased CuO nanofluid. It is observed that for the particle concentration of ϕ =0.2% and temperature of 60 °C, the nanofluid prepared with NDFe3O4 nanoparticles has 1.398-times and 1.556-times higher viscosity than that for Fe3O4 and CuO nanofluids.. Table 2 reports on the thermal conductivity and viscosity of single nanoparticle (Fe3O4/water)-based nanofluid [70] and hybrid nanofluids (MWCNT-Fe3O4/water; ND-Fe3O4/water) for which the data of Sundar et al. [38,50] is used for comparison. They observed the thermal conductivity of single nanoparticles (Fe3O4/water) nanofluids increases with increasing particle volume concentration and temperature; similar trend is observed for the thermal conductivity of hybrid nanofluids (MWCNT-Fe3O4/water; ND-Fe3O4/water). From the data, it can be noticed that the thermal conductivity enhancement is 8.3% and 16.67% for ϕ =0.2% of Fe3O4/water nanofluid at temperatures of 20 °C and 60 °C, respectively. Similarly, at temperatures of 20 °C and 60 °C, the thermal conductivity enhancement is 12.8% and 24.65%, respectively, for ϕ =0.2% of MWCNT-Fe3O4/water nanofluid and 9.15% and 17.76%, respectively, for ϕ =0.2% of ND-Fe3O4/water. The thermal conductivity of the hybrid-nanofluids presents higher values than that of single-nanoparticle (Fe3O4) nanofluids. Sundar et al. [38] used 26% of MWCNT and 74% of Fe3O4 for the preparation of MWCNT-Fe3O4/ water hybrid nanofluids; with the use of MWCNT for the preparation of hybrid nanofluids, the thermal conductivity is further enhanced to 4.15% and 6.83% at temperatures of 20 °C and 6.83% at 60 °C, respectively, at ϕ =0.2% compared to that of the Fe3O4 nanofluid. The

Fig. 8. Thermal conductivity ratio of ND-Fe3O4 hybrid nanofluids compared to Fe3O4 nanofluids (a) water-based (b) 60:40% EG/W-based (Sundar et al. [50]).

Fig. 9. Experimental viscosity of MWCNT-Fe3O4 hybrid nanofluids at different volume concentrations and temperatures (Sundar et al. [38]).

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Fig. 11. Heat transfer coefficient of Ag/(MWNT-HEG)/EG-based hybrid nanofluids at different volume fractions and Reynolds numbers (Baby and Ramaprabhu [43]).

6. Heat transfer characteristics of hybrid nanofluids 6.1. Carbon nanotubes based hybrid nanofluids Prior to the use of hybrid nanofluids in heat exchange applications, the estimation of their heat transfer characteristics are very essential. The research works shows the higher heat transfer enhancements with hybrid nanofluids compared to base fluids as well as single particle nanofluids. Relevant work on heat transfer of carbon nanotubes-based hybrid nanofluids are given below. Baby and Ramaprabhu [41] measured heat transfer of Ag/(MWNT-HEG)- ethylene glycol (EG) based nanofluid for volume fractions of 0.005% and 0.01%, respectively, and their data set is reported in Fig. 11.. Aravind and Ramaprabhu [44] prepared water and EG based graphene/carbon nanotubes nanofluids and estimated the heat transfer coefficient in the Reynolds number range from 2000 to 10000 and their data is shown in Fig. 12. Sundar et al. [38] observed heat transfer enhancements of 20.62% and 31.10% for 0.1% and 0.3% volume concentration of MWCNT-Fe3O4/water hybrid nanofluids and the data is shown in Fig. 13... Labib et al. [64] numerically studied the heat transfer performance of water and EG based CNT/water and mixture of Al2O3 into CNT using two phase mixture model and observed EG based nanofluids gives better heat transfer rates compared with water. Baby and Ramaprabhu [42] prepared the hybrid nanostructure (fMWNT+f-HEG) of functionalized MWNT (f-MWNT) and functionalized HEG (f-HEG) by using the post mixing technique and prepared water-based nanofluids. They observed heat transfer enhancement of 289% at a Reynolds number of 15,500 for a 0.01% volume concentration compared to water data.

Fig. 10. Viscosity ratio of ND-Fe3O4 hybrid nanofluids compared to Fe3O4 nanofluids (a) water-based (b) 60:40% EG/W-based (Sundar et al. [50]).

synergistic effect of the presence of a nanocomposite is clearly demonstrated in this study. Sundar et al. [70] observed that the viscosity of single nanoparticle (Fe3O4/water)-based nanofluid increases with increasing particle volume concentration but it decreases with increasing temperature (Table 2). Similar trend is observed for hybrid nanofluids (MWCNTFe3O4/water; ND-Fe3O4/water) [38,50]. In the measured temperatures and volume concentrations range, it should be mentioned that the use of MWCNT or ND for the preparation of hybrid nanofluids causes increase in viscosity as compared to the Fe3O4 nanofluid.

Table 2 The thermal conductivity and viscosity comparison between single particle nanofluid (Fe3O4/water [70]) and hybrid nanofluids (MWCNT-Fe3O4 [38] and ND-Fe3O4 [50]). Property

Thermal conductivity (W/ m K) Viscosity (mPa s)

Temperature T (°C)

20 40 60 20 40 60

Base fluid

Fe3O4/water nanofluid [70]

MWCNT-Fe3O4/water hybrid nanofluid [38]

ND-Fe3O4/water hybrid nanofluid [50]

ϕ =0.0%

ϕ =0.2%

ϕ =0.1%

ϕ =0.3%

ϕ =0.05%

ϕ =0.10%

ϕ =0.20%

0.602 0.631 0.653 0.79 0.52 0.24

0.6524 0.7167 0.7619 0.84 0.55 0.33

0.6734 0.72 0.7891 0.91 0.61 0.39

0.6856 0.7656 0.8389 1.01 0.76 0.45

0.6158 0.6489 0.6842 1.2 0.58 0.37

0.6345 0.6706 0.7177 1.23 0.61 0.39

0.6571 0.7036 0.769 1.26 0.64 0.41

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Fig. 14. Nusselt number comparison between Al2O3-Cu/water hybrid nanofluid and Al2O3/water nanofluid at 0.1% volume concentration (Suresh et al. [35]).

bottom wall; they reported the higher heat transfer rates for hybrid nanofluids compared to nanofluids with the same volume concentration. Aravind and Ramaprabhu [44] synthesized graphene and graphene/multi walled carbon nanotube composite material and prepared water-based nanofluids and found heat transfer enhancement of 193% at Reynolds number of 2000 for 0.02% volume concentration and suggested these nanofluids are beneficial for cooling of electronic circuits. Fig. 12. Heat transfer coefficient at different volume fraction of nanofluids and Reynolds number (a) GC/DI water (b) GC/EG CG/DI water (Aravind and Ramaprabhu [44]).

6.2. Oxides based hybrid nanofluids The relevant literature related to heat transfer of oxides-based hybrid nanofluids is surveyed in what follows. Suresh et al. [35] experimentally estimated the heat transfer coefficient of Al2O3-Cu/ water hybrid nanofluids flow in a tube and observed Nusselt number enhancement of 13.56% at 0.1% volume concentration at Reynolds number of 1730 compared with water and the data is shown in Fig. 14. Suresh et al. [61] also used Al2O3-Cu/water hybrid nanofluid flow in a tube under turbulent flow conditions and observed heat transfer enhancement of 8.02% at 0.1% volume concentration.. Han and Rhi [63] prepared hybrid nanofluids with different volume concentrations of Ag/Al2O3-H2O and used as working fluid in a grooved heated pipe and they determined the heat transfer coefficient in the heat transfer rate for a power range of 50–300 W with 50 W intervals, volume concentrations of 0.005%, 0.05%, 0.1%, and hybrid combinations, inclination of 5°, 45° and 90°, and cooling water temperatures of 1 °C, 10 °C, and 20 °C and obtained better thermal performance with hybrid nanofluids in a grooved heat pipe. Madhesh et al. [36] obtained heat transfer coefficient, Nusselt number and overall heat transfer coefficient enhancements of 52%, 49% and 68% at 2.0% volume concentration of Cu-TiO2 hybrid nanofluids with a pumping penalty of 14.9%. Ho et al. [60] prepared hybrid water-based suspension of Al2O3 and microencapsulated phase change material (n-eicosane) particles (MEPCM) and estimated the convective heat transfer coefficient and observed effective cooling. Baby and Ramaprabhu [55] prepared CuO/HEG water and EG based hybrid nanofluids and observed at 0.005% nanofluid heat transfer enhancement is 81% and 202% at entrance of the pipe, 39% and 156% at the exit of the pipe for Reynolds numbers of 4500 and 15500. At same Reynolds numbers range for 0.01% nanofluid the heat

Fig. 13. Nusselt number of MWCNT-Fe3O4 hybrid nanofluids estimated from Eq. (10) with effects of particle concentrations and Reynolds number (Sundar et al. [38]).

Baby and Ramaprabhu [43] synthesized multi-walled carbon nanotubes (MWNT), hydrogen exfoliated graphene (HEG) and silver nanoparticles and prepared EG-based nanofluids; they observed convective heat transfer enhancement of 570% for 0.005% volume concentration at Re=250. Takabi and Salehi [65] numerically investigated laminar natural convection for Al2O3-Cu/water hybrid nanofluids in a sinusoidal corrugated enclosure with a discrete heat source on the

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8. Applications of hybrid nanofluids

transfer enhancement is 138% and 232% at the entrance of the pipe, respectively. Baby and Ramaprabhu [41] prepared silver/hydrogen induced exfoliated graphene (Ag/HEG) water and EG based nanofluids. They observed for water-based nanofluids, the heat transfer enhancement is 157% at 0.005% vol of nanofluid at the entrance of the heated section and 105% at the exit of the heated section at a Reynolds number of 15000. At same Reynolds number (Re=15500), for 0.01% vol of nanofluid the enhancement is 188% and 122% at the entrance and at the exit of the heated section. Similarly they observed for EG-based nanofluids at 0.005% vol, the enhancement is 291% and 188% at the entrance and exit of the tube at Re=250. At same Reynolds number (Re=250), for 0.01% vol of nanofluid, the enhancement is 327% and 289% at entrance and exit of the pipe.

The single phase fluids such as water (W), ethylene glycol (EG), propylene glycol (PG) and engine oil (EO) are primarily used in, among many other applications, in electronic cooling, engine cooling and vehicle thermal management, generator cooling, in machining coolant, welding, power systems, lubrication, thermal storage, solar heating, cooling and heating in buildings, biomedical, spacecraft devices and defense equipment. To achieve improved heat transfer rates with these single-phase fluids in heat exchange applications is practically impossible due to their relatively low thermal conductivities; moreover, their use with phase change is often precluded for technological or operational reasons. Therefore, one path to enhance their heat transfer performance is to disperse small quantities of nanometer-size particles. Researchers have observed higher heat transfer rates by using a variety of nanoparticles in different base fluids [5,7,8,10,12,13,19 and 21–28]. Over the recent past, the advancement of this area of nanotechnology, particularly in what concerns the synthesis of nanocomposite materials and their characterization, and the preparation of hybrid nanofluids, has been making major strides. Consistently, the published work demonstrates that the heat transfer enhancement obtained with hybrid nanofluids is consistently higher than that of single phase nanoparticles nanofluids [35,38,61,41–43 and 55], as a result of synergistic thermal conductivity property of the hybrid nanofluids. The use of hybrid nanofluids still is very limited; however, with heat transfer enhancement values as high as 31.10% [38], hybrid nanofluids may become the heat transfer fluid of the future for various potential applications. Even so, the number of review articles dealing with the heat transfer characterization of hybrid nanofluids is scarce [66].

7. Friction factor characteristics of hybrid nanofluids 7.1. Carbon nanotubes based hybrid nanofluids The eventual augmentation friction factor for hybrid nanofluids may critical in determining the viability of their use in commercial applications. The increase of friction factor beyond a certain threshold may require an amount of pumping power that will offset the benefits of enhanced heat transfer. Ideally, a hybrid nanofluid should satisfy simultaneously moderate increase or even reduced friction factor and substantially enhanced heat transfer coefficient. Sundar et al. [38] conducted friction factor experiments for MWCNT-Fe3O4/water hybrid nanofluids flowing in a tube and they observed for 0.3% vol that the nanofluid friction factor increased by 1.18-times at a Reynolds number of 22000; the experimental values are presented in Fig. 15..

8.1. Heat sink Selvakumar and Suresh [62] used Al2O3-Cu/water hybrid nanofluid as coolant for thin-channeled copper heat sink and observed heat transfer enhancement of 25.2% at a mass flow rate of 0.0178 kg/s at 0.1% vol compared to distilled water.

7.2. Oxides based hybrid nanofluids Suresh et al. [35] determined the friction factor of Al2O3-Cu/water hybrid nanofluids flow in a tube and observed friction factor enhancement of 16.97% at 0.1% volume concentration when compared with water; for the same volume concentration the friction factor enhancement of Al2O3/water nanofluid is 6%. This result indicates that the Al2O3-Cu/water hybrid nanofluid will cause an extra penalty in terms of pumping power, when compared to Al2O3/water nanofluid; the data set is presented in Fig. 16. Suresh et al. [61] also determined the friction factor of 0.1% of Al2O3-Cu/water hybrid nanofluids flowing in a tube under turbulent flow conditions and they observed that the friction factor of Al2O3-Cu/water slightly higher than that for Al2O3/ water nanofluid.. Selvakumar and Suresh [62] observed pressure drop enhancement of 14.25% for 0.1% Al2O3-Cu/water hybrid nanofluid flow in a thin channeled copper heat sink at volume flow rate of 1.347 L/min (2.24×10−5 m3/s). Sundar et al. [12,38] conducted heat transfer and friction factor for MWCNT-Fe3O4 hybrid nanofluid flow in a tube and compared with Fe3O4 nanofluid data. For the same Reynolds number of 22000, the heat transfer enhancement is 30.96% for 0.6% vol of Fe3O4/water nanofluid and 31.10% for 0.3% vol of MWCNT-Fe3O4 hybrid nanofluid, when both are compared to water. Also for the Reynolds number of 22000, the friction factor increase, when compared to water, is 1.10times for 0.6% vol of Fe3O4/water nanofluid and 1.18-times for 0.3% vol of MWCNT-Fe3O4. For hybrid nanofluids, taking into consideration the heat transfer enhancement, the increase in friction factor is practically negligible. The available heat transfer and friction factor data for hybrid nanofluids are summarized in Table 3. The available Nusselt number and friction factor correlations for hybrid nanofluids in laminar and turbulent flow regimes are reported in Table 4.

8.2. Boiling Bhosale and Borse [69] prepared Al2O3-Cu/water hybrid nanofluids by considering 2.5 mg of CuO and 2.5 mg of Al2O3 and observed 90% of critical heat flux enhancement for 1.0% vol of nanofluids using 36 gauge nicrome wire.

Fig. 15. Experimental friction factor of MWCNT-Fe3O4 hybrid nanofluids from Eq. (16) with effects of particle concentrations and Reynolds number (Sundar et al. [38]).

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particles is achieved with well-established techniques, for nanocomposite based nanofluids, the dispersion of two different materials in the base fluid poses a considerable problem due to the surface charge (positive or negative), which varies from one-particle to another one. Therefore, when preparing hybrid nanofluids it should be given to the following aspects: (i) selection of appropriate materials, (ii) synthesis of the nanocomposite materials, (iii) bonding between the materials involved in the composite, and (iv) use of adequate surfactant. An unfortunate drawback associated with the hybrid nanofluids is their increased of viscosity, when compared to that of the base fluid. The nanofluids, when used in internal flows, lead to an increase of pressure drop with the consequent increase in pumping power. The optimized selection and synthesis of materials for the preparation of stable hybrid nanofluids still are an elusive goal; therefore, they should be a research priority in the immediate future with the additional aim of reducing their cost and in this way facilitate their utilization in commercial engineering applications. For design purposes, development of theoretical models for the thermophysical properties, such as thermal conductivity and viscosity, for hybrid nanofluids should be considered as an important research priority. Fig. 16. Variation of friction factor with Reynolds number for the different fluids (Suresh et al. [35]).

10. Conclusions

8.3. Micro power generation

The thermal properties, such as thermal conductivity and viscosity, of hybrid-nanofluids present higher values than those for singlenanofluids; the thermal conductivity and viscosity of water-based Fe3O4, MWCNT-Fe3O4 and ND-Fe3O4 are used for the purpose of comparison. The thermal conductivity of Fe3O4 nanofluid is enhanced up to 8.3% and 16.67% for ϕ =0.2% vol at temperatures of 20 °C and 60 °C, respectively; while, for ϕ =0.2% vol, the thermal conductivity is enhanced by 12.8% and 24.65% with MWCNT-Fe3O4 and 9.15% and 17.76% withND-Fe3O4/water at temperatures of 20 °C and 60 °C, respectively. The viscosity of Fe3O4 nanofluid is increased by 6.32% and 37.5% for ϕ =0.2% vol at temperatures of 20 °C and 60 °C, respectively; while, for ϕ =0.2% vol, the viscosity is increased by 39.87% and 75% with MWCNT-Fe3O4 and 59.49% and 70.83% with ND-Fe3O4 at temperatures of 20 °C and 60 °C, respectively. These values indicate that hybrid nanofluids exhibit higher thermal conductivity than the nanoparticle (Fe3O4) nanofluid; unfortunately, this enhancement of thermal conductivity is accompanied by an increase in viscosity. The use of 26% of MWCNT for the preparation of the

He et al. [67] prepared single walled carbon nanotube-copper sulfide nanoparticles (SWCNT-CuS NPs) dispersed in poly-styrene sulfonate solution and used in micro-power generation and observed the voltage enhancement for 1.6 mV at 900 μL nanofluid. 8.4. Solar energy Xuan et al. [68] prepared TiO2/Ag hybrid nanofluids and studied the solar energy absorption features with solar irradiation spectrum and they compared the optical properties of hybrid nanofluids with individual TiO2 and Ag nanofluids. 9. Challenges of hybrid nanofluids The stability of nanocomposite nanoparticles in the base fluid is a major challenge. While for single-phase nanofluids, stability of the Table 3 Heat transfer and friction factor enhancements of hybrid nanofluids. Hybrid nanofluids

Al2O3-Cu/water Al2O3-Cu/water Cu-TiO2/water Ag/(MWNT-HEG) dispersed in EG Graphene wrapped multiwalled carbon nanotubes (MWNT)/ water and EG Ag/HEG dispersed in water and EG MWCNT/Fe3O4 dispersed in water CuO/HEG dispersed water and EG

Flow conditions

700 < Re