Modeling and optimization of hydrophobic surfaces for a two-phase closed thermosyphon

Abstract

Promoting dropwise condensation on condensers has the potential to significantly improve cooling and heat recovery performance of two-phase closed thermosyphons (TPCTs). If it is well designed, the dropwise condensation can provide an order of magnitude higher heat transfer coefficient than filmwise condensation of conventional metal condensers. The thermal design of an ideal dropwise condensing surface is so important in the rationale for the advantage of dropwise condensation in the TPCTs. However, the effect of the hydrophobic promoter thickness on the thermal characteristics of the TPCT and its optimal value are still unclear. Here, we develop a unified model framework for the TPCT with the dropwise condenser and investigate how much the dropwise condensation can improve the thermal performance of the TPCT. As the hydrophobic promoter for the TPCT, we proposed a nanoscopically smooth Teflon-based hydrophobic coating on theoretical insights, experimentally evaluated its condensation heat transfer, and then developed its dropwise model with high accuracy by combining a recent dropwise model with the optimized parameters. Subsequently, the developed model was unified with the TPCT model to investigate the thermal characteristics of the TPCT system and the effect of the promoter thickness. The model results show that the Teflon-coated condenser with a thin thickness (≤ 10 nm) can significantly reduce the overall thermal resistance of the TPCT by up to 4.5 times compared to filmwise condensation. However, an increase in film thickness limited the performance enhancement of the TPCT due to the dominant thermal resistance through the film. By considering both the durability and the thermal resistance of the TPCT, we suggested the optimal Teflon film thickness as approximately 60 nm. This work provides physical insights and guidelines to obtain ideal TPCTs combined with dropwise condensers.