Characterizing condensation from humid air on wettability-engineered inclined surfaces through non-intrusive mapping of boundary layer

Abstract

Vapor condensation from humid air is encountered in numerous industrial applications, often necessitating non-intrusive techniques to characterize and estimate heat and mass transfer performance. Schlieren imaging has emerged as a promising non-intrusive method for visualizing the density boundary layer that forms near the condensing surfaces. Eliminating the need for complex optical equipment, Background Oriented Schlieren (BOS) distinguishes itself from other Schlieren imaging techniques due to its simplicity and suitability for large-scale implementation. However, the technique warrants careful consideration of the pertinent flow configuration. Here we investigate the influence of surface inclination (relative to the vertical) and surface-wettability of the condensing surfaces on condensation from humid air using BOS. We map the density boundary layer during condensation on inclined surfaces with three distinct surface wettability, viz., superhydrophilic (SHPL), hydrophilic (HPL, the control) and superhydrophobic (SHPB). Our findings reveal that the SHPB surface exhibits enhanced condensation performance at higher inclination angles, while the SHPL surface demonstrates a decline in performance with increasing inclination. In contrast, the control surface displays minimal variation in density gradient fields across different inclinations. We further propose empirical correlations to predict the mass flux and condensation heat transfer coefficient (CHTC) based on the density boundary layer data obtained from the BOS images for various condenser surface wettability and inclination angles. These correlations show agreement within ±20% when validated against independently conducted condensate collection experiments, highlighting the potential of BOS as a reliable, non-intrusive tool for predicting condensation performance across a wide range of industrial applications.