Experiments on the local heat transfer characteristics of a circulating fluidized bed

The role of particle diameter in the heat transfer of a gas-solid suspensio n to the walls of a circulating fluidized bed was studied for particles of uniform size. This work reports and analyzes new experimental results for t he local bed to wall heat transfer coefficient. not including the radiation component, in a long active heat transfer surface length laboratory bed, w hich extend previous findings and clear up some divergences. The research i ncluded determining the effects of extension and location of the heat trans fer surface. circulating solids mass flux and average suspension density. A n experimental set-up was built, with a 72.5 mm internal diameter riser. 6. 0 m high, composed of six double pipe heat exchangers, 0.93 m high, located one above the other. Five narrow sized diameter quartz sand particles - 17 9. 230, 385. 460 and 545 mu m - were tested. Temperature was kept approxima tely constant at 423 K and the superficial gas velocity at 10.5 m/s. The ma jor influence of suspension density on the wall heat transfer was confirmed , and contrary to other authors. a significant effect of particle size was found. which becomes more relevant for smaller particles and increasing sus pension density. It was observed that the extension of the heat transfer su rface area did not influence the heat transfer coefficient for lengths grea ter than 0.93 m. The heat transfer surface location did not show any effect , except fur the exchanger at the botton of the riser. A simple correlation was proposed to calculate the heat transfer coefficient as a function of p article diameter and suspension density. (C) 2000 Elsevier Science Inc. All rights reserved.