Heat transfer characteristics of a vertical tube in a fluidized-bed combuster

Abstract

The heat transfer coefficient between a coal fluidized-bed combustor and a vertical immersed tube was determined in a 15-cm-diameter, 1-m-high fluidized bed with a freeboard zone of height 0.5 m and diameter 25 cm. The effects of the particle size (0.33-0.78 mm), the air velocity (60-190 cm/s), the bed temperature (800-950°C), and the radial position of the tube on the heat transfer coefficient were studied. The heat transfer coefficient increased with increasing air velocity to a maximum and then decreased, and increased with increasing bed temperature and decreasing particle size. The heat transfer coefficient was a maximum when the tube was located at 1/4 of the bed diameter from the wall. The thickness of the gas film on the tube was determined using a model which takes into account the effect of radiation on the heat transfer coefficient. Radiation accounted for 10% of the overall heat transfer.