| Summary: | 碩士 === 國立中正大學 === 化學工程研究所 === 89 === Abstract
This paper is to study the relationship between macroscopic flow structure and heat transfer through the instantaneous qualitative and quantitative analysis in bubble columns. Due to the limitation of measuring techniques, it only can measure the time-average heat transfer and flow structure independently. Therefore, using a two-dimensional bubble column (220x50x1.2cm), the hydrodynamics and heat transfer coefficient (h) are studied using particle image analyzer (PIA) system and heat transfer probe.
With increasing gas velocity, the flow structures changes from dispersed regime to coalesced bubble regime that is divided into the 4-region and 3-region flows. When the gas velocities lower 1cm/sec, it is uniform distributions of gas holdup, average bubble size, liquid velocity, and heat transfer coefficient in the radial direction except near sidewalls. For coalesced bubble regime, the 4-region flow region comprise descending, vortical, fast bubble, and central plume regions at the gas velocity between 1 and 3 cm/sec. When gas velocity is over 3 cm/sec, the regime becomes 3-region flow, no central plum. In coalesced bubble regime, the fast bubble flow region has the maximum gas holdup, average bubble size, liquid velocity as well as heat transfer rate because of bubble inducing the strongest turbulent intensity. The vortex in the vortical flow region acts like close cell to prevent interacting from its surrounding, and is unfavorable to heat transfer. 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. The heat transfer coefficient of three-phase is higher than that of two-phase due to the introduction of solid particles. It is found the h first increases, achieve a maximum, and then decreases as a function of gas, liquid velocity and the diameter of the particle.
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