Heat transfer characteristics in bubble columns and three-phase fluidized beds

Multiphase flow reactors have been applied widely to catalytic reactions, biological waste water treatment and several petroleum processes. High heat transfer rate is one of the important characteristics in the operation of bubbling multiphase flow systems (bubble columns and three-phase fluidized beds). However, little has been known in the past about the intrinsic mechanism underlying the enhanced heat transfer in bubbling systems (gas-liquid or gas-liquid-solid) over non-bubbling systems (liquid or liquid-solid). The present study has unravelled this unknown which bears not only fundamental but also practical significance. The technique employed is a novel one which involves high speed video camera for direct flow visualization and quantification, and the development of a small heat transfer probe for synchronized instantaneous heat transfer coefficient measurements.;It is believed that the heat transfer property in the bed is intimately associated with the bubble motion, bubble size, and phase holdups, which are affected by fluid flow including wake flow. The bubble wake effect on heat transfer is quantified by the measurement of the instantaneous local changes in the heat transfer coefficients due to the passage of gas bubbles in liquid and liquid-solid systems. A special heat transfer probe is located within the bed to trace the instantaneous local heat transfer rate during the passage of gas bubbles. The heat transfer probe accurately measures the heat flux and the surface temperature over a small area instantaneously. Simultaneous visualization is performed to establish the correspondence between the visual and sensor signals, and hence relate the local instantaneous hydrodynamics to the heat transfer rate. The heat transfer coefficient exhibits a sharp peak in the bubble wake. In both liquid and liquid-solid systems, the observed local maximum in heat transfer coefficient behind a rising bubble is due to the bubble-wake induced surface renewal. Enhancement in heat transfer due to the bubble increases with the size because of increased surface renewal caused by larger bubble wake and stronger vortices. However, the local maximum in heat transfer due to bubbles is more pronounced in liquid compared to that in liquid-solid systems.;A mechanistic model is developed based on the consecutive film and surface-renewal theory to account for the heat transfer behavior in bubble columns and three-phase fluidized beds. The model is validated in systems of practical significance including simulated bioreactors and petroleum processing systems. The findings provide effective means for heat transfer control in bubbling systems through controlling the bubble-wake dynamics by varying the bubble size.