Air-Side surface wettability effects on the performance of slit-fin-and-tube heat exchangers operating under wet-surface conditions

A study of condensate retention and the attendant thermal-hydraulic effect associated with changes in air-side surface wettability is presented for a series of slit-fin-and-tube heat exchangers of identical geometry with controlled wettability covering a wide range of contact angles. An experiment in which the retained mass of air-side condensate was measured under dynamic conditions is described, and the results are analyzed using conventional thermal-hydraulic measurements of j and f. The data demonstrate that for the heat exchangers used in this study, the j factor is not strongly influenced by condensate retention, but the friction factor is significantly reduced for surfaces of increased wettability. Hydrophilic heat exchangers retain much less air-side condensate than do the hydrophobic heat exchangers, and the amount of retention is found to depend on the air-side Reynolds number (Re_dh) and the rate of latent heat transfer (Ql). Based on an assumption of filmwise condensation, a new model for predicting the mass of retained condensate is described and compared with the steady-sate retention data. The model is successful in predicting retained condensate over a wide range of tested conditions. The potential of this new approach and possible refinements that will add engineering value are discussed.