To the right in Fig. 3.1.1 the radial profiles of the axial and tangential
gas velocity components are sketched. The former shows the outer region of
downwardly directed axial flow and the inner one of upwardly directed flow.
As mentioned, the downward velocity at the wall is the primary mechanism
for particle transport out the dust outlet. The axial velocity often shows a
dip around the center line. Sometimes this is so severe that the flow there
is downwardly directed. The tangential velocity profile resembles a Rankine
vortex: a near loss-free swirl surrounding a core of near solid-body rotation.