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

Abstract 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.