On the rupture of bubble foam in subcooled flow boiling of seawater in a vertical annulus

This paper investigates the nature of foam rupture and its effects on heat transfer during subcooled flow boiling of 3.5 wt% artificial seawater in a vertical annulus. Flow visualizations near the outlet of the heated section using a high-speed camera disclose the rupture of foam flow and the following development of slug bubble for the inlet temperatures of 55°C, 65°C, and 75°C and boiling numbers ranging from 3 × 10⁻⁴ to 11 × 10⁻⁴. The slug flow, otherwise, is absent for the case with inlet temperature of 85°C in this study and 90°C, as reported in our previous study. The nature of such foam rupture and evolution of slug flow is studied through the image processing and measurements in Image J. Geometric characteristics including the Feret diameters, equivalent diameters, angles, and aspect ratios are measured and discussed. Motion characteristics including the bubble growth rates, velocities and trajectories of slug bubbles are also measured and analyzed. It reveals that the slug bubble grows approximately linearly. However, the bubble growth rate demonstrates large variations and crucial effect on heat transfer. The trajectories of the slug bubbles reveal zigzag motions with large radial displacements. A theory is proposed to explain the ruptures of bubble foam. Such coexistence of foam flow and slug flow enhances significantly heat transfer in subcooled flow boiling, leading to a large wall temperature drop up to 10°C, compared with 1°C in those cases with stable foam flow in artificial seawater and slug flow in de-ionized water.