*3.3. Surface Superheat*

The authors Gajghate et al. [35] studied the effect of the ZrO<sup>2</sup> layer settled during poolboiling experiments on the heat transfer enhancement. The obtained boiling curves showed the conjugated influence of the nanoparticle layer thickness and surface roughness on the layer superheat value under different heat fluxes. The researchers observed a decrement in the wall superheat value with increasing heat flux and the maximum reduction was verified at 5.8 K with a heat flux of 109.8 kW/m<sup>2</sup> using a 200 nm ZrO2-coated copper substrate having 227 nm of average roughness. The greatest reduction in the surface superheat value was of 31.52% compared with a smooth copper substrate. This effect is caused by the thickness increase in the ZrO<sup>2</sup> layer deposited onto the copper substrate during pool boiling, which incremented the effective heat transfer surface area and the nucleation core point density. The bubble dynamic at different heat fluxes was also studied showing the nucleation of an isolated vapor bubble on the copper surface at low heat fluxes. With the imposition of high heat fluxes, the bubble diameter was enhanced and lowered the bubble departure frequency. In addition, the authors [42] studied the superheat value of the deposited layer and concluded that it depended mainly on the diameter of the bubbles at the departure stage of evolution and on the boiling time. The layer superheat value was also found to decrease from using the 0.0025% nanofluid to the 0.005% nanofluid. It was observed that the bubble diameter at departure and the boiling time dramatically increased from 0.0025 to 0.005% nanofluid, while keeping almost the same growth time of the nucleated vapor bubbles. With these alterations, the magnitude of the evaporation superheat layer increased steadily with the third power of the diameter of the bubbles at departure and decreased almost linearly with increasing pool-boiling time. Therefore, the conjugated effect of the alteration in the departure diameter of the bubbles and ebullition time led to an increase in the deposition layer superheat value when using the 0.005% nanofluid rather than the 0.0025% nanoparticle concentration nanofluid. The researchers also stated that the HTC associated with each and every heat transfer mechanism was affected by the changes in the bubble dynamics and ITR. These factors

are probably determined by the structure in the boiling-induced deposition layer at the active nucleation points. Furthermore, the researchers Hadzic et al. [33] investigated the influence of the nanoparticle dimension and concentration of titanium oxide nanofluids on the pool-boiling heat transport from laser-textured copper substrates. In their work, nanofluid pool boiling was evaluated with 4–8 nm titanium oxide nanoparticles with 0.001 wt. % and 0.1 wt. %. The boiling curve obtained for the 0.001 wt. % nanofluid was stable, while the boiling curve for the 0.1 wt. % nanofluid was shifted toward higher superheat values after the completion of each consecutive pool-boiling experiment. Such a shifting effect may be due to the decrement in the number of nucleation points through the deposition of titanium oxide nanoparticles. Employing the 0.1 wt. % nanofluids, the microcavities on the laser-textured surface were filled with nanoparticles during the initial experiment, which led to a decrease in the number of nucleation sites and an increase in the layer superheat value during the following experiments. With the increase in the number of consecutive experiments, the layer superheat value reached unpractical values and the boiling heat transfer degraded. The boiling experiments were repeated for large (490 nm) titanium oxide nanoparticles. At this time, the authors found with the 0.001 wt. % nanofluid, an appreciable shift in the boiling curve toward lower layer superheat values, while with the 0.1 wt. % nanofluid, the respective boiling curve was observed to be shifted toward higher superheat values. These findings were consistent with those using the smaller nanoparticles. Nevertheless, it was reported that after more than four hours of boiling, the more concentrated nanofluid boiling curve became unstable because of the thick layer settled on the surface that also locally flaked off. An additional study was conducted by mixing 0.05 wt. % small titanium oxide nanoparticles with 0.05 wt. % larger titanium oxide nanoparticles. At this time, the boiling curve was shifted toward a higher layer superheat value in accordance with the behavior observed in the case of the 0.1 wt. % nanofluid having either smaller or larger nanoparticles. It was also found that after the completion of the initial experiment, the layer superheat value was considerably enhanced, which is likely due to the filling of the surface microcavities and channels with the smaller sized nanoparticles that reduced the nucleation points. Moreover, as the latter decreased, the same happened with the boiling HTC. Overall, the authors arrived at the following conclusions:

