*1.2. Boiling of Nanofluids*

A common method of modify the properties of the coolant in boiling applications is the addition of various types of nanoparticles to the base fluid [30–32]. This changes its thermophysical properties, but more importantly, the nanoparticles are deposited onto the boiling surface during boiling, creating micro- and nanostructures that can favorably affect the boiling process [33]. The available literature mostly suggests that the boiling of nanofluids leads to an improvement of the HTC and the CHF, while some few studies make the opposite conclusion [34]. The increase in HTC and CHF tends to be correlated with increasing the concentration of nanoparticles, up to a certain point. At excessive

concentrations, the HTC begins to decrease, while the CHF will remain unaltered [35]. At high nanoparticle concentration, the deposited layer on the surface becomes thicker, which leads to greater thermal resistance and decreases the HTC [35]. Due to the deposition of nanoparticles on the surface and their large effect on heat transfer during boiling, Fang et al. [31] suggested that further experiments on nanofluid boiling should be combined with a comprehensive study of the influence of nanoparticle deposition time for different concentration ranges. Long-term experiments on the variation of nanoparticle deposition time should be performed, and correlations to predict the evolution of deposited nanoparticle layers should be proposed. In addition to the concentration, the material and size of the nanoparticles and the preparation of the nanofluid also affect the boiling performance. Dadhich et al. [36] and Fang et al. [31] pointed out that there is a significant need for a database of thermal properties for different materials in different size ranges of nanoparticles.

The deposition of nanoparticles from the nanofluid during boiling is the most common enhancement strategy involving modified fluids. Manetti et al. [37] conducted an experimental study of HTC in pool boiling of deionized water and Al2O3-water nanofluid with low (0.0007 vol.%) and high (0.007 vol.%) volume concentration on smooth and rough copper surfaces within a heat flux range of 100 to 800 kW m−<sup>2</sup> . They observed that HTC increased at low concentrations at average heat flux values, which was related to increasing the radius of the cavities through deposition of nanoparticles on the surfaces due to boiling. Increasing the heat flux led to a decrease in the HTC caused by the increased deposition rate of nanoparticles on the rough surface and the filling of the cavities with nanoparticles. Ahmed et al. [38] investigated pool boiling heat transfer performance on horizontal flat copper surfaces using nanofluid and pure water. They used 40–50 nm alumina nanoparticles to prepare three different nanoparticle concentrations (0.01 vol.%, 0.1 vol.%, and 1 vol.%). After performing boiling experiments, the nanoparticle-coated surfaces were used for pool boiling experiments using pure water. During the nanofluid boiling experiments, the authors concluded that the concentration of nanoparticles has a major effect on the heat transfer performance. At lower concentrations, the rate of deposition of the particles is lower, resulting in a greater enhancement of heat transfer, which can be attributed to the fact that the increased thermal conductivity of nanofluids has a greater dominance than the effect of nanoparticle deposition on the surface. On the other hand, the boiling of pure water on the surface coated with nanoparticles showed that high deposition rates lead to an improvement in heat transfer, which was explained by a less uniform deposition layer on the surface. Huang et al. [39] studied the enhancement of boiling on nickel wires coated with TiO<sup>2</sup> nanoparticles in pure water. The coating was produced by electrical heating of the wire in nanofluids with concentrations from 0.01 to 0.1 wt.% and heat flux up to 1000 kW m−<sup>2</sup> . Experimental results of pure water boiling showed an enhancement of the CHF of up to 82.7% for coated nickel wire prepared in 0.1 wt.% nanofluids. On all nickel-coated wires, the HTC deteriorated due to the higher thermal resistance caused by the deposition of nanoparticles. Kiyomura et al. [40] investigated boiling heat transfer performance of surfaces coated with Fe2O<sup>3</sup> nanoparticles in Fe2O<sup>3</sup> water-based nanofluid at a high (0.29 g/L) and a low concentrations (0.029 g/L). The results showed the highest HTC values on coated copper surfaces with low mass concentration, and with increasing the concentration, the roughness of the surfaces increased. Salimpour et al. [41] performed boiling experiments on smooth and rough copper surfaces using iron-oxide-water-based nanofluid at low and high heat fluxes. They found that at low heat fluxes on smooth surfaces, and at high heat fluxes on rough surfaces, the deposition of nanoparticles on the surface enhanced the heat transfer during boiling. According to other research conducted in this field, there is no clear understanding of how the boiling heat transfer performance is changed by the various effects of nanoparticle deposition during nanofluid boiling [42,43].

In addition to in situ nanoparticle deposition, dip coating and drop casting techniques were also proposed for boiling applications. An example is the study by Yim et al. [44], who investigated the surface wettability in nucleate pool boiling on aluminum surfaces coated with TiO<sup>2</sup> nanoparticles from 1 wt.% TiO<sup>2</sup> ethanol-based nanofluids, using a drop casting technique. The obtained results showed that the performance of nucleate pool boiling was 64.1% higher for TiO2-coated surfaces than for the bare surfaces.
