Improvement of heat transfer by promoting dropwise condensation using electrospun polytetrafluoroethylene thin films†
Vapor condensation is a crucial part of a broad range of industrial applications including power generation, water harvesting, and air conditioning. Hydrophobic and superhydrophobic surfaces promote dropwise condensation in vapor-filled environments and increase their heat transfer coefficients more than filmwise condensation on hydrophilic surfaces. Although dropwise condensation can lead to energy-efficient transfer, it is hard to achieve stable dropwise condensation in high-temperature environments. To decide the best conditions for achieving higher heat transfer is also difficult because the heat transfer coefficient is influenced by not only surface wettability but also surface structures of thin films and substrates. Herein, we fabricated thin films with different wettabilities and surface structures using polytetrafluoroethylene (PTFE) which show high heat resistance to determine the best conditions for heat transfer. Several different films were prepared by electrospinning a mixed solution of PTFE and polyvinyl alcohol on aluminum (Al) and copper (Cu) tubes. After annealing them, the PTFE thin films enhanced heat transfer performance and showed stable dropwise condensation in high-temperature environments. The films fabricated by electrospinning a solution containing 66 wt% PTFE displayed the highest heat transfer coefficients, with heat transfer coefficients 64% and 61% greater than those of uncoated Al and Cu tubes, respectively. That is because homogeneous superhydrophobic surfaces that showed the highest departure frequency of condensed water droplets were fabricated using 66 wt% PTFE. The results suggest that these electrospun PTFE thin films would demonstrate excellent potential for use on the surface of heat exchangers in various industries.