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

Measurements of pressure gradient, local void fraction and interfacial velo city were taken in R-134a flowing through a vertical duct to characterize t he transition from slug to fully developed annular flow. The measurements w ere taken in both heated and adiabatic conditions for two system pressures and two mass now 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 th e same time, the local void profiles showed an inversion from a classical c enter-peaked slug distribution to a wall-peaked annular distribution. The e ffect was less evident at the lower pressure and flow rater where transitio n was prolonged and less noticeable, and where a thicker transverse film ex isted in higher average void fractions. The heated data showed similar tren ds as the adiabatic data except that the heating action induced a more dram atic effect on the frictional pressure gradient near the slug-to-annular tr ansition and caused the transition to occur at lower average void fractions . Published by Elsevier Science Ltd.