Within a gas/particle two-phase testing facility, multi-level concentrators were investigated by obtaining gas/particle two-phase characteristics of each concentrator within a centrally fuel-rich swirl burner. Measurements of velocities, particle volume flux profiles and relative particle number concentrations were obtained and analyzed to assess the performance of these concentrators. For concentrators with fewer levels, peak values of the axial particle volume flux were observed to move away from the central axis of the primary air duct. The rich/lean air ratios near the exit of the last ring were much larger for the single-level concentrator than for the 2- and 3-level concentrators. Conversely, the concentration ratio is always smaller for the single-level concentrator than for the others. In addition, the performance ratio R-CR near the exit is always larger than 2, indicating that the 2- and 3-level concentrators can achieve a stable flame. The resistance coefficient increased with fewer rings making up the concentrator. (C) 2011 Elsevier Ltd. All rights reserved.