2012 JHT Ahn Kim Boiling Review

A Review on Critical Heat Flux

Enhancement With Nanofluids and

Surface Modification

Ho Seon AhnDepartment of Mechanical Engineering,

POSTECH, Pohang 790-784,

Republic of Korea

Moo Hwan Kim1

Division of Advanced Nuclear Engineering,

POSTECH, Pohang 790-784,

Republic of Korea

e-mail: [email protected]

Recently, there has been increasing interest in boiling nanofluidsand their applications. Among the many articles that have beenpublished, the critical heat flux (CHF) of nanofluids has drawnspecial attention because of its dramatic enhancement. This arti-cle includes recent studies on CHF increasing during the pastdecade by various researchers for both pool boiling and convec-tive flow boiling applications using nanofluids as the workingfluid. It presents a review of nanofluid critical heat flux researchwith the aim of identifying the reasons for its enhancement andthe limitations of nanofluid applications based on various pub-lished reports. In addition, further research required to make useof the CHF enhancement caused by nanofluids for practical appli-cations is discussed. Finally, the surface modification method withmicro/nanostructures to increase the CHF is introduced and rec-ommended as a useful way. [DOI: 10.1115/1.4005065]

Keywords: nanofluid, critical heat flux, nanostructure,microstructure

1 Introduction

Nanofluids are dilute liquid suspensions of nanoparticles withat least one of their principal dimensions at the nanoscale level.From previous investigations, nanofluids have been found to pos-sess enhanced thermo-physical properties, such as thermal con-ductivity, thermal diffusivity, viscosity, and convective heattransfer, compared to base fluids, such as oil or water [1–3]. Theseparticles can be metallic (Cu, Au), metal oxides (Al2O3, TiO2,SiO2, ZnO2), carbon (diamond, nanotubes), or other materials.The typical base fluid alone has a low thermal conductivity. Nanopar-ticles can be dispersed in the base fluid and remain suspended in thefluid to a much greater extent than was previously achieved withmicroparticles or larger-sized particles. Brownian motion of nanopar-ticles in the base fluid allows the nanoparticles to maintain their dis-persed state and to enhance the thermo-physical properties of the fluid.

There are several review articles concerning nanofluids: some con-cern their potential benefits for heat transfer applications [4–11] andsome are about their enhanced thermal conductivity [3,12].

Keblinski et al. [13] classified research on the thermal transportof nanofluids into three categories, based on the type of heat trans-fer mode:

• thermal conductivity• convective heat transfer• boiling heat transfer

Using a very small volume fraction of nanoparticles signifi-cantly enhances the thermal conductivity and convective heattransfer capabilities of the suspensions without encountering prob-lems found in common slurries, such as clogging, erosion, sedi-mentation, and increasing pressure drop [1,14]. However, thereare still many conflicting results for the boiling heat transfer ofnanofluids, with some groups postulating that the heat transferincreases during boiling [15] and others arguing that it does not[16]. This apparent contradiction can be explained as follows.During nanofluid boiling heat transfer, the nanoparticles depositedon the heating surface can increase the active cavities of nucleateboiling, thereby enhancing boiling heat transfer. However, whenenough particles are deposited, they can fill up the active cavitiesand reduce nucleate boiling, thereby degrading the boiling heattransfer.

Boiling heat transfer has a peak heat flux below which a boilingsurface can stay in the nucleate boiling regime. This is called thecritical heat flux (CHF), and it is the point beyond which there is atransition from a nucleate boiling regime to a film boiling regimeunder pool boiling. This transition is an undesirable phenomenon,causing an excessive increase in the temperature of the boilingsurface. Such an increase in temperature can exceed the meltingpoint of the construction material and lead to a crisis in variousthermal systems, such as boilers and fuel in a nuclear reactor.Thus, an enhanced CHF is very important to the safety margin ofa thermal system.

You et al. [20] measured the CHF during pool boiling on a flat,square Cu surface immersed in an Al2O3–water nanofluid andshowed an unprecedented three-fold increase in the CHF over thatof pure water. It is intriguing that nanoparticles at volume concen-trations as low as 3–10% can trigger such a dramatic increase inthe CHF. Since these results were reported, several researchershave tried to understand why the CHF increases in nanofluids.

This article offers a critical review of CHF enhancement causedby nanofluids. Therefore, the authors introduce the preparation ofnanofluids and the CHF enhancement in pool and flow boiling.Basically, the problems caused by nanofluids in real applicationsare discussed, and future research directions are proposed. In addi-tion, further research topics to overcome some problems of nano-fluids were introduced and some recommendations weresuggested.

2 Pool Boiling CHF Enhancement With Nanofluids

Since the concept of nanofluids was first postulated by Choi [1],researchers have studied the heat transfer characteristics of engi-neered nanofluids extensively. Choi stated that the enhanced ther-mal properties of nanofluids make them attractive for coolingapplications. The heat transfer process remains in the boiling re-gime when using nanofluids for cooling in high heat flux applica-tions. Because nanoparticles increase the thermal conductivity ofconventional fluids (Fig. 1), many researchers expected that nano-particles would also enhance boiling heat transfer. However, theresults presented by Das et al. [16] were somewhat contrary to theexpectations. The boiling curves of their nanofluids indicated thatthe boiling performance of the water deteriorated with the addi-tion of nanoparticles (i.e., the boiling curves were shifted to theright, as shown in Fig. 2). The shifts of the curves were propor-tional to the particle concentration and depended on the tuberoughness. The deterioration in heat transfer performance waslarger with smoother surfaces.

From the unmatched results with the prediction based on theincrease of thermal conductivity, there was a doubt about the ap-plicable possibility of nanofluids. In addition, even though thethermal conductivity increased, there were still some problemssuch as the increase of viscosity, the decrease of effective specificheat, and the variation of wettability [17–19].

Compared to the contradictory nucleate boiling results withnanofluids under pool boiling, more qualitatively consistent obser-vations have been reported on the enhancement of the critical heat

1Corresponding author.Contributed by the Heat Transfer Committee of ASME for publication in the

JOURNAL OF HEAT TRANSFER. Manuscript received March 28, 2011; final manuscriptreceived August 2, 2011; published online December 19, 2011. Assoc. Editor: SatishG. Kandlikar.

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flux (CHF) by nanofluids. You et al. [20] studied Al2O3–waternanofluids at a pressure of 2.89 psia (Tsat¼ 60 �C) and tested con-centrations of nanoparticles ranging from 0 to 0.05 g/l. The meas-ured pool boiling curves of these nanofluids saturated at 60 �C,demonstrating that the nucleate boiling heat transfer coefficientsof all concentrations, including pure water, were about the same;however, the CHF of pure water increased dramatically due to theaddition of nanoparticles. Adding an extremely small amount ofnanoparticles (0.001 g/l) to pure water resulted in a sizableincrease in the CHF values from 540 to 670 kW/m2 (see Fig. 3).When the concentration was greater than 0.005 g/l, the CHFincreased consistently by about 200% compared with that of purewater. From these results, they concluded that the unusual CHFenhancement found using nanofluids containing alumina nanopar-ticles at volume concentrations as low as 10�3% could not beexplained by any existing CHF model. A continuing increase inthe CHF was not observed at concentrations higher than 0.01 g/l,which is significantly less than what is usually used in nanofluidsto enhance thermal conductivity. Using this result, they pointed

out that the use of nanoparticles to enhance the liquid-to-vaporphase-change heat transfer was not related to the increased ther-mal conductivity provided by nanofluids. Based on these results,many researchers have attempted to understand why the CHFincreases when using nanofluids. Mainstream literature publishedin this field since 2003 will now be discussed in chronologicalorder.

Vassallo et al. [21] studied the pool boiling heat transfer of sol-utions with 0.5% silica particles with diameters ranging from 15to 3000 nm. The pool boiling experiments were carried out usinga 0.4-mm-diameter NiCr wire heater at atmospheric pressure.Figure 4 shows a marked increase in the CHF for both nanosolu-tion and microsolution compared with water, but no appreciabledifferences in the heat transfer for temperatures less than theCHF. Stable film boiling at temperatures close to the wire melting

Fig. 1 Thermal conductivity enhancement of Cu and Al2O3

nanofluids [3]

Fig. 2 Pool boiling characteristic of nanofluids on a smooth and a roughened heater [16]

Fig. 3 Boiling curves at different concentrations of aluminananofluids [20]

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point was achievable with the nanosolutions, but not with themicrosolutions. At the end of the tests, a thick silica coating(0.15–0.2 mm) was observed on the wire for nanosolutions, indi-cating some possible surface interaction with the wire at higherfluxes, but only a thin silica coating (0.025–0.05 mm) wasobserved on the wire for microsolutions. Based on the observedsilica coatings, they suggested a possible surface roughness effectthat changed the nucleation site density and improved heat trans-fer. To test this theory, they measured the CHF of pure water onNiCr wires artificially roughened with emery paper. Indeed, theyobserved a higher attainable heat flux at a given wire superheatwithin the nucleate boiling regime, and even a CHF point that wasabout 30% higher than that of virgin wire. However, the effect ofthe surface roughness alone did not explain the dramatic heatfluxes attained with the nanosolutions. They indicated that morework was needed to fully understand the mechanisms responsiblefor the large gains in heat transfer at high heat fluxes.

Dinh et al. [22] conducted a nanofluid boiling experiment on ananoscopically smooth heater by passing a direct electric currentthrough a 460-nm thin titanium film vapor deposited on 130-lmborosilicate glass substrate. The nanofluid used in the experimentwas made of a water-based 37-ppm solution of Al2O3 nanopar-ticles (38 nm in average diameter). Thermal images of the heatersurface during boiling were recorded using a high-speed high-re-solution infrared camera. They reported an outstanding enhance-ment in the CHF and identified an increased nucleation sitedensity based on the thermal images of the heater surface. Theydiscussed their experimental findings in terms of surface super-heating and nucleation site density and described how the nano-particles may have affected the onset of nucleate boiling andincreased the critical heat flux. A high concentration of nanopar-ticles in the coolant favored particle deposition on the heater sur-face; these particles then served as nucleation sites. A highnucleation site density, associated with small bubbles, decreasedthe size of the dry spots formed beneath the bubbles and increasedthe curvature of the liquid meniscus to promote liquid inflow to-ward the contact line. These factors may have been responsiblefor the high resistance to burnout observed in their pool boilingexperiments. Additionally, they suggested that the super-spreading behavior of nanofluids reported by Wasan and Nikolov[23] as well as the effect of capillary wicking due to microcurva-ture (i.e., surface morphology) could increase the CHF.

Milanova and Kumar [24] observed that the CHF increasedmore at higher pH levels (up to 12.3) when using SiO2–waternanofluids, but there was a relatively little influence on the nucle-

ate boiling regime. Nanoparticle deposition on the heater surfacewas also observed, but the role of the deposition layer wasunclear. They postulated that nanofluids in a strong electrolyte(i.e., with a high ionic concentration) allowed a higher criticalheat flux than buffer solutions because of the difference in surfacearea. The formation and surface structure of the depositionaffected the thermal properties of the liquid.

Considerable conjecture and guesswork emerged about the rea-son behind the observed CHF enhancement for nanofluids. Someresearchers reported nanoparticle deposition on the heating sur-face, but it was not thoroughly investigated. Wen and Ding [25]observed up to a 40% enhancement in the nucleate boiling ofalumina–water nanofluids. They did not report CHF results butinvestigated the heat transfer behavior of nanofluids under nucle-ate pool boiling conditions. Based on their results, they suggestedthat the reported controversies about pool boiling heat transferbehavior could be associated with the properties and behavior ofboth nanofluid and boiling surface, as well as their interactions.Their interest in the boiling surface turned out to be important.

Bang and Chang [26] investigated the boiling heat transfercharacteristics of nanofluids using nanosolutions with differentvolume concentrations of alumina nanoparticles, from 0% to 4%.The size of the nanoparticles had a normal distribution, rangingfrom 10 to 100 nm, and their average diameter was 47 nm. Theexperiments were conducted using a smooth horizontal flat sur-face with a roughness of a few tens of nanometers under atmos-pheric pressure. The experimental results showed that nanofluidshad poor heat transfer performance compared with pure water dur-ing natural convection and nucleate boiling (see Fig. 5). However,the CHF was enhanced not only in horizontal pool boiling but

Fig. 4 Boiling curves of NiCr wire (D¼0.4 mm) in silica–waternanofluids [21]

Fig. 5 Boiling curves of pure water and nanofluids, and boilingheat transfer coefficients [26]

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also in vertical pool boiling. They observed a roughness changeon the heater surface and hypothesized that the reason for thechanged CHF performance might be due to the nanoparticle sur-face coating on the heater, but they could not explain the exactreason for this change.

At this point, an important experiment was performed that indi-cated that the CHF in nanofluids increased not due to the effect ofthe nanofluids but due to the nanoparticle deposits on the heatersurface. Kim and Kim [27] performed pool boiling experimentwith TiO2–water nanofluids at various concentrations. The CHFenhancement increased with the nanofluid concentration in ac-cordance with previously observed trends. However, they alsotested a nanoparticle-coated heater with pure water. Thenanoparticle-coated heater was prepared from a preconductednanofluid boiling test, for which the temperature had remainedbelow the CHF. The CHF performance of the nanoparticle-coatedheater with pure water was not less than that of a bare heater witha nanofluid. The CHF data of both experiments are shown inFig. 6. From this graph, there is no doubt that the CHF enhance-ment is dependent not on the nanofluid itself, but solely on thechanged heater surface condition, due to the nanoparticle coating.This brilliant experiment made clear the role of nanoparticledeposits on the heating surface, which previously had been just aconjecture [28–31].

The CHF enhancement in nanofluids is the result of nanopar-ticle deposits that form on the heater surface during boiling. Animportant question since Kim and Kim [27] is which surface pa-rameters influence the CHF enhancement in nanofluids. Kim et al.[32,33] conducted pool boiling experiments with nanofluids con-taining alumina, zirconia, and silica. They characterized thenanoparticle-coated surfaces according to the contact angle, whichis a measure of wettability (see Fig. 7). It is well known that thecontact angle decreases with the increasing CHF. Kim et al. [32]reported that a thin layer of deposited nanoparticles changed thesurface energy and surface morphology, and that this was closelyrelated to the observed contact angle. Later, other researchers[34–36] reconfirmed that the wettability effect of nanoparticles onthe heater surface influenced the CHF enhancement. They alsoexplained that the CHF was increased by the enhancement of sur-face wettability using the prediction of Kandlikar [37], whichexpressed the effect of surface wettability on the CHF value asfollows:

q00CHF ¼ hfgq1=2g

1þ cosb16

� �2

pþ p

41þ cosbð Þcos/

� �1=2

� rg ql � qg

� �h i1=4

(1)

Liu and Liao [34] and Coursey and Kim [35] performed pool boil-ing experiments with water-based and alcohol-based nanofluidson a plain heated surface. Both studies confirmed, from static con-tact angle measurements, that the nanoparticle layer formed onthe heater surface significantly improved the wettability. Theyalso showed that surface treatments, such as oxidation alone,resulted in a CHF enhancement similar to nanofluids. Finally,they postulated that the CHF increased in nanofluids due to thewettability effect. From these experimental results, the improvedwettability of heating surface by nanoparticles was revealed as themain reason that CHF increases. However, the mechanism ofCHF enhancement was not investigated by the improved wettabil-ity, and the reason that the CHF in nanofluids increased more thanthe prediction of Kandlikar [37] with wettability effect was notrevealed.

Kim et al. [30,38] suggested that the CHF enhancement ofnanofluids was due to a change in the surface microstructure andtopography of the heater due to the nanoparticle surface coatingformed during pool boiling. They based this on the experimentalresults with nanoparticle-coated heaters in pure water and scan-ning electron microscopy (SEM) image analysis. Figure 8 showsSEM images of a nanoparticle-coated surface. They proposed aquantitative characterization of three surface characteristics thatcould reveal the CHF enhancement mechanism: roughness, con-tact angle, and capillary wicking. Kim et al. [30] characterized theheated surface and demonstrated the dominant contribution of sur-face wettability and capillary wicking but found that the surfaceroughness was unimportant. They noticed fast and wide liquidspreading on nanoparticle-coated wire surfaces. Capillary wickingwas measured by the water rising length on vertically erectedwires in a water reservoir. Kim and Kim [38] examined the effectof the surface wettability and capillarity of the nanoparticle-coated layer on the CHF. They focused on the capillary wickingon a porous surface caused by the enhanced wettability from thenanoparticle coating. Figure 9(a) shows that the effective liquidsupply delayed the irreversible growth of dry patches. Figure 9(b)shows that the CHF enhancement could not be explained by thecontact angle alone (i.e., a wettability effect). Kim and Kimexplained the CHF difference in Figs. 9(a) and 9(b) based on theattainable heat flux gain due to the capillary wicking liquid sup-ply. After accounting for the capillary wicking effect, the remain-ing data, which could not be explained by the wettability, becameclearer. Consequently, both wettability and capillary wicking are

Fig. 6 CHF data for a bare heater immersed in a nanofluid, anda nanoparticle-coated heater immersed in pure water [27]

Fig. 7 (a) Macrolayer concept. (b) Macrolayer thickness versuscontact angle [32].

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important parameters that contribute to the CHF increase fornanoparticle-coated surfaces. Kim et al. [39] reported the CHFenhancement mechanism on nanoparticle-deposited surfaces.Based on Kandlikar’s CHF model with wettability effects [37],they determined that the recoil force of the liquid evaporationnear the liquid–solid–vapor triple line was affected by the surfacewettability and that the CHF increased due to the force balance.They conducted a simple experiment to verify the relationshipbetween the surface temperature and the recoil force of liquidevaporation. This was a small droplet dynamic wetting test on ahot surface; both bare and nanoparticle-deposited surfaces wereexamined. The CHF enhancement mechanism was revealed bythe relationship between the surface temperature and the recoilforce of liquid evaporation. Through high-speed visualization of asmall droplet dynamic wetting test on a hot surface (Fig. 10), theypostulated that the nanoparticle layer increased the stability of theevaporating microlayer underneath a growing bubble on a heatedsurface, and thus the irreversible growth of hot spots was inhib-ited, resulting in CHF enhancement when boiling nanofluids.From these experimental results, the capillary wicking of heatingsurface by nanoparticles was revealed as another reason that CHFincrease, besides of wettability. It could be also explained as theheat flux gain on the nanoparticles deposited heater by the capil-lary wicking of liquid, which could not be explained only by thewettability effect.

The wettability and capillarity of a nanoparticle-deposited sur-face have been revealed as the main parameters that increase theCHF in pool boiling from previous researches. However, otherinvestigations have also been conducted to explain the CHFenhancement in nanofluids using other approaches. The resultsshowed that the surface wettability and capillarity were not theonly main parameters to influence CHF directly.

First, Arik and Bar [40] and Arik et al. [41] postulated that thesurface properties of the heater were related to the thermaleffusivity

S ¼ dh

ffiffiffiffiffiffiffiffiffiffiffiffiffiqhchkh

p(2)

which can strongly influence the CHF. The higher the thermaleffusivity, the more effectively conduction can dissipate throughthe hot/dry spots. A highly effusive layer on the heater surfacedelays the CHF. Kim et al. [39], Raykar and Singh [42], and Kimet al. [33] also used the concept of enhanced thermal effusivity onthe nanoparticle-deposited heater surface to explain the CHFenhancement in nanofluid boiling. Even though it is clear that thenanoparticles on the heating surface affect the CHF enhancement,they suggested the thermal effusivity as the parameter of CHFenhancement, not the wettability and the capillarity.

Second, Sefiane [43] reported a basic experiment that was usedto propose a different approach toward understanding the

Fig. 8 SEM image of the surface after pool boiling 10�1% nanofluids [30]

Fig. 9 (a) Relationship between CHF phenomena and capillary wicking: (upper) capillaryspreading of a liquid drop over thin porous layers with a small apparent contact angle, (lower)capillary rewetting flow toward a dry spot region during bubble growth on porous layers. (b)CHF of pure water versus contact angle on a nanoparticle-deposited surface [38].

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mechanism, through which the presence of nanoparticles affectsthe heat transfer during nanofluid boiling. Pure ethanol and etha-nol with aluminum nanoparticles evaporating on a hot polytetra-fluoroethylene surface were investigated. The results indicatedthat the nanoparticles promoted pinning of the contact line of themeniscus and the evaporating drops. Structural disjoining pres-sure, stemming from the ordered layering of nanoparticles in theconfined wedge of the evaporation meniscus, was thought to bethe principal mechanism behind the observed pinning behavior.Sefiane [43] suggested that analysis of boiling nanofluid heattransfer should account for the important effects of nanoparticleson the contact line region through the structural disjoining pres-sure and provide accurate interpretation of the results. Wen [44]postulated a mechanism for CHF enhancement in nanofluids usingthe disjoining pressure. He reviewed experiments and possiblemechanisms of enhancing the CHF using thermal nanofluids andidentified the important role of the structural disjoining pressure atthe meniscus of dry patches. He then explained that the structuraldisjoining pressure due to the nanoparticles increased the wettabil-ity and inhibited dry patch development. Eventually, the structuraldisjoining pressure was also induced by the nanoparticle-deposited heater surface, which influenced the enhanced wettabil-ity and increased the CHF.

Third, Park et al. [45] performed pool boiling CHF experimentswith nanofluids made from graphene and graphene-oxide nano-sheets. The CHF increased significantly by 150–250% comparedwith pure water. They checked the wettability, capillarity, andeffusivity of the heater surface; however, there was no change thatcould explain the increased CHF. They focused on hydrodynamicinstability theory (or the hydrodynamic liquid-chocking limit)[46] to interpret the CHF enhancement. By measuring theRayleigh–Taylor instability wavelength of the heater, they postu-lated that the graphene and graphene-oxide nanosheets on theheater surface modulated the shorter instability wavelengths,which dramatically increased the CHF (Fig. 11) [47]. Theseresults suggest the possibility of CHF enhancement up to maxi-mum 500%. As the theoretical approaches of CHF enhancement

on the artificial porous media of Liter and Kaviany [47], the CHFwas increased by the modulated hydrodynamic instability wave-length. Results of Park et al. represented that the wavelength onthe heating surface could be controlled using the nanofluids on thepool boiling. It means that the further research about wavelengthcontrolling by nanoparticles should be examined.

Several previous works have shown that the surface effect bythe nanoparticles deposition on the heating surface is a criticalreason for CHF increase. The suggested parameters were the wett-ability, the capillarity, the thermal effusivity of heating surface,the RT wavelength, and structural disjoining pressure due to thenanoparticles, which are related with one another (The wettabilityinfluences the capillarity and the Rayleigh–Taylor wavelength. Thestructural disjoining pressure influences the wettability. The structure

Fig. 10 (a) Wetting of a water droplet on a water-boiled copper surface at 120 �C, 140 �C, and160 �C. (b) Wetting of a water droplet on a titania nanoparticle-fouled copper surface at 140 �C,160 �C, 180 �C, and 200 �C [39].

Fig. 11 CHF phenomenon and comparison between modelsand experimental data [45]

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by the nanoparticles also influences the capillarity and the disjoiningpressure.) The relationships between these parameters with each othershould be examined carefully by a parametric study, in order tounderstand CHF enhancement due to nanoparticles more clearly.

3 Flow Boiling CHF Enhancement With Nanofluids

Research in convective flow boiling of nanofluids has becomemore popular in the past three years, perhaps due to the recentdemand for high heat flux cooling of electronic components andnuclear reactor applications. Lee and Mudawar [48] conducted thefirst study to estimate flow boiling CHF of nanofluids. They usedalumina nanoparticles in a water-based fluid for a microchannelcooling system. They found an enhanced heat transfer coefficientfor single-phase laminar flow; however, in the two-phase regime,the nanofluids caused surface deposition in the microchannel, andlarge clusters of agglomerated nanoparticles were formed. Thisclogging problem is a serious issue if nanofluids are to be incorpo-rated in microchannel cooling systems, where any temperatureexcursions can result in temperature hot spots and possible ther-mal failure of the device.

Kim et al. [49,50] conducted internal flow boiling CHF experi-ments with a 1/4 in. SUS tube, 100 mm in length, and a 3/8 in.SUS tube, 240 mm in length, using dilute alumina, zinc oxide,and diamond water-based nanofluids. They varied the concentra-tion of the nanofluids from 0.001 vol. % to 0.1 vol. %, and themass flux from 1000 kg/m2�s to 2500 kg/m2�s. They used a directheating method to apply heat to the tube. The nanofluids exhibiteda significant CHF enhancement with respect to pure water at highmass fluxes (2000–2500 kg/m2�s), but there was no enhancementat a lower mass flux (1500 kg/m2�s). They suggested that somenanoparticles were deposited on the boiling surface during theexperiments. Such particle deposition increased the wettability ofthe boiling surface. Meanwhile, Ahn et al. [51] conducted convec-tive flow boiling CHF experiments on a copper heater with a10-mm diameter using 0.01-vol. % alumina–water nanofluids.They also changed the flow velocity from 0 m/s (pool boiling) to4 m/s (Fig. 12). They determined that nano/microstructures wereformed on the surface during the nanofluid flow boiling, signifi-cantly changing the surface morphology. However, the roughnesschange alone was not enough to explain the observed CHFenhancement. The change in the surface wettability due to nano-particle deposition was identified as a key parameter. The normal-ized CHF enhancement data (CHFnanofluids/CHFpurewater at a givenflow velocity) and static contact angles (enhanced wettability dueto nanoparticle deposition) independently were in good agreementwith the pool boiling CHF model of Kandlikar [37] (Eq. (1)).Additionally, ad hoc tests were performed to assess the effect of

nanoparticle deposition on the heater surface. The CHF of thenanoparticle-coated heater at a given flow velocity in pure waterincreased more than that of a bare heater in pure water (seeFig. 12). Kim et al. [52] reported nanoparticle deposition on theheater surface after nanofluid flow boiling and considered this tobe the main cause behind the observed CHF enhancement. Theyfound CHF enhancement of up to 70%, with a nanoparticle con-tent of less than 0.01% by volume of alumina in water. This againshows that only a small nanoparticle concentration is required toobtain dramatic CHF enhancements during nanofluid flow boiling.Finally, Ahn et al. [53] conducted CHF visualization of pure waterflow boiling on a nanoparticle-coated heater to interpret the effectof the nanoparticles on CHF enhancement. They postulated thatthe enhanced surface wettability of the nanoparticle-coated heaterinfluenced the flow boiling regime entirely, and delayed the CHF,based on classical models. Additionally, Khanikar et al. [54] per-formed flow boiling experiments in a carbon nanotube (CNT)-coated copper microchannel. They used just water as the workingfluid. Appreciable differences in the influence of the CNT coatingwere observed at high rather than low mass velocities. The CHFwas repeatable at low mass velocities, but degraded followingrepeated tests at high mass velocities, demonstrating that highflow velocities caused appreciable changes in the morphology ofthe CNT-coated surface. While the CHF was enhanced by theincreased heat transfer area associated with the CNT coating, theenhancement decreased following repeated tests because the CNTfin effect was compromised by the bending. This result also sup-ported the relationship between flow boiling CHF enhancementand the nanoparticle-deposited surface.

Flow boiling CHF enhancement in nanofluids is stronglyrelated to the surface wettability, which is similar to pool boilingCHF enhancement. Further experimental data need to be collectedon the flow boiling of nanofluids to obtain a more substantial data-base and a better understanding of nanofluid flow boiling mecha-nisms. In contrast with pool boiling, the flow boiling CHF innanofluids is still being investigated and strongly needed.

4 Nanofluids Application

Nanofluid boiling results in a build-up of a porous layer ofnanoparticles on the heater surface. This layer has been shown tosignificantly improve the surface wettability and the capillarity. Itis hypothesized that this surface wettability improvement may beresponsible for the CHF enhancement observed by almost allresearchers so far. From reports of pool and flow boiling CHF innanofluids with very small amounts of nanoparticles in the litera-ture, the wettability of a nanoparticle-deposited heater surfaceincreases the CHF for both boiling conditions. This creates possi-bilities for real applications of nanofluids, such as using them inthe cooling fluids of nuclear reactors.

Recently, Bang and Heo [55] suggested the first protocol forapplying nanofluids to real-life problems. They used axiomaticdesign theory to systemize the design of nanofluids to bring theirpractical use forward. At a parametric level, the design of a nano-fluid system is inherently coupled due to the characteristics of athermal-fluid system; design parameters physically affect eachother and share sublevel parameters for the nanoparticles, creatinga feedback loop. Their approach allows nanofluids to be designedfor a specific purpose. Buongiorno et al. [56] explored the poten-tial use of nanofluids to enhance the in-vessel retention capabil-ities of advanced light-water reactors. Their nanofluid systemenabled a 40% decay heat removal enhancement for a given CHFmargin and was physically and functionally separated from theprimary system and other safety systems (i.e., the emergency corecooling system or ECCS). This facilitated its inspection and main-tenance and also eliminated any undesirable interference with thereactor operation. To apply nanofluids to real applications, thesystemic design method was suggested as a means of designingnanofluids well matched with the needs of the specific targetapplication.

Fig. 12 Comparison of CHF values for pure water and nano-fluid on a clean surface, and pure water on a nanoparticle-coated surface [51]

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Despite the observed CHF enhancement and the proposeddesign methods for real applications, nanofluids still have someproblems. First, the nanoparticle layer is also considered by someresearchers to be responsible for the deterioration observed in theboiling heat transfer coefficient. Nanoparticle deposits create a re-sistance in the heat transfer from the heater surface to the fluid,caused by a decrease in the contact angle, and reduce the nuclea-tion site density. Kwark et al. [57] postulated that the observeddecrease in the boiling heat transfer coefficient with increasednanoparticle concentration could be attributed to the correspond-ing thicker coating that was formed, which offered increased ther-mal resistance. The CHF is affected by the increased wettabilityprovided by nanoparticles deposited on the heater surface and notby the thickness of the nanoparticle coating. Kwark et al. con-cluded that there was an optimal nanofluid concentration forwhich the CHF enhancement is at a maximum without any degra-dation of the boiling heat transfer coefficient. They also investi-gated possible mechanisms behind the deposition and adhesion ofnanoparticles to the heater surface during nanofluid boiling. Bycontrolling the heat flux and boiling time, they identified the opti-mal thickness of the nanoparticle layer that would increase theCHF and not decrease the boiling heat transfer coefficient. Thisresult makes it possible to overcome the heat transfer deteriorationproblem caused by nanofluid boiling.

Second, as previously described, nanoparticles can cause clog-ging problems for pumps, pipes, and other components of thermalcooling systems due to agglomeration. This problem also can bedescribed in terms of the stability of nanofluids and whether thenanoparticles in the base fluid maintain a well-dispersed state ornot. Formulating stable nanoparticles in liquid suspensions (nano-fluids) is difficult, as is controlling their properties, such as ther-mal conductivity, viscosity, and wettability, for heat transferapplications. There are some concerns over the dispersion stabilityof nanofluids. Factors such as time, temperature, concentration,particle type, dispersion medium, and pH all play important rolesin the dispersion stability, and poor nanoparticle dispersion in thebase fluid possibly results in poor heat transfer enhancement. Thepreparation of nanofluids is very important in ensuring the desiredperformance. Lee and Mudawar [48] reported clogging problemswith nanoparticles in microchannels, as previously described.Recently, however, Khanikar et al. [54] performed flow boilingexperiments in a CNT-coated copper microchannel. Instead ofusing nanofluids, they used water as the working fluid and thenanoparticles (CNT) were coated on the surface of microchannel.They obtained good results in terms of enhanced CHF and boilingheat transfer coefficient, but the CHF values were not experimen-tally repeatable.

Third, the process used to coat nanoparticles on a heated sur-face still leads to some particles detaching from the heater. Ahnet al. [51] reported that the CHF of a nanoparticle-coated speci-men at a given flow velocity in pure water increased more thanthat of a bare specimen in pure water; however, this increase wasless than what was observed in nanofluids (Fig. 12). This was dueto nanoparticle detachment, as determined by SEM, along withroughness and static contact angle data. Thus, nanoparticledetachment during flow boiling with water could lower the CHFbelow that obtained with nanofluids.

Fourth, the increased viscosity of nanofluids compared with thepure fluid leads to lack of pumping power under flow boilingapplication. As previously mentioned, the aggregation of nanopar-ticles would increase the viscosity of nanofluids [17–19]. Eventhough the surfactant added into the nanofluids makes the nano-fluids stable, prevents the aggregation, and reduces the increasedviscosity [18], the clear mechanism was not revealed.

The problems mentioned above still occur in nanofluids,even fluids that exhibit a strong effective enhancement of theCHF. Thus, several researchers are working to overcome thesedifficulties. To overcome these difficulties, surface modifica-tion with micro/nanostructures were widely suggested andresearched.

5 Surface Modification With Micro/Nanostructures

Suspended nanoparticles in a nanofluid enhance its thermalconductivity, which has led to much interest in nanofluids as engi-neered fluids. Moreover, findings that even a small amount ofnanoparticles can noticeably increase the CHF have emphasizedthe importance of nanofluids. However, there are still severalproblems using the fluids, such as the decrease in heat transfer andinstability of the fluids. The mechanism behind how the nanopar-ticles affect the heating surface characteristics and enhance theCHF is well known, along with the fact that the boiling heat trans-fer performance can be characterized by the surface condition.Thus, development of a permanent heating surface for ideal condi-tions is conceptually reasonable.

Several studies have examined surface modifications as ameans of enhancing the CHF and boiling heat transfer; these aresummarized in Table 1 [58–72]. Recently, the developments ofnano/microstructure manufacturing techniques have made trials forenhancing the boiling heat transfer performance possible. There aremany techniques that can be utilized to make nano/microscaledstructures, including nanocoating, anodic oxidation, and MEMSfabrication. The wettability and capillarity of the heater surface areimportant parameters affecting CHF enhancement. Many methodsfor making heater surfaces hydrophilic and hydrophobic have beendeveloped by microfluidics research groups.

Some research groups studying boiling heat transfer have triedusing applied nano/microstructures on the heater surface. Vemuriand Kim [73] used a nanoporous surface consisting of a 70-lm-thick layer made from aluminum oxide. They demonstrated areduction of about 30% in the incipient superheat for a givenapplied power for a nanoporous surface compared with a plainsurface. SEM photographs of the nanoporous coating were takento determine the size of the pores (Fig. 13(a)). Ahn et al. [74] fab-ricated multiwall carbon nanotubes on silicon substrates with 9-and 25-lm-tall CNT forests (Fig. 13(b)). The heat transfer in poolboiling was augmented to the same degree in both cases comparedwith a plain silicon surface. However, the taller sample resulted ina 28% improvement in the CHF compared with a plain silicon sur-face, but the smaller sample had only a 25% improvement. Thetaller nanotubes provided better pathways for liquid flow to thenucleation sites. They also provided a mechanistic description forthe enhancement in terms of the pinned contact line on the CNTsurfaces and a reduction in the critical Rayleigh–Taylor instabilitywavelength. Launay et al. [75] conducted boiling heat transferexperiments on hybrid nano/microstructured thermal interfaces.The silicon substrate was modified by etching during fabrication,and coated with CNTs, as shown in the nano/microhybrid surfacein Fig. 13(c). Experimental results indicated that use of the CNT-enabled purely nanostructured interfaces improved the boilingheat transfer only at very low superheats compared with the baresurface. However, the CHF value was highest on the surface onlywith microstructures. Figure 13(d) shows the vertically aligned Sinanowires, 20–300 nm in diameter and 40–50 lm in length, usedby Chen et al. [76] for boiling heat transfer experiments. Theyobserved increases of more than 100% in the CHF and heat trans-fer coefficients. A 3D porous structure of metallic material wasalso demonstrated. Li et al. [77] developed porous structures (Fig.14(a)) that enhanced the boiling performance by providing anincreased surface area and an increased number of active nuclea-tion sites for boiling. Liter and Kaviany [47] designed wicked po-rous structures (Fig. 14(b)) with Cu particles to increase the CHF.They postulated that the capillary ability of porous structures withnanoparticles increased the CHF nearly three times versus a plainsurface. The modulation of 3D structures separated the liquid andvapor phases, thus reducing the liquid–vapor counterflow resist-ance adjacent to the surface. They proposed using a modulatedwavelength induced by the surface structures to increase the CHF,based on the theory of Zuber [46]. Thus, these studies focused onthe structural effects of nanoscaled and microscaled surfaces onthe enhanced boiling heat transfer and CHF.

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Table 1 Summary of modified surface on CHF enhancement reported in the literature

Reference HeaterWorking fluid and

conditionModification

methodSurface

characteristics Results

Costello and Frea [58] SUS cylinder Distilled, tap water, 1 bar Capillary wicking materialinserting and deposition

Wettability/capillarityimprove

CHF 160%

Hahne and Dieselhorst [59] Pt, NiCr tube Deionized water, 1 bar Oxidized surface by repeatedexperiment

Wettability improve CHF 50%

Messina and Park [60] Cu plate Freon 113, 1 bar Etching with pit arrays, sand-ing, mirror by polishing

Surface microgeometry CHF increase

Marto and Lepere [61] Cu tube FC-72, Freon 113, 1 bar Copper enhanced surfacewith cavities

Cavity size control DT decrease

Chowdlhury andWinterton [62]

Al, Cu cylinder Water, methanol, 1 bar Roughness, surface energy(anodic oxidation)

Wettability, Ra improve CHF increase

Liaw and Dhir [63] Flat copper Water, Freon 113, 1 bar Oxidation and fluorosiliconecoating

Wettability improve CHF increase

Anderson and Mudawar [64] Cooper FC-72, 1 bar Roughness, artificial cavity(fins, studs, grooves)

Roughness improve CHF 50%

Golobic and Ferjancic [65] Ti, Fe ribbon FC-72, 1 bar Roughness (sandpaper),etching on low roughness,particle coating

Roughness improve CHF 29–130%

Fong et al. [66] Zircaloy-2 tube Water, 1 bar Created microsize cavities,contact angle (glass beadpined)

Wettability, Ra improve CHF 60%

Liter and Kaviany [47] Not reported Not reported Modulated porous layercoating

Wettability improve CHF 211%

Ferjancic and Golobic [67] Fe ribbon Water, FC-72, 1 bar Roughness change, etchedsurface (sanding)

Ra improve CHF 51%

Kim et al. [68] Pt wire FC-72, 1 bar Diamond particles—micropo-rous layer coating

N/C CHF 130%

Honda et al.[69] Si chip FC-72, 1 bar Dry etching (microfins, sub-micron roughness)

Microstructure CHF 130%

Fong et al. [70] Zr-2 tube Water, 1 bar Oxidation Wettability, Ra improve CHF 70%Takata et al. [71] Copper plate Water, 1 bar TiO2 coating and UV

irradiationWettability improve CHF 120%

Ujereh et al. [72] Si and Cu Water, 1 bar Aged with UV irradiation Well aged improve CHF 75.5%Kim et al. [78] Si Water, 1 bar ZnO structures using MEMS

techniqueWettability CHF 90%

Ahn et al. [79] Zircaloy Water, 1 bar Anodic oxidation Wettability, spreading CHF 105%

Fig. 13 SEM photograph of a nanoporous surface that reduced the wall superheat. (a) [73], (b)[74], (c) [75], and (d) [76].

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There were many efforts to increase the CHF and the boilingheat transfer coefficient under pool boiling through the surfacemodification using the MEMS technique. However, there are noparametric studies to investigate and find the optimal condition ofheater surface based on the consistent parameters which stronglyaffected the CHF and the boiling heat transfer. Recently, Kimet al. [78] focused on the results of the dramatic CHF enhance-ment in nanofluids and designed an artificial surface with surfacemodifications using a nanoparticle-coated heater surface and aMEMS technique on a silicon surface. Based on a literaturereview, they identified three parameters that would increase theCHF: the structure, wettability, and capillarity properties of thesurface. They prepared four surfaces: plain (F), microstructured(M), nanostructured (N), and micro/nanohybrid structured (NM)(Fig. 15). Their CHF values were 1121 kW/m2 for the F surface,1652 kW/m2 for the M surface, 2003 kW/m2 for the N surface,and 2326 kW/m2 for the NM surface. They described the reasonfor the CHF enhancement of each surface: structural effects on themicrostructured surface, and wettability and liquid spreading

(capillarity) effects on the nanostructured and micro/nanostruc-tured surfaces. Additionally, Ahn et al. [79] made surface modifi-cations on a metallic zirconium alloy heater using an anodicoxidation method, in order to apply the nuclear power plant as thenuclear fuel cladding which has the capability of increasing CHF.They controlled the time of the anodic oxidation and found theycould control the surface wettability. They obtained the micro-,nano-, and micro/nanostructured surfaces shown in Fig. 16. Theypostulated that the spreading (capillarity) effect below a contactangle of 10 deg influenced the CHF enhancement more than pre-dicted by Kandlikar [37]. They had a maximum CHF enhance-ment of 100% on the micro/nanostructured surface. In addition,the boiling heat transfer coefficients did not changed as comparedwith the bare surface. The quantitative analysis of Ahn et al. [80]of the CHF enhancement effect of liquid spreading on the nano/microstructured surfaces was nearly the same as the nanofluidresults of Kim and Kim [38] and the results of Kim et al. [78].

From MEMS technique to anodic oxidation, the surface modifi-cation for the enhanced boiling has been conducted; however,

Fig. 14 SEM photographs of Cu particle structures (3D). (a) Enhanced boiling heat transfer[77] and (b) enhanced critical heat flux [47].

Fig. 15 SEM photographs of artificial nanoparticle-coated surfaces. (a) Microstructures, (b) nanostructures, and (c) micro/nanohybrid structures [78].

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there are only a few results for the surface modification. To under-stand the enhanced boiling heat transfer and CHF on a modified sur-face, a well-designed experiment and a brilliant approach are neededto clearly reveal the mechanism of enhanced boiling and CHF.

6 Conclusion and Recommendations

Since Choi first described the concept of nanofluids [1], manystudies have been performed to investigate the possibility of usingnanofluids in practical applications. Nanofluids increase the CHFsignificantly, even when the nanofluids contain a very smallamount of nanoparticles. This enhancement is caused by the sur-face morphology, surface wettability, and capillarity of the heatersurface, which are changed by nanoparticle deposition duringboiling. CHF enhancement in nanofluids has been confirmed notonly for pool boiling but also for flow boiling. These essentialCHF enhancement results have made nanofluids attractive forapplications such as electric cooling systems and nuclear powerplants. However, there remain some problems that must be solved,such as the deterioration of the boiling heat transfer, cloggingissues due to the agglomeration of nanoparticles, and the robust-ness of the deposited nanoparticle layer on the heater surface. Weauthors recommend future research in using nanofluids as engi-neered fluids for two-phase heat transfer as follows:

(1) We need to understand the nanoparticle coating process onthe heater surface and the characteristics of the nanoparticlelayer to optimize the coating process for boiling heat trans-fer applications.

(2) We need nanofluids that maintain their stability overincreased operating times.

(3) A nanoparticle-coated surface has many merits thatincrease the CHF, but the robustness of these surfaces isweak.

(4) There is no clear understanding of the deterioration and theenhancement of boiling heat transfer for various nanofluids.

(5) There are only a few results for CHF enhancementobserved during flow boiling with nanofluids.

(6) Recently, graphene nanofluids have been developed. Theirability to modulate the Rayleigh–Taylor instability wave-length is quite interesting. Shorter wavelengths increase theCHF.

Additionally, we have recommended and described furtherresearch for increasing the CHF in this article. Modified artificialsurfaces have been created to overcome some of the problems ofnanofluids. These are based on the main parameters (structural,wettability, and capillarity effects) that increase the CHF in nano-fluids. An optimized boiling surface has been the goal of manyresearchers when developing nano/microstructure manufacturingtechniques. Surface modifications are also attractive from thepoint of view of applications. However, there are still some ques-tions regarding the nano/microstructures on the heater surface.We recommend future research in surface modifications toimprove two-phase heat transfer as follows:

(1) There is no clear understanding of the enhanced boilingheat transfer and CHF of different nano/microscaledstructures.

(2) An optimal surface should be determined, based on a well-designed and predicable process. Such a surface could dra-matically increase both the CHF and the boiling heattransfer.

(3) A new method for surface modifications must be developedto apply nanofluids to practical applications. Thus, we needto develop a designable surface modification method to re-alize intentionally an optimal heater surface. The MEMStechnique is not suited to mass production.

(4) Modified surfaces have not yet been applied to flowboiling.

(5) From the literature, the CHF enhancement mechanism hasbeen well characterized for modified surfaces where there

Fig. 16 SEM photographs of anodic oxidation results on a zirconium alloy with completewetting [80]

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are structural, wettability, and capillarity effects. Toincrease the CHF further, the Rayleigh–Taylor wavelengthon the heater surface should be modulated to thehydrodynamic-chocking limit [47].

Acknowledgment

This research was supported by WCU (World Class University)program through the National Research Foundation of Koreafunded by the Ministry of Education, Science and Technology(R31-30005).

Nomenclatureq00 ¼ heat flux (w/m2)h ¼ latent heat (kJ/kg)g ¼ acceleration due to gravity (m/s2)S ¼ thermal effusivity (J/(m�K�s1/2))c ¼ specific thermal capacity (J/kg�K)k ¼ thermal conductivity (W/m�K)

Greek Lettersr ¼ surface tension (N/m)q ¼ density (kg/m3)b ¼ contact angle (deg)u ¼ orientation angle (deg)d ¼ nonevaporating region thickness (m)

Subscriptsl ¼ liquid stateg ¼ vapor state

CHF ¼ critical heat fluxh ¼ heater

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