This study proposes two different approaches to describe the dynamics of a transient gas-solid flow inside a modified Hartmann tube or a similar apparatus. An experimental analysis was performed to analyze the main influencing parameters during a typical characterization test of combustible solids. For this purpose, the development of dust clouds of different solid materials inside the tube has been evidenced by high-speed videos and particle-size measurements. The two-phase flow induces variations in particle size distributions and segregation levels that have been identified during the dispersion process. Furthermore, a computational fluid dynamics (CFD) simulation, based on an Euler-Lagrange approach, has been developed with ANSYS FLUENT (TM), It was directed towards the assessment of flow conditions pertaining to the agglomeration and fragmentation of particles dispersed in the standard setup. The experimental data have been used to adjust a fragmentation model in the CFD simulation in order to describe the variations in particle size distributions accurately. These approaches have evidenced three different stages of solids dispersions, which can be classified according to variations in particle size distributions: a fragmentation phase, stabilization of the dust cloud and sedimentation phase. These facts suggest that, for the micrometric aluminum powders which have been tested, the minimum ignition delay must be 60 ms, after considering the distribution of the discrete phase inside the tube and the variations in size distribution at the elevation of ignition electrodes. This delay being greatly influenced by powders properties, this paper suggests combining both simulation and experimental approaches in order to determine the most relevant or the most conservative conditions for the determination of dust flammability.