Heat transfer to a stationary and moving sphere immersed in a fluidized bed

• Main Author: Desai, Chetan Jitendra
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-1989
• Subjects: Mechanical engineering
• ISBN: 9798207428413

The motion of submerged objects in fluidized beds and the corresponding improvement in the heat transfer rate has been largely unexplored. This improvement in heat transfer rate is significant in the design and operating cost of industrial fluidized beds that are used for heat treatment of metal objects, solid waste to be burned, gasification of coal particles, freezing of food grains, and coating operation. In the present study the heat transfer to a stationary sphere, a vertically downward moving sphere, and an oscillating copper sphere submerged in an air fluidized bed of glass particles was studied with the bed at standard pressure and temperature conditions. The purposes of this study were; first to investigate several of the important variables known to influence the heat transfer rate to a stationary sphere in an air fluidized bed, and secondly to determine the effect of forced linearly downward motion and oscillating motion of the sphere on heat transfer rate. Average heat transfer coefficients were determined from transient cooling measurements made on either a stationary or moving copper sphere with an embedded thermocouple at its center. A data acquisition system was developed for this experiment. The heat transfer coefficient for a stationary sphere was found to increase rapidly with an increase in the superficial air velocity above that of a packed bed, then leveled off at higher superficial velocities. Heat transfer coefficient was improved by a factor of 4 to 13 compared to the stationary sphere near incipient fluidization as a result of the linearly downward motion of the sphere with a diminished effect in heat transfer observed at the higher superficial air velocities. The surface contact residence time of the emulsion packet was estimated for an incipient fluidization condition from the linear velocity of the sphere and the sphere diameter. This residence time was used to predict an average heat transfer coefficient from a modified packet theory of heat transfer including the effect of wall thermal resistance. The predicted theoretical heat transfer coefficient agreed well with the experimental results. For an oscillating sphere the heat transfer coefficient increased 7.5 to 14 times that observed for a stationary sphere near incipient fluidization and then leveled off at the higher superficial air velocities. Contrary to the general trend, the heat transfer coefficient for the case of 355-420 $\mu$m glass particles was found to decrease at the equivalent average sphere velocities corresponding to the higher frequency and peak-to-peak amplitudes. Heat transfer correlations were developed for each of three cases, namely, for the stationary sphere, the linearly moving sphere, and the oscillating sphere.