Transient dynamics of solid concentration in downer fluidized bed

Abstract This paper presents new results from experiments performed in a 15 cm diameter down-flow fast-fluidized bed. Tests were conducted at room temperature and near atmospheric pressure, with 125 μm glass beads. Superficial gas velocities range from 0 to 6 m/s. A needle capacitance probe was used to obtain measurement of instantaneous solids concentration ( e s ) in a small sampling volume of approximately 0.1 cm 3 . With the downer operating at steady-state, the capacitance probe provided a time-trace of the local solid fraction, indicating that e s was generally less than 0.01. However the measurements indicated that the local concentration did have excursions to significantly higher concentrations for short durations. This behavior is indicative of clusters in the down-flow fast-fluidized bed. While clusters are known to be a dominant feature of upward fast fluidization in risers, their presence has been questioned for solid/gas down flows. Sample data are presented to indicate the density, duration, and time fraction of clusters observed in this downer fluidized bed. Preliminary comparisons are made to the limited data available for comparable operating conditions in a riser.