Visualized experimental study of steam condensation on a vertical tube with non-condensable gas (air) under free convection

Abstract

The presence of non-condensable gas (NCG) significantly reduces the condensation heat transfer coefficient (HTC) during steam condensation. However, the impact of condensate modes and dynamic behavior on NCG during steam condensation remains poorly understood. Moreover, prior research on the effect of wall subcooling on HTC has conflicting results. This study addresses these gaps through a visualized experimental investigation of steam condensation on the outer surface of a vertical tube under free convection with NCG (air). The experiment covered a range of parameters, including air mass fraction (0.08–0.4), wall subcooling (4–23 K), and system pressure (0.1–0.4 MPa). Visualization revealed three distinct condensate modes: film, droplet-film, and droplet-film-stream. At an air mass fraction of 0.08, a steam pressure of 0.15 MPa, and a subcooling of 10.0 K, observations on the tube wall surface revealed four dynamic features: droplet merging, droplet shedding and flushing, droplet dynamic growth, and film flow. At a steam pressure of 0.1 MPa and a subcooling of 8.0 K, a 1 % increase in air mass fraction resulted in an average decrease of 136.9 W/(m² K) in the condensation HTC below 30 % air mass fraction, and 42.9 W/(m² K) above 30 %. In particular, the HTC exhibited an increasing trend followed by a decrease with increasing wall subcooling. Based on the experimental data, an experimental correlation for the average HTC was developed as a function of air mass fraction, wall subcooling, and system pressure. The correlation accurately predicted 90 % of the experimental data within a 10 % uncertainty range.