Enhancement of critical heat flux in nucleate boiling of nanofluids: a state-of-art review
© Kim; licensee Springer. 2011
Received: 29 October 2010
Accepted: 9 June 2011
Published: 9 June 2011
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© Kim; licensee Springer. 2011
Received: 29 October 2010
Accepted: 9 June 2011
Published: 9 June 2011
Nanofluids (suspensions of nanometer-sized particles in base fluids) have recently been shown to have nucleate boiling critical heat flux (CHF) far superior to that of the pure base fluid. Over the past decade, numerous experimental and analytical studies on the nucleate boiling CHF of nanofluids have been conducted. The purpose of this article is to provide an exhaustive review of these studies. The characteristics of CHF enhancement in nanofluids are systemically presented according to the effects of the primary boiling parameters. Research efforts to identify the effects of nanoparticles underlying irregular enhancement phenomena of CHF in nanofluids are then presented. Also, attempts to explain the physical mechanism based on available CHF theories are described. Finally, future research needs are identified.
Nanofluids are a new class of nanotechnology-based heat-transfer fluids, engineered by dispersing and stably suspending nanoparticles (with dimensions on the order of 1-50 nm) in traditional heat-transfer fluids. The base fluids include water, ethylene, oil, bio-fluids, and polymer solutions. A variety of materials are commonly used as nanoparticles, including chemically stable metals (e.g., copper, gold, silver), metal oxides (e.g., alumina, bismuth oxide, silica, titania, zirconia), several allotropes of carbon (e.g., diamond, single-walled and multi-walled carbon nanotubes, fullerence), and functionalized nanoparticles.
Nanofluids originally attracted great interest because of their abnormally enhanced thermal conductivity . However, recent experiments have revealed additional desirable features for thermal transfer. Key features of nanofluids that have thus far been discovered include anomalously high thermal conductivity at low nanoparticle concentrations [2, 3], a nonlinear relationship between thermal conductivity and concentration for nanofluids containing carbon nanotubes , strongly temperature-dependent thermal conductivity , and a significant increase in nucleate boiling critical heat flux (CHF) at low concentrations [5, 6]. State-of-the-art reviews of major advances on the synthesis, characterization, thermal conductivity, and single-phase and two-phase heat transfer applications of nanofluids can be found in [7–17]. However, the available reviews have paid much more attention to thermal properties and single-phase convective heat transfer than to two-phase heat transfer, and even reviews including two-phase heat transfer have only briefly touched upon important new research on the significant increase of CHF in nanofluids.
This paper presents an exhaustive review and analysis of CHF studies of nanofluids over the past decade. The characteristics of CHF enhancement in nanofluids are systemically reviewed according to the effects of boiling parameters. Efforts to reveal the key factors leading to nanofluid CHF enhancement are summarized. Attempts to understand the precise mechanism of the phenomenon on the basis of existing CHF theories are also presented. Finally, future research needs are identified in the concluding remark.
Summary of studies on CHF of nanofluids in pool boiling
Al2O3 in water
Cu plate (10 × 10 mm2)
200%, (19.9 kPa)
SiO2 (15, 50, 3,000 nm) in water
NiCr wire (ϕ = 1 mm)
Al2O3 (38 nm) in water
Ti layer on glass
TiO2 (27, 85 nm) in water
Al2O3 (70-260 nm), ZnO in water; Al2O3 in ethylene glycol
Al2O3 (47 nm) in water
SS plate (4 × 100 mm2)
Gold (3 nm) in water, 2.3 kPa
Cu disk (1 cm2)
SiO2 (10-20 nm) in ionic solution of water
NiCr wire (ϕ = 0.32 mm)
TiO2 (23 nm)
NiCr wire (ϕ = 0.2 mm)
Al2O3 (47 nm) in water
Ti wire (ϕ = 0.25 mm)
SiO2 (10 nm)
Al2O3 (110-210 nm)
SS wire (ϕ = 0.381 mm)
ZrO2 (110-250 nm) in water
SiO2 (20-40 nm)
CuO (30 nm) in water
Cu plate (40 × 40 mm2); with grooves
50%, (100 kPa)
140%, (31.2 kPa)
220% (7.4 kPa)
Al2O3 (45 nm) in water and ethanol
Glass, Au, and Cu surfaces
CuO (59 nm) and SiO2 (35 nm) in water and alcohol (C2H4OH) with SDBS surfactant
Cu disk (ϕ = 20 mm)
Al2O3 (22.6, 46 nm) in water
NiCr wire (ϕ = 0.64 mm)
BiO2 (38 nm)
Al2O3 (<25 nm) in water
Cu disk (ϕ = 10 and 15 mm)
Ag (3, 10, 80, 150, 250 nm)
Single-walled CNT in water with hydrochloric acid
NiCr wire (ϕ = 0.32 mm)
Multi-walled CNT in water with PVP polymer
10-4-10-2, 0.05 vol.%
Cu plate (9.5 × 9.5 mm2)
Ti wire (ϕ = 0.25 mm)
200% (19.9 kPa)
140% (19.9 kPa)
Cu (10-20 nm) in water
0.25, 0.5, 1.0 wt.%
Plate (30 × 30 mm2)
w/ SDS surfactant
w/o SDS surfactant
TiO2 (45 nm) and Al2O3 (47 nm) in water
Cu and Ni disks (ϕ = 20 mm)
Al2O3 (139 nm), CuO (143 nm), Diamond (86 nm) in water
Cu plate (10 × 10 mm2)
CNT in water with nitric acid for pH 6.5;
Cu plate (40 × 40 mm2)
60% (100 kPa)
140% (31.2 kPa)
200% (7.4 kPa)
Graphene in water
Graphene-oxide in water
Al2O3 in water
Summary of studies on CHF of nanofluids in flow boiling
Al2O3 (40-50 nm) in water
SS316 tube (5.45 and 8.7 mm I.D.)
ZrO2 (50-90 nm)
Diamond (4 nm)
Inlet subcooling: <20 K
Al2O3 (50 nm) in water
SS316 tube (11 mm I.D.)
Inlet subcooling: 25 and 50 K
Al2O3 (47 nm) in water
Rectangular channel (10 × 5 mm2)
Inlet subcooling: 0 K (saturated)
Single side heating: Cu disk (ϕ = 10 mm)
Al2O3 (25 nm) in water
SS tube (ϕ = 510 μm)
Inlet temperature: 30-404C
nanoparticle material and size,
existence of additives, and
Pressure affects nucleate boiling heat transfer and CHF by influencing physical properties such as the vapor density, latent heat of vaporization, and surface tension of the working fluids. Liu et al. [20, 27] investigated the effect of system pressure on the CHF enhancement of nanofluids, including those with alumina nanoparticles and carbon nanotubes. They found that CHF enhancement in nanofluids is a strong function of system pressure and the enhancement effect is more significant at lower pressures. This discovery is consistent with the system pressure vs. CHF trend of the experimental results obtained by You and his coworkers [5, 22, 28, 29] with an identical heater geometry and experimental setup.
Ionic additives and surfactants can significantly distort the nucleate boiling heat transfer and CHF phenomena in nanofluids by influencing the stability of the particles and their mutual interactions near the heated surface. Kumar and his coworkers [32–35] primarily investigated the effects of ionic additives. Their experimental results demonstrated that when the surface tension of a nanofluid is carefully controlled with ionic additives such as HCl and NaOH, its performance can be further intensified, resulting in a CHF nearly three or four times higher than that of pure water. On the other hand, Kathiravan et al.  conducted pool-boiling CHF experiments on Cu-water nanofluids with and without sodium lauryl sulfate (SDS) anodic surfactant. Although the nanofluid without surfactant exhibited CHF increases of up to 50% (which is consistent with the results of previous studies), the CHF of the nanofluid with surfactant was severely diminished, presumably due to the reduction in surface tension. In conclusion, previous studies reveal that the effect of additives such as ionic additives and polymer surfactants on the CHF performance of nanofluids can be strong, but our current understanding of the effect is very limited. Additional research will be required to understand the role of additives in the nucleate boiling heat transfer and CHF of nanofluids.
Although most CHF experiments with nanofluids have been carried out under pool-boiling conditions, there have been a very limited number of CHF studies in forced convection condition. A group at MIT (USA) reported for the first time that nanofluids can significantly enhance the CHF under subcooled flow boiling conditions [37, 38]. They conducted subcooled flow boiling experiments in a stainless steel tube with an internal diameter of 8.7 mm at a pressure of 0.1 MPa for three different mass fluxes (1,500, 2,000, and 2,500 kg/m2 s). The maximum CHF enhancements were 53%, 53%, and 38% for nanofluids with alumina, zinc oxide, and diamond, respectively, all obtained at the highest mass flux. Kim et al.  performed similar flow boiling CHF experiments in a stainless steel tube with an internal diameter of 10.98 mm at relatively low mass fluxes ranging from 100 to 300 kg/m2 s and inlet subcooling temperatures of 25°C and 50°C. The results for alumina nanofluids confirmed a significant flow boiling CHF enhancement of up to about 70% under all experimental conditions.
Later, a group at POSTECH (South Korea) investigated the flow boiling CHF of nanofluids under saturated conditions [40, 41]. To visualize liquid-vapor two-phase structures in nanofluid flow boiling, they used a rectangular channel made of transparent strengthened acryl with a cross-sectional area of 10 × 5 mm (width × height). The working fluid was heated only on a short-heated surface (a disk with a diameter of 10 mm) placed at the bottom of the flow channel, and a maximum CHF enhancement of 40% was achieved. It was reported using the visualization results that the existence of nanoparticle deposition alters the wetted fraction of the heating surface by cooling liquid under forced convection, delaying the occurrence of the CHF.
Recently, some research tried to assess feasibility of the use of nanofluids for small-sized cooling systems utilizing flow boiling heat transfer. Vafaei and Wen  investigated subcooled flow boiling of alumina-water nanofluids in small single circular microchannels with a diameter of 510 μm and reported an increase of approximately 51% in the CHF at 0.1 vol.%. On the other hand, in similar experiments conducted by Lee and Mudawa  with alumina-water nanofluids at 1.0 vol.%, the CHF point could not be reached due to severe clogging of the circular flow channel (500 μm diameter). Obviously, good stability of nanoparticles in nanofluids is a critical requirement for application to cooling systems with small flow channels.
All the experimental studies listed in Tables 1 and 2 have produced some enhancement in CHF under both pool and flow boiling conditions. To account for the observed phenomena, all probable factors associated with nanoparticles have been thoroughly examined, focusing on the physical properties of nanofluids and nanoparticle-surface interactions. In this section, these investigations and the resulting advances are reviewed to understand the key factors responsible for the increased CHF of nanofluids.
The application of nanofluids to boiling heat transfer was first motivated by their abnormally enhanced thermal conductivity at nanoparticle concentrations on the order of a few percent by volume . However, You et al., in their pioneering research  on CHF enhancement in nanofluids, reported that continued increases in CHF were not observed at concentrations higher than approximately 0.01 vol.%, which is significantly lower than the usual concentration of nanoparticles used for the enhancement of thermal conductivity in nanofluids. Thus, the observed CHF increases could not be explained in terms of the effect of nanoparticles on thermal conductivity enhancement. In addition to thermal conductivity, it was revealed that all other physical properties of dilute nanofluids, including surface tension, vapor and liquid density, viscosity, heat of vaporization, and boiling point temperature, are almost the same as the corresponding properties of pure water [28, 45, 46]. In summary, the transport and thermodynamic properties of nanofluids at low concentration (<0.01 vol.%) are very similar to those of pure water. It can be concluded that changes in the properties of nanofluids do not account for the enhancing effect of nanoparticles on liquid-to-vapor phase-change heat transfer.
To interpret the mechanism of CHF enhancement in nanofluids, two kinds of hypotheses on the roles of nanoparticle during nanofluid boiling were suggested in the early stage of research.
The preceding conclusion on the role of nanoparticle deposition is compatible with the recent work of Kim et al. , who studied pool boiling heat transfer during the quenching of a hot sphere in a nanofluid. They reported that the CHF remained unchanged when a clean sphere was cooled in the nanofluid, and it was only enhanced during the cooling of a sphere with a nanoparticle layer. This result, therefore, confirmed that the deposition layer of the nanoparticles plays a critical role in effectively enhancing the CHF by modifying the heater surface. In conclusion, an understanding of the underlying mechanism should be sought to study the influence of the nanoparticle-deposited surface on CHF.
A consensus explanation of the cause of CHF enhancement in nanofluids seems to be obtainable via an intense study focused on the effect of the nanoparticle layer. In other words, the CHF of a nanofluid is enhanced by its improved ability to actively wet the heater surface, thanks to the porous structure of the thin nanoparticle sorption layers.
hydrodynamic instability model,
macrolayer dryout model,
bubble interaction model,
hot/dry spot model.
In this section, the previous studies aiming to understand the physical mechanism of CHF enhancement in nanofluids in terms of the above predominant CHF theories is reviewed.
where hfg is latent heat of evaporation. However, the initial research of the nanofluid CHF (for example, You et al.  and Kim et al. ) discarded hydrodynamic instability theory as an interpretative tool for CHF in nanofluids because of its inability to account for surface effects. In fact, the CHF correlation of Eq. 4 does not depend on the fluid properties at all, whereas the primary reason for increased CHF in nanofluids is the change in surface characteristics associated with the deposition of nanoparticles during nanofluid boiling. You et al.  concluded using their nanofluid CHF studies that some important unknown factors, potentially missing from Zuber's theory, might be responsible for the increased CHF in nanofluids.
On the other hand, Golubovic et al.  suggested a possible approach to interpreting the CHF mechanism in nanofluids by modifying the hydrodynamic instability models of Lienhard and Dhir  and Ramilison et al. . They hypothesized that a change in the surface contact angle alters the size and spacing of the vapor jets above the heater surface, so that the surface effect on the nanoparticle deposition can be incorporated into the hydrodynamic CHF model proposed by Lienhard and Dhir .
Recently, Park et al.  reported that the CHF of water-based nanofluids containing graphene/graphene-oxide nanoparticles was as high as that of alumina nanofluid, even though neither the wettability nor the capillarity of the surface was improved on the nanoparticle layers. Alternatively, they measured the dewetting wavelengths of water on heater wires and reported that the wavelength change corresponds to the CHF enhancement tendency for all tested nanofluids. Although a direct correlation between the critical instability wavelength obtained from Zuber's theory and the dewetting wavelength of the liquid is questionable, they concluded that the wavelength modulation most adequately supports the CHF enhancement of nanofluids.
Haramura and Katto  proposed the macrolayer dryout model. In this model, CHF occurs due to macrolayer dryout if the heat flux is sufficient to evaporate the macrolayer before the departure of the mushroom bubble. Kim et al.  examined the impact of the improved wettability of a nanoparticle layer (or reduction of the contact angle) on the equivalent thickness of the macrolayer. When the macrolayer thickness was calculated using the model of Sadasivan et al. , they found that a contact angle reduction due to nanoparticle deposition could produce an increase in the thickness of the liquid layer enough to result in a significant increase in the CHF.
where Δτ W and Δτ d are the bubble wait time and departure time at the heated surface, respectively. In Eq. 5, the bubble departure time is a strong function of the nucleation site density (n"). According to the Wang and Dhir  correlation, the site density decreases with the contact angle. Thus, the intensity of the shear stress generated by the mutual interactions of bubbles grows slowly on a heated surface with a low contact angle compared with a surface with a large contact angle. Kim et al.  found that according to the Kolev  model, a change in the contact angle θ can have a major impact on the CHF. Therefore, it could be concluded that the bubble-interaction theory supports the notion of surface wettability improvement as a plausible cause of CHF enhancement in nanofluids.
Hot spot model was first proposed by Unal et al. . This model suggests that the temperature at the center of a dry patch on the heater surface is an important parameter that can trigger CHF. Ability of cooling liquid to rewet the heated dry area should make the strong impact on CHF. In accordance with this idea, Theofanous and Dinh  proposed the modified hot spot model with focus on the micro-hydrodynamics of the solid-liquid-vapor line at the boundary of a hot/dry spot. In their model, CHF occurs when the evaporation recoil force (which drives the liquid meniscus to recede) becomes larger than the surface tension force (which drives the meniscus to advance and rewet the hot/dry spot).
Kim et al.  semi-quantitatively showed that the improved wettability on the nanoparticle-fouled surface significantly increases the surface tension force to rewet the hot/dry spot, suggesting higher CHF. In this regard, they concluded that the hot/dry spot model incorporating the micro-hydrodynamics of a liquid meniscus corroborates the link between increased wettability and CHF enhancement in nanofluids. In addition, Kim et al.  conducted sessile-drop wetting experiments focused on the effect of a nanoparticle layer on the stability of an evaporating meniscus. They found that an individual liquid meniscus is more stable on an alumina nanoparticle layer and hence can sustain the evaporation recoil force at a higher heat flux. The evaporative heat-flux gain attainable on the nanoparticle layer was of the same order of magnitude as the CHF increases in nanofluids. Thus, these experimental results also supported the hot/dry spot theory based on the micro-hydrodynamics of a liquid meniscus.
Several recent studies have demonstrated that the CHF model proposed by Kandlikar and Steinke  is reasonably well correlated with measured CHF data in nanofluids as a function of contact angle (see, for example, [40, 71]). This model utilizes the evaporation momentum force and receding contact angle β as parameters. Fundamentally, this model also focuses on the micro-hydrodynamics of the vapor-liquid interface of a single bubble at the heater surface based on the force balance at the solid-vapor-liquid triple-contact line. The accuracy of this model in predicting the CHF of nanofluids supports the argument that the hot/dry spot model incorporating the micro-hydrodynamics of an evaporating meniscus is a plausible mechanism.
Over the past decade, a considerable amount of research has been carried out in the area of nucleate boiling critical heat flux (CHF) in nanofluids. It is now known that in both pool and flow boiling, the CHF capability of conventional heat transfer fluids (such as water or alcohol) is significantly improved by suspending nanoparticles in the base liquids even at small particle concentrations (less than 0.1 vol.%).
The present review of available studies indicated that there is a general consensus in the key cause of CHF enhancement in nanofluid boiling: the thin nanoparticle layer formed on the heater surface, during nucleate boiling of nanofluids, increases the CHF via their improved ability to wet the heater surface. Although appropriate modifications of all the traditional CHF theories succeed in demonstrating approaches to and possibilities for incorporating the impact of microscale deposition of nanoparticles with nanoscale pores, a sufficiently definite theory to link the improved wettability and the increase of CHF on the nanoparticle layer has not yet emerged owing to the complexity of the phenomenon of CHF and to the lack of information about microscale two-phase flow underneath bubbles. It is very difficult to figure out the underlying mechanism leading to CHF from on relatively large-scale conventional nucleate boiling experiments, which only yield time- and space-averaged information of the complex phenomenon of CHF. In this regard, to understand the fundamental mechanism of CHF enhancement in nanofluids, the efforts by researchers have to focus on obtaining the full details of two-phase heat transfer near the heater surface (for example, direct measurement of the time-dependent temperature and liquid-vapor phase distributions on the heater surface in high heat-flux nucleate boiling).
Another area that merits further study is the effect of pressure and heater geometry. A systematic review of available data in literature revealed that the magnitude of the CHF enhancement in nanofluids is very strongly dependent on system pressure and heater geometry. These parametric effects must be carefully considered when assessing the potential of nanofluids for various industrial applications. For example, it is doubtable if nanofluids significantly enhance CHF even in the high-pressure condition, such as nucleate reactor core. Experiments are needed to extend the nanofluids' usability to many high-flux systems with a wide diversity of heater geometry and pressure conditions.
From a practical point of view, considering application of nanofluids to actual thermal-flow systems, good stability of nanoparticles is one of the critical necessary conditions, as indicated in the review of the microchannel flow boiling applications. Adding ionic additive to control electrostatic condition of solution is one of the simplest options to improve dispersion stability of nanoparticles in nanofluids, but it can severely alter characteristic structures of nanoparticle deposition on a heater surface, resulting in the distorted nucleate boiling CHF performance. There is no systematic study available in literature that describes the effects of additives on nucleate boiling CHF in nanofluids.
This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MEST; grant no. 2010-0018761).
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