In Dilute Riser Flow (DRF), the solids are predominantly moving upwards with negligible downward flow.
In Core-annulus Flow (CAF), the solids motion is an upward core flow and unsteady downward (cluster or streamer) flow in the annulus.
The regime of the Core-annulus Flow (CAF) with Turbulent Fluidised Bottom Bed (TFBB) is characterized by an axial voidage profile of typical S-nature, due to the appearance of a Turbulent Fluidised Bottom Bed (TFBB).
The residence time for CAF with TFBB is significantly longer than CAF itself and DRF due to the existence of the fully mixed TFBB.
Chan et al. [11] demonstrated that the residence time for solids in TFBB alone can range from 10 to 20 s. The voidage of the TFBB ranges from 0.7 to 0.9 and can be predicted by the empirical Equation (3) [15]: