INSTANTANEOUS LOCAL HEAT-TRANSFER AND HYDRODYNAMICS IN A CIRCULATING FLUIDIZED-BED

Local heat transfer mechanisms and hydrodynamics are studied in a 9.3 m tall, 152 mm i.d. transparent cold model circulating fluidized bed for 171-mu-m Ottawa sand. Instantaneous measurements at the wall are made with platinum-coated heat transfer probes. For some conditions, simultaneous local voidages are determined using a capacitance probe. Results show that the sudden and dramatic peaks in the measured instantaneous heat transfer coefficients are directly caused by the arrival of strands of particles at the heat transfer surface. Analyses of the capacitance probe signals indicate that these strands possess wide distributions of voidages which vary with the local time-averaged area-averaged suspension density. Simultaneous heat transfer probe measurements further suggest the existence of characteristic residence lengths for these strands. The average falling velocity of the strands is 1.26 m s-1 using high-speed cinematography.