Wettability-defined frosting dynamics between plane fins in quiescent air

Abstract

Finned heat exchangers used in energy applications regularly undergo condensation frosting. Understanding the effects of fin spacing on frost growth with surfaces having differing wettability is important for the design of next-generation heat exchangers. This study presents an experimental investigation using high-resolution optical imaging of frost growth between two parallel vertical plain aluminum fins as a function of surface wettability (hydrophilic, superhydrophilic, superhydrophobic), fin surface temperature (-5 to -15 °C), fin spacing (2 to 8 mm), and relative humidity (≈ 50 and 70%). We observe two distinct regimes of frost growth for all test conditions. The effect of surface wettability on frost growth rate becomes significant for fin spacing ≤ 6 mm at fin surface temperatures below -10 °C. The fin surface temperature plays a significant role for fin spacing > 4 mm. Furthermore, for hydrophilic surfaces, the effect of relative humidity was found to be significant for fin spacing > 4 mm. To explain the frosting physics and mass transfer effects, we developed a three-dimensional finite volume method simulation to characterize the moist air flow field between fins at varying fin spacings. For similar humidity, fin surface temperature, and wettability, larger fin spacing resulted in higher frost growth rate due to higher access to moist air. We use the experimental data to develop an empirical correlation for predicting frost thickness. Furthermore, we develop an artificial neural network independent of predetermined functional form that can be trained with the experimental data to predict frost thickness with high accuracy.