Effects of macroscopic hydrodynamics on heat transfer in bubble columns

The hydrodynamics and heat transfer of two-dimensional bubble columns opera ted in Various flow regimes are simultaneously studied using particle image velocimetry and heat transfer probe, respectively. With increasing gas vel ocity, the flow structures change from dispersed bubble regime to coalesced bubble regime divided into the 4- and 3-region flows. At low gas velocity (< 1 cm/s), the main low structure is the dispersed bubble regime with the uniform distributions of gas holdup, average bubble size, liquid velocity, and heat transfer in the radial direction except near sidewalls. For gas ve locity between I and 3 cm/s, the 4-region flow regime begins to take over w ith the bubble coalescence and interaction in the flow. Macroscopic structu res operated in the 4-region flow regime comprise descending, vortical, fas t bubble, and central plume regions. In this regime, the fast bubble region has the maximum gas holdup, average bubble size, liquid velocity as well a s heat transfer rate due to bubble inducing the strongest turbulent intensi ty. The vortex in the vortical flow region acts like a close cell to preven t interacting with its surrounding, and leads to unfavorable heat transfer. The heat transfer in the central plume region is better than that in the d ispersed bubble regime because it has higher gas velocity to generate large r bubbles from the injector. When gas velocity is over 3 cm/s, the regime b ecomes the 3-region flow with a large fast bubble stream, no central plume. The path of the center of descending Vortex has the minimum heat transfer rate. On the whole, the 3-region flow regime has the highest heat transfer rate in all three regimes. It is found that the flow and heat transfer are profoundly dominated by the macroscopic hydrodynamics structures. (C) 2001 Published by Elsevier Science Ltd.