Experimental study of developing free-falling annular flow in a large-scale vertical pipe

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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 × 104, 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.
Original languageEnglish
Article number111064
JournalExperimental Thermal and Fluid Science
Early online date25 Sept 2023
Publication statusPublished - 1 Jan 2024


  • Annular flow
  • Droplet entrainment
  • Film velocity
  • Free falling
  • PIV
  • Tee junction

ASJC Scopus subject areas

  • General Chemical Engineering
  • Nuclear Energy and Engineering
  • Aerospace Engineering
  • Mechanical Engineering
  • Fluid Flow and Transfer Processes


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