**3. Effects of the Nanoparticle Deposition on the Boiling Heat Transfer Parameters** *3.1. Heat Transfer Coefficient*

The deposited nanoparticles together with the suspended remaining nanoparticles in the base fluid influence the boiling HTC. The conjunction of the substrate properties such as its base material, the vapor/liquid interface features, and the characteristics of the nanoparticles themselves play a relevant role in the pool-boiling HTC enhancement. This role is performed by the force balance changing and dynamics of the three-phase contact line, bubble growth stage, bubble frequency at departure, and wetting and rewetting. It was predictable that the poor thermal conductivity of the deposition layer increased the conduction thermal resistance and, hence, degraded the HTC. However, the experimental data denied it. In this sense, the experimental work conducted by White et al. [10] showed that the pool-boiling HTC increased with increasing thickness of the deposited layer of titanium oxide nanoparticles. The authors stated that no significant deterioration in the HTC of nanofluids was verified in the course of the deposition. Such a fact may reveal that the impact of the thermal resistance of the deposition layer is not significant when compared with that of the pool-boiling heat transfer by convection, even though the thermal conductivity of the titanium oxide layer is relatively low. Moreover, in the work conducted by Kathiravan et al. [60], the authors used high thermal conductivity nanoparticles of copper and observed that the deposited nanoparticle layer deteriorated the boiling HTC. The already published scientific articles proposed that the influence of the thermal resistance of the deposition layer is not significant and the deterioration of the HTC using nanofluids might be linked with the heating surface wettability and roughness alterations. Additionally, it can be stated that the heat flux directly affects the HTC since at low heat fluxes, the impact of the concentration of the nanoparticles on the pool-boiling HTC is negligible given that at low heat fluxes the larger heating surface cavities are the only ones that are active. Nevertheless, at high heat fluxes, the smaller cavities of heating surface are activated as well but the HTC is reduced with the increasing concentration of the nanoparticles [79]. This effect may be caused by the nanoparticle filling of the smaller surface cavities and by the decrease in the nucleation site's density.

Moreover, in specific studies concerning the impact on the pool-boiling HTC of the surface roughness alterations provoked by the deposition of nanoparticles, it has been proposed that the smaller nanoparticles fill the cavities of relatively rough surfaces and, consequently, decrease the surface roughness and number of active nucleation points, which in turn, demises the pool-boiling HTC and increases the wall superheat value [80,81]. Nevertheless, in the study carried out by Das et al. [7], pool-boiling tests were conducted on a considerably rough heating surface using a 0.005 vol. % zirconia nanofluid. The authors reported that in spite of the heating surface roughness decreasing, the HTC increased. On the other hand, the authors Narayan et al. [9] found that in the case of the average size of alumina nanoparticles being similar to the average surface roughness level, the HTC deteriorated with increasing nanoparticle fraction. In addition, Bang and Chang [11] verified that the pool-boiling HTC became lower when the average surface roughness was inferior to the size of the nanoparticles, and although the surface roughness became higher with the increasing concentration of the nanoparticles, the HTC deteriorated. Furthermore, many researchers reported the increment [79], decrement [41], and lack of reaction [81] of the deposited nanoparticles on the boiling HTC. In conclusion, it should be emphasized that the active nucleation site's density directly depends on the surface roughness and wettability, and on the average size of the nanoparticles. If the nanoparticles are small compared with the valleys of the surface roughness profile, the active nucleation site density decreases, and if the nanoparticles are not too small when compared with the average roughness of the surface, the nanoparticles will fill and split the surface cavities and the number of available nucleation points will increase. In the cases where the deposited nanoparticles are bigger than the valleys encountered in the surface roughness profile, the nucleation site density might change differently. The flooded cavities are not able to nucleate bubbles. In fact, cavities that are not completely flooded are the ones able to initiate the nucleation of the vapor bubbles and, hence, augment the HTC. The reduction in the surface wettability may avoid flooding its cavities and, in turn, create a higher number of nucleation sites. The authors Forrest et al. [18] studied the influence of the wettability of the heating surface on the HTC using hydrophilic, super hydrophilic, and hydrophobic heating-coated wires. The wires were coated with different surface-treated silica nanoparticles to generate different wettability effects and it must be stated that no visible change was found in the surface roughness after coating, which might indicate that the nanoparticles coated conformably to the micro-scaled surface deformities. The hydrophobic surface was found to possess a higher number of active nucleation sites and higher HTC, the super hydrophilic surface deteriorated the HTC using water, and in the hydrophilic surface, no alteration was observed in the HTC compared with that of the bare heating surface.
