Investigating the effect of environmental conditions and surface type on condensation heat transfer coefficient and droplet departure time

This study investigates the impact of surface characteristics—hydrophilic (copper) and hydrophobic (Teflon-coated copper) surfaces—and environmental conditions such as relative humidity (RH) ranging from 80 % to 96 %, temperature differences (DT) from 4 °C to 10 °C, and airflow velocities (V) from 2 to 8 m/s during 180 min on humid air condensation heat transfer coefficient (HTC) and droplet departure time. The research utilizes a Design of Experiments (DOE) strategy, utilizing the Response Surface Methodology (RSM) paired with a Central Composite Design (CCD) to evaluate the influence of these parameters and provide a correlation relationship between the HTC of each surface and the applied environmental conditions. Hydrophilic surfaces generally exhibited higher average HTCs than hydrophobic ones. However, at a temperature difference of 10 °C, relative humidity of 96 %, and air velocities of 2 and 8 m/s, hydrophilic surfaces significantly decreased HTC due to a condensation regime transition from dropwise to filmwise. The highest recorded average HTC was 1.16 and 1.13 kW/m²°C on the hydrophobic surface under these conditions. The temperature difference had the most significant effect on increasing the HTC. Additionally, it was observed that the relative humidity played a more critical role than the flow velocity. There is a similar process for droplet exit, with the difference that in some experiments, the heat flux of hydrophobic surfaces was slightly higher than that of hydrophilic surfaces. Still, the drop fell on it later and left the surface because of the nature of the hydrophobic surface, which prevents droplets from spreading and coalescence with other droplets.