Heat transfer in a fluidized bed. Part I. The effect of thermal driving force on the heat transfer coefficient

Abstract

The influence of the thermal driving force on the coefficient for heat exchange between a fluidized bed (diam. 30 cm) and part of a submerged vertical cylinder (diam. 7 cm) is described. In the experiments both negative and positive driving forces have been applied (the heat flow being considered positive if directed towards the bed). Beds of glass beads, quartz sand and silica-alumina catalyst powders have been used to cover a range of particle size and density. For powders exhibiting dense phase expansion, heat transfer coefficients are found which are significantly affected by the driving force. At driving forces of about − 70°C heat transfer coefficients in a 40 μm catalyst bed were only half of those at driving forces approaching zero. The behaviour of a transparent ‘two-dimensional’ fluidized bed showed that this might be the result of a reduction of solids mobility in the vicinity of the relatively cold surface. In an attempt to explain this reduction three mechanisms were considered; of these the decrease of the dense phase expansion due to a temperature gradient proved to be the most important one. Powders not exhibiting dense phase expansion show heat transfer coefficients which are only slightly affected by the driving force, the differences resulting entirely from changes in physical properties with temperature.