Anumberof industrial flows, such as pneumatic conveying andmillduct flows in power stations, involve gas-particle with solids fraction low enough to ignore particle concentration effects. The solids volume fraction is quite low in mill-duct flows. However, the introduction of bends and complicated flowpatterns can result in regions of dense particle rope where solids volume fraction can to be no longer negligible. Roping is a phenomenon which has been of great interest to many industrial applications [1–3]. With regard to numerical modelling of the gas-solid systems, very little is known about the force exerted on the individual particles when a fluid flows through an assembly of particles, i.e. the drag force [4]. A good understanding of particle–fluid interactions is essential to predict more accurately the dynamics of these systems. In previous studies, it was found that the particulate mass loading increases the slip velocity of the particles when the superficial gas velocity decreases [5]. An increases in particle concentration, without increasing the mass loading, can also lead to an increase in particle slip velocity [6–8]. It was also reported that the use of drag coefficient developed for particles in isolation, not in a cluster environment might cause discrepancies the numerical prediction results [9–11]. Haider and Levenspiel [12] reported experimental data on terminal velocity of different non-spherical particles and generated a drag formula that accounts for shape factor. Their drag formula did not account for particleconcentration in cluster environment.