  Annular flow is the primary characteristic of unsteady wastewater flow, which
initiates entrained air and sets up the air pressure regime within the system -
an important design consideration. This paper reports on an experimental
investigation of free-falling annular flow in a vertical pipe with different
inlets at extended flow ranges up to Re = 3 x 10e4, similar to those in
Building Drainage Systems (BDS). In the experimental setup, a vertical pipe
system (5 m) was used to record velocity profiles and film thickness in the
developing region through Particle Image Velocimetry (PIV) measurements.
Entrained droplets were collected through a separator, and the entrainment
fraction was calculated at different flow conditions. The study reports on the
development process of the film velocity and thickness along the vertical pipe,
which agrees well with empirical predictions. The results of the droplet
entrainment of a vertical annular flow show the development process to the
steady state. Additionally, a Tee-junction inlet in drainage system generates a
higher and different entrainment profile.
