Effect of acoustic field on heat transfer in a sound assisted fluidized bed of fine powders

Abstract

The fluidized beds are widely used in a variety of industries where heat transfer properties of the fluidized system become important for successful operation. Fluidized are preferred in heat recovery processes because of their unique ability of rapid heat transfer and uniform temperature. Fine powders handling and processing technologies have received widespread attention due to increased use of fine powders in the manufacture of drugs, cosmetics, plastics, catalysts, energetics and other advanced materials. A better understanding of fluidization behavior of fine powders is of great importance in applications involving heat transfer, mass transfer, mixing, transporting and modifying surface properties etc. The difficulty in putting the fine powders in suspension with the fluidizing gas is related to the cohesive structure and to the physical forces between the primary particles. The sound waves agitate bubbling and this results in improving solids mixing in the fluidized bed. The improved solids mixing results in uniform and smooth fluidization, which leads to better heat transfer rates in the fluidized bed. The heat transfer for different sized particles at different acoustic conditions, gas velocities and angular positions around the circumference of heat transfer surface was investigated. Optimum fluidization velocities and acoustic conditions to obtain maximum heat transfer rates from the heat transfer surface to the bed material were found out. Most of the research work in heat transfer in bubbling fluidized beds is focused on Geldart B and D particles. Very little information is available for heat transfer with fine powders belonging to Geldart groups C and A category. The heat transfer between a bubbling fluidized bed of fine powders with an immersed heating surface in absence and presence of acoustic waves was investigated. Acoustic energy of sufficient intensity and sound pressure level significantly improved the quality of fluidization of fine powders. Heat transfer rates could be improved by the oscillation of the particles at certain frequencies. The heat transfer intensity strongly depended upon the angular position along the circumference of the heat transfer surface. The location for maximum heat transfer rates was at the sides of the tube and shifted upward at higher gas velocities. Acoustic waves had very little effect on heat transfer for coarse grained particles. However, heat transfer rates improved appreciably for fine powders under acoustic conditions. The data for local and average heat transfer coefficients is presented as a function of excess air velocity and sound pressure level.