HEAT-TRANSFER CHARACTERISTICS IN VISCOUS GAS-LIQUID AND GAS-LIQUID-SOLID SYSTEMS

Local heat-transfer measurements are performed using a special heat-tr ansfer probe in gas-liquid and gas-liquid-solid systems with viscous N ewtonian liquids as the continuous phase. Effects of viscosity on bubb le-liquid and bubble-liquid-solid interactions affecting local heat tr ansfer are studied through heat-transfer experiments with simultaneous flow visualization in a simplified system involving single bubbles or a chain of gas bubbles moving in viscous liquids and liquid-solid sys tems. Effects of viscosity on bubble wake and local heat transfer are examined with reference to heat transfer in freely-bubbling beds (bubb le columns and three-phase fluidized beds). The kinematic viscosity of the fluid greatly influences the nature of flow in the wake which aff ects local heat transfer in the bed. The local heat transfer decreases with the viscosity due to the rapid decay in the circulation strength of the bubble wake caused by increased viscous dissipation of vortici ty. Local heat transfer due to cyclic/periodic injection of bubbles is significantly enhanced due to increased bubble-wake interactions whic h rapidly accelerate bubbles and increase average bubble rise velocity . Heat transfer in simplified liquid and liquid-solid systems with sin gle- and chain-bubble injections characterizes the local heat-transfer performance of freely-bubbling beds (bubble columns and three-phase f luidized beds). A mechanistic model developed accounts for the heat-tr ansfer behavior in bubble columns and three-phase fluidized beds with viscous liquids.