• Journal of the Korean Society of Visualization
• /
• v.22 no.3
• /
• pp.70-77
• /
• 2024

The objective of this study is to experimentally measure condensation heat transfer at the vapor-liquid interface of boiling bubbles generated in subcooled water. Rainbow Schlieren Deflectometry (RSD) is an optical technique that allows for the non-intrusive visualization of the temperature gradient field in liquid. It is known that the refractive index of a liquid changes as a function of temperature. When light passes through a liquid medium, the degree of deflection is proportional to the spatial gradient of the temperature field. The deflected light passes through a rainbow filter with a continuous variation in color from the center to the outer boundary. As a result, the temperature gradient field in the liquid can be visualized as a color contour. Boiling experiments in a pool of subcooled water were conducted to visualize the temperature gradient field near the vapor-liquid interface of a vapor bubble. A set of RSD images was obtained for the entire life cycle of a boiling bubble, from nucleation through growth to departure from the heating wall. The temperature gradient field near the vapor-liquid interface around the bubble was carefully analyzed. The condensation heat transfer coefficient was calculated and compared to existing prediction models.