Active venting mechanisms for accurate heat transfer measurement during steam dropwise condensation

The presence of non-condensable gases (NCGs) can significantly affect condensation heat transfer by creating a vapor-diffusion barrier near the condensing surface. While quantitative analysis of condensation heat transfer with steam-NCG mixtures has been studied at higher NCG concentrations, few have investigated condensation mechanisms at low (sub-atmospheric) steam pressures with trace NCG levels (mass fractions below 0.1%). In this work, we study dropwise condensation at absolute steam pressures below 12 kPa and demonstrate that even minute NCG concentrations in the steam, below 20 parts per million by mass (ppm), can still significantly degrade heat transfer. Although enhanced degassing procedures can initially reduce NCG levels and improve heat transfer, they are wholly insufficient to sustain measurement accuracy over time due to the inexorable buildup of NCGs near the condensing surface. To address this challenge, we introduce an active venting strategy that continuously removes NCGs from the vicinity of the condensing surface. Experimental results show that this strategy can sustain higher HTCs by increasing flow velocity to promote the more effective removal of NCGs. This approach provides a practical and scalable method to mitigate measurement error due to NCG accumulation in condensation heat transfer studies, supporting efforts toward standardized testing for accurate results, as well as contributing to a better understanding of steam condensation mechanisms.