Thermal resistance at a surface in contact with fluidized bed particles

Abstract

While most investigators agree that heat transfer from the dense phase of a fluidized bed is the dominant mode of heat transfer from bed to surface, models of the process have taken many disparate forms. The differences among these models are most evident with beds of large particles and at short dense phase residence times. An experimental apparatus has been developed which permits measurement of the thermal resistance between a surface and adjacent particles during a rapid transient process. Meaningful results are obtainable for periods as short as 10 ms. Large heat transfer coefficients for particles with diameters between 0.65 and 1.0 mm were observed in the initial 20 ms. During an ensuing period of about 80 ms the measured heat transfer coefficients were lower and nearly invariant with time. After that the coefficients began to decline. This behavior is most consistent with a discrete particle model with initial heat transfer by asperities in direct contact with the surface followed by heat transfer through gas layers in areas adjacent to the contact points. No significant difference was observed between minimally fluidized and packed beds of the same batch of particles, refuting the notion of a gas layer separating the particles from the surface in fluidized beds.