Effect of degree of fluidization on wall-to-bed high temperature heat transfer with a single, vertical heater
This work is a theoretical and experimental study of combined convective and radiative heat transfer from an inner tube to an annular fluidized bed which absorbs, emits, and scatters energy. The theoretical model of radiative heat transfer was based on the initial radiation intensity at the emitter and the degree of fluidization in the bed. Gas composition was found to be less important to radiative heat transfer 'than the solid particle characteristics.
This is because the gas phase has a relatively low extinction coefficient for radiation compared with the solid phase. In this kind of system, the gas can be treated as transparent. The gas viscosity affects radiative heat transfer indirectly by changing the extent of particle dispersion in the fluidized bed.
The diffusion approximation for radiation, combined with conduction, was successful in predicting the temperature distribution and the heat flux at the outer reactor wall in the bed at the minimum fluidization gas velocity. The use of an effective thermal conductivity and a radiant conductivity was adequate to represent the thermalradiative characteristics of the particulate phase in the fluidized bed. The sum of both conductivities was used to define the Nusselt number which was used to correlate the wall-to-bed high temperature heat transfer in an aggregative fluidized bed.
It was found experimentally that the particle concentration in the particulate phase has a substantial effect on the wall-to-bed heat transfer in a fluidized bed. The degree of particle dispersion was then introduced in terms of particle size, particle concentration in the particulate phase, and bed geometry to describe the wall-to-bed high temperature heat transfer in an aggregative fluidized bed.
The maximum wall-to-bed high temperature heat transfer coefficient in an aggregative fluidized bed was correlated with the fluidization characteristics. The regression analysis showed that the correlation is excellent between the Nusselt number and the Archimedes number and the degree of particle dispersion.