Bubble Induced Heat Transfer in Two-Phase Systems

Abstract

The two-phase theory of fluidization has been extended to cover heat transfer at surfaces by introducing the concept of a property boundary layer in the vicinity of the surface. The property boundary layer is a consequence of changed voidage and hence of changed thermophysical properties of the emulsion phase at the surface. A method of defining and calculating this boundary layer has been developed. A model of heat transfer from a surface to a gas fluidized bed based on the extended theory has been developed. Aggregative gas fluidized beds and bubbling gas-liquid systems can be unified from the point of view of heat transfer by defining a general two-phase system consisting of a discrete gas bubble phase and a continuous dense phase. The thickness of the property boundary layer of the dense phase, which is in the vicinity of the surface, differentiates between bubbling liquids and aggregative gas fluidized beds since it is zero in the former case and non-zero in the latter one. To investigate the mechanism of the bubble induced heat transfer, the multi-bubbling system has been simplified by generating a single continuous stream of gas bubbles in a stationary dense phase. Furthermore, a special probe which can be used to discriminate between conductive and convective modes of heat transfer has been designed. A model of the bubble induced heat transfer in the simplified system based on the surface renewal and penetration theory has been developed. It has been found that transient conduction into the dense phase is the most important mechanism of the bubble induced heat transfer. In the case of aggregative gas fluidized beds of small particles operating below the radiative temperature level, it is responsible for at least 90% of heat transfer; in the case of bubbling gas-liquid systems for about 75% of heat transfer. In the former case the remainder is contributed by superimposed gas convection, and in the latter case by liquid convection.