MECHANISM OF HEAT-TRANSFER IN BUBBLY LIQUID AND LIQUID-SOLID SYSTEMS - SINGLE BUBBLE INJECTION

Local instantaneous changes in heat-transfer coefficients due to the passage of gas bubbles in liquid and liquid-solid systems are measured. A special heat-transfer probe is developed and located within the bed to trace the instantaneous local heat-transfer rate during the passage of single gas bubbles. A microfoil heat flow sensor is attached to a foil heater, and the sensor-heater probe assembly can accurately measure the heat flux and the surface temperature over a small area. Signals from the sensor are amplified and interfaced with the microcomputer data acquisition system. Simultaneous visualization is performed using a high-speed video camera and a borescope to establish the correspondence between the visual and sensor signals, and hence relate the local instantaneous hydrodynamics to the heat-transfer rate. Local heat-transfer coefficient vs. time traces are analyzed in conjunction with visual signals. 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. The local maximum in heat transfer, however, is more pronounced in liquid than in liquid-solid systems.