2.3.12. Re-Suspension of the Nanoparticles

The re-suspension of the deposited nanoparticles of the nanofluids can mitigate or, in some cases, even eliminate the potential deterioration of the nanofluids over time. Indeed, this re-suspension promotes the long-lasting heat transfer amelioration of the nanofluids. The particle re-suspension occurrence is usually observed in nature in, for instance, the re-suspension of the sediments of a riverbed that are carried away by water flow and the re-suspension of sand on the ground, which is blown away by the wind. In particular, the re-suspension of wall-surface-adhering particles by the action of turbulent flows has received great attention over the past few years from researchers. Nevertheless, most of the published scientific articles concerned with nanoparticle suspension comprise single-phase working fluids, and only a few deal with the two-phase fluid condition as it occurs, for example, in boiling. Since the boiling process may induce a very high fluid flow and disturbance of the vapor bubbles, the re-suspension of the nanoparticles deposited onto the heating surface is a very probable phenomenon. If those nanoparticles can be re-suspended, it generally denotes that the time-dependent deterioration of the nanofluids can be relieved without the help of any additional means or methods. This degradation mitigation of the nanofluids over time is a very promising way to achieve long-lasting heat transfer performance. Under pool-boiling scenarios, the movement of the working fluid is induced by convection and movement of the bubbles provoked by buoyancy action. A large part of the existing boiling research is derived from pool-boiling experiments. Consequently, pool boiling seems to be a suitable starting point for the indepth study of the particle re-suspension behavior that can separate the effect of boiling

from the action of the convection-driven flows on the nanoparticle re-suspension. In the work developed by [76], pool-boiling experiments were performed using surfaces with deposited nanoparticles produced by nanofluids. The researchers found the re-suspension ratio of these nanoparticles for different boiling times, densities of the deposition layers, and varying heat fluxes to infer on the impact of the experimental parameters on the re-suspension of the nanoparticles. The authors reported that a certain fraction of the deposited nanoparticles was re-suspended into the liquid bulk and, after that, migrated along with the liquid flow during the boiling process. Since the re-suspension does not take place before boiling, the disturbance suffered by the vapor bubble disturbance in the course of boiling must be an important factor for the re-suspension of the nanoparticles. Besides that, the main difference in the deposition surface between before and after boiling is the porosity of the surface. Hence, it can be stated that the porosity of the deposition layer is the major contributor for the re-suspension of the nanoparticles in the liquid bulk during the boiling process. For a better understanding, it can be noted that when a vapor bubble is in its nucleation state of evolution, the fluid near the bubble will be vaporized because of the superheat. The porous structure of the deposition layer enables the fluid to be vaporized in the layer to be enlarged from the outside of the layer, resulting in a fluid flow from outside toward the inside of the layer. This flow will steadily impinge on the deposited nanoparticles. In the cases where there is no fluid flowing through the deposition layer porous structure, the forces acting on the deposited nanoparticles are gravity, adhesive forces from the interacting neighboring nanoparticles, and brace force. In the cases where the outside-to-inside flow occurs through the pores, the operating fluid will create a dragging force on the nanoparticles around the pore. This force provokes the motion of the nanoparticles that tend to follow the fluid flow. As the generated vapor bubble continues to grow, the drag force increases. At the departure stage, when the vapor bubbles begin their raising from the surface, an extreme disturbing effect at the back region of the departing bubbles occurs. Under these conditions, the drag force overcomes the gravitational effect and forces of adhesion and provokes the detachment of the nanoparticles from the deposition layer, which moves together with the flowing fluid. Given that the fluid flow at the wake of the rising vapor bubbles is also upward driven, the detached nanoparticles will rise upwards together with the liquid flow and are, by this way, resuspended. Hence, it should be emphasized that two main factors can be assumed for the re-suspension of the nanoparticles: the first is the effect of the fluid flow through the porous structure of the deposition layer during the growth stage of the bubbles and the second is the disturbing effect at the departure of the bubbles. Apart from these factors, many other factors may impact on the nanoparticle re-suspension trend. That is, in the case of the growing and rising process of the vapor bubbles, the nanoparticles dragged from the deposition layer can be re-suspended in the working fluid as a consequence of the departure of the rising bubbles or the breaking-up of the bubbles at the liquid and vapor phases interface. Nevertheless, further experimental and numerical works should be carried out to provide a better understanding of the governing mechanisms of the nanoparticle re-suspension. In the above-mentioned work, the authors plotted the re-suspension ration versus the deposition area density using 20 KW/m<sup>2</sup> , 50 KW/m<sup>2</sup> , 80 KW/m<sup>2</sup> , and 100 KW/m<sup>2</sup> of heat flux. Moreover, the authors arrived at the following conclusions:


conclusions:

increased with increasing density of the deposition zone. When there were imposed moderate heat fluxes ranging between 50 kW/m<sup>2</sup> and 80 kW/m<sup>2</sup> , the re-suspension ratio initially increased and then decreased with increasing deposition area density. When high heat fluxes of around 100 kW/m<sup>2</sup> were applied, the re-suspension ratio decreased with the density of the deposition area. posed moderate heat fluxes ranging between 50kW/m2 and 80 kW/m2, the re-suspension ratio initially increased and then decreased with increasing deposition area density. When high heat fluxes of around 100 kW/m2 were applied, the re-suspension ratio decreased with the density of the deposition area. Moreover, Chen et al. [77] studied the re-suspended nanofluid pool boiling under

case of the growing and rising process of the vapor bubbles, the nanoparticles dragged from the deposition layer can be re-suspended in the working fluid as a consequence of the departure of the rising bubbles or the breaking-up of the bubbles at the liquid and vapor phases interface. Nevertheless, further experimental and numerical works should be carried out to provide a better understanding of the governing mechanisms of the nanoparticle re-suspension. In the above-mentioned work, the authors plotted the re-suspension ration versus the deposition area density using 20KW/m2, 50KW/m2, 80KW/m2, and 100 KW/m2 of heat flux. Moreover, the authors arrived at the following

1. During boiling, a fraction of the deposited nanoparticles may be re-suspended within the operating fluid and, after that, migrate along with the fluid flow. 2. The nanopores of the deposition layer have a cardinal part in the re-suspension phenomenon and fluid flow through the pores at the growing stage of the vapor bubbles and the disturbance at the rear zone of the departing bubbles should be

3. The re-suspension ratio increased with increasing heat flux, given that the former increased near 300% when the heat flux was enhanced from 20 to 100 kW/m2. Moreover, when low heat fluxes near 20 kW/m2 were applied, the re-suspension ratio increased with increasing density of the deposition zone. When there were im-

Moreover, Chen et al. [77] studied the re-suspended nanofluid pool boiling under the action of an electric field. The investigation team also discussed the difference in the thermal behavior between the re-suspended nanofluid and the base fluid alone. Figure 7 schematically illustrates the distribution of the deposited nanoparticles before and after the re-suspension phenomenon. Figure 8 shows the fluid flow and heat transfer of the nanoparticle re-suspension. the action of an electric field. The investigation team also discussed the difference in the thermal behavior between the re-suspended nanofluid and the base fluid alone. Figure 7 schematically illustrates the distribution of the deposited nanoparticles before and after the re-suspension phenomenon. Figure 8 shows the fluid flow and heat transfer of the nanoparticle re-suspension.

*Nanomaterials* **2022**, *12*, x FOR PEER REVIEW 27 of 45

taken as the main factors for the re-suspension.

**Figure 7.** Boiling-induced nanoparticle deposition time-dependent features. **Figure 7.** Boiling-induced nanoparticle deposition time-dependent features.

**Figure 8.**Fluid flow and heat transfer of the nanoparticle re-suspension. **Figure 8.** Fluid flow and heat transfer of the nanoparticle re-suspension.

The authors stated the following conclusions:


process for the creation of a semiconductor titanium oxide nanoparticle film deposited onto a FTO (F-doped tin oxide) glass conductive substrate. A pool-boiling apparatus was employed to deposit the titanium oxide 20 nmsized nanoparticle nanofluid. The boiling of the nanofluid directly on the FTO glass substrate enables the deposition of the nanoparticles onto its surface. Using the as-deposited films, the crystal growth of the titanium oxide nanoparticles was controlled by altering the temperature, duration, and ramping rate of post-sintering. A densely packed titanium oxide layer was obtained for the as-deposited substrate through the pool-boiling process. For the maximum temperature at 550 °C, the titanium oxide grain sizes became larger and near 50 nm and more round-shaped titanium oxide nanostructures were observed. This work demonstared for the first time how the sintering of titanium oxide nanoparticles proceeds for the nanoporous totanium oxide films. It was observed that the titanium oxide nanoparticles fused with each other and crystal growth happened through the neighboring 2 to 4 nanoparticles at 550 °C.Hence, an extra beneficialfeature of the pool-boiling-induced nanoparticle deposition is that the heating surface will begin to sinter the thin deposited layer. Although this heat treatment is not enough to obtain the required properties, it is enough to increase the stability of the deposited layer. By taking advantage of the stability of the film and the sintering properties of the titanium oxide nanoparticles, a post-sintering treatment after the nanoparticle deposition considerably impacts on the prodictionof a uniform, robust, and dense film. In conclusion, this work demonstrated for the first time that sequential pool-boiling and sintering processes are

alternative procedures to create uniform porous titanium oxide layers.

2.3.13. Sintering of the Nanoparticles

ductivity enhancement of the heat transfer media.
