Effects of air-side surface wettability on the performance of dehumidifying heat exchangers

摘要

Wettability manipulation is investigated as a method to enhance condensate drainage from the air-side surface of dehumidifying heat exchangers. The attendant impact on the thermal-hydraulic performance of a heat exchanger is also explored, with the goal of improving energy efficiency of heat exchangers through appropriate surface technologies. Three groups of heat exchangers with identical geometry and different controlled wettability are tested in a closed-loop wind tunnel under conditions typical to air-conditioning applications. Under the same operating conditions, the amount of condensate retention on a specimen is significantly reduced after a hydrophilic surface treatment (up to 43.5% lower). Due to more filmwise manner of retention and a reduced possibility of bridging, a super-hydrophilic surface shows improved drainage performance for these tested heat exchangers, over the entire range of fin spacing (more than 30% mass reduction for all cases of Fs = 1.1 mm, Fs = 2.0 mm, and Fs = 5.2 mm). For a very wide fin spacing, a hydrophobic treatment may also help decrease the retention due to reduced droplet size, but it is not as effective as the hydrophilic treatment. It is also observed that hydrophilic treatment causes a slight heat-transfer degradation (a trend within the experimental uncertainty), due to the filmwise mode of condensation, and the degradation is relatively lager for specimens with wide fin spacing because the benefits of dropwise condensation are more pronounced under conditions of little or no bridging (~ 15% Colburn j factor degradation for specimens with Fs = 5.2 mm). Nevertheless, pressure drop across the heat exchanger is more sensitive to the enhancement of surface wettability (~ 40% reduction for Fs = 1.1 mm and ~ 20% reduction for wider fin spacing). The results suggest that the advantages of hydrophilic treatment should be fully exploited for compact heat exchangers, due to the fact that the pressure drop and pumping power consumption can be managed while heat-transfer performance does not deteriorate. As part of the effort to understand the underlying mechanisms for condensate retention, a model for predicting the mass of retained condensate is proposed based on laminar filmwise condensation assumptions. The model suggests that mass retention on a heat exchanger increases with latent heat transfer rate (Ql) and decreases with air-side Reynolds number (Redh), which are also reflected by the experimental data. The model is successful in predicting the magnitude and trends of condensate retention for the heat exchanger specimens over a wide range of tested conditions. A surface embossing technique that can inexpensively impart micro-grooved topographical features on aluminum fin stock to enhance water drainage is introduced, as another direction to explore for manipulating fin wettability. The parallel-groove surface features serve to increase the apparent contact angle of water droplets placed onto the surface. A consistent reduction of critical sliding angle is observed on these embossed surfaces (grooves were aligned to the direction of gravity), which is due to the contact line discontinuities and contact line pinning effects induced by groove structure on the surface. Water droplets exhibit an elongated shape along with the direction of micro-grooves, and this anisotropic wetting is attributed solely to the roughness anisotropy.