A cold-model circulating fluidized bed is used to investigate gas–solid flow behavior in tapered-in risers with inclined angles of 0.3°, 0.5°, and 0.7°, with a conventional cylindrical riser as a reference. Local solids holdup and particle velocity are measured using optical fiber probes. The tapered-in risers exhibit a distinct S-shaped axial solids holdup profile and a continuously decreasing particle velocity along the axial direction, in contrast to the typical C-shaped solids holdup profile and bottom particle-acceleration region observed in the cylindrical riser. The bottom section of the tapered-in riser remains in a dilute fast-transport state because of the high local gas velocity, whereas the middle section becomes the main flow-transition region. In this region, the decrease in local gas velocity caused by cross-sectional expansion is not followed immediately by particle deceleration, leading to near-wall accumulation, back-mixing, local internal circulation, and the highest solids holdup. Radially, the cylindrical riser shows a core–annulus structure that weakens after the bottom acceleration region, while the tapered-in risers develop stronger core–annulus structures in the middle and upper sections. A mechanistic analysis based on the Kutta–Joukowski (K-J) force further indicates that the radial non-uniformity of local solids holdup is closely related to the K-J force acting on particles. The K-J force analysis helps explain the development of near-wall solids accumulation and core–annulus structures, and its magnitude is jointly affected by slip velocity and particle-velocity gradient. Mesoscale analysis further shows that near-wall clusters in the middle and upper sections of the tapered-in riser are denser and more persistent, although their occurrence frequency is lower. These results clarify the geometry-induced flow restructuring in tapered-in risers and provide hydrodynamic guidance for the design of tapered-in riser–turbulent bed coupled reactors for PDH.
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