Annular Flow

Annular flow is a two-phase flow regime where the liquid forms a film of varying thickness along the wall of the pipe and the vapor phase flows at a higher speed down the middle of the pipe.  The interface between the vapor and liquid phase is not entirely well defines.  Part of the liquid is sheared off from the film by the vapor and is carried along in the core as entrained droplets.  At the same time, turbulent eddies in the vapor deposit droplets on the liquid film.

Due to the different forces on the fluid, the thickness of the liquid film is not constant across the cross section of the pipe.  The effects of gravity can cause the thickness of the fluid film towards the bottom of the annulus to be bigger than the top.  Downstream of bends, most of the liquid will be at the outer wall.

Key Points about Annular Flow

Gas Core  -  The gas phase flows as a continuous core in the center of the conduit.
Liquid Film  -  A thin liquid layer or film coats the inner wall of the pipe or channel.
Droplets  -  Small liquid droplets may be entrained within the gas core due to shear forces.
High Gas Velocity  -  Annular flow typically occurs at relatively high gas velocities, where the gas phase dominates the dynamics of the flow.
Shear Forces  -  The gas core exerts shear forces on the liquid film, which can cause wave-like instabilities on the liquid surface.

Applications for Annular Flow

Oil and Gas Industry  -  Annular flow is significant in gas-lift operations, production tubing, and multiphase pipeline transport. Understanding this flow is needed for optimizing production and avoiding operational issues like erosion or hydrate formation.
Heat Transfer Systems  -  In heat exchangers, annular flow ensures efficient heat transfer due to the high surface area of the liquid film in contact with the wall.
Nuclear Engineering  -  Annular flow can occur in nuclear reactor cooling systems, where understanding flow behavior is critical for safety and efficiency.