Slug-to-annular regime transitions in R-134a flowing through a vertical duct

Measurements of pressure gradient, local void fraction and interfacial velocity were taken in R-134a flowing through a vertical duct to characterize the transition from slug to fully developed annular flow. The measurements were taken in both heated and adiabatic conditions for two system pressures and two mass flow rates. The adiabatic data showed that droplet entrainment was enhanced at the higher pressure so that a definite transition occurred between thick film flooding-type flow and thin film annular flow; when this situation occurred, a small decrease in pressure gradient was noted. At the same time, the local void profiles showed an inversion from a classical center-peaked slug distribution to a wall-peaked annular distribution. The effect was less evident at the lower pressure and flow rate, where transition was prolonged and less noticeable, and where a thicker transverse film existed in higher average void fractions. The heated data showed similar trends as the adiabatic data except that the heating action induced a more dramatic effect on the frictional pressure gradient near the slug-to-annular transition and caused the transition to occur at lower average void fractions.