The present study reports a scaled-up down flow reactor study using 2.54-cm and 5.08-cm internal diameter units both utilizing Computational Particle Fluid Dynamics (CPFD) modeling and experimental data. The solid particles used are Fluid Catalytic Cracking (FCC) catalyst particles with a 84.42 mu m mean diameter and a 1722 kg/m(3) particle density. The gas velocity and solid mass flux are varied from 1.1 to 2.0 m/s and from 7 to 50 kg/m(2) s, respectively. Experimental data is obtained using a CREC-GS-Optiprobe** sensors to be able to account for individual particle clusters. Data analysis involves the setting of the data baseline in compliance with solid mass balances. On the other hand, a CPFD method is applied to simulate the complex CFB downer fluid dynamics. With the CPFD method, cluster behaviour is extensively studied for a wide set of operating conditions. Cluster sizes are obtained experimentally for each of the operating conditions considered. CPFD simulations are compared with experimental results. Good agreement is observed in all cases, except in the near-wall region. It is also proven in the present study that, near-wall discrepancies between experimental findings and hybrid CPFD modeling predictions are significantly reduced in the 5.08-cm ID downer unit. Thus, it is anticipated that this hybrid CPFD model provides a fully trustable computational method for the scaling-up of industrial down flow reactor units with diameters larger than 5.08-cm ID. (C) 2017 Elsevier Ltd. All rights reserved.