Experimental study on hydrodynamic characteristics of tapered-in risers--颗粒学报PARTICUOLOGY

a State Key Laboratory of Heavy Oil Processing, China University of Petroleum Beijing, Beijing, 102249, China
b China University of Petroleum Beijing at Karamay, Karamay, 834000, China

10.1016/j.partic.2026.08.008

Volume 118, November 2026, Pages 1-15

Received 8 June 2026, Revised 5 August 2026, Accepted 9 August 2026, Available online 22 August 2026, Version of Record 31 August 2026.

E-mail: mengxiliu@sina.com; liumx@cup.edu.cn

Highlights

• Tapered-in risers exhibit an S-shaped axial solids-holdup profile and continuously decreasing particle velocity.

• The middle section acts as the main flow-transition region, with back-mixing and internal circulation.

• The core–annulus structure weakens axially in cylindrical risers but develops from bottom to upper sections in tapered-in risers.

• Kutta–Joukowski transverse force analysis provides a mechanical interpretation for radial solids-holdup non-uniformity.

• Mesoscale analysis reveals denser and more persistent near-wall clusters in tapered-in risers.

Abstract

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.

Graphical abstract

Keywords

Tapered-in riser; Circulating fluidized bed; Gas-solid hydrodynamics; Propane dehydrogenation reactor

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