*4.1. Concentration of the Nanoparticles*

The published results showed different characteristics of the natural thermo-convection of the nanofluids according with the concentration of the nanoparticles in the base fluid. One was the boiling heat transfer degradation for, in the majority of cases, volume fractions greater than 0.1. Another was the occurrence of optimum heat transfer rate and coefficient at a certain concentration level, beyond which a decrement was reported. Hence, each and every nanofluid may enhance theboiling heat transfer in an exact volume fraction of included nanoparticles for an exact case depending on the stability of the nanofluid, nature of the base fluid, thermal condition and cavities morphology, and density of the heating surface [87]. As was previouslymentioned throughout the present work, nanoparticle deposition on the pool-boiling heating surface is enhancedwith afraction of the nanoparticles

suspended in the base fluid. An excessive nanoparticle deposition may cause the thickening of the deposition layer, which in turn, may lead to an increase in the thermal resistance and, consequently, to the pool-boiling heat-transfer deterioration. This fact was confirmed through the experimental work conducted by the researchers Mukherjee et al. [88] using silica nanofluids having different volume fractions. The authors reported that with lower concentrations of 0.0001 vol. % and 0.001 vol. %, the used nanofluids exhibited limited nanoparticle deposition, inducing a great number of nucleation sites and improving the heat transfer performance. Additionally, when concentrations of 0.01 vol. % and 0.1 vol. % were employed, a greater nanoparticle deposition occurred. The development of a thick deposit further impeded the pool-boiling heat transfer. The results confirmed an increment in the HTC and CHF for 0.0001 vol. % and 0.001 vol. % and a decrement with 0.01 vol. and 0.1 vol. fractions. Furthermore, and according tothe authors Kim et al. [40], the deposition layer produces a continuous modification in the surface morphology that directly influences the boiling heat transfer. Such surface morphology continuous alteration depends strongly on the nanoparticle concentration. In the cases where low nanoparticle volume fractions of 0.0001 and 0.001 were employed, only a slight modification of the heating surface was observed. However, when the concentration was increased, the nanoparticle deposit thickened, and more micro-scaled structures developed on the surface caused by the clustering of the silica nanoparticles. Figure 9 schematically represents the nanoparticle deposition process using different weight fractions of nanofluids. Mukherjee et al. [89] reported that the deposited nanoparticles fill the surface cavities, producing a smoother final surface and also concluded that a grater nanoparticle deposition led to a smoother heating surface. The authors noted that the surface modification is less pronounced at lower concentrations of 0.01 vol. % and 0.1 vol. %, due to the smaller amount of available nanoparticles and the higher stability of the used nanofluids that avert any further settlement of the nanoparticles. Nevertheless, such a tendency changes when the volume fraction is high at 1%. At this concentration, the amount of suspended nanoparticles is considerable and their deposition rate is greater and sufficient to fill up the cavities and alter the texture of the heating surface. Owing to a lesser change inthe surface and improved stability, the nanofluids exhibited enhancements in the HTC and CHF and the opposite trend was found at higher nanoparticle fractions.The effect of nanoparticle concentration in the base fluid was also investigated by Kole and Dey [90]. Two diverse concentrations of zinc oxide nanoparticles having a size of 30–40 nm in ethylene glycol were evaluated. It was observed that, after pool boiling, the nanoparticles were deposited over the heating surface, which prevented the active nucleation sites, and, therefore, the HTC decreased. By measuring the CHF values through the use of a thin copper–nickel alloy, the authors reported a considerable increase in the CHF by increasing the concentration of zinc oxide. The investigation team verified a maximum CHF enhancement of 117% for zinc oxide nanoparticle volume fractions of 2.6%. Moreover, the authors Minakov et al. [36] demonstrated that even at the 0.25 vol. % concentration of nanoparticles, the CHF increased by more than 50% and continued to grow with further increases in the nanoparticle fraction. It was stated that at high concentrations of nanoparticles, the growth rate of CHF slowed down and reached a constant value. Such behavior was due to the stabilization of the deposit size on the heat transfer surface. Moreover, the researchers Ahmed and Hamed [91] studied the pool-boiling heat transfer on smooth copper surfaces using nanofluid and water. Alumina nanoparticles of 40–50 nm were employed to prepare 0.01 vol. %, 0.1 vol. %, and 1 vol. % nanoparticle concentrations, and after performing the boiling experiments, the nanoparticle-coated surfaces were employed for pool-boiling experiments using pure water. The authors found that the concentration of the nanoparticles had great impact on the heat transfer behavior. At lower concentrations, the rate of deposition of the nanoparticles was lower, resulting in a greater improvement of the heat transfer, which can be attributed to the fact that, at low fractions of nanoparticles, the superior thermal conductivity of the nanofluids prevailed over the effect of the nanoparticle deposition onto the heating surface. In addition, the boiling of water on the surface coated with nanoparticles demonstrated that

high deposition rates lead to an improvement in the heat transfer, which may be due to the less uniform deposited layer on the surface. Moreover, the authors Coursey and Kim [92] examined the potential of dispersed alumina nanoparticles in water and ethanol. The researchers demonstrated that at low concentrations of alumina, the CHF value remained unchanged, whileat higher nanoparticle concentrations, the CHF value was improved by 37%. In addition, the published results revealed that the carbon nanotube-based nanofluids exhibited appreciably higher thermal features, including thermal conductivity, boiling convective HTC, and CHF compared with the base fluids themselves, as well as other types of nanofluids. These enhanced properties further increased with increasing carbon nanotube concentration and temperature [93]. *Nanomaterials* **2022**, *12*, x FOR PEER REVIEW 34 of 45

**Figure 9.**Schematic representation of the pool-boiling-induced nanoparticle deposition for different concentrations and effects on the heat transfer parameters. **Figure 9.** Schematic representation of the pool-boiling-induced nanoparticle deposition for different concentrations and effects on the heat transfer parameters.
