Comparative evaluation of four-phase interaction models for solid-gas separation and flow dynamics in a square cyclone under high loading conditions

This study presents a comparative evaluation of four interphase coupling models: one-way, two-way, four-way, and four-way with agglomeration - applied to square cyclone separators under high solid-phase loading. The objective was to assess how coupling strategies influence separation efficiency, pressure drop, and internal flow structures under circulating fluidized bed conditions. Simulations employed the Reynolds Stress Model for turbulence closure across a wide range of solid fractions. The results show that the one-way coupling underestimates fine-particle separation and does not capture the effect of solid loading on pressure drop. The two-way coupling improves predictions by including gas–solid feedback but neglects inter-particle dynamics. The four-way coupling further enhances accuracy by accounting for particle collisions, while the agglomeration variant provides the most reliable results. Including agglomeration increases the effective aerodynamic size of fine particles, improves collection, stabilizes pressure drop, and reduces turbulence intensity. Vortex structure analysis confirmed that agglomeration suppresses chaotic instabilities and strengthens flow stability. The findings demonstrate the novelty of applying agglomeration modeling to square cyclones and show that four-way coupling with agglomeration is the most accurate and robust approach for realistic CFD simulations. Beyond methodological advances, the results provide practical guidance for the design and optimization of CFB cyclones and improve understanding of gas–solid flow dynamics in high-load separation systems. • First report of agglomeration-enabled coupling in square cyclone separators. • Solid loading governs vortex structure and pressure drop in high-solid flows. • Coupling choice directly drives fine-particle separation efficiency. • Findings guide design of efficient square cyclones for CFB applications. • Dense Discrete Phase Model enables robust simulation of dense-phase cyclones.