Measurements of Cluster-Wall Contact Times in a Scale-Model Circulating Fluidized Bed

Abstract

Abstract One of the mechanisms of heat transfer at the wall of the riser in a circulating fluidized-bed (CFB) is conduction from the clusters of material moving adjacent to the wall; this mechanism is also called particle convection. Relative to each cluster, particle convection is a transient phenomenon, so that the the amount of time that the clusters spend in contact with the wall influences the rate of heat transfer. The motion of clusters at the wall has been observed and quantified using thermal imaging as a flow-visualization technique. By heating the clusters at the wall and observing their motion with an infrared camera, the average amount of time that a cluster spends at the wall can be determined. The average contact times measured are between 0.35 s and 0.48 s, and some variation with operating conditions in the CFB is observed. These contact times can also be used to calculate bed-to-wall heat-transfer coefficients around 100 W/m2-K, which is in agreement with other heat-transfer measurements in CFBs. These experiments were conducted in a cold scale-model CFB with a riser that has a 0.159 m square cross-section and is 2.44 m tall.