Investigation On Dynamic Behaviors And Heat Transfer Mechanism Of Condensed Droplets On Directional Transported Hydrophobic Surfaces

Hydrophobic surfaces with excellent hydrophobicity are widely used in integrated heat dissipation,aerospace,and nuclear industries.Moreover,droplet growth process is intimately related to the wetting characteristics,macroscopic and microscopic structures of condensed surfaces.Therefore,hydrophobic surface and SLIPSs(Slippery liquid-infused porous surfaces,SLIPSs)were prepared by simple thermal annealing and chemical processing,respectively,to achieve continuous and steady droplet condensation,and helical grooves were fabricated to induce directional transport of condensed droplets.The relationship of dynamic behavioral characteristics and enhanced heat transfer of condensate droplets on smooth hydrophobic tubes,helically-fined hydrophobic tubes,smooth SLIPS tubes and helically-fined SLIPS tubes were systematically studied,and the effects of helical fin structure,lubricant viscosity and their interactions on droplet growth process and heat transfer performance were thoroughly analyzed.It was demonstrated that the helical fin structure not only increases the condensation heat transfer area but also provides directional transport channels for condensate droplets,which promotes the coalescence and sliding of condensate droplets under the synergistic effect of the hydrophobic surface wettability,achieving rapid droplet growth and efficient removal.The renewal time period of smooth SLIPS impregnated with GPL 103 is 5.80 s,and that of helically-fined hydrophobic surface is only 4.88 s.However,the dropping frequency of condensate droplets on helically-fined SLIPS impregnated with GPL 107 is as high as 75.16±5.70 N/(m·s),and the condensed surface renewal time period is further shortened to 2.99 s.In addition,the synergistic effect of the highly viscous lubricant and the helical fin structure on the SLIPS achieved high mobility of condensate droplets.The condensing heat transfer coefficient of helically-fined SLIPS tube impregnated with GPL 107 was 35.8%and 55.9%higher than that of the smooth SLIPS tubes impregnated with GPL 103 and GPL 107,respectively,while it was98.1%and 204.4%higher than that of the smooth and helically-fined hydrophilic tubes.When the heat flux is 145 k W/m~2,the condensation heat transfer coefficient of helically-fined hydrophobic tube is approximately 327%higher compared to that of smooth hydrophilic tube.However,the condensation heat transfer coefficients of both hydrophobic and SLIPS tubes decreased with increasing subcooling temperature due to the mismatch between the droplet growth rate and dropping frequency and the droplet nucleation frequency.Furthermore,durability verification revealed that stable droplet condensation and excellent condensation heat transfer were still maintained on helically-fined hydrophobic tube after continuous working for80 h and on helically-fined SLIPS tube impregnated with GPL 107 after continuous working for 12 h.However,the condensation heat transfer performance of helically-fined SLIPS tube impregnated with GPL 103 was significantly weakened after 7.5 h continuous working because the low viscosity lubricant of GPL 103 accumulated at the helical groove roots failed to prevent lubricant loss caused by the shear force during massive condensate droplet sliding.