**2. Nanoparticle Deposition over Boiling Time**

*2.1. Causing Mechanisms*

The causing mechanism for the nanoparticle deposition during pool boiling has been pointed out by Modi et al. [38] to be the evaporation in the microlayer beneath the vapor bubbles, where most of the heat and mass transfer is carried out. In this direction, the researchers Li et al. [39] also reported that the microlayer accumulates the nanoparticles that are deposited in the heating surface when the microlayer fully evaporates. The authors found that the microlayer evaporation was the key factor to promote the growing of the bubbles and that the nanoparticle deposition process would continue for as long as the duration of boiling. Figure 2 schematically represents the deposition of the nanoparticles in the microlayer of the vapor bubble. Moreover, Kim et al. reported in another study [40] that the growth of the deposition layer was promoted by the natural convection or by the gravitydriven sedimentation. Furthermore, as the heating surface temperatures increase beyond the operating fluid saturation point, the vapor bubbles are generated at the nucleation sites and start to grow because of the evaporation of the fluid at the contact line, nearby the zone of the bubbles, and inside the liquid microlayer underneath the vapor bubbles [41].

The buoyancy force is responsible for pushing the bubbles upwards until their detachment occurs. In addition, similar mechanisms generate bubbles within the nanofluids during pool boiling. Nevertheless, due to the evaporation rate of the microlayer, the fraction of the nanoparticles in the region around the heating substrate will be enhanced and the distance between the suspended nanoparticles will be shorter. With these conditions, more collisions are likely to happen between the nanoparticles, agglomeration, and sedimentation over the heat transfer surface. *Nanomaterials* **2022**, *12*, x FOR PEER REVIEW 14 of 45

**Figure 2.** Schematic representation of the nanoparticle deposition in the vapor bubble microlayer. Adapted from [42]. **Figure 2.** Schematic representation of the nanoparticle deposition in the vapor bubble microlayer. Adapted from [42].

### *2.2. Characteristics of the Deposition Layer 2.2. Characteristics of the Deposition Layer*

The first characteristic that can be highlighted for the deposited layer of nanoparticles during the pool boiling with nanofluids is the bonding strength with the heating surface. In this sense, the researchers Kwark et al. [43] reported that even after 16 consecutive pool-boiling experiments, the deposition layer kept a noticeable bonding strength with the substrate. Moreover, the deposition layer of nanoparticles has a porous structure over the heating surface [12,17,44] in the nanofluid pool boiling. The thickness of the porous structure and the associated capillarity effect are also relevant characteristics of the deposited nanoparticle layer. The thickness accommodates the extra liquid microlayer and the capillary wicking drives the fluid toward the dry spots at the base of the growing vapor bubbles. Throughout the pool-boiling process, the characteristics of the heating surface covered by the nanoparticle layer are continuously modified as the thickness of the layer tends to increase over time. The authors Kim et al. [45] investigated the evolution of the CHF under pool-boiling conditions on a heating NiCr wire covered with porous layers of nanoparticles. The authors obtained different deposit structures by changing the heat flux during the boiling of 0.01 volume fraction of titanium oxide nanoparticles suspended in water. The surface properties of the testing wires were measured to identify the parameters closely linked with the appreciable CHF increase. The investigation team noted that the heat capacity of the surface of the wires was altered due to the nanoparticle deposition and, hence, the heat capacity was not the main factor in interpreting the observed CHF enhancement. This experimental work may lead to the conclusion that when the CHF occurs on a very small heat transfer surface, such as a thin wire, it is described as being the consequence of the liquid dryout beneath the vapor patch associated with the dramatic increment in the surface superheat value caused by The first characteristic that can be highlighted for the deposited layer of nanoparticles during the pool boiling with nanofluids is the bonding strength with the heating surface. In this sense, the researchers Kwark et al. [43] reported that even after 16 consecutive pool-boiling experiments, the deposition layer kept a noticeable bonding strength with the substrate. Moreover, the deposition layer of nanoparticles has a porous structure over the heating surface [12,17,44] in the nanofluid pool boiling. The thickness of the porous structure and the associated capillarity effect are also relevant characteristics of the deposited nanoparticle layer. The thickness accommodates the extra liquid microlayer and the capillary wicking drives the fluid toward the dry spots at the base of the growing vapor bubbles. Throughout the pool-boiling process, the characteristics of the heating surface covered by the nanoparticle layer are continuously modified as the thickness of the layer tends to increase over time. The authors Kim et al. [45] investigated the evolution of the CHF under pool-boiling conditions on a heating NiCr wire covered with porous layers of nanoparticles. The authors obtained different deposit structures by changing the heat flux during the boiling of 0.01 volume fraction of titanium oxide nanoparticles suspended in water. The surface properties of the testing wires were measured to identify the parameters closely linked with the appreciable CHF increase. The investigation team noted that the heat capacity of the surface of the wires was altered due to the nanoparticle deposition and, hence, the heat capacity was not the main factor in interpreting the observed CHF enhancement. This experimental work may lead to the conclusion that when the CHF occurs on a very small heat transfer surface, such as a thin wire, it is described as being the consequence of the liquid dryout beneath the vapor patch associated with the dramatic increment in the surface superheat value caused by the merging of the vapor bubble rather than the hydrodynamic instability. The researchers stated that the thickness of the

the merging of the vapor bubble rather than the hydrodynamic instability. The researchers stated that the thickness of the surface porous structures holding an extra fluid

increment. The thickness and structure of the deposition layer is affected by the concentration, size, and shape of the nanoparticles, and by the temperature of the heating substrate, rate of evaporation, and finally, induced heat flux. For instance, in the cases when the surface presents a very high temperature and, hence, enhanced evaporation rate, the fects.

from [47].

ticulate fouling.

surface porous structures holding an extra fluid macrolayer and capillary wicking effect to conduct the working fluid toward the dry regions underneath the vapor bubbles were the key parameters responsible for the CHF increment. The thickness and structure of the deposition layer is affected by the concentration, size, and shape of the nanoparticles, and by the temperature of the heating substrate, rate of evaporation, and finally, induced heat flux. For instance, in the cases when the surface presents a very high temperature and, hence, enhanced evaporation rate, the porous layer becomes thicker and more condensed having larger agglomerations. A higher fraction of nanoparticles dispersed in the base fluid usually conducts to a thicker and more condensed deposition layer. Moreover, the morphology and intrinsic characteristics of the nanoparticles affect the deposition pattern during boiling. Another important characteristic is the roughness of the deposited layer, given that its modification during the boiling process will affect the nucleation site density [15–17]. Furthermore, it was already found that not only were the nanoparticles constituents of the deposited layer, since this one also contains solvable salts sedimented in the course of the pool boiling using nanofluids [46] and the influence of these compounds needs further understanding. The formation of a deposition layer during boiling requires engineering to minimize, or even eliminate, the negative consequences on the boiling heat transfer of nanofluids. Nevertheless, there are positive impacts encountered in most cases in the nanofluids and the boiling-induced nanoparticle deposition relative to the conventional thermal fluids such as, for instance, water. Figure 3 presents some of these positive effects. porous layer becomes thicker and more condensed having larger agglomerations. A higher fraction of nanoparticles dispersed in the base fluid usually conducts to a thicker and more condensed deposition layer. Moreover, the morphology and intrinsic characteristics of the nanoparticles affect the deposition pattern during boiling. Another important characteristic is the roughness of the deposited layer, given that its modification during the boiling process will affect the nucleation site density [15–17]. Furthermore, it was already found that not only were the nanoparticles constituents of the deposited layer, since this one also contains solvable salts sedimented in the course of the pool boiling using nanofluids [46] and the influence of these compounds needs further understanding. The formation of a deposition layer during boiling requires engineering to minimize, or even eliminate, the negative consequences on the boiling heat transfer of nanofluids. Nevertheless, there are positive impacts encountered in most cases in the nanofluids and the boiling-induced nanoparticle deposition relative to the conventional thermal fluids such as, for instance, water. Figure 3 presents some of these positive ef-

*Nanomaterials* **2022**, *12*, x FOR PEER REVIEW 15 of 45

**Figure 3.** Positive effects of the nanofluids and nanoparticle deposition relative to water. Adapted **Figure 3.** Positive effects of the nanofluids and nanoparticle deposition relative to water. Adapted from [47].
