Visualization of fluidized-bed heat exchanger in upward/downward flow condition by neutron radiography

Heat transfer characteristics of a fluidized-bed heat exchanger are dominated by the characteristics of bed-material movement, especially, in the neighboring region of the heat transfer tube. In the present experiment, a simulated fluidized-bed heat exchanger model was partitioned into two channels to form an upward and downward flow simultaneously in the same fluidized-bed model. Then the total amount of circulating material was kept at the same value even at the different void fractions between two channels. The flow pattern of bed material was visualized by neutron radiography with introducing tracers into the fluidized bed. The simulated fluidized bed consisted of aluminum plates, and the bed materials were sands of 96% SiO2 (mean particle diameter: 0.154-0.321 mm, density: 2550 kg/m3). Bed materials were almost transparent for neutrons. On the contrary, tracer particles of about 1 mm diameter made of B4C with clay were opaque. Thus, the tracer particles were detected clear enough for PTV (Particle Tracking Velocimetry). The fluidized-bed behavior was then discussed in relation to the heat transfer characteristics around the heated tubes submerged in the bed.