Boiling heat transfer is used in a variety of industrial processes and applications, such as refrigeration, power generation, heat exchangers, cooling of high-power electronics components and cooling of nuclear reactors. Enhancements in boiling heat transfer processes are vital, and could make these typical industrial applications, previously listed, more energy efficient. The intensification of heat-transfer processes and the reduction of energy losses are hence important tasks, particularly with regard to the prevailing energy crisis.
In terms of boiling regimes, nucleate boiling is an efficient heat-transfer mechanism; however, for the incorporation of nucleate boiling in most practical applications, it is imperative that the critical heat flux (CHF) is not exceeded. CHF phenomenon is the thermal limit during a heat-transfer phase change; at the CHF point the heat transfer is maximised, followed by a drastic degradation after the CHF point. Basically, the boiling process changes from efficient nucleate boiling to lesser-efficient film boiling at the CHF point. The occurrence of CHF is accompanied by localised overheating at the heated surface, and a decrease in the heat-transfer rate. An increase in the CHF of the boiling system would therefore allow for more compact and effective cooling systems for nuclear reactors, air-conditioning units, etc. For decades, researchers have been trying to develop more efficient heat-transfer fluids, and also to increase the CHF of the boiling system which would, in turn, improve process efficiency and reduce operational costs. This is where nanofluids could play a key role; nanofluids could potentially revolutionise heat transfer.
Nanofluids are colloidal suspensions of nanoparticles (length scales 1-100 nm) in a base fluid. These particles can be metallic (Cu, Au) or metal oxides (Al2O3, TiO2, ZrO2), carbon (diamond, nanotubes), glass or another material, with the base fluid being a typical heat-transfer fluid, such as water, light oils, ethylene glycol (radiator fluid) or a refrigerant. The base fluids alone have rather low thermal conductivities. Suspending particles in a base liquid to improve the thermal conductivity is not a new idea; previously the set back for scientists was the particle size. Manufacturing limitations in the past allowed only the creation of microparticles, and these particles quickly settled out of the fluid, and deposited in pipes or tanks, clogging flow passages, causing damage and erosion to pumps and valves, and increasing pressure drop. Nanoparticles, however, can be dispersed in base fluids and remain suspended in the fluid to a much greater extent than was previously achieved with microparticles. This is mainly thought to be due to Brownian motion preventing gravity settling and agglomeration of particles, resulting in a much more stable, suspended fluid.
] first used the term 'nanofluids' in 1995, where he provided results of a theoretical study of suspended copper nanoparticles in a base fluid; he indicated abnormal improved thermal properties of the nanofluids. Further experimental investigations have reported that suspensions containing nanoparticles have substantially higher thermal conductivities than those of the base heat-transfer fluids [1
]. This was initially considered abnormal since such a large enhancement in the CHF, as large as 200% in some cases [4
], could not be interpreted through the existing CHF theories and models. What is also exciting is that only very small volume fractions, i.e. <1%, are required to show enhancement of the thermal base fluid.
Already, there has been significant research into the enhancements in nucleate boiling CHF by the use of nanofluids for pool boiling applications. Research on enhancements of CHF using nanofluids under convective flow conditions have been investigated, but to a lesser extent. It is also interesting to note that the majority of the experimental data provided in the literature are for enhancement effects of nanoparticles or nanofluids on the CHF condition. There is a significant gap in the data presented of the enhancement, which nanofluids have on the boiling heat transfer (BHT) coefficient, which is also a vital piece of information to know for their incorporation in heat-transfer applications. The BHT coefficient is a measure of the heat transfer due to phase change of a liquid during boiling. It is related to the heat flux that is a heat flow per unit area, and the thermodynamic driving force for the heat flow, i.e. a temperature difference.
An interesting advantage of using nanofluids for heat transfer applications is the ability to alter their properties. That is, the thermal conductivity and surface wettability, for example, can be adjusted by varying the particle concentration in the base fluid, and hence allowing nanofluids to be used for a variety of different applications. However, it is also important to note that addition of nanoparticles to a base fluid also changes the viscosity, density and even the effective specific heat; these properties also have a direct effect on the heat transfer effectiveness.
An enhancement of the CHF offers the potential for major performance improvement in many practical applications that use nucleate boiling as their primary heat transfer mode. To implement such heat transfer enhancements in the various applications previously listed, it is of paramount importance to better comprehend the fundamental BHT characteristics of nanofluids and the mechanisms that are at play in both convective and pool boiling regimes.
Nanofluids enhancement on boiling
There are several review articles concerning nanofluids; some on their potential benefits on heat-transfer applications [5
] and also some on their thermal conductivity enhancement [3
]. The use of nanofluids for boiling enhancement is a promising area that is currently being explored by many researchers for pool boiling applications [4
], and more recently, albeit to a lesser extent, in convective boiling applications [17
]. Figure shows the rapid growth in nanofluid boiling research in recent years. The articles shown in the bar chart of Figure are those that have been published in journals between 2003 and 2010; before 2003, there were no published journal articles found using both keywords 'nanofluid' and 'boiling'. (The authors would like to point out that there have been conference articles concerning 'nanofluids' and 'boiling', but only published journal articles have been considered in Figure ). There is a sharp increase in nanofluid boiling research in recent years; this is most likely due to the reported enhanced thermal conductivity of nanofluids, and the relatively large gap in the knowledge that exists, concerning the mechanisms involved in nanofluid boiling enhancement.
Bar chart to illustrate the increasing trend in journal articles dedicated to nanofluid boiling in the last seven years.
This review article has tried to incorporate all dominant pool boiling and convective boiling articles using nanofluids to date. A summary of the main convective and pool nanofluid boiling studies has been provided in Table . It is hoped that this article provides a concise and fair account of the advantages and of the limitations of nanofluids in respect of their boiling performance and application.
Summary of the main convective and pool boiling nanofluid journal articles in the last seven years
Convective flow boiling
Research in convective flow boiling of nanofluids has become more popular in the past two years, perhaps because of the recent demand for high-heat flux cooling of microelectronics components and other compact cooling processes. An experimental study was conducted by Lee and Mudawar [18
] to explore the benefits of using alumina (Al2
) nanoparticles in a water base fluid for microchannel-cooling applications. They found enhancement of the heat-transfer coefficient for single-phase laminar flow; however, in the two-phase regime, the nanofluids caused surface deposition in the microchannels, and large clusters, agglomerates of nanoparticles, were formed. This clogging problem is a serious issue if nanofluids are to be incorporated in microchannel cooling of microelectronics components, where any temperature excursions can result in temperature hot spots and possible thermal failure of the device.
As stated previously in the Introduction
, only low volume concentrations of nanoparticles are required to significantly alter the thermal properties of the base fluids. Ahn et al. [17
] investigated aqueous nanofluids with a 0.01% concentration of alumina nanoparticles; CHF was distinctly enhanced under forced convective flow conditions compared to that in pure water; see Figure . They conducted experiments with varying flow velocities, starting from 0 m/s (effectively pool boiling) up to 4 m/s. A CHF enhancement of 50% was found at 0 m/s, which is consistent with pool boiling CHF enhancement found by previous researchers [30
]. After the boiling experiments, these authors used a scanning electron microscope (SEM) to examine the heater surfaces, and the contact angle was also measured. They determined that the enhancement was mainly due to nanoparticle deposition on the heater surface during vigorous boiling. This deposition caused the contact angle to decrease from 65° to about 12°, illustrating an evident enhancement in the wettability of the heater surface. The experiments performed by Ahn et al. illustrated that nanofluids caused significant CHF enhancements for both pool boiling and convective flow boiling conditions. Figure shows the comparison between the CHF values for water boiling on both a clean surface and on a nanoparticle-fouled surface. Flow boiling CHF enhancement in nanofluids is strongly related to the surface wettability, which is similar to the pool boiling CHF enhancement as will be discussed in the following section on 'Pool boiling'.
Figure 2 Comparisons of CHF values for pure water and nanofluid on the clean surface, and pure water on a nanoparticle-coated surface .
Another investigation by Kim et al. [23
] also resulted in a similar nanoparticle deposition on the heater surface after nanofluid boiling. Kim et al. [23
] investigated the subcooled flow boiling using dilute alumina, zinc oxide and diamond water-based nanofluids. They measured both the CHF and the heat transfer coefficient during their flow boiling experiments. CHF enhancement was found to increase with both mass flux and nanoparticle concentration for all nanoparticle materials; an increase as great as 53% was observed for CHF. The experimental data obtained for the heat transfer coefficient showed little enhancement for the nanofluids at low heat fluxes; a slight enhancement was seen at higher heat fluxes. They also arrived at the same theory as Ahn et al. [17
]; that is, the nanoparticle deposition on the heater is one of the main contributors to the CHF enhancement. In relation to how this nanoparticle deposit can affect the heat transfer coefficient, they came to two conclusions: firstly, that the deposit changes the number of micro-cavities on the surface, and secondly that the surface wettability is also changed. They measured the number of micro-cavities on the surface and the contact angle of the fluid on the surface, and hence obtained an estimation of the nucleation site density at the heater surface. However, whether the nucleation site density was enhanced or found to deteriorate, the heat transfer coefficient remained largely unchanged as that obtained for pure water. They concluded from this that there must be other mechanisms offsetting the effect of nucleation site density enhancement, possibly changes in the bubble departure diameter and/or bubble departure frequency.
Again, Kim et al. [24
] noticed a nanoparticle deposition on the heater surface after nanofluid flow boiling, and considered this to be the main cause behind the CHF enhancement that they observed. They found a CHF enhancement of up to 70%, with a nanoparticle content of less than 0.01% by volume of alumina in water. This again shows that only a small nanoparticle concentration is required to obtain rather dramatic CHF enhancements during flow boiling of nanofluids.
Further experimental data need to be obtained on flow boiling of nanofluids, so as to have a more substantial database, and a better understanding on nanofluid flow boiling mechanisms. In contrast, there is a much greater number of nanofluid pool boiling experiments available in the literature, which are discussed in the following section on 'Pool boiling'.
Pool-boiling experiments with water-based nanofluids containing Al2
nanoparticles were conducted by Kim et al. [32
]. Again, nanoparticle deposition was observed on the heater surface soon after nanofluid boiling was initiated; an irregular porous structure was formed at the surface. This is very similar as to the one that was observed during the convective flow boiling of nanofluids presented in the previous section. Kim et al. [32
] investigated this surface deposition further and noted an enhancement in wettability. They analysed the modified Young's equation and came to the conclusion that wettability enhancement is caused by two combined effects; the first effect they thought to be an increase in adhesion tension; and the second, an increase in the surface roughness. Activation of micro-cavities on the heater surface is inhibited by the nanoparticle deposition (since there is a decrease of contact angle), which leads to a decrease in bubble nucleation in nanofluids. The surface wettability affects the CHF; CHF occurs when dry patches (hot spots) develop on the heater surface at high heat fluxes; these dry spots can be rewetted or can irreversibly overheat, causing CHF. Therefore, an increase in surface wettability promotes dry-spot rewetting, thus delaying CHF.
As presented previously in the section on 'Convective flow boiling', the addition of just a small volume concentration of nanoparticles can provide a significant CHF enhancement, and the same has been achieved during pool boiling of nanofluids as observed by You et al. [4
] in 2003. You et al. measured the CHF in pool boiling using a flat, square copper heater submerged with nanofluids at a sub-atmospheric pressure of 2.89 psia. It should be noted here that in the literature, the pressure has been shown to have a great impact on the BHT and CHF enhancement, with both increasing significantly with a decrease in the system pressure [47
]. The graph in Figure evidences the effect of nanoparticle concentration on the CHF compared to a pure water case. You et al. noted that a 200% CHF increase was measured for a nanofluid containing just 0.005 g/l (approx. 10-4
vol.%) of alumina nanoparticles.
Figure 3 Graph illustrating CHFnanofluids/CHFwater at different concentrations (g/l) of nanoparticles .
Nanofluids were also found by Kim et al. [45
], to significantly enhance the CHF, creating a large wall superheat during pool boiling of water-based nanofluids with 0.01% alumina and titanium nanoparticles. Once again, nanoparticle deposition was observed on the heater surface after vigorous nanofluid boiling. The enhancement of the CHF was found to be of the same magnitude when both nanofluids and pure water were later boiled on the already nanoparticle-fouled heater surface. This implies that the surface modification due to the deposition is the reason behind the CHF enhancement, and that perhaps the working fluid has little effect on the CHF, once the heater surface has already been nanoparticle-fouled. They went on to postulate that the nanoparticle layer increases the stability of the evaporating microlayer underneath a growing bubble on a heated surface, and thus irreversible growth of a hot spot is inhibited, resulting in CHF enhancement when boiling nanofluids.
Further nanoparticle deposition was observed by Bang and Chang [30
], who also measured a CHF enhancement of 50%, with alumina-water nanofluids on a stainless steel plate. They determined that the nanoparticle deposition on the heater after boiling was a porous layer that led to increased surface wettability. However, they also noted a deterioration in the BHT coefficient, which could have been an unfortunate result of the nanoparticle-fouled surface. Das et al. [13
] also observed nanoparticle deposition on the heater surface after boiling. They too noted an increase in wall superheat with increasing nanoparticle concentration, and again degradation in the BHT with the alumina-water nanofluid that they investigated. Kwark et al. [15
] postulated that the decrease in the BHT coefficient with increased nanoparticle concentration, which they observed, can be attributed to the corresponding thicker coating created, which offers increased thermal resistance. CHF, on the other hand, is not dictated by the thickness of the nanoparticle coating, but by the increased wettability that the nanoparticle deposit provides at the heater surface [36
]. They concluded that there is an optimal nanofluid concentration, at which point the CHF enhancement is at a maximum, and without any degradation of the BHT coefficient. They found the optimal concentration to be about 0.025 g/l, and this is also consistent with data found in other studies [4
]. They also demonstrated how the nanofluid boiling performance shows transient-like behaviour dependent on both heat flux and experiment duration, that is prolonging the nanofluid experiments adversely affects the BHT coefficient. Kwark et al. [15
] also investigated possible mechanisms behind the deposition and adhesion of nanoparticles to the heater surface during boiling of nanofluids. Figure illustrates the mechanism as proposed by Kwark et al. [15
], where it is the boiling itself that appears to be the mechanism responsible for the nanoparticle coating formation. This is also consistent with Kim et al. [36
], who postulated that nanoparticles are deposited on the heater surface during nanofluid boiling, hence creating a nanoparticle coating. They assumed that the nanoparticle coating was formed by nucleated vapour bubbles growing at the heater surface and the evaporating liquid that is left behind, inducing a concentrated micro-layer of nanoparticles at the bubble base.
Figure 4 Mechanism of nanoparticle deposition during the boiling process (micro-layer evaporation) .
CHF enhancement in nanofluids has been widely observed by almost all researchers in convective boiling [17
] and in pool boiling [4
]. On the other hand, the BHT coefficient database is fairly inconsistent, and the data are rather scattered. Some researchers report no change of heat transfer in the nucleate boiling regime, some report heat transfer deterioration, and others heat transfer enhancement. Several studies (Kim et al. [36
], Coursey and Kim [40
], Kim et al. [34
], Ahn et al. [17
], Kim et al. [32
], to name but a few) have attributed the CHF enhancement seen during both pool and convective boilings of nanofluids to the improved wettability at the heater surface after the deposition of a nanoparticle layer. Figure clearly shows the nanoparticle deposit left on a NiCr wire after pool boiling of TiO2
nanoparticles, taken from Kim et al. [34
Figure 5 TiO2 nanoparticle-coated NiCr wire after pool boiling CHF experiment of nanofluids with different particle volume concentrations .
The roughness of the nanoparticle-fouled surface is significantly greater than that of the clean surface, due to the nature of the peak-and-valley structure of the deposit. This surface roughness can affect the vapour bubble growth because of the distribution and activation of the nucleation sites.
Kwark et al. [15
] performed two tests to investigate the effect of nano-coated surfaces on pool boiling performance. They used a clean heater with alumina (Al2
) in water nanofluid, and also a nanoparticle-coated heater (this heater had been coated in a previous nanofluid boiling experiment) with pure water. Effectively, the first test built up the nanoparticle coating on the heater surface, and the second test investigated the effect of this coating on the boiling performance in pure water. They found that when the nano-coated heaters were tested in pure water, boiling on the surface may detach some of the nanocoating from the heater surface. However, the overall results showed that pure water with a pre-coated-nanoparticle heated surface provided the same CHF enhancement as nanofluids with the same nanoparticle-pre-coated heated surface, thus demonstrating that it is the surface coating and the enhanced wettability that cause the CHF enhancement that they observed, and not the suspended nanoparticles in the fluid (the nanofluid).
Nanofluid use in BHT has been shown in most cases to contribute to CHF enhancement. Research on surface characteristics indicates that deposition of nanoparticles on the heating surface is one of the main causes behind the CHF enhancement. Surface wettability, liquid spreadability and morphology are some of the heater surface properties altered by the nanoparticle deposition. Figure illustrates how the contact angle drastically changes, dependent on whether the heated surface has been exposed to nanofluid boiling or not. The wettability also changes depending on the nanoparticle concentration in the base fluid, with a two-fold increase in the concentration of Al2O3 nanoparticles in water decreasing the contact angle from 46.5° to 33°.
Figure 6 Water and Al2O3 nanoparticle drops of different particle concentrations on heater surfaces boiled in corresponding nanoparticle concentration nanofluid . (a) θ = 90°, water on clean heater wire; (b) θ = 46.5°, droplet (more ...)
Particle image velocimetry (PIV) has been used to help better comprehend the effects of nanofluids upon boiling. Dominguez-Ontiveros et al. [49
] investigated Al2
nanoparticles in water, and visually observed their effect on nucleate boiling. They noted a change in the hydrodynamic behaviour of bubbles with the addition of nanoparticles to the pure water. Fluid velocities were depressed with nanofluids relative to the pure water case, and they also observed an increase in fluid circulation because of the nanoparticles. A relationship between wall temperature and nanoparticle concentration was found, and the complexity of the nanofluid pool boiling was highlighted. Further research of this nature, that is, the use of high-speed imaging, infrared thermography, PIV techniques, are required to fully comprehend the mechanisms of nanofluid boiling and the role of nanofluids on the enhancement phenomena observed by researchers.