2.3.5. Roughness

With increasing concentration of the nanofluids, the deposited layer of nanoparticles grows on the heat transfer surface. However, the published results do not always confirm the direct relation between surface roughness and nanofluid concentration. This may be caused by the lack of uniformity of the deposited nanoparticle layer on the heating surface. Such a fact might conduct to higher heat transfer rates with the pool-boiling nanoparticle-deposited surfaces as compared with the ones obtained through the pool boiling of water alone. Nevertheless, it is possible to settle a well-defined correlation between the roughness of the surface and the deposited layer. Moreover, the authors Wen et al. [62] investigated the impact of surface roughness using rough and smooth surfaces. For the smooth surface, the increase in particle deposition on the heating surface led to an increased surface roughness. In the case of the rough surface, no considerable variation in the sur-

face roughness was reported with increasing nanofluid concentrations. The researchers argued that the surface modification was performed by the features associated with the nanoparticles after the boiling process. Furthermore, the surface roughness was found to increase with an increasing wall superheat value for rough surfaces. In addition, the pool-boiling heat transfer of rough surface using nanofluids was around two-fold greater than that of the water alone. A reduction of around 30% in the heat transfer was reported for the pool boiling with nanofluids for a plain surface when compared with the base fluid itself. Although the surface roughness suffers a decrement from 0.167 to 0.099 µm after boiling completion, it achieved a CHF amelioration, which was likely due to alterations in the surface microstructure and topography. Moreover, Ham et al. [3] further investigated the boiling heat transfer of aluminum oxide nanoparticles dispersed in water on heating surfaces with different average roughness. The volume fraction of the nanoparticles ranged between 0 and 0.05 and the average surface roughness was of 177.5 nm and 292.8 nm. The researchers found that when the volume fraction was increased from 0 to 0.05, the CHF increased by 224.8% and 138.5% on the surfaces with roughness of 177.5 nm and 292.8 nm, respectively. The heat transfer capability of the aluminum oxide nanofluid with a 0.05 vol. % of concentration was lower than the one obtained with deionized water at R<sup>a</sup> = 177.5 nm, but the maximum HTC value increased due to the increase in the CHF. Furthermore, Figure 5 schematically illustrates the boiling-induced nanoparticle deposition on two different rough heating surfaces. the surface roughness was reported with increasing nanofluid concentrations. The researchers argued that the surface modification was performed by the features associated with the nanoparticles after the boiling process. Furthermore, the surface roughness was found to increase with an increasing wall superheat value for rough surfaces. In addition, the pool-boiling heat transfer of rough surface using nanofluids was around two-fold greater than that of the water alone. A reduction of around 30% in the heat transfer was reported for the pool boiling with nanofluids for a plain surface when compared with the base fluid itself. Although the surface roughness suffers a decrement from 0.167 to 0.099 µm after boiling completion, it achieved a CHF amelioration, which was likely due to alterations in the surface microstructure and topography. Moreover, Ham et al. [3] further investigated the boiling heat transfer of aluminum oxide nanoparticles dispersed in water on heating surfaces with different average roughness. The volume fraction of the nanoparticles ranged between 0 and 0.05 and the average surface roughness was of 177.5 nm and 292.8 nm. The researchers found that when the volume fraction was increased from 0 to 0.05, the CHF increased by 224.8% and 138.5% on the surfaces with roughness of 177.5 nm and 292.8 nm, respectively. The heat transfer capability of the aluminum oxide nanofluid with a 0.05 vol. % of concentration was lower than the one obtained with deionized water at Ra = 177.5 nm, but the maximum HTC value increased due to the increase in the CHF. Furthermore, Figure 5 schematically illustrates the boiling-induced nanoparticle deposition on two different rough heating surfaces.

face. Such a fact might conduct to higher heat transfer rates with the pool-boiling nanoparticle-deposited surfaces as compared with the ones obtained through the pool boiling of water alone. Nevertheless, it is possible to settle a well-defined correlation between the roughness of the surface and the deposited layer. Moreover, the authors Wen et al. [62] investigated the impact of surface roughness using rough and smooth surfaces. For the smooth surface, the increase in particle deposition on the heating surface led to an increased surface roughness. In the case of the rough surface, no considerable variation in

*Nanomaterials* **2022**, *12*, x FOR PEER REVIEW 20 of 45

**Figure 5.** Schematic illustration of the boiling nanoparticle deposition on two different rough surfaces: (**a**) Ra and Rz with very different values and (**b**) Ra and Rz with approximate values. **Figure 5.** Schematic illustration of the boiling nanoparticle deposition on two different rough surfaces: (**a**) Ra and Rz with very different values and (**b**) Ra and Rz with approximate values.

The authors Ji et al. [63] made a qualitative analysis of the boiling-induced nanoparticle deposition on two different rough heating surfaces. The authors stated that it was easier to obtain a uniform deposition layer with a higher Ra and lower RZ rough surface in the corresponding 5 (b) case. The mass concentration lines at the same height in the two different cases represent the equality of the concentration. The authors observed too that the heating surface with lower Ra and larger RZ is not so easy to be uniformly coated by the nanoparticles. With lower weight fractions of the nanoparticles, it is possible to observe higher peaks outside the surface touching the water. Therefore, the more roughed surface was the one that exhibited improved heat transfer behavior. It can be stated that the number of deposited nanoparticles induces variations in the surface roughness of the heating surface. The continuous growth of the deposited layer produces greater thermal insulation, which leads to the decrement of the CHF. Further experimental studies are The authors Ji et al. [63] made a qualitative analysis of the boiling-induced nanoparticle deposition on two different rough heating surfaces. The authors stated that it was easier to obtain a uniform deposition layer with a higher R<sup>a</sup> and lower R<sup>Z</sup> rough surface in the corresponding 5 (b) case. The mass concentration lines at the same height in the two different cases represent the equality of the concentration. The authors observed too that the heating surface with lower R<sup>a</sup> and larger R<sup>Z</sup> is not so easy to be uniformly coated by the nanoparticles. With lower weight fractions of the nanoparticles, it is possible to observe higher peaks outside the surface touching the water. Therefore, the more roughed surface was the one that exhibited improved heat transfer behavior. It can be stated that the number of deposited nanoparticles induces variations in the surface roughness of the heating surface. The continuous growth of the deposited layer produces greater thermal insulation, which leads to the decrement of the CHF. Further experimental studies are needed to establish the optimum value of the deposition layer thickness that maximizes the CHF.
