*3.4. Comparison of the Overall Boiling Performance*

It was observed that regardless of the size of the nanoparticle, an increase in concentration had the same effect on the CHF and HTC, which agrees with the findings of previous research [33,35,62–64]. The research groups in this field concluded that an increase in nanoparticle concentration leads to the creation of a fouling layer on the surface, which deteriorates HTC due to a decrease in active nucleation site density. In our case, the laser-induced microcavities were filled with nanoparticles, leading to a decrease in contact angle and a reduction in the number of cavities suitable for bubble nucleation [65,66]. Additionally, the adhesion energy, which is defined as the horizontal component of the surface tension force acting against the bubble growth, significantly increased with nanoparticle deposition. This caused the bubble departure frequency to decrease and prolonged their growth times. On top of that, the fluid was wetting the entire deposited layer, which lead to deterioration of the active nucleation sites on the surface [67]. On the other hand, the layer formed on the surface improved the wettability, capillary wicking action, and constitution of inflow liquid inside the fouling layer [62,63,68], which effectively increased the achievable heat flux values without CHF incipience.

A comparison of the boiling performance of nanofluids with small nanoparticles with the performance of twice-distilled water on the reference and LT surface is shown in Figure 13. Nanofluids at both concentrations deteriorated the heat transfer parameters, in comparison with the boiling of pure water, on the LT surface. Additionally, at the low concentration, the boiling performance also decreased compared to the boiling performance of water on the untreated reference surface. The highest deterioration in HTC recorded at CHF was measured to be 60% at the low and 89.5% at the high nanoparticle concentration compared to the HTC recorded on the LT surface with the base fluid (water) at CHF.

**Figure 13.** Comparison of boiling characteristics of TiO2-water nanofluids with small size nanoparticles with the performance of pure water on reference and LT surface (1st and 5th run): boiling curves (**a**) and heat transfer coefficients (**b**)*.* **Figure 13.** Comparison of boiling characteristics of TiO<sup>2</sup> -water nanofluids with small size nanoparticles with the performance of pure water on reference and LT surface (1st and 5th run): boiling curves (**a**) and heat transfer coefficients (**b**). **Figure 13.** Comparison of boiling characteristics of TiO2-water nanofluids with small size nanoparticles with the performance of pure water on reference and LT surface (1st and 5th run): boiling curves (**a**) and heat transfer coefficients (**b**)*.*

Furthermore, Figure 14 shows a comparison of the boiling performance of nanofluids with the performance of twice-distilled water on the reference and LT surface. Here, a deterioration of the HTC when using the nanofluid instead of pure water is also recorded. Furthermore, Figure 14 shows a comparison of the boiling performance of nanofluids with the performance of twice-distilled water on the reference and LT surface. Here, a deterioration of the HTC when using the nanofluid instead of pure water is also recorded. Furthermore, Figure 14 shows a comparison of the boiling performance of nanofluids with the performance of twice-distilled water on the reference and LT surface. Here, a deterioration of the HTC when using the nanofluid instead of pure water is also recorded.

**Figure 14.** Comparison of boiling characteristics of TiO2-water nanofluids-small size NPs with base fluid on the reference and LT surface (1st and 5th run): boiling curves (**a**) and HTCs (**b**). **Figure 14.** Comparison of boiling characteristics of TiO2-water nanofluids-small size NPs with base fluid on the reference and LT surface (1st and 5th run): boiling curves (**a**) and HTCs (**b**). **Figure 14.** Comparison of boiling characteristics of TiO<sup>2</sup> -water nanofluids-small size NPs with base fluid on the reference and LT surface (1st and 5th run): boiling curves (**a**) and HTCs (**b**).

Figure 15 shows a comparison of the performance of the laser-textured surface, when used either with water (denoted as LT), or with nanofluids, relative to the boiling performance of pure water on the untreated reference surface (REF). At low and medium heat flux values, the reference surface provides superior performance during the first experimental run and only at high heat fluxes, the laser-texturing and nanoparticle deposition enhance the HTC (Figure 15a). However, after 5 experimental runs were performed on each surface, the performance of the laser-textured surface used with pure water or 0.001 wt.% large nanoparticle nanofluid significantly exceeds the performance of the untreated surfaces. This is mainly attributed to the increased number of potential active nucleation sites on both surfaces, as shown in the SEM images and discussed previously. On the other hand, the deterioration of HTC is exacerbated on other surfaces, with its values being up to 75% lower compared to those of pure water boiling on the untreated surface. The main cause for the deterioration of boiling performance is the growth of a thick nanoparticle deposit layer, which raises the surface temperature due to increased thermal resistance of Figure 15 shows a comparison of the performance of the laser-textured surface, when used either with water (denoted as LT), or with nanofluids, relative to the boiling performance of pure water on the untreated reference surface (REF). At low and medium heat flux values, the reference surface provides superior performance during the first experimental run and only at high heat fluxes, the laser-texturing and nanoparticle deposition enhance the HTC (Figure 15a). However, after 5 experimental runs were performed on each surface, the performance of the laser-textured surface used with pure water or 0.001 wt.% large nanoparticle nanofluid significantly exceeds the performance of the untreated surfaces. This is mainly attributed to the increased number of potential active nucleation sites on both surfaces, as shown in the SEM images and discussed previously. On the other hand, the deterioration of HTC is exacerbated on other surfaces, with its values being up to 75% lower compared to those of pure water boiling on the untreated surface. The main cause for the deterioration of boiling performance is the growth of a thick nanoparticle deposit layer, which raises the surface temperature due to increased thermal resistance of Figure 15 shows a comparison of the performance of the laser-textured surface, when used either with water (denoted as LT), or with nanofluids, relative to the boiling performance of pure water on the untreated reference surface (REF). At low and medium heat flux values, the reference surface provides superior performance during the first experimental run and only at high heat fluxes, the laser-texturing and nanoparticle deposition enhance the HTC (Figure 15a). However, after 5 experimental runs were performed on each surface, the performance of the laser-textured surface used with pure water or 0.001 wt.% large nanoparticle nanofluid significantly exceeds the performance of the untreated surfaces. This is mainly attributed to the increased number of potential active nucleation sites on both surfaces, as shown in the SEM images and discussed previously. On the other hand, the deterioration of HTC is exacerbated on other surfaces, with its values being up to 75% lower compared to those of pure water boiling on the untreated surface. The main cause for the deterioration of boiling performance is the growth of a thick nanoparticle deposit layer, which raises the surface temperature due to increased thermal resistance of the deposited titanium dioxide layer. The obtained results are in accordance with the

the deposited titanium dioxide layer. The obtained results are in accordance with the findings of other authors for large nanoparticles and mixed sizes of nanoparticles [69], and

the deposited titanium dioxide layer. The obtained results are in accordance with the findings of other authors for large nanoparticles and mixed sizes of nanoparticles [69], and

findings of other authors for large nanoparticles and mixed sizes of nanoparticles [69], and also for nanofluids with small nanoparticle diameters [70]. *Nanomaterials* **2022**, *12*, x FOR PEER REVIEW 18 of 22

**Figure 15.** Comparison of HTC for different boiling media (pure water or nanofluids) relative to the performance of the untreated reference surface during the 1st run (**a**) and during the 5th run (**b**)*.* **Figure 15.** Comparison of HTC for different boiling media (pure water or nanofluids) relative to the performance of the untreated reference surface during the 1st run (**a**) and during the 5th run (**b**).

Significant enhancement of the CHF was not observed at 0.001 wt.% compared to the CHF for pure water on either the laser-textured or untreated surface, while the highly concentrated nanofluid evidently caused thicker deposits, resulting in significant increases in the CHF value. Enhanced CHF can be mainly be attributed to the increased wettability [69,71] of the surfaces and their porosity; this also agrees with the contact angle measurements after boiling, shown in Figure 12. Significant enhancement of the CHF was not observed at 0.001 wt.% compared to the CHF for pure water on either the laser-textured or untreated surface, while the highly concentrated nanofluid evidently caused thicker deposits, resulting in significant increases in the CHF value. Enhanced CHF can be mainly be attributed to the increased wettability [69,71] of the surfaces and their porosity; this also agrees with the contact angle measurements after boiling, shown in Figure 12.

Finally, a comparison of all boiling curves for the first and last experimental run is made in Figure 16a,b, respectively, to elucidate the effect of both nanoparticle concentration and size on boiling performance. Additionally, the corresponding plots comparing Finally, a comparison of all boiling curves for the first and last experimental run is made in Figure 16a,b, respectively, to elucidate the effect of both nanoparticle concentration and size on boiling performance. Additionally, the corresponding plots comparing HTC values at selected heat fluxes are shown in Figure 16c,d.

HTC values at selected heat fluxes are shown in Figure 16c,d. During the first experimental run, an enhancement of boiling performance in terms of increased HTC at medium and high heat flux values was observed for the laser-textured surface tested with pure water, for both mixed nanoparticle size nanofluid and for both low-concentration nanofluids (0.001 wt.%). A noticeable CHF increase was obtained with highly concentrated nanofluids (0.1 wt.%), but high surface superheat values were already recorded for the nanofluid with large nanoparticles, hinting at thicker deposits and problems during future testing. It can be concluded that large nanoparticles agglomerate much faster, which improves the deposition rate, leading to a reduction in the number of active nucleation sites. This observation is not in accordance with previous research, which concluded that if the size of nanoparticles is much smaller than the roughness of the surface, an enhancement of boiling performance will be achieved. While even the large nanoparticles are approximately two orders of magnitude smaller than the laser-induced roughness, a deterioration in boiling performance was observed, matching previous reports for larger ratios of nanoparticle size to surface roughness [70,72]. Here, we report that regardless of the sizes of nanoparticles, the HTC of nanofluids at the higher concentration on lasertextured surface decreases compared to the boiling of water on the same surface. The last experimental run on each surface revealed that highly concentrated nanofluids provided a notable CHF enhancement, but universally at the expense of notable surface superheat increase, which reached 100 K for nanofluid containing 0.1 wt.% of small nanoparticles. Overall, CHF enhancement due to the boiling of nanofluid on laser-textured surfaces was not observed at the low concentration.

(**a**) and during the last run (**b**) on each surface; HTC values during the first run (**c**) and during the

last run (**d**) on each surface.

**Figure 15.** Comparison of HTC for different boiling media (pure water or nanofluids) relative to the performance of the untreated reference surface during the 1st run (**a**) and during the 5th run (**b**)*.*

measurements after boiling, shown in Figure 12.

HTC values at selected heat fluxes are shown in Figure 16c,d.

Significant enhancement of the CHF was not observed at 0.001 wt.% compared to the CHF for pure water on either the laser-textured or untreated surface, while the highly concentrated nanofluid evidently caused thicker deposits, resulting in significant increases in the CHF value. Enhanced CHF can be mainly be attributed to the increased wettability [69,71] of the surfaces and their porosity; this also agrees with the contact angle

Finally, a comparison of all boiling curves for the first and last experimental run is made in Figure 16a,b, respectively, to elucidate the effect of both nanoparticle concentration and size on boiling performance. Additionally, the corresponding plots comparing

**Figure 16.** Summary of boiling performance for all tested cases: boiling curves during the first run (**a**) and during the last run (**b**) on each surface; HTC values during the first run (**c**) and during the last run (**d**) on each surface. **Figure 16.** Summary of boiling performance for all tested cases: boiling curves during the first run (**a**) and during the last run (**b**) on each surface; HTC values during the first run (**c**) and during the last run (**d**) on each surface.
