Enhanced heat transfer on hollow hierarchical nanowired surface during transient spray cooling of liquid nitrogen

Abstract

The liquid spreading and replenishment has a key effect on the performance of transient spray cooling. In this study, we used liquid nitrogen as the working fluid and conducted transient spray cooling experiments on four modified copper surfaces, including superhydrophilic surface, hydrophilic surface, smooth copper surface, and hollow hierarchical nanowired surface. The results show that increasing the single-scale surface hydrophilicity of the copper surface can effectively increase the critical heat flux (CHF) of liquid nitrogen transient spray cooling, due to the effective enhancement of evaporation heat transfer coefficient h. The multi-scale hollow hierarchical structure surface can further enhance the heat transfer of liquid nitrogen transient spray cooling, its cooling rate is accelerated 1.3 times, the CHF is increased 1.2 times and the maximum h_MAX is increased 1.11 times that of the superhydrophilic surface. Through the calculation of the liquid film climb theory model, it was found that the liquid nitrogen film climbs at a speed of 48.10 m/s in the nanowired clusters. The ultrafast climb of liquid nitrogen film in hierarchical nanowires may be one of the main reasons for enhanced heat transfer. This study broadens the experimental database of hierarchical structure surfaces and new ideas on enhanced heat transfer during cryogenic spray cooling.