Influence of Surface Modification on the Transient Dehumidification Performance of Fin-and-tube Heat Exchanger

• Published in: International journal of heat and mass transfer Vol. 173; p. 121202
• Main Authors: Yang, Kai-Shing, Lin, Hong-Yi, Wu, Yu-Lieh, Wu, Shih-Kuo, Wang, Chi-Chuan
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 01.07.2021; Elsevier BV
• Subjects: Condensation; Contact angle; Copper; Dehumidification; Density; Droplets; Fin-and-tube heat exchanger; Flow visualization; Heat exchangers; Humidity; hydrophilic; Hydrophilicity; hydrophobic; Hydrophobicity; Polydimethylsiloxane; Pressure drop; Relative humidity; Steady state; transient performance; Tube heat exchangers; Wettability; hydrophilic; dehumidification; hydrophobic; Fin-and-tube heat exchanger; transient performance
• ISSN: 0017-9310; 1879-2189
• DOI: 10.1016/j.ijheatmasstransfer.2021.121202

•Performance of hydrophobic fin-and-tube heat exchanger is conducted in dry/wet conditions.•Surface modification has no significant influence on the heat transfer performance.•Hydrophobic surface promotes dropwise condensation and shows 3.4 times higher pressure drop.•The transient pressure drop depends on the relative humidity (RH) and coating considerably.•The droplet distribution density was less uniform and the surface contains fewer droplets. In this study, surface modifications subject to hydrophilic and hydrophobic conditions are made for copper fin-and-tube heat exchanger to examine the dehumidification performance. Polydimethylsiloxane was used to coat on the surface of heat exchangers to increase the hydrophobicity with a contact angle of 105° while the uncoated copper surface is 76°. The effects of fin spacing and relative humidity on the transient dehumidifying performance are examined in details. The experimental results indicate that surface modification imposes negligible influence on the heat transfer performance. Hydrophobic surface promotes dropwise condensation and may form water bridging which may impede the droplets from falling, thereby showing some 3.4 times higher pressure drop than the hydrophilic one. Upon condensation on the hydrophilic surface, the size of the droplets plays pivotal influence on the transient pressure drop of the heat exchangers. The flow visualization indicates that liquid films prevails on the fin surfaces due to high surface wettability. However, the behavior of transient pressure drop depends on the relative humidity (RH) considerably. For RH = 50%, the required time to reach steady state is much longer than that of RH = 80%. Interestingly, an overshoot of pressure drop vs. time before reaching the steady state is clearly seen for RH = 65%. This phenomenon is associated with the dynamic condensate removal from the fin surface. The droplet distribution density is comparatively uniform at a low frontal velocity (i.e. 0.5 m s−1). Upon raising frontal velocity, the droplet distribution density becomes less uniform with fewer droplets adhering on the fin surfaces. Hence, the wet and dry pressure drop ratio exhibits a decreasing trend at a higher frontal velocity.