Wall-to-Bed Heat Transfer Characteristics in a Fluidized Bed of CNT Particles

Carbon nanotubes (CNTs) with high thermal and electrical conductivities are very promising materials in a wide range of potential applications. In recent years, fluidized bed reactors have been increasingly applied for mass production in the CNT synthesis using catalytic chemical vapor deposition technology. Heat supply to the catalyst or CNT particles in the bed for the CNT synthesis is achieved by using heated reactor wall. The wall-to-bed heat transfer coefficients are needed to design the reactor, where temperature control plays an important role.

The effect of gas velocity on the average and local heat transfer coefficients between a heat transfer probe (20 mm-OD) at wall and a fluidized bed has been firstly determined in a fluidized-bed (0.15 m i.d. X 2.6 m high) of the CNT particles (d_p=0.485 mm; bulk density=92 kg/m³). The average heat transfer coefficient exhibits a maximum value (692 W/m²K) at 2.8 U_mf with variation of gas velocity. The variation of instantaneous heat transfer coefficients reflects well the entangled CNT aggregates behavior near wall region in the dense phase. The obtained average heat transfer coefficients have been correlated with the experimental parameters from present results and previous literature.