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.

Choi 1 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 123. 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.

Boiling with nanofluids enables certain properties to be adjusted by varying the nanoparticle concentration or nanoparticle material, such as the thermal conductivity of the working fluid and the surface wettability of the heater surface. The benefit of less pumping power required for the same heat transfer, compared to just using the base liquid, is also applicable. Nanofluid boiling also results in a build-up of a porous layer of nanoparticles on the heater surface. This layer has been shown to significantly improve the surface wettability; see Figure 6 where the measured changes in the static contact angle on the nanofluid-boiled surfaces compared with the pure-water-boiled surfaces are shown. It is hypothesised that this surface wettability improvement may be responsible for the CHF enhancement observed by almost all of the researchers so far. However, this nanoparticle layer is also considered by some researchers to be also responsible for the deterioration found in the BHT coefficient. Since the nanoparticle deposit creates a resistance in the heat transfer from the heater surface to the fluid, caused by a decrease in the contact angle, and/or produces a reduction in the nucleation site density. The heat transfer mechanisms responsible for the CHF and BHT enhancements and/or deteriorations have not been fully comprehended.

An article by Keblinski et al. 50 is a good overview of enhanced heat conduction in nanofluids, and the possible mechanisms involved. Several mechanisms for the enhancement of thermal conductivity are presented in their article such as Brownian motion of the particles, molecular-level layering of the liquid at the liquid-particle interface and the clustering effect of nanoparticles leading to direct solid-solid paths. Boiling enhancement in nanofluids is thought to be due to several mechanisms: firstly an enhancement via nanoparticle interactions with bubbles 46; secondly, an improvement in the thermal conductivity at the heater surface due to the accumulation of highly conductive nanoparticles forming a porous deposit there 32. Several researchers have noticed this nano-deposition at the heater surface, which can alter the surface area, the surface wettability and the bubble nucleation. Conversely, the nanoparticles gathering at the heater surface as a deposit results in a decrease in the number of nanoparticles available to interact with bubbles. Also the nanoparticle deposit at the heater can result in a loss of nucleation sites at the surface, since the nanoparticles may fill the micro-cavities, resulting in a loss of boiling performance 1323303251. The nucleation site density, bubble departure diameter and bubble frequency are all affected by nanofluid boiling. It has been found by several researchers 432 that bubble diameters increase during boiling with nanofluids, but the nucleation site density decreases with the addition of nanoparticles to the base fluid. Further studies focusing on bubble dynamics and bubble parameters will provide valuable insight into the mechanisms by which nanoparticles affect the heat transfer coefficient.

The research in the literature points to the fact that there is indeed a critical limit for the concentration of nanoparticles in a base fluid that will provide both CHF and BHT enhancements through particle interaction and nanoparticle deposition at the heater surface, but before too many boiling cavities are filled with nanoparticles. Previously illustrated in Figure 3 were the experimental data obtained by You et al. 4, which clearly indicated that there was a certain concentration (<0.01 g/l) after which no further CHF enhancement was found. The same conclusion was identified by Liu et al. 47, who found that an optimal carbon nanotubes mass concentration existed, which provided a corresponding maximum heat transfer enhancement in their experiments.

Formulating stable nanoparticle-in-liquid suspensions (nanofluids) is difficult, and so too is the control of their properties such as thermal conductivity, viscosity and wettability for heat transfer applications. There are some concerns over the dispersion stability of nanofluids 6255253 and of a particle migration effect occurring 29. Certain approaches in preparation of nanofluids can lead to instability problems caused by particle agglomeration in the base fluid. Several researchers have experienced poor stability of nanofluids with sedimentation characteristics occurring. The addition of surfactants (or stabilisers) to nanofluids during the formulation process has been shown to effectively disperse nanoparticles in the base fluids. However, the addition of a surfactant can greatly change the properties of the nanofluid. For example, the surface tension, viscosity and wettability can all be altered, and so the properties of the nanofluid should include the effect of addition not only of the nanoparticles to the base fluid, but also of the surfactant. This could be a reason for the scattering of data found in Table 2 as it is difficult to differentiate whether it is the nanoparticles or the surfactant, which have altered the thermal properties of the base fluid, and it was also not clear in some articles in the literature if a stabiliser or surfactant had been added to the nanofluid. Factors, such as time, temperature, concentration, particle type, dispersion medium and pH, all play important parts in the dispersion stability, with poor dispersion of nanoparticles in the base fluid possibly resulting in poor heat transfer enhancement. It is also essential to have a uniformly dispersed nanofluid when obtaining heat transfer data; otherwise the data will not necessarily be easy to reproduce. It could hence be beneficial to validate the dispersion of nanoparticles in their base fluids with the use of scattering techniques, hence providing a characterisation of the particle distribution.

The scatter of nanofluid boiling data, as shown in Table 2 could hence be due to the nature of the nanofluids used and to what extent the nanoparticles remained suspended in the base fluid, as discussed previously. It has already been shown in the literature that during two-phase cooling in a microchannel 18, nanoparticles can cause catastrophic failure by depositing into large clusters near the channel exit due to localised evaporation once boiling commences. There is some uncertainty over whether degradation over time occurs on the enhancement effect of nanofluids and nanocoatings on the BHT. Table 2 clearly illustrates the conflicting data existing in the literature on the effect of nanofluids on the BHT coefficient. However, it is almost conclusive that the presence of nanoparticles suspended in a base fluid does increase the critical heat flux of the boiling system.

To better understand the use of the terms 'enhancement', deterioration' and 'unchanged' as used in Table 2 boiing heat transfer experimental data have been provided for each of these three terms, see Figures 7, 8 and 9.

Figures 7, 8 and 9 illustrate BHT 'enhancement, 'deterioration' and 'unchanged', respectively. It can be seen in Figure 7 that the BHT is enhanced with the TiO₂-water nanofluid at the two smallest concentrations of 0.00005 and 0.0001 vol.%, as investigated by Suriyawong and Wongwises 48. After 0.0001 vol.%, the BHT starts to deteriorate. It is interesting to note that Figure 8 shows experimental data from the same researchers 48, except that the TiO₂ nanofluid was boiled on an aluminium surface as opposed to a copper surface as seen in the previous figure, Figure 7. The combination of the TiO₂ nanofluid with the aluminium surface led to deterioration in the BHT for all the nanoparticle concentrations investigated. Finally, Figure 9 shows experimental data of You et al. 4, whose investigations of Al₂0₃-water nanofluids on copper surfaces showed no evident change in the BHT over that obtained for pure water. The results presented in the literature are inconsistent even for nanoparticles under similar experimental conditions.

Nanofluids have been shown by nearly all researchers to enhance the CHF during boiling. However, there are conflicting experimental results regarding the effect that nanofluids have on the BHT coefficient, as shown in Table 2. Some researchers have shown that nanofluids provide an enhancement 2029 on the BHT coefficient, others a deterioration 1330, and some others no change at all 428. Further systematic experimental study needs to be performed to understand the mechanisms behind BHT enhancement, and to comprehend why such contradictory data exist among researchers. The BHT coefficient is an important factor, particularly if nanofluid boiling is to be incorporated in the design of engineering systems, such as the cooling of nuclear reactors.

Figure 10 summarises pictorially the main factors affecting nanofluid boiling enhancement. It has been shown by researchers that there are several factors that individually or in combination can play an important role in the nanofluid boiling enhancement. For example, Suiyawong and Wongwises 48 noted an enhancement in the BHT of up to 15% when they investigated TiO₂ pool boiling on copper surfaces, but a deterioration in the BHT when they boiled the same TiO₂ nanofluid on an aluminium heater, see Figures 7 and 8.

Nanoparticle deposition on the heater surface has been observed by nearly all the researchers who have conducted nanofluid boiling, both pool and convective. This is thought to be the main reason behind the critical heat flux enhancement. This nanoparticle layer increases the surface roughness, the surface area, and the surface wettability. The mechanisms underlying this CHF enhancement have still not been clarified, and they remain under discussion and investigation.

Water has been the most commonly used working fluid with nanoparticles so far in the literature. It would be interesting to compare water-based nanofluids with heat-transfer data to be obtained for refrigerant-based nanofluids in the future. There exist already a few experimental studies using refrigerant-based nanofluids, e.g. Peng et al. 2021, Henderson et al. 25 and Park and Jung 38. More experimental data with varying base fluids is required. However, there have already been some reports 25 that water has the greatest aptitude to suspend non-coated nanoparticles, in comparison with other base fluids such as ammonia, hydrocarbons, HFCs and HCFCs.

Boiling performance is dependent on the combined effect of particle concentration, surface properties, and the nature of base fluid (i.e. if it is highly wetting), as indicated by Coursey and Kim 40. If CHF enhancement is due to nanofluids reducing the contact angle, and due to improving wetting, then it might be advisable to simply provide surface treatment (nanocoatings) to the boiling surfaces as opposed to using nanofluids, since already surface oxidation alone has been shown to provide slightly higher heat transfer than nanofluids at a lower wall superheat by 20°C 40.

It has been shown in the literature that the use of nanofluids in boiling is a relevant and pertinent topic. There are many benefits of nanofluid boiling, particularly, in terms of increasing the CHF of the boiling system. However, further research is required before conclusive findings can be presented on the effect of nanofluid boiling on the BHT. It is also important to perform experiments over a long time period, to see if there are any time-dependent effects on the nanoparticle suspensions. Nanofluid boiling has resulted in most researchers finding a porous nanoparticle deposit on the heater surface after vigorous boiling. This deposit is considered by most researchers to be responsible for the CHF enhancement. If this is the case, then it could prove to be just as advantageous to simply pre-coat heater surfaces with nano-deposits instead of boiling with nanofluids, where possible flow passage blockages, particularly in convective flow boiling applications, could be prevented.

BHT: boiling heat transfer; CHF: critical heat flux; PIV: particle image velocimetry.