Bubble Dynamical Behavior and Flow Boiling Characteristics of Slug Flow in Hairpin Tubes.

A series of experiments was conducted to investigate the flow boiling of R141b in hairpin tubes. A focus was addressed on visually observing flow boiling and two-phase flow in U-turn bends of a vertical upward hairpin tube with inner diameter of 6 mm at liquid velocities of 0.098, 0.147 and 0.196 m/s and heat fluxes in the range of 6191 W/m² to 24763 W/m², respectively. The bend could break up large vapor slugs in two ways, bubble tail fracture and bubble rupture, which induced significant effects on local heat and mass transfer. Under certain experimental conditions, small bubbles formed in the bend could not coalesce to be large vapor slugs but accumulate to form a unique flow pattern and greatly release thermal non-equilibrium in the rest part of hairpin the tube. [ABSTRACT FROM AUTHOR]