Scaling and optimizing dense fluidized bed reactors remain challenging because gas–solid interactions are complex and highly heterogeneous, most experimental observations are limited to pseudo-2D systems, and validating the numerical models used for design is difficult. This challenge is particularly pronounced for geometries with diameter-transformed structures (i.e., the bottom section of transported risers and two- and multizone fluidized bed reactors) and for Geldart A solids, whose fine particle size and cohesive behavior produce complex, chaotic flow patterns. In this work, we developed an approach that integrates cold-flow testing with CPFD simulations to regulate gas–solid fluidization in a 3D diameter-transformed dense fluidized bed reactor. We modified the Ergun-based drag model using pressure-drop measurements and implemented it in CPFD simulations. The model accurately reproduces experimental pressure drop and local solids holdup in a dense bed reactor containing 1.2 kg of Geldart A particles, demonstrating its predictive capability. We employed the validated model to evaluate hydrodynamics (i.e., void fraction, solid flux, and residence time distribution) under various operating conditions, enabling flexible application across different reactions.