Rheological behaviour of nanofluids: A review Anuj Kumar Sharma a,n , Arun Kumar Tiwari b , Amit Rai Dixit a a Department of Mechanical Engineering, Indian School of Mines, Dhanbad 826004, India b Department of Mechanical Engineering, Institute of Engineering & Technology, GLA University, Mathura 281406, India article info Article history: Received 1 October 2014 Received in revised form 30 June 2015 Accepted 17 September 2015 Keywords: Nanofluids Nanoparticles Rheological behaviour Newtonian Non-Newtonian Viscosity Pressure drop abstract A colloidal mixture of nanometre-sized ( o100 nm) metallic and non-metallic particles in conventional fluid is called nanofluid. Nanofluids are considered to be potential heat transfer fluids because of their superior thermal and tribological properties. In recent period, nanofluids have been the focus of attention of the researchers. This paper presents a summary of a number of important research works that have been published on rheological behaviour of nanofluids. This review article not only discusses the influ- ence of particle shape and shear rate range on rheological behaviour of nanofluids but also studies other factors affecting the rheological behaviour. These other factors include nanoparticle type, volumetric concentration in different base fluids, addition of surfactant and externally applied magnetic field. From the literature review, it has been found that particle shape, its concentration, shear rate range, surfactant and magnetic field significantly affect the rheological behaviour of any nanofluid. It has been observed that nanofluids containing spherical nanoparticles are more likely to exhibit Newtonian behaviour and those containing nanotubes show non-Newtonian flow behaviour. Furthermore, nanofluids show New- tonian behaviour at low shear rate values while behave as non-Newtonian fluid at high shear rate values. Authors have also identified the inadequacies in the research works so far which require further investigations. & 2015 Elsevier Ltd. All rights reserved. Contents 1. Introduction ........................................................................................................ 779 2. Rheological behaviour of nanofluids..................................................................................... 786 2.1. Newtonian and non-Newtonian behaviour of nanofluids .............................................................. 786 2.2. Rheological behaviour of ferrofluids ............................................................................... 786 2.3. Effect of surfactants on rheological behaviour of nanofluids ............................................................ 786 3. Conclusions ........................................................................................................ 786 4. Recommendations for future work ...................................................................................... 789 References ............................................................................................................. 789 1. Introduction Conventional fluids, such as, mineral oils, have excellent lubrication properties but poor thermal properties that restrict their use as coolants in industrial applications. Nowadays a num- ber of methods are available to enhance the heat transfer rate of any conventional fluid. One such method may be the addition of small-sized solid particles (millimetre and micrometre) in con- ventional fluid that can improve its thermal properties. But use of these fluids has shown serious problems such as clogging, high erosion, pressure drop in pipelines and poor stability of Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/rser Renewable and Sustainable Energy Reviews http://dx.doi.org/10.1016/j.rser.2015.09.033 1364-0321/& 2015 Elsevier Ltd. All rights reserved. Abbreviations: ASCH, Al 2 O 3 –SiO 2 clay hybrid; BCA, 2-Butoxyethylacetate; CA, Diethylene glycol monethyl ether acetate; CMC, Carboxy methyl cellulose; CNT, Carbon nanotube; DEG, Diethylene glycol; EG, Ethylene glycol; GNP, Graphene nanoplatelets; HTPB, Hydranxy terminated polybutadiene; ITO, Indium Tin Oxide; ICH, Iron oxide clay hybrid; MWCNT, Multi walled carbon nanotube; PG, Propylene glycol; PPG, Poly propylene glycol; PSi, Poly siloxane; TNT, Titanate nanotube n Corresponding author. Tel.: þ91 9711037075; fax: þ91 5422368157 E-mail address: [email protected](A.K. Sharma). Renewable and Sustainable Energy Reviews 53 (2016) 779–791
Transcript
Renewable and Sustainable Energy Reviews 53 (2016) 779–791
Contents lists available at ScienceDirect
Renewable and Sustainable Energy Reviews
http://d1364-03
AbbreDiethyleCarbonnanoplaICH, Iroglycol; P
n CorrE-m
journal homepage: www.elsevier.com/locate/rser
Rheological behaviour of nanofluids: A review
Anuj Kumar Sharma a,n, Arun Kumar Tiwari b, Amit Rai Dixit a
a Department of Mechanical Engineering, Indian School of Mines, Dhanbad 826004, Indiab Department of Mechanical Engineering, Institute of Engineering & Technology, GLA University, Mathura 281406, India
a r t i c l e i n f o
Article history:Received 1 October 2014Received in revised form30 June 2015Accepted 17 September 2015
Keywords:NanofluidsNanoparticlesRheological behaviourNewtonianNon-NewtonianViscosityPressure drop
x.doi.org/10.1016/j.rser.2015.09.03321/& 2015 Elsevier Ltd. All rights reserved.
A colloidal mixture of nanometre-sized (o100 nm) metallic and non-metallic particles in conventionalfluid is called nanofluid. Nanofluids are considered to be potential heat transfer fluids because of theirsuperior thermal and tribological properties. In recent period, nanofluids have been the focus of attentionof the researchers. This paper presents a summary of a number of important research works that havebeen published on rheological behaviour of nanofluids. This review article not only discusses the influ-ence of particle shape and shear rate range on rheological behaviour of nanofluids but also studies otherfactors affecting the rheological behaviour. These other factors include nanoparticle type, volumetricconcentration in different base fluids, addition of surfactant and externally applied magnetic field. Fromthe literature review, it has been found that particle shape, its concentration, shear rate range, surfactantand magnetic field significantly affect the rheological behaviour of any nanofluid. It has been observedthat nanofluids containing spherical nanoparticles are more likely to exhibit Newtonian behaviour andthose containing nanotubes show non-Newtonian flow behaviour. Furthermore, nanofluids show New-tonian behaviour at low shear rate values while behave as non-Newtonian fluid at high shear rate values.Authors have also identified the inadequacies in the research works so far which require furtherinvestigations.
Conventional fluids, such as, mineral oils, have excellentlubrication properties but poor thermal properties that restricttheir use as coolants in industrial applications. Nowadays a num-ber of methods are available to enhance the heat transfer rate ofany conventional fluid. One such method may be the addition ofsmall-sized solid particles (millimetre and micrometre) in con-ventional fluid that can improve its thermal properties. But use ofthese fluids has shown serious problems such as clogging, higherosion, pressure drop in pipelines and poor stability of
Fig. 2. Pumping power versus particle volume concentration [48].
A.K. Sharma et al. / Renewable and Sustainable Energy Reviews 53 (2016) 779–791780
suspension. About a decade ago, nanometre-sized particlesreplaced these milli- and micro-sized particles in the suspension,leading to the development of a new class of fluids called ‘nano-fluids’. These nanofluids have a number of advantages, such as,better stability, greater thermal conductivity and lower pressuredrop compared to the base fluid. Also, use of these nanofluids hasshown a remarkable improvement of performance parameters inmachining, such as, milling [1–8], grinding [9–18], drilling [19] andturning [20–23] of various metals and their alloys. Sharma et al.[24] reviewed the literature available on nanofluid application invarious machining processes as cutting fluid and observed thatnanofluid improved machining performance significantly.
Nanofluids are colloidal mixtures of nanometre-sized particles(1–100 nm) in a base fluid. The nanoparticles can be metallic, non-metallic, oxide, carbide, ceramics, carbonic, mixture of differentnanoparticles (hybrid nanoparticles) and even nanoscale liquiddroplets. The base fluid may be a low viscous liquid like water,refrigerant or a high viscous liquid like ethylene glycol, mineral oilor a mixture of different types of liquids (EGþwater, water-þpropylene glycol etc.). The term ‘nanofluid’ was first coined byDr. Stephen Choi (Energy Technology Division, Argonne NationalLaboratory, USA) in 1995 [25]. However, there was an earlier andindependent report by Masuda et al. [26] which dealt with thesimilar subject. At the initial stage, research on nanofluids wasmainly conducted at Argonne National Laboratory, USA. At thisstage, the key area of research was thermal conductivity undermacroscopically static conditions. Few researchers have observedin their investigations that addition of nanoparticles in conven-tional fluids remarkably enhanced their thermal conductivity [27–39,139–141] in comparison to the base fluids. Saidur et al. [40]observed that the thermal conductivity of nanofluids increasedwith the increase of particle volumetric concentration in basefluid. The mixing of nanoparticles with base fluid may alter thethermo-physical properties of fluids as the nanoparticles possesshigher thermal conductivity than base fluids [25,41]. However,various experiments have shown that the increase of thermalconductivity might be offset by an increase of viscosity and noticeda little penalty in pressure drop [42–45]. Tiwari et al. [46,47]observed that an increase of nanoparticle volume concentrationincreased the viscosity and density of fluid, which, in turn, causeda pressure drop, and hence, increased the pumping power. Incre-ment of shear viscosity of nanofluids as a function of nanoparticleconcentration can also be seen in Fig. 1. Vajjha and Das [48]observed that increase of nanoparticle loading in the base fluidincreased viscosity and density significantly. They observed anincrement of �91% in viscosity and �13.9% in density at 6 vol%Al2O3/EGþwater (60:40) nanofluid. They also found that at a giventemperature, all three nanofluids at 2 vol% (CuO, Al2O3 and SiO2)required less pumping power than the base fluid. However, CuO
Fig. 1. Shear viscosity of nanofluids as a function of nanoparticle concentration[57].
nanofluid required more pumping power than the base fluid at ahigher volumetric concentration (43 vol%). This can be wellexplained by Fig. 2. Thus, an increase of viscosity may incur apenalty in pressure drop and rise in pumping power. Hence,viscosity of nanofluids can play a vital role in selection of thenanofluid for a particular application.
The pressure drop in any fluid is also affected by Reynoldsnumber. A few researchers [49–55] have observed that an increaseof Reynolds number of any fluid flow increased its pressure drop.Moreover, as shown in Fig. 3, there is a small increase in pressuredrop with the increasing particle volume concentrations. In thepresent paper, however, the review is restricted to summarizingthe effects of nanoparticle type, shape, shear rate range andvolumetric concentration on rheological flow behaviour. Further-more, the rheological behaviour of a nanofluid can also give anidea of viscosity variation with shear rate. The detailed rheologicalanalysis of nanofluids [62–135] is sufficient to explain that theycan exhibit either or both Newtonian and non-Newtonian beha-viours. This behaviour depends on various factors such as nano-particle shape [28,56–57], size [64], nanoparticle concentration[58–60], nanoparticle structure [61], surfactants [70,75,77–78,80,83,104–106,112,116,125,128], shear rate range [70,96,114]and even magnetic field [125–135]. Furthermore, Wang et al. [137]reviewed literature available on rheology of nanofluids and foundBrownian motion and nanoparticle aggregation to be the majormechanisms for rheological properties of nanofluids. The rheolo-gical behaviour of nanofluid, being an important factor in itsapplication, might be helpful in understanding the viscosity profile
Fig. 3. Variation of pressure drop versus Reynolds number for different particlevolumetric concentrations [53].
Table 1Summary of rheological behaviour of different nanofluids.
SiO2, TiO2/deionized water 0.468 0.16–1.73 mm 0–500 SiO2 alone exhibited Newtonian behaviour while all SiO2/TiO2 mixed sus-pensions showed Bingham plastic behaviour. Due to the addition of a smallamount of TiO2, the plastic viscosity increased remarkably compared topure SiO2 suspension.
Richmond et al. [62]
TiO2/pure water 0.05–0.12 7–20 10–1000 A shear thinning behaviour was observed in all suspensions over all shearrate values. As solid concentration exceeded 0.1%, the flow curves of sus-pensions became apparently thixotropic.
Tseng and lin [63]
TiO2/distilled water 0.24, 0.6 and 1.18 Primary size 20, 95 0.1–1000 All the suspensions showed strong shear thinning behaviour till the shearrate reached 100 s�1 and after this it showed Newtonian behaviour. Also,shear viscosity increased with increasing nanoparticle loading and size.
He et al. [64]
TiO2/EG 0.1, 0.21, 0.42, 0.86 and 1.8 25 0–200 All suspensions behaved as Newtonian fluid. The relative viscosity depen-ded on nanoparticle concentration in a non-linear manner but was inde-pendent of temperature.
Chen et al. [65]
TiO2/EG 0.5, 2 and 8.0 wt% 70-100 0.5–104 Both pure EG and EG-based nanofluids showed Newtonian behaviour. Also,viscosity increased with temperature and nanoparticle concentration.
Chen et al. [66]
TiO2/distilled water 0.12, 0.24, 0.6 10 1–104 Suspensions showed shear thinning behaviour. The intensity of shearthinning behaviour of suspension increased with an increase in particleconcentration. The viscosity decreased with an increase of temperature.
Chen et al. [67]
TiO2/water 0–2 Dia¼10 0.03–3000 TiO2/EG showed Newtonian behaviour while rest of the three nanofluidsexhibited non-Newtonian behaviour.
Chen et al. [68]TiO2/EG Length¼100TNT/waterTNT/EGTiO2/deionized water 0.2–3 21 n Suspension having lower volumetric concentration (0.2) showed almost
Newtonian behaviour but for higher concentrations it exhibited non-New-tonian behaviour.
Turgut et al. [69]
TiO2/water 0.013, 0.020, 0.029, 0.050 5–6/80–90 0–100 In the shear rate range 1–100 s�1, the suspension showed Newtonianbehaviour while for higher shear rate (4100 s�1), it showed shear thinningbehaviour.
Alphonse et al. [70]
TiO2/water 0.05–0.65 20, 25, 40, 100 n Nanofluids showed Newtonian behaviour. Penkavova et al. [71]Al2O3 0.1, 0.2, 0.5, 25 350–1000 Base fluid as well as all the suspensions showed non-Newtonian (shear
thinning) behaviour. The relative apparent viscosity of TiO2 and Al2O3
nanofluids increased with an increase of nanoparticle concentration, whilefor CuO nanofluid, it was found to be almost independent of concentration.
Hojjat et al. [72]TiO2 1.0, 1.5, 3.0, 10CuO/aqueous solution (0.5 wt%) of CMC 4.0 30–50
Anatose TiO2/EG 1.51–8.83 35717 0.1–1000 Both types of suspensions exhibited non-Newtonian (shear thinning)behaviour. Viscosity decreased with increasing volumetric concentration. Atapprox. 10 s�1 shear rate, the viscosity of both nanofluids was found to beindependent of temperature.
Cabaleiro et al. [73]Rutile TiO2/EG 1.36–8.08 47718
MWCNT/polycarbonate 0.5, 1, 2 and 5 wt% 10–15 n Composites having more than 2 wt% MWCNT showed Non-Newtonianbehaviour at lower frequencies while 0.5 and 1% exhibited Newtonianbehaviour.
Potschke et al. [74]
MWCNT/poly α-olefin (PAO6) oil 0.12 n 100 The suspensions with lowest (0.3%) and highest (8%) dispersant con-centrations reported strong thinning behaviour while the suspension with3 wt% dispersant showed Newtonian behaviour. This suspension with lowerparticle loading (o0.09 vol%) showed Newtonian behaviour, while for0.09 vol% and 0.13 vol%, it showed slight shear thinning at low stress.
Yang et al. [75]
MWCNT/vinyl ester-polyster 0.05, 0.1, 0.3 wt% 15 10�1–103 Neat resin suspension showed almost Newtonian behaviour but MWCNTenriched base fluid showed shear thinning behaviour.
Seyhan et al. [76]
MWCNT/poly α-olefin (PAO6) oil 1 wt% 100 100 At 25 °C, suspensions without dispersant behaved slightly like shear thin-ning, but, at 75 °C, suspension showed very strong shear thinningbehaviour.
Yang et al. [77]
MWCNT-Al2O3/glycerol(10 wt%) and water 20, 25, 30, 35, 40, 45 Al2O3(27.5) 1–200 Both Al2O3 and MWCNT-Al2O3 suspensions exhibited shear thinningbehaviour. With an increase of nanoparticles, loading in suspensionincreased its viscosity.
0.1 wt% Dia 20–40 10�2–103 Suspensions at low concentrations (o¼0.04 wt%) showed shear thinningbehaviour at low shear rate but behaved as Newtonian fluid at high shearrate. The nanofluids containing higher concentration (40.06 wt%) exhib-ited shear thinning behaviour. Interestingly, the viscosity of nanofluids wasfound to be lower than that of the base fluid at much higher shear rate.Viscosity of nanofluids reduced with an increase of temperature.
Wang et al. [79]Length¼5–15 mm
MWCNT/deionized water 0.24–1.43 20–30 0–200 Suspension with low CNT concentration (o0.24 vol%) and 0.1–0.2 wt%,chitosan behaved as Newtonian fluid while with high CNT concentrationand 0.1–0.2 wt%, chitosan exhibited shear thinning behaviour.
Phuoc et al. [80]
MWCNT/EG 0.5–4% mass fraction 10–120 Nanofluids exhibited Newtonian behaviour. Viscosity decreased with anincrease in temperature. Viscosity of 4% MWCNT nanofluid at 55 °C wasfound to be much lower than that of pure EG at 25 °C.
Meng et al. [81]
MWCNT/EG 0.5 wt% 10–30 0.1–100 Suspension showed shear thinning behaviour. Suspension with sonicationtime of 40 min. exhibited the highest viscosity. Furthermore, the suspensionwith sonication time of 1355 min. showed a flat curve comparable to that ofpure EG on stress–shear rate plot approaching Newtonian behaviour.Interestingly, the viscosity at a fixed shear rate first increased thendecreased with an increase in sonication time.
Ruan and Jacobi [82]
MWCNT/deionized water 0.05, 0.24, 1.27 20–30 1–120 At a high volumetric concentration, nanofluid showed a clear shear thin-ning behaviour. Viscosity increased with rise of concentration anddecreased with rise of temperature. Surfactant produced better stability ofnanofluid.
Wang et al. [83]
MWCNT/distilled water 600, 1400, 2200 ppm n 0.01–100 All the suspensions showed shear thinning flow behaviour. Surfactantincreased the viscosity slightly as compared to distilled water and alsomildly increased its friction factor.
Ko et al. [84]
SiO2/ethanol 1.1–7.0 35, 94, 190 0–5�104 Suspensions showed Newtonian behaviour over a wide range of shear rates.Also viscosity increased with increase of nanoparticle concentration.
Chevalier et al. [85]
SiO2/deionized water 0.45, 1.85, 4 12 n Viscosity of nanofluids increased with an increase of nanoparticle con-centration and decreased by the increase in temperature.
Tavman et al. [86]Al2O3/deionized water 0.5, 1.5 30SiO2/ethanol 0.4, 0.7, 1, 1.1, 1.3, 1.6, 2.6, 3.1 10–100 10–1000 Rheology of suspensions was investigated under very high shear rates. All
the nanofluids showed Newtonian behaviour.Chevalier et al.[87]
Hybrid ICH and ASCH/aqueous 5 wt% bentonitefluid (5B)
0.5 wt% 2 1–200 All the clay-based fluids showed a strong shear thinning behaviour. Jung et al. [88]
Silica/distilled water 0.002–0.132 12 0.1–1000 Suspensions containing low concentration (o0.069 vol%) behaved asNewtonian fluids while nanofluids with higher particle concentrationbecame shear thinning in nature.
Mondragon et al.[89]
SiO2/paraffinic mineral oil 1.0, 2.0 20 1–1000 Both base fluid and suspensions showed Newtonian behaviour at �30 °C.However, shear thinning behaviour in both was observed at elevated tem-perature (�100 °C) and pressure. Viscosity of both the base fluid andnanofluids increased with an increase of pressure.
Anoop et al.[90]
Al2O3/pure water 0.01–0.16 37 1–1000 The suspension generally showed a transition from shear thinning to shearthickening as the shear rate exceeded a certain critical level ofapprox.100 s�1. Also, this critical value of shear rate increased with a rise ofnanoparticle concentration. However, pH 11 suspensions showed shearthinning behaviour over the entire range of shear rate and no transition inflow behaviour could be seen.
Tseng and Wu [91]
Al2O3/double distilled water 0.01–0.15 0.2 mm 1–1000 All suspensions containing Al2O3 micro-sized particles (�0.2 mm) showedshear thinning flow behaviour at low shear rate values followed by shearthickening behaviour as shear rate surpassed a critical value. For all sus-pensions, this critical value increased with an increase of solid concentra-tion (ϕ).
Tseng and Wu [92]
Alumina/PG 0.5, 2.0 and 3.0 27, 40, 50 0–100 All the suspensions were found to be Newtonian in nature. Prasher et al. [93]Al2O3/water 0.5, 1, 2, 4, 6 50 1–1000 All nanofluids exhibited Newtonian behaviour. Anoop et al. [94]Al2O3/EGCuO/EG
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Al2O3/EG 0.005–0.066 40–50 123 Suspension exhibited Newtonian behaviour at tested shear rate (123 s�1).Viscosity increased with concentration of nanoparticles and decreased withtemperature rise.
Gallego et al. [95]
Al2O3/water 1 wt% 30 nm 10�1–5000 CNT nanofluid behaved almost as Newtonian fluid for high shear rates(4100 s�1) and as non-Newtonian fluid for low shear rates. Al2O3 sus-pension exhibited non-Newtonian shear thickening behaviour. Also, boththe nanofluids showed thixotropic nature. The viscosity decreased with thetemperature rise.
Aladag et al. [96]CNT/water 9 mm
CuO/deionized water n 30, 75 and 150 0–20 Suspensions having small particles (30 and 75 nm) exhibited pseudoplasticnature while with large sized particles (150 nm) showed pseudoplasticbehaviour with a yield stress. Also, for very small shear rate values(o5 s�1), a rapid drop in viscosity was observed and no apparent change inviscosity could be seen when shear rate was higher than 5 s�1. Moreover,suspensions containing smaller sized particles had higher viscosity.
Chang et al. [97]
CuO/EG 10�5�10�1 (dilute limit¼0.002)
10–30 10�2–103 All the suspensions exhibited very strong shear thinning character. For veryhigher shear rate values, all suspensions had almost the same viscosity asthat of the base fluid.
Kwak and Kim [98]
CuO/EG and water mixture (60:40 by wt.) 1, 2, 3, 4, 5 and 6.12 29 0–8 All suspensions showed Newtonian behaviour. The viscosity of suspensionsincreased with an increase of nanoparticle loading and decreased expo-nentially with temperature.
Namburu et al. [99]
CuO/PGþwater (60:40) 0.025, 0.1, 0.4, 0.8 and 1.2 o50 500–700 Nanofluids exhibited Newtonian behaviour. Viscosity decreased exponen-tially with the increase of temperature.
Naik et al. [100]
CuO/water 0–0.018 23–37 Nanofluids exhibited Newtonian behaviour. Viscosity increased with anincrease of nanoparticle concentration and decreased with rise oftemperature.
Pastoriza et al. [101]1173
CuO/oil (SN-500) 0.2, 0.5, 1.0, 2.0 wt% 50 1–17 All suspensions exhibited Newtonian behaviour at different temperatures.Viscosity decreased with an increase in temperature and increased withnanoparticle concentration.
Saeedinia et al. [102]
CuO/0.4 wt% Xanthan gum aqueous solution 0.1, 0.3, 0.5 wt% o50 10–1200 All suspensions showed shear thinning behaviour. Viscosity of nanofluidsdecreased with an increase of temperature.
William et al. [103]o50ZnO/0.4 wt% Xanthan gum aqueous solution
BaTiO3/distilled water 0.1–0.55 0.8 mm 1–1000 Suspension without NH4PA appeared to be shear thinning. As shear rateexceeded�400 s�1, it followed Bingham plastic nature. But adding NH4PA(2 wt%), the suspension appeared to be close to Newtonian behaviour at alow shear rate (�100 s�1), while for higher values of shear rate, the stress–shear rate curve deviated from linearity and revealed dilatant flowbehaviour.
Tseng and Li [104]
BaTiO3/ethanol–isopropanol 0.3–0.6 0.58 mm 1–1000 All suspensions showed shear thinning behaviour at a low shear rate (�20–60 s�1) and moved towards Bingham plastic nature as shear rate increasedfurther and followed by shear thickening behaviour at high shear ratevalues.
Tseng and Lin [105]
Nickel/terpineol 0.03–0.1 0.3 mm 1–1000 All suspensions containing nickel powder (average particle size �0.3 mm)appeared to have shear thinning flow behaviour over the entire range ofshear rate.
Tseng and Chen [106]
Nickel/α-terpineol 0.01–0.28 90 1–1000 The Ni–terpineol suspensions generally exhibited shear thinning behaviour.But for lower solid concentrations
Tseng and Chen [107]
(o0.05 vol%), the suspensions followed shear thickening flow behaviour.Aluminium/HTPB, PPG and PSi 2.1, 6.25 and 10 120 10�3–103 The rheology of polyethylene coated and uncoated aluminium nano-
particles was investigated in this work. The uncoated HTPB suspensiondisplayed Newtonian behaviour while HTPB coated suspensions showedshear thinning behaviour for 6.25 and 10 vol%. Both the PPG and PSi sus-pensions reported shear thinning behaviour even for uncoated particles.
Mary et al. [108]
Aluminium(ALEX)/paraffin oil 2–45 10�3–104 ALEX-paraffin oil suspension showed shear thinning behaviour while ALEX-HTPB suspension exhibited almost Newtonian behaviour for a wide range ofnanoparticle concentration.
Teipel and Barth [109]2–47Aluminium/HTPB
Silver/DEG 0.11–4.38 40 1–200 The suspensions exhibited non-Newtonian (pseudoplastic) flow behaviour.Viscosity increased with an increase of particle concentration.
Tamjid and Guenther [110]
Silver/BCA and CA in weight ratio 5:1 1.0–16 30–50 1–4000 All suspensions showed shear thinning behaviour. Chen et al. [111]Graphite/oil 0.17–1.36 10–30 0.1–1000 Suspensions exhibited shear thinning flow behaviour. Addition of surfactant
and graphite nanoparticles increased the viscosity of nanofluids. Viscosityalso decreased with an increase of temperature.
Graphite/deionized water 1–4 3–4 1–100 Suspension behaved as shear thinning fluid. The viscosity increased withthe increased loading of nanoparticles. Enhancement of viscosity of sus-pensions, held for 3 days, was much higher than the freshly preparednanofluids for the same volume of concentration.
Duan et al. [113]
Graphene/glycerol 0.0025–0.02 15–50 1–180 At low shear rates, the suspensions showed shear thinning behaviour for alltemperatures. But for high shear rates, the nanofluids behaved as New-tonian fluids. Shear thinning behaviour became more prominent withincreasing nanoparticle concentration.
Moghaddam et al. [114]
GNP/distilled water 0.025, 0.05, 0.075, 1.0 wt% Thickness-2 nm 1–200 All the suspensions exhibited shear thinning behaviour. The shear thinningbehaviour was found to be more pronounced at higher concentrations.Viscosity of all nanofluids decreased with an increase of temperature.
Mehrali et al. [115]Dia-2 mm
ITO/deionized water 0.2–0.3 60 10–500 All the suspensions showed Newtonian flow behaviour over a range ofshear rates. Besides this, suspensions behaved as Bingham fluid for lowershear rates, while for very high shear rates, it showed shear thickeningbehaviour.
Tseng and Tzeng [116]
TNT/EG 0.1, 0.21, 0.42, 0.86, 1.80 10 10�1–103 TNT–EG nanofluids exhibited shear thinning behaviour. The shear viscosityincreased with the TNT concentration.
Chen et al. [117]
ZnO/EG 0.01–0.05 10–20 0–100 Nanofluid with lower particle concentration (o0.02) exhibited Newtonianbehaviour while suspension having higher volumetric concentrations(40.03) nanofluids possessed shear thinning behaviour.
Yu et al. [118]
TNT/EG 0.5, 1.0, 2.0, 4.0, 8.0 Dia¼10 10�1–103 All suspensions behaved as slightly shear thinning fluid for low particleconcentration (r2.0%), while for higher TNT concentration, they exhibitedvery strong shear thinning behaviour. Nanofluids with higher temperaturesexhibited stronger shear thinning behaviour as compared to suspensions atlower temperatures.
Chen et al. [119]Length¼100
The viscosity showed an increasing trend with an increase of temperaturefor lower shear rates (o10 s�1 ), while for higher shear rates (410 s�1),a reverse trend was noticed.
CaCO3/distilled water 0.12, 0.48, 1.40, 2.05, 4.11 20–50 5–100 All suspensions showed Newtonian behaviour. Tao et al. [120]MgO/EG 0.5–5 20 10–150 Suspensions showed Newtonian behaviour. Viscosity of suspensions
increased with an increase of nanoparticle concentration and decreasedwith a rise in temperature.
Xie et al. [121]
Gold/water 0.01 10, 20, 50 200–2000 The nanofluids exhibited Newtonian behaviour. Nanofluids with larger-sized (50 nm) nanoparticles possessed higher viscosity as compared to thesuspensions having smaller sized (10 and 20 nm) nanoparticles. Also,viscosity decreased with the rise of temperature.
Abdelhalim et al. [122]
Carbon black powder (N115)/EG 2.2, 5.6, 7.8 20 6–120 Suspensions showed shear thinning behaviour and the extent of thisbehaviour increased with an increase of carbon black inclusion into EG.Furthermore, shear viscosity decreased with an increase of temperature atthe same shear rate.
Meng et al. [123]
Yttrium oxide(Y2O3)/EG 1, 5, 10, 15, 20 wt% 3171 0.01–2000 Y2O3 nanofluid showed non-Newtonian behaviour. MgAl2O4 suspensionshowed Newtonian behaviour for low particle concentration while itexhibited shear thinning behaviour for higher (15 and 20 wt%) particleconcentration. Temperature did not have significant effect on the viscosityof MgAl2O4 suspension at high shear rate.
Cholewa et al. [124]4071Magnesium–aluminium spinel(MgAl2O4)/EG
n not mentioned.
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Table 2Summary of magneto-rheological behaviour of different nanofluids.
Fe3O4/deionized water Low 0.7, 1.6, 2.0, 0.6, 0.5, High 37.3,33.1, 30.8, 28 wt%
1.5�107–4.5�107 All suspensions followed shear thinning behaviour. Among low concentrationsuspensions, i.e., sample 1–5, the sample with 0.5 wt% possessed the highestviscosity while among higher concentration samples (6–9), the suspension with37.3 wt% exhibited the highest viscosity.
coated Fe3O4)30 wt% 10 0–100 Suspension showed slight departure from Newtonian behaviour without magnetic
field. Under magnetic field, the suspension exhibited Bingham plastic behaviour.Wei et al. [127]
Fe2O3/deionized water 0.01, 0.02, 0.03, 0.04 20–40 13.2–264 Suspension having 0.2 wt% PEO behaved as Newtonian fluid when ϕ was less than0.02 and exhibited shear thinning behaviour for higher nanoparticle concentration.The same behaviour was noticed with 0.2 wt% PEO. However, the latter suspensionswitched to shear thinning behaviour at ϕ as low as 0.02.
Phuoc and Massoudi etal. [128]
Fe2O3/EG 6.6 29718 1–1000 Suspensions showed shear thinning behaviour and also thixotropy. Gallego et al. [129]Fe2O3/glycerol 0.25–0.8 26 0.01–264 Nanofluids exhibited shear thinning behaviour. Viscosity increased with con-
centration of nanoparticles and decreased with temperature rise.Abareshi et al. [130]
Magnetite/transformer oil 0.8–21 6–7 1–1000 Except for the most concentrated suspension (20.8%), all the suspensions showedNewtonian behaviour. Viscosity increased with particle loading.
Resiga et al. [131]
Fe–Ni/paraffin oil 2–12 wt% (optimum 10 wt%) o15 1–100 Without magnetic field, the nanofluid with 10 wt% loading possessed pseudoplasticnature. As magnetic field increased, the fluid behaved as Bingham plastic fluid.
Katiyar et al. [132]
Fe3O4/polyethylene glycol (PEG) 0.48, 1.0, 3.05, 3.6 30 0.01–1000 All suspensions showed shear thinning behaviour. Moattar and Cagincara[133]
γ-Fe2O3/iso-butanol 0.05–0.6 o10 1–1000 Iso-butanol-based nanofluid showed Newtonian behaviour while MEK-basednanofluid exhibited shear thinning behaviour. The magnetic field did not affect therheological flow behaviour of iso-butanol-based nanofluid but an increase inmagnetic induction to the MEK-based nanofluid changed its flow behaviour fromNewtonian to strongly non-Newtonian.
Vekas et al. [134]γ-Fe2O3/methyl-ethyl-ketone(MEK)
Fe3O4/transfer oil(TR30) 1.8, 2.0, 3.6, 6.4 5.94– 6.8 * At low magnetic field value, a slight increment and at 0.04 T a strong increment inviscosity was observed. Viscosity increased further, but, at about 0.07 T, the visc-osity decreased. Further on, the viscosity decreased even with the increase inmagnetic field.
Vekas et al. [135]
* not mentioned.
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of any nanofluid, whether varying or constant with shear rate.Also, the non-Newtonian fluid behaviour and shear rate-dependent viscosity of nanofluid require careful consideration inits processing and application.
A significant amount of research work has already been com-piled and reviewed on heat transfer and viscosity of nanofluids.However, very few studies on rheological behaviour of nanofluidshave been published. In this paper, the authors attempt to reviewimportant research works available on rheological behaviour ofnanofluids at different shear rate ranges and particle volumeconcentrations. A brief discussion on the effect of surfactants andmagnetic field on rheological behaviour of nanofluids has alsobeen included here.
2. Rheological behaviour of nanofluids
The rheological flow behaviour of any fluid is explained interms of the relationship between shear stress (τ) and shear rate(γ). The shear stress is defined as the tangential force applied perunit area and the shear rate is stated as the change of shear strainper unit time. The ratio of shear stress to shear rate is known asviscosity (η), which can also be defined as a measure of resistanceoffered by the adjacent layers to one another during the flow ofliquid suspension. The fluid behaviour can be categorised asNewtonian and non-Newtonian (pseudoplastic, Bingham plastic,Bingham and Dilatant). For Newtonian behaviour, the viscosityremains constant with shear rate and the stress exhibits linearrelation with shear rate while for non-Newtonian behaviour, theviscosity may vary with shear rate and correlation between stressand shear rate follows Bingham plastic behaviour.
2.1. Newtonian and non-Newtonian behaviour of nanofluids
Rheological behaviour of nanofluids affects pressure drop ofnanofluids. Additionally it gives an idea of nanoparticle structur-ing, which can be helpful in predicting the thermal conductivity ofnanofluids. The rheological behaviour can be measured by rhe-ometers [64–68,70,72]. Some researchers [62–63,69,71] have usedviscometers to measure the viscosity. Nowadays viscometers areconsidered inadequate as they are not capable to read the featureof shear dependence, especially for low viscosity liquid-basednanofluids containing non-spherical particles. Richmond et al.[62] have found that mixing of TiO2 in SiO2/water nanofluidchanges its flow behaviour from Newtonian to non-Newtonian.One can conclude that except for the behaviour as observed byPenkavova et al. [71], the TiO2/water nanofluid shows shear thin-ning behaviour [63,64,67–68]. The TiO2/EG nanofluid exhibitsNewtonian behaviour even for high shear rate. MWCNT nanofluidshowed Newtonian and non-Newtonian behaviour both, depend-ing upon the type of base fluid. A clear shear thinning behaviourwas reported by MWCNT mixed in water, oil, resin and EG [74–80,82–84] except in the case of MWCNT/EG [81]. The glycerolbased and silicone oil based fluids behave Newtonian manner inall studied MWCNT volume fractions and temperatures [138].Nanofluids containing MWCNT with high volumetric concentra-tion show non-Newtonian behaviour, while with lower con-centration, nanofluids exhibit Newtonian behaviour [74,75]. Allthe suspensions containing SiO2 nanoparticles show Newtonianbehaviour [85–90]. Al2O3/water nanofluid show non-Newtonianbehaviour [91,92] while Al2O3/EG and Al2O3/PG behave as New-tonian fluid [93–95]. Water-based nanofluid containing micro-sized Al2O3 particle exhibits shear thinning behaviour. The rheo-logical behaviour of various nanofluids enriched with nano-particles, such as, CuO, BaTiO3, Ni, Al, Ag, graphite, grapheme,
CaCO3, TNT, Gold, Carbon black powder and Yttrium oxide hasbeen systematically summarized and analysed in Table 1.
2.2. Rheological behaviour of ferrofluids
Ferrofluids are suspensions of ferromagnetic and ferromagneticnanoparticles (Fe3O4, Fe2O3, Fe, γ-Fe2O3 etc.) in polar and non-polar carrier liquids. The ferrofluids are better known for theirapparent viscosity, capable of experiencing a rapid reversal uponapplication of external magnetic field. The rheological behaviour ofdifferent ferrofluids has been summarized in Table 2. Hong et al.[125,126] observed that Fe3O4/water nanofluid showed Newtonianbehaviour for low particle concentration but exhibited shearthickening behaviour followed by shear thinning behaviour forhigher concentration. Fe2O3 mixed in EG [129] and glycerol [130]showed shear thinning behaviour while magnetite in transformeroil exhibited Newtonian behaviour. Under the influence of externalmagnetic field, Fe3O4–TiO2/silicone oil [127] and Fe–Ni/paraffin oilnanofluids [132] showed a departure from the Newtonian toBingham plastic behaviour. Vekas et al. [135] observed an incre-ment in the viscosity with the increase of magnetic field but upto acertain value. After that, the viscosity decreased further even withan increase of magnetic field.
2.3. Effect of surfactants on rheological behaviour of nanofluids
Surfactants are often used in the preparation of stable nano-fluids. Stabilization of the nanofluids is generally considered vitalin achieving uniform particle packing structure throughout thesuspension. The influence of mixing surfactant on the rheologicalbehaviour of nanofluid is summarized and analysed in Table 3.Phuoc et al. [80] and Wang et al. [83] observed that addition ofsurfactant in MWCNT/water nanofluid changed its flow behaviourfrom Newtonian to non-Newtonian. The mixing of NH4PA as sur-factant in BaTiO3/distilled water nanofluid [104] changed its flowbehaviour from Newtonian to dilatant for higher shear rate values.The addition of surfactant changed the rheological behaviour ofBaTiO3/ethanol isopropanol from pseudoplastic to dilatant [105]even though it did not affect the flow behaviour of Ni/terpineolnanofluid [106].
3. Conclusions
This review work has focussed on the rheological behaviour ofnanofluids and ferrofluids under wide range of shear rate andnanoparticle volumetric concentration. It has studied importantliterature available on rheological behaviour of nanofluids. It hasalso attempted a brief analysis of the influence of magnetic fieldand surfactant on rheological behaviour of nanofluid. The effect ofdifferent base fluids, shear rate range, nanoparticle concentrationand nanoparticle shape has also been compiled and analysed. Thefollowing conclusions are drawn from this study:
� Most of the nanofluids containing low nanoparticle concentra-tion behave as Newtonian fluid and nanofluid with high con-centration exhibits non-Newtonian behaviour as shown inTable 4.
� Nanofluids show Newtonian behaviour at low shear rate andexhibit non-Newtonian behaviour at high shear rates as shownin Table 4.
� Spherical nanoparticles are more likely to exhibit Newtonianbehaviour while nanoparticles having tubular and tetragonalshapes show non-Newtonian behaviour as shown in Table 4.
Table 3Summary of effect of dispersant on rheological behaviour of nanofluids.
TiO2/water Poly ethylene glycol (PEG) 2000 The addition of PEG 2000 first reduced the viscosity to a minimumvalue (about 50–60% of the viscosity of nanofluid without PEG). Onfurther addition of PEG 2000, the viscosity increased steadily.
Alphonse et al. [70]
MWCNT/poly α-olefin(PAO6) oil
Polyisobutene succinimide (PIBSI) (0.3–8 wt%)
The suspensions with the lowest (0.3%) and the highest (8%) dis-persant concentrations reported strongly thinning behaviour whilethe suspension with 3 wt% dispersant followed Newtonian behaviour.
Yang et al. [75]
MWCNT/poly α-olefin(PAO6) oil
PIBSI 1000 and PIBSI 500 3 wt% At 25 °C, the suspensions without dispersant displayed slightly shearthinning behaviour but at 75 °C dispersion showed very strong shearthinning behaviour. Suspension with PIBSI 1000 showed mild shearthinning behaviour, while by adding PIBSI 500, the suspension dis-played very strong thinning behaviour.
Yang et al. [77]
MWCNT-Al2O3/glycerol(10 wt%) and water
Poly-acrylic acid (PAA) On adding PAA in suspension, it was absorbed on particle surface. Thesubstantial colloidal interactions were observed when the nano-particle loading was 435 vol% and CNT content was 41.3 vol%.
Lu [78]
MWCNT/deionized water 0.2 wt% Chitosan Suspension with low CNT concentration (o0.24 vol%) and 0.1–0.2 wt% chitosan behaved as Newtonian fluid while with high CNTconcentration and 0.1–0.2 wt% chitosan exhibited shear thinningbehaviour. Adding low wt% of chitosan (0.1, 0.2 wt%) in waterincreased its viscosity significantly while adding 0.5 wt% chitosan inwater decreased its viscosity and changed the flow behaviour ofsuspension toward non-Newtonian behaviour.
Phuoc et al. [80]
MWCNT/deionized water Sodium dodecyl benzene sulphonate(SDBS) and TritonX-100
Viscosity increased with the rise of concentration and decreased withthe rise of temperature. Surfactant produced better stability ofnanofluid.
Wang et al. [83]
BaTiO3/distilled water Ammonium polyacrylate (NH4PA) Adding NH4PA (2 wt%), the suspension appeared to be near New-tonian behaviour at a low shear rate (�100 s�1), while for highervalues of shear rate, the stress–shear rate curve deviated from line-arity and revealed dilatant flow behaviour.
Tseng and Li [104]
BaTiO3/ethanol–isopropanol
Polymeric dispersant (anionic and catio-nic) KD1,2,6,7 and PS-2
Addition of dispersant also changed the flow behaviour of suspensionfrom pseudoplastic to dilatant as shear rate surpassed �800 s�1. Theviscosity of suspension reduced to the lowest level by the addition ofa polymeric dispersant KD-6 (3 wt%).
Tseng and Lin [105]
Nickel/terpineol Polymeric dispersant (anionic and catio-nic) KD1,2,4,5,6,7 and PS-2
The addition of dispersant did not affect the flow behaviour. Also,addition of polymeric dispersant (KD-6 at 2 wt%) to suspensionreduced the viscosity to minimum, almost 60% of the suspensionhaving no dispersant.
Tseng and Chen [106]
Silver/BCA and CA in weightratio 5:1
Polymeric 910, 9250 and KD-6 All suspensions showed a shear thinning behaviour. Polymeric 9250surfactant alleviated the agglomeration of silver nanoparticles insuspension.
Chen et al. [111]
Graphite/oil CH-5 Addition of surfactant and graphite nanoparticles increased theviscosity of nanofluids, Viscosity decreased with an increase oftemperature.
Wang et al. [112]
ITO/deionized water Ammonium polyacrylate (NH4PA) Addition of 0.5–2 wt% NH4PA in the suspension reduced its viscosityapproximately by 99% as compared to original suspension. The sus-pension behaved as Bingham fluid at low shear rates and showedchanging nature toward shear thickening flow behaviour when shearrate exceeded a critical level.
Tseng and Tzeng [116]
Fe3O4/deionized water Oleate sodium, PEG-4000 Viscosity of suspension decreased with increasing PEG dosage. Theviscosity of suspension was the lowest for 2.9% oleate sodium and2.9% PEG (Maximum solid content of 2.0 wt%). The viscosity reachedthe maximum for suspension containing 2.8% oleate sodium and 4.7%PEG (least solid content of 0.5 wt%). The surfactants could not onlyhold the sedimentation and aggregation of nanoparticles but alsointroduced thixotropic effect.
Hong et al. [125]
Fe2O3/deionized water Polyvinylpyrrolidone (PVP) or Poly-ethylene oxide (PEO)
Suspension having 0.2 wt% PEO behaved as Newtonian fluid when ϕ
was less than 0.02 and exhibited shear thinning behaviour for highernanoparticle concentration. The same behaviour was noticed with0.2 wt% PEO. However, the latter suspension switched to shearthinning behaviour at ϕ as low as 0.02.
Phuoc and Massoudiet al. [128]
A.K. Sharma et al. / Renewable and Sustainable Energy Reviews 53 (2016) 779–791 787
� TiO2 mixed in viscous liquid like EG shows Newtonian beha-viour, while with low viscous liquid like water, it exhibits non-Newtonian behaviour.
� MWCNT nanofluid mostly exhibits shear thinning behaviour forlow shear rates. However, sometimes it shows Newtonianbehaviour at high shear rate range.
� SiO2 nanofluids, irrespective of base fluid, display Newtonianbehaviour over a wide range of shear rates.
� Al2O3 nanofluids show a transition from shear thinning beha-viour to shear thickening as shear rate exceeds certain critical
level. This critical value increases with an increase in nano-particle concentration.
� CuO nanofluid exhibits almost Newtonian behaviour. However,with xanthan gum, it shows shear thinning behaviour.
� Nanofluids with highly viscous base fluid, such as, EG and PG,are more likely to possess Newtonian behaviour than thosemade of low viscous fluid like water. The conclusion is con-sistent with the results observed in the study by Chen et al. [57].
� Addition of surfactant in nanofluid increases its viscosity andmay also change its flow behaviour to dilatant.
Table 4Summary of nanoparticle type, shape, size and rheological behaviour.
SiO2, TiO2/deionized water 0.16–1.73 mm Prismatic irregular shape SiO2 Newtonian Richmond et al. [62]SiO2/TiO2 Bingham plastic
TiO2/distilled water 20 Spherical Shear thinning He et al. [64]95
TiO2/EG 25 Spherical Newtonian Chen et al. [65]Titania/EG 70–100 Spherical Newtonian Chen et al. [66]Titanate/distilled water 10 Tube Shear thinning Chen et al. [67]TiO2/water Dia¼10 Spherical and tube both Newtonian Chen et al. [68]TiO2/EG Length¼100 Non-NewtonianTNT/water Non-NewtonianTNT/EG Non-NewtonianTiO2/deionized water 21 Spherical Low conc. Newtonian while high conc.
Non-NewtonianTurgut et al. [69]
TiO2/water 5–6/80–90 Spherical Newtonian for low shear rate andshear thinning for high shear rate
Dia 20–40 Tube Shear thinning Wang et al. [79]Length¼5–15 mm
MWCNT/EG Tube Newtonian Meng et al. [81]MWCNT/EG 10–30 Tube Shear thinning Ruan and Jacobi [82]MWCNT/deionized water 20–30 Tube At high conc. Shear thinning but at low
conc. NewtonianWang et al. [83]
MWCNT/distilled water * Tube Shear thinning Ko et al. [84]SiO2/ethanol 35, 94, 190 Spherical Newtonian Chevalier et al. [85]SiO2/ethanol 10–100 Spherical Newtonian Chevalier et al. [87]Silica/distilled water 12 Spherical Low conc. Newtonian while high conc.
Shear thinningMondragon et al. [89]
SiO2/paraffinic mineral oil 20 Spherical Newtonian Anoop et al. [90]Al2O3/pure water 37 spherical Shear thinning Tseng and Wu [91]Al2O3/double distilled water 0.2 mm Spherical Non-Newtonian Tseng and Wu [92]Alumina/PG 27, 40, 50 Spherical Newtonian Prasher et al. [93]Al2O3/water 50 Spherical Newtonian Anoop et al. [94]Al2O3/EGCuO/EGAl2O3/EG 40–50 Spherical Newtonian Gallego et al. [95]Al2O3/water 30 nm Spherical Non-Newtonian for low shear rates
and Newtonian for high shear ratesAladag et al. [96]
CNT/water 9 mm TubeCuO/deionized water 30, 75 and 150 Spherical Pseudoplastic Chang et al. [97]CuO/EG 10–30 Rod like Shear thinning Kwak and Kim [98]CuO/EG and water mixture (60:40 by wt.) 29 Spherical Newtonian Namburu et al. [99]CuO/PG and water (60:40) o50 Spherical Newtonian Naik et al. [100]CuO/water 23–37 Spherical Newtonian Pastoriza et al. [101]
1173CuO/oil (SN-500) 50 Spherical Newtonian Saeedinia et al. [102]BaTiO3/distilled water 0.8 mm Spherical Non-Newtonian Tseng and Li [104]BaTiO3/ethanol–isopropanol 0.58 mm Spherical Shear thinning Tseng and Lin [105]Nickel/terpineol 0.3 mm Spherical Shear thinning Tseng and Chen [106]Nickel/α-terpineol 90 Spherical Shear thinning Tseng and Chen [107]Aluminium/HTPB, PPG and PSi 120 Spherical Shear thinning Mary et al. [108]Silver/DEG 40 Spherical Pseudoplastic Tamjid and Guenther
[110]Silver/BCA and CA in weight ratio 5:1 30–50 Spherical Shear thinning Chen et al. [111]Graphite/oil 10–30 Not spherical but complex
crystalline shapeShear thinning Wang et al. [112]
Graphite/deionized water 3–4 Not Spherical but complexshape
Shear thinning Duan et al. [113]
Graphene/glycerol 15–50 Platelets Non-Newtonian for high shear rateand Newtonian for low shear rates
Moghaddam et al.[114]
GNP/distilled water Thickness-2 nm Platelets Shear thinning Mehrali et al. [115]Dia-2 mm
ITO/deionized water 60 Spherical Newtonian but Bingham plastic forvery high shear rates
Tseng and Tzeng [116]
(TNT)/EG 10 Rod like Shear thinning Chen et al. [117]ZnO/EG 10–20 Spherical Low conc. Newtonian while high conc.
Shear thinningYu et al. [118]
TNT/EG Dia¼10 Tube Shear thinning Chen et al. [119]
A.K. Sharma et al. / Renewable and Sustainable Energy Reviews 53 (2016) 779–791788
Length¼100CaCO3/distilled water 20–50 Spherical Newtonian Tao et al. [120]MgO/EG 20 Spherical Newtonian Xie et al. [121]Gold/water 10, 20, 50 Spherical Newtonian Abdelhalim et al. [122]Carbon black powder (N115)/EG 20 Spherical Shear thinning Meng et al. [123]Fe3O4/deionized water �10 Newtonian for low conc. and Shear
thinning for high conc.Hong et al. [126]
Fe2O3/EG 29718 Spherical Shear thinning Gallego et al. [129]α-Fe2O3/glycerol 26 Spherical Shear thinning Abareshi et al. [130]
* not mentioned.
A.K. Sharma et al. / Renewable and Sustainable Energy Reviews 53 (2016) 779–791 789
� With the application of external magnetic field, nanofluidshows a deviation in rheological behaviour from Newtonian tonon-Newtonian.
4. Recommendations for future work
Researchers so far have given more attention to nanofluidscontaining a single nanoparticle while few studies have beencarried out on hybrid nanofluids. Therefore, further investigationscan be attempted focussing on combinations of different nano-particles (i.e., hybrid nanoparticles). The authors have investigatedthe influence of a few parameters, such as, nanoparticle shape,size, volumetric concentration and shear rate range on the rheo-logical behaviour of nanofluids and have observed some incon-sistencies regarding the shape and size of particles. A fewresearchers have found that while nanofluids containing sphericalnanoparticles can exhibit both types of behaviour (i.e., Newtonianand non-Newtonian), nanofluids with tubular-shape particlesexhibit non-Newtonian behaviour. These findings can further beadvanced by unfolding the influence of nanoparticle shape andsize on rheological behaviour with more quality work in future.The optimization of the above mentioned parameters can also becarried out for various nanofluids which may be helpful in syn-thesizing a new class of nanofluids with better rheological prop-erties. Sidik et al. [136] reviewed literature on challenges ofnanofluids and observed that it is unavoidable to prevent sedi-mentation of particles without using surfactants. For betterunderstanding of the influence of surfactants in nanofluids, it isnecessary to carry on quality investigations further.
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