A review of experimental investigations on thermal phenomena in nanofluids
 Shijo Thomas^{1} and
 Choondal Balakrishna Panicker Sobhan^{1}Email author
DOI: 10.1186/1556276X6377
© Thomas and Balakrishna Panicker Sobhan; licensee Springer. 2011
Received: 13 January 2011
Accepted: 9 May 2011
Published: 9 May 2011
Abstract
Nanoparticle suspensions (nanofluids) have been recommended as a promising option for various engineering applications, due to the observed enhancement of thermophysical properties and improvement in the effectiveness of thermal phenomena. A number of investigations have been reported in the recent past, in order to quantify the thermofluidic behavior of nanofluids. This review is focused on examining and comparing the measurements of convective heat transfer and phase change in nanofluids, with an emphasis on the experimental techniques employed to measure the effective thermal conductivity, as well as to characterize the thermal performance of systems involving nanofluids.
Introduction
The modern trends in process intensification and device miniaturization have resulted in the quest for effective heat dissipation methods from microelectronic systems and packages, owing to the increased fluxes and the stringent limits in operating temperatures. Conventional methods of heat removal have been found rather inadequate to deal with such high intensities of heat fluxes. A number of studies have been reported in the recent past, on the heat transfer characteristics of suspensions of particulate solids in liquids, which are expected to be cooling fluids of enhanced capabilities, due to the much higher thermal conductivities of the suspended solid particles, compared to the base liquids. However, most of the earlier studies were focused on suspensions of millimeter or micron sized particles, which, although showed some enhancement in the cooling behavior, also exhibited problems such as sedimentation and clogging. The gravity of these problems has been more significant in systems using mini or microchannels.
A much more recent introduction into the domain of enhancedproperty cooling fluids has been that of nanoparticle suspensions or nanofluids. Advances in nanotechnology have made it possible to synthesize particles in the size range of a few nanometers. These particles when suspended in common heat transfer fluids, produce the new category of fluids termed nanofluids. The observed advantages of nanofluids over heat transfer fluids with micron sized particles include better stability and lower penalty on pressure drop, along with reduced pipe wall abrasion, on top of higher effective thermal conductivity.
It has been observed by various investigators that the suspension of nanoparticles in base fluids show anomalous enhancements in various thermophysical properties, which become increasingly helpful in making their use as cooling fluids more effective [1–4]. While the reasons for the anomalous enhancements in the effective properties of the suspensions have been under investigation using fundamental theoretical models such as molecular dynamics simulations [5, 6], the practical application of nanofluids for developing cooling solutions, especially in miniature domains have already been undertaken extensively and effectively [7, 8]. Quantitative analysis of the heat transfer capabilities of nanofluids based on experimental methods has been a topic of current interest. The present article attempts to review the various experimental techniques used to quantify the thermal conductivity, as well as to investigate and characterize thermal phenomena in nanofluids. Different measurement techniques for thermal conductivity are reviewed, and extensive discussions are presented on the characterization of thermal phenomena such as forced and free convection heat transfer, circulation in liquid loops, boiling and two phase flow in nanofluids, in the sections to follow.
Thermal conductivity
The techniques employed for measurement of thermal conductivity can be broadly classified into transient and steady state methods. The transient measurement techniques frequently used are the hot wire method, the hot strip method, the temperature oscillation method and the 3ω method. Steadystate measurement using a 'cutbar apparatus' has also been reported. These methods are reviewed below.
The short hot wire (SHW) method
The transient short hot wire (SHW) method used to measure the thermal conductivity and thermal diffusivity of nanofluids has been described by Xie et al. [9, 10]. The technique is based on the comparison of experimental data with a numerical solution of the twodimensional transient heat conduction applied to a short wire with the same lengthtodiameter ratio and boundary conditions as in the experimental setup.
In the calculation method, the dimensionless volumeaveraged temperature rise of the hot wire, θ_{v} [= (T_{v}  T_{i})/(q_{v}r^{2}/λ)] is approximated by a linear equation in terms of the logarithm of the Fourier number Fo [=αt/r^{2}], where T_{i} and T_{v} are the initial liquid temperature and volume averaged hotwire temperature, q_{v} the heat rate generated per unit volume, r the radius of the SHW, t is the time, and λ and α the thermal conductivity and the thermal diffusivity of liquid, respectively. The coefficients of the linear equation, A and B, are determined by the least squares method for a range of Fourier numbers corresponding to the measuring period. The measured temperature rise of the wire ΔT_{v} [=T_{v}  T_{i}] is also approximated by a linear equation with coefficients a and b, determined by the least square method for the time range before onset of natural convection. Thermal conductivity (λ) and thermal diffusivity (α) of nanofluids are obtained as λ = (VI/πl)(A/a) and α = r^{2} exp[(b/a)  (B/A)], where l is the length of the hotwire, and V and I are the voltage and current supplied to the wire. The uncertainties of the thermal conductivity and thermal diffusivity measurements using SHW have been estimated to be within 1.0 and 5.0%, respectively.
Temperature oscillation technique
where G is the phase shift, u amplitude in Kelvin, and L thickness of fluid sample in meter.
The phase and amplitude of temperature oscillation at the two surfaces as well as at the central point C, gives the thermal diffusivity of the fluid, from Equations 1 or 2.
The temperature oscillation in the reference layer at the two boundaries of the test fluid yields the thermal conductivity. The frequency of temperature oscillation in the reference layer, in the Peltier element and that in the test fluid are the same.
where λ is the thermal conductivity of the fluid.
The thermal diffusivity of the reference layer being known either from Equations 7 or 8, the thermal conductivity of the specimen can be evaluated from Equation 6.
3ω method
where Q' is the heating power per unit length generated at the metal heater, k the thermal conductivity of the substrate, q the complex thermal wave number, ω the angular frequency of the input current, and ρ and C_{p} the substrate density and heat capacity, respectively.
ΔT_{sub} is the heater temperature oscillation due to the heat transfer in the quartz substrate alone (measured in vacuum). The nanofluid thermal conductivity k_{nf} is obtained from a least squares fit of ΔT_{nf} calculated from Equation 10.
Microlitre hot strip devices for thermal characterization of nanofluids
where T_{o} is the intercept on the temperature axis of the T vs. ln(t) graph. The thermal diffusivity, α_{f} depends on the thermal conductivity k, the density, and the specific heat capacity of the fluid. As a firstorder approximation, it is possible to obtain the thermal conductivity from the measurement of α_{f}.
Steady state measurement using cutbar apparatus
where q is the heat flux, k_{copper} the thermal conductivity of copper bars, ΔT_{bar} the temperature difference along the copper bars, and ΔZ_{bar} the distance along the copper bars.
where k_{eff} is the effective thermal conductivity of the nanofluid, q the heat flux, ΔT_{cell} the average temperature difference between the two surfaces of the test cell, ΔZ_{cell} the distance between the two cell surfaces, k_{Oring} the thermal conductivity of the rubber Oring, A_{Oring} the crosssectional area of the rubber Oring, and A_{cell} the crosssectional area of the test cell. Baseline experiments using ethylene glycol and distilled water showed an accuracy of measurement within +/2.5%.
Comparison of thermal conductivity results
Thermal conductivity values
Sl.no.  Base fluid  Nanoparticle  Avg particle size (nm)  Conc. (vol.%)  Sonication time (h)  Temp. (°C)  Enhancement  Method of measurement  Uncertainty % 

1  Distilled water  Al_{2}O_{3}  36  10  3  27.534.7  1.3 times  Steady state  2.5 
2  Distilled water  CuO  29  6  3  34  1.52 times  Steady state  2.5 
3  Distilled water  Al_{2}O_{3}  28.6  1  12  2151  210.8%  Temperature oscillation  2.7 
4  Distilled water  Al_{2}O_{3}  28.6  4  12  2151  9.424.3%  Temperature oscillation  2.7 
5  Distilled water  CuO  38.4  1  12  2151  6.529%  Temperature oscillation  2.7 
6  Distilled water  CuO  38.4  1  12  2151  1436%  Temperature oscillation  2.7 
7  Distilled water  Al_{2}O_{3}  20  1  NA  550  10%  SHW method  1 
8  Distilled water  Al_{2}O_{3}  45  1  15  NA  4.4%  3ω method  NA 
Viscosity
Viscosity, like thermal conductivity, influences the heat transfer behaviour of cooling fluids. Nanofluids are preferred as cooling fluids because of their improved heat removal capabilities. Since most of the cooling methods used involve forced circulation of the coolant, modification of properties of fluids which can result in an increased pumping power requirement could be critical. Hence, viscosity of the nanofluid, which influences the pumping power requirements in circulating loops, requires a close examination. Investigations [3, 4, 15–22] reported in the literature have shown that the viscosity of base fluids increases with the addition of nanoparticles.
Praveen et al. [15] measured the viscosity of copper oxide nanoparticles dispersed in ethylene glycol and water. An LV DVII+ Brookfield programmable viscometer was used for the viscosity measurement. The copper oxide nanoparticles with an average diameter of 29 nm and a particle density of 6.3 g/cc were dispersed in a 60:40 (by weight) ethylene glycol and water mixture, to prepare nanofluids with different volume concentrations (1, 2, 3, 4, 5, and 6.12%). The viscosity measurements were carried out in the temperature range of 35 to 50°C. The variation of the shear stress with shear strain was found to be linear for a 6.12% concentration of the nanofluid at 35°C, which confirmed that the fluid has a Newtonian behavior. At all concentrations, the viscosity value was found to be decreasing with an increase in the temperature and a decrease in concentration of the nanoparticles. The suspension with 6.12% concentration gave an absolute viscosity of around 420 centiPoise at 35°C.
Viscosity values
Sl.no.  Reference  Nanoparticle used  Basefluid  Concentration  Temp range  Percentage enhancement in viscosity 

1  Praveen et al. [15]  CuO (29 nm)  60:40 (in weight) ethylene glycol and water mixture  1, 2, 3, 4, 5, 6.12%  35 to 50°C  For 6.12% conc: 4.5 times @ 35°C and 3 times @ 50°C 
2  CuO (29 nm) Al_{2}O_{3}(36 and 47 nm)  Water  112%  22 to 75°C  CuO @ 9%: 710 times Al_{2}O_{3}(36 nm) @ 9%: 4.53.5 times Al_{2}O_{3}(47 nm) @ 9%: 5.44.4 times  
3  Chen et al. [17]  Titanate nanotubes (diameter approx. 10 nm, length approx. 100 nm, aspect ratio approx. 10)  Ethylene glycol  0.5, 1.0, 2.0, 4.0, and 8.0% by weight  2060°C  @ 8%: High shear viscosity is in the range of 1035 m Pa s 
4  Phuoc et al. [18]  Fe_{2}O_{3} (2040 nm)  Deionized water containing 0.2% polymer by weight as a dispersant.  1, 2, 3, 4%  25°C  @ 2%: Infinite viscosity is 12.25 cP for 0.2% PEO (Polyethylene oxide) surfactant, and 2.58 cP for 0.2% PVP (Polyvinylpyrrolidone) surfactant 
5  Garg et al. [19]  MWCNT (multiwalled carbon nanotube) (diameter of 1020 nm, length of 0.540 μm)  Deionized water with 0.25% by mass of gum Arabic  1% by mass  15 and 30°C  Viscosity of nanofluids increases with sonication time. Beyond a critical sonication time it decreases due to increased breakage of CNTs 
6  Murshed et al. [20]  TiO_{2} (15 nm)/Al_{2}O_{3} (80 nm)  Deionized water with Cetyl Trimethyl Ammonium Bromide (CTAB) surfactant (0.1 mM)  15% by volume    @ 5% of Al_{2}O_{3} viscosity increases by 82% @ 4% of TiO_{2} viscosity increases by 82% 
7  Chena et al. [21]  TiO_{2} (25 nm) and TNT (Titanate nanotubes) (diameter approx. 10 nm, length approx. 100 nm, aspect ratio approx. 10)  Water, ethylene glycol  0.11.8% by volume    @ 0.6% of waterTNT 80% increase in viscosity @ 1.8% EGTNT 70% increase in viscosity @1.8% EGTiO_{2} 20% increase in viscosity 
8  Duangthongsuk et al. [4]  TiO_{2} (21 nm)  Water  0.2, 0.6, 1.0, 1.5, and 2.0% with pH values of 7.5, 7.1, 7.0, 6.8, and 6.5,  15, 25 and 30°C  @ 15°C for the conc. range of 0.22% viscosity increases by 415%. 
9  Lee et al. [22]  Al_{2}O_{3} (30 ± 5 nm)  Deionized water (DI)  0.010.3 vol.%  2139°C  @ 21°C for the conc. range of 0.010.3% viscosity is enhanced by 0.082.9% 
Forced convection in nanofluids
Forced convection heat transfer is one of the most widely investigated thermal phenomena in nanofluids [23–35], relevant to a number of engineering applications. Due to the observed improvement in the thermal conductivity, nanofluids are expected to provide enhanced convective heat transfer coefficients in convection. However, as the suspension of nanoparticles in the base fluids affect the thermophysical properties other than thermal conductivity also, such as the viscosity and the thermal capacity, quantification of the influence of nanoparticles on the heat transfer performance is essentially required. As the physical mechanisms by which the flow is set up in forced convection and natural convection are different, it is also required to investigate into the two scenarios individually. The case of the natural convection (thermosyphon) loops is another problem in itself, because the characteristic of the flow is similar to that of the forced convection loop, though the mechanism is buoyancy drive. Some of the important investigations on forced convection in nanofluids are reviewed in this section. Studies on free convection and thermosyphon loops will be discussed in the sections to follow.
Convective heat transfer in fully developed laminar flow
where P, , and C_{p} are the surface perimeter, the mass flow rate, and the heat capacity, respectively.
Convective heat transfer under constant walltemperature condition
where (T_{w}  T_{b})_{LM} is the logarithmic mean temperature difference, A, D, and L crosssectional area, diameter, and heated length of the pipe and is the average flow velocity. The uncertainties of the calculated heat transfer coefficient, pressure drop, Peclet number, Nusselt number, and Reynolds number were 3, 3, 3, 4, and 2.5%, respectively. The convective heat transfer coefficient was measured for nanofluids in the laminar flow regime at constant wall temperature condition, for the volume concentration in the range of 0.2 to 2.5%. The experimental results were compared with the SiederTate correlation. Addition of nanoparticles showed a deviation from the values obtained by the correlation, which was particularly significant at higher values of the Peclet number. Typically, at a Peclet number of 6000, the heat transfer coefficient was found to be enhanced by 1.16 times for 0.2% concentration and 1.41 times for 2.5% concentration.
Convective heat transfer in thermally developing region
Singlephase and twophase heat transfer in microchannels
Convective heat transfer in confined laminar radial flows
where T_{b,r} and T_{b,i} are the bulk temperatures at a given radius and at the inlet.
Considering all the uncertainties on experimental measurements, the average relative errors on Nusselt number calculations were estimated as 12.1, 11.5, and 11% for cases with particle volume concentrations of 2, 4, and 6%, respectively. The experiments were aimed at investigating the effect of nanofluids in a steady laminar flow between the disk and a flat plate, with axial entry and radial exit. The heat transfer coefficient was found to increase with the particle concentration and the flow rate and decrease with an increasing gap between disks.
Summary
Convective heat transfer coefficient and frictional effects
Sl. no.  Reference  Nanoparticle  Base fluid  Flow regime  Wall boumdary condition  Concentration  Enhancement in heat transfer coefficient  Pressure drop/friction factor 

1  Hwang et al. [23]  Al_{2}O_{3} (30 ± 5 nm)  Water  Fully developed laminar flow with  Constant heat flux  0.010.3 vol.%  @ Re = 700 for 0.3%, heat transfer coeff., h increases by 8%  Friction factor follows f = 64/Re_{D} 
2  Heris et al. [24]  Al_{2}O_{3}  Water  Laminar, Re:7002050  Constant wall temp.  0.2, 0.5, 1.0, 1.5, 2.0, 2.5% volume  @ Peclet no., Pe = 6000 for 2.5%, h increases by 41%  ΔP = 200 Pa/m @ Re = 700 ΔP = 700 Pa/m @ Re = 2000 
3  Anoop et al. [25]  Al_{2}O_{3} (45 and 150 nm)  Water  Laminar thermally developing flow  Constant heat flux  1, 2, 4, and 6 wt%  @ x/D = 147, Re = 1550 and 4%, for 45 nm h increases by 25% and for 150 nm h increases by 11%   
4  Lee et al. [26]  Al_{2}O_{3} (36 nm)  Water  Laminar flow in microchannels, Re_{Dh} = 140941  Constant heat flux  1, 2% by volume  @ Q = 300 W, Re = 800 for 2%, h increases by 17%  @ Re = 800 ΔP = 21000 Pa for 2 vol.% ΔP = 15000 Pa for water. 
5  Gherasim et al. [27]  Al_{2}O_{3} (47 nm)  Water  Laminar radial flow  Constant heat flux  2, 4, and 6% by volume  @q" = 3900 W/m^{2}, disk spacing of 2 mm and Re = 500 for 4%, heat transfer is doubled   
6  Kim et al. [28]  Al_{2}O_{3} (2050 nm), amorphous carbonic nanofluids (20 nm)  Water  Laminar and turbulent flows  Constant heat flux  Amorphous carbonic nanofluids @3.5 vol.%, Al_{2}O_{3} nanofluids @3 vol.%.  @x/D = 50, Re = 1460 for 3% Al_{2}O_{3}, h increases by 25% @x/D = 50, Re = 6020 for 3% Al_{2}O_{3}, h increases by 15%   
7  Heris et al. [29]  CuO (5060 nm), Al_{2}O_{3} (20 nm)  Water  Laminar flows  Constant wall temp.  0.23 vol.%  @Pe = 6500 for 3% Al_{2}O_{3} Nu = 8.5 @Pe = 6500 for 3% CuO Nu = 8   
8  Jung et al. [30]  Al_{2}O_{3} (170 nm)  Water, WaterEthylene glycol 50:50  Laminar flow in rectangular microchannel  Constant heat flux  0.6, 1.2, 1.8% by volume  @x/D = 0, Re = 284 for 1.8% in water, h increases by 40%. @x/D = 0, Re = 32 for 1.8% in waterEG, h increases by 14%.  Friction factors comparable with that of water 
9  Ding et al. [31]  Titanate (20 nm), CNT, titanate nanotubes (d = 10 nm and l = 100 nm), nano diamond (250 nm)  Water  Thermally developing laminar and turbulent flow  Constant heat flux  04 vol.%  Heat transfer deteriorates for ethylene glycolbased titania and aqueousbased nanodiamond nanofluids. WaterCNT nanofluids give max enhancement  
10  Sharma et al. [32]  Al_{2}O_{3} (47 nm)  Water  Hydrodynamically and thermally developed Transition flow.  Constant heat flux  0.02, 0.1% by volume  For 0.1% in the range of Re = 35008000 heat transfer enhanced by 1424%   
11  Duangthongsuk et al. [33]  TiO_{2} (21 nm)  Water  Turbulent flow, Re400017000  Double pipe counter flow heat exchanger  0.2 vol.%  h increases by 611% for the flow range of Re = 400017000  Pressure drop and friction factor of the nanofluid are close to those of water 
12  Ding et al. [34]  MWCNT  Water  Laminar flow  Cosntant heat flux  0.1, 0.25, and 0.5% by volume  @x/D = 150, Re = 1200 for 0.1% h increases by 150%   
13  Yu et al. [35]  SiC (170 nm)  Water  Re = 330013000  Constant heat flux  3.7 vol.%  @Re = 10000 h is enhanced by 60%  The pumping power penalty for SiCwater is lesser than for Al_{2}O_{3}water 
Almost all of the above investigations have shown that the performance of nanofluids in forced convection heat transfer is better than that of the base fluid. However, there have been studies which reported deterioration in convective heat transfer in ethylene glycol based titanate nanofluids [31]. It generally is noticed that the percentage enhancement in heat transfer is much more than the individual enhancement in thermal conductivity. This fact is often attributed to the effect of the disruption of the thermal boundary layer due to particle movement [25].
The enhancement of heat transfer capabilities of fluids results in accomplishing higher heat transfer rates without incorporating any modifications to existing heat exchangers. It also effectively leads to a reduction in the pumping power requirements in practical applications, as a lower flow rate will produce the required amount of heat transfer. These, in general makes the use of nanofluids for forced circulation loops attractive, leading to better performance and the resulting advantage in energy efficiency.
Natural convection loops using nanofluids
Many of the investigations on natural convection phenomena in nanofluids deal with stagnant columns of the liquid, and in these studies, a possibility of reduction of the heat transfer coefficient has been observed [36]. Some investigators have discussed on the reasons for this behavior, and have suggested that this may be due to the reduction in the gradients of temperature within the fluid, resulting from the enhancement of the fluid thermal conductivity. However, natural circulation loops present a different scenario compared to convection in liquid columns, as the circulation is developed due to thermosyphon effect. It is of interest to look into some of the investigations on natural circulation loops with nanofluids, and understand the heat transfer performance under the influence of the nanoparticles. A few important articles on this topic are reviewed below. Some investigations on natural convection in stagnant fluid columns and pool boiling heat transfer are also reviewed.
where T_{e} and T_{c} are average values of the temperatures measured by the thermocouples.
The basic mechanisms of heat transfer, in a gravityassisted thermosyphon, are nucleate pool boiling in the evaporator and filmwise condensation in the condenser section [14]. The boiling and condensation heat transfer rates are influenced by the thermophysical properties of the working fluid and the characteristic features of the solid substrate. Major limitations of the gravity assisted thermosyphon are the dryout limitation, counter current flow limitation (CCFL) or flooding, and the boiling limitation. It was noticed that if the filling ratio (FR) is more than 40%, dry out phenomenon is not observed and the maximum heat flux is limited by the CCFL/flooding or the boiling limitation (BL).
The thermal performance of the system was found to be deteriorating when nanofluids were used as working fluids. The deterioration was maximum with laponite and minimum for aluminum oxide suspended nanofluids. Increased thermal conductivity of the nanofluids showed no effect on the nucleate pool boiling heat transfer coefficient. It was suggested that physical interaction of nanoparticles with the nucleating cavities has been influencing the boiling characteristics of the nanofluids. The deterioration of the thermal performance of the nanofluid in closed twophase thermosyphon was attributed to the improvement in wettability due to entrapment of nanoparticles in the grooves present on the surface. Improved critical heat flux values were also observed, which effect was also attributed to the increased wettability characteristics of nanofluids.
Natural convection heat transfer is a preferred mode as it is comparatively noise less and does not have pumping power requirement. The use of Al_{2}O_{3}/water nanofluids in closed twophase thermosyphon systems [37] has shown to increase its efficiency by 14.7% when compared to water. In rectangular loops [38] with waterbased nanofluids, the flow instabilities were found to decrease and the circulation rates improved, compared to the base fluid. At the same time, there have been observations [39] that in twophase thermosyphon loops, waterbased nanofluids with suspended metal oxides have inferior thermal performance compared to the base fluids, which was explained as due to the increased surface wettability of nanofluids.
Studies in stagnant columns
Experimental investigations have been reported on natural convection in stagnant columns, as well as pool boiling heat transfer in nanofluids. Measurement of critical heat flux (CHF) has also been reported in pool boiling.
Putra et al. [40] experimentally investigated the natural convection inside a horizontal cylinder heated from one side and cooled from the other. The effects of the particle concentration, the material of the particles, and the geometry of the containing cavity on natural convection were investigated. A systematic and definite deterioration of natural convection was observed and the deterioration increased with particle concentration. Copper oxide nanofluids showed larger deterioration than aluminum oxide nanofluids. With 4% Al_{2}O_{3} concentration, an L/D ratio of 1.5 showed a higher value of Nusselt number compared to an L/D ratio of 0.5.
Liu et al. [41] studied the boiling heat transfer characteristics of nanofluids in a flat heat pipe evaporator with a microgrooved heating surface. The nucleate boiling heat transfer coefficient and CHF of waterCuO nanofluids at different operating pressures and particle concentrations were measured. For a nanoparticle mass concentration less than 1%, the heat transfer coefficient and CHF were found to increase. Above 1% by weight, the CHF was almost constant and the heat transfer coefficient deteriorated. This was explained to be due to a decrease in the surface roughness and the solidliquid contact angle. Heat transfer coefficient and CHF of nanofluids were found to increase with a reduction in the pressure. At the atmospheric pressure, the heat transfer coefficient and CHF showed 25 and 50% enhancement, respectively, compared to 150 and 200% enhancement at a pressure of 7.4 kPa.
Boiling heat transfer on a hightemperature silver sphere immersed in TiO_{2} nanofluid was investigated by Lotfi et al. [42]. A 10 mm diameter silver sphere heated to 700°C was immersed in the nanofluid at 90°C to study the boiling heat transfer and quenching capabilities. Film boiling heat flux in the TiO_{2} nanofluid was found to be lower than that in water. The accumulation of nanoparticles at the liquidvapor interface was found to reduce the vapor removal rate from the film, creating a thick vapor film barrier which reduced the minimum film boiling heat flux. Experiments by Narayan et al. [43] showed that surface orientation has an influence on pool boiling heat transfer in nanoparticle suspensions. A smooth heated tube was suspended at different orientations in nanofluids to study the pool boiling performance. The pool boiling heat transfer was found to be maximum for the horizontal inclination. Al_{2}O_{3}water nanofluids of 47 nm particles and 1% by weight concentration showed enhancement in pool boiling heat transfer performance, over that of water. With increase in concentration and particle size, the performance decreased for nanofluids. For vertical and 45° inclination orientations, nanofluids showed inferior performance compared to pure water. Coursey and Kim [44] investigated the effect of surface wettability on the boiling performance of nanofluids. In the experiments, heater surfaces altered to varying degrees by oxidization or by depositing metal were investigated by measuring the surface energy measurements and boiling heat transfer (CHF). It was found that the CHF of poorly wetting systems could be improved by up to 37% by the use of nanofluids, while surfaces with good wetting characteristics showed less improvement.
Conclusion
Suspending nanoparticles in base fluids has proven to show considerable effects on various thermophysical properties, which influences the heat transfer performance. This article focused on some of the recently reported investigations on convective heat transfer and phase change in nanofluids. It also presented some discussions on the experimental techniques employed to measure the effective thermal conductivity, as well as to characterize the thermal performance of systems involving nanofluids.
The thermal conductivity of nanofluids has been measured using transient and steadystate methods, of which the transient hot wire method is found to be more versatile, accurate, and reliable. A review of the important investigations on forced convection heat transfer at various flow and heat transfer conditions have shown that the performance of nanofluids in forced convection is better than that of the base fluid. It has also been noticed that the percentage enhancement in heat transfer is much more than the individual enhancement in thermal conductivity.
The use of nanofluids in thermosyphon loops has shown an increase in the efficiency, a decrease in flow instabilities, and an increase in the flow rates. There have also been observations that in twophase thermosyphon loops, the increased wettability of nanofluids may adversely affect the thermal performance compared to that of the base fluid.
Investigation on the natural convection inside a horizontal cylinder heated from one side and cooled from the other has shown deterioration in heat transfer while nanofluids are used. At low nanoparticle mass concentrations, the CHF was found to increase in a flat heat pipe. In pool boiling heat transfer in nanoparticle suspensions, the orientation of the heater surface is found to have an influence on the heat transfer rate, the maximum being for horizontal orientation. It has been noticed that for poorly wetting surfaces, the CHF can be increased by the use of nanofluids.
Of the various applications proposed, the use of nanofluids in closed circulation loops for sensible heat removal is found to be the most attractive, and these can become part of steadystate heat exchange systems. The enhancement of the heat transfer capability of fluids with suspended nanoparticles makes their use in convection loops and thermosyphons an interesting option, leading to better system performance and the resulting advantage in energy efficiency.
Abbreviations
 BL:

boiling limitation
 CCFL:

counter current flow limitation
 CHF:

critical heat flux
 MEMS:

micro electromechanical systems
 PDMS:

polydimethylsiloxane
 PECVD:

plasma enhanced chemical vapor deposition
 POM:

polyoxymethylene
 SHW:

short hot wire
 THS:

transient hot strip
 TPCT:

twophase closed thermosyphon.
Declarations
Authors’ Affiliations
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