Recent studies have indicated that ZnO superhydrophilic surfaces on copper and aluminum substrates enhance surface wettability and evaporation processes in droplet evaporation and pool boiling experiments. At slightly superheated surface temperatures, droplet evaporation on a nanoporous superhydrophilic surface exhibits onset of nucleation and nucleate boiling effects found in pool boiling processes. This study explores water droplet evaporation and quenching experiments conducted on nanostructured surfaces of a 45° downward facing pyramid copper, aluminum, and 304 stainless steel substrate. The nanostructured surfaces were used to conduct adiabatic droplet deposition, droplet evaporation, and quenching experiments. Through the three experiments and through surface characterization, by means of scanning electron microscopy, the underlying heat transfer performance and mechanism of droplet evaporation and pool boiling on various host metal substrates can be understood. The six surfaces tested were the following: bare copper, aluminum, and 304 stainless steel surfaces; and ZnO nanostructured surfaces on copper, aluminum, and 304 stainless steel. The experimental variables in this study were temperature and time. Through temperature and time measurements, the mean heat flux at varying superheats could be determined. Experimental results indicate the ZnO nanostructure enhances wettability and water evaporation on a variety of metal substrates. The presence of the ZnO nanostructure increases the mean heat flux and critical heat flux (CHF) in comparison to the bare metal surfaces. These results provide insight into the potential of enhancing spray cooling systems by growing a ZnO nanostructure on a heat exchanger fabricated of various metals.

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