PARTICLE MIGRATION AT SOLID-SURFACES AND HEAT-TRANSFER IN BUBBLING FLUIDIZED-BEDS .1. PARTICLE MIGRATION MEASUREMENT SYSTEMS

Heat transfer between submerged surfaces and fine particle fluidized beds is affected by particle migration to and from the exchanger surface. Owing to the lack of appropriate measurement techniques up to now, however, modelling of particle migration to and from heat exchanger surfaces was based primarily on speculation and less on experimental evidence. The present work has therefore the following two objectives. In this part of the paper, the measuring and analysing systems are described. A collection of luminous particles adjacent to a transparent solid surface is marked by a light pulse transmitted via fibre optics. The fate of the initially bright spot is recorded on videotape. Digital image analysis of spot migration gives statistically determined migration velocities along the heating surface, whereas the decay in luminosity defines the particle exchange frequency in the direction perpendicular to the solid surface. Comparison of the measured particle exchange frequencies with the simultaneously measured heat transfer coefficients reveals a direct correspondence between particle migration and heat transfer. The measured velocities of particles parallel to the wall provide an explanation for the well-known effect of probe size on the measured heat transfer coefficients. Details of fine particle behaviour and visualized by inspection of microscopic video images. Part 2 of this paper deals with the prediction of heat transfer in bubbling fluidized beds, using the results obtained with the experimental technique, described in Part 1.