Thermal Radiation Characteristics of Fluidized Solids.

Abstract

Radiative heat transfer from an emulsion of high temperature fluidized solids is visualised, utilizing a continuum approach, as that from a volume emitter of voidage greater than zero. The initial considerations were of a simplified mathematical model which for an homogeneous constant property, emitting, absorbing and scattering medium may be described by the exact formulation for one dimensional, steady-state radiative transfer. From the analysis the dependancy of the emitted radiative flux levels upon the emissivity of the bed material emerged. Such observations were verified experimentally. Following a study of the hydrodynamic behaviour of the bed surface, a model describing transient radiative transfer was evolved based upon the packet model with initial isothermal conditions. The small residence times of the surface eruptions, found experimentally, constrained the flux levels to be almost invariant with time. The analysis was theoretically and experimentally extended to include the conductive heat transfer mode in consideration of surfaces immersed into a fluidized bed. Again, using the packet model the dominant conductive mode was apparent for early particle residence times. Radiation plays an increasingly significant role as the time history of the particle contact period proceeds. Contrary to previous suggestions, radiation contributes significantly to the overall energy transfer for the normal operating temperature range of fluidized bed heat exchangers unless sufficiently low particle or transfer surface emmisivities are encountered. Throughout the work a number of problems have been highlighted where further effort is required.