Pool boiling enhancement using hierarchically structured ZnO nanowires grown via electrospraying and chemical bath deposition

This study entailed the fabrication of hierarchically structured ZnO nanowires via electrospraying and chemical bath deposition for pool boiling applications. Nanoscale ZnO seeds were patterned on a metal substrate by electrospraying, after which the seeds were grown into ZnO nanowires via chemical bath deposition. Next, the effect of the patterned ZnO nanowires on the pool boiling performance was investigated. In addition, the optimal nanowire pattern that yielded the highest critical heat flux (CHF) and effective heat transfer coefficient (h_eff) was identified. The numerous nanoscale cavities that existed among the ZnO nanowires acted as nucleation sites, thereby facilitating an efficient boiling process. The hierarchical structure of the ZnO nanowires increased the CHF by 40% compared with that of the non-coated, bare surface. Furthermore, the cooling effect increased owing to the ZnO nanowires; this in turn decreased the superheat and increased h_eff. In addition, the ZnO nanowires exhibited surface wettability owing to their hierarchical structure. The optimal combination of a bare and hydrophobic surface and a hydrophilic surface covered with ZnO nanowires yielded the highest CHF and h_eff. Moreover, the hydrophilic and hydrophobic surfaces promoted capillary pressure and rapid bubble departure, respectively, and their combination yielded the optimal pool boiling condition. Bubble formation and dynamics were observed using a CCD camera, and the patterned ZnO nanowires were characterized via scanning electron microscopy, optical profilometry, and optical microscopy. Moreover, the theoretically predicted heat transfer was found to be consistent with the experimental data.