Simulation of the Bubble Behaviors for Gas–Liquid Dispersion in Agitated Vessel

抄録

The present study mainly investigates bubble behaviors in an agitated vessel. Computational fluid dynamics provides a method for exploring the complex fluid flow in an agitated vessel. A stirred tank model with transport equation for the interfacial area concentration and force equilibrium equation for bubbles under multi-forces is created for a 43-dm³ agitated vessel. The model considers the breakage and coalescence mechanism of the bubbles. The local gas holdup is measured by fiber-optical probe. After this, the bubble size distributions are calculated by the interfacial area concentration model. The simulations with validated models show good agreement with the experiments. From the simulation results, the area in which the gas holdup value is greater than 0.01 extends 37% in the upper circulation region when the rotation speed is increased to 40 rad/s from 20 rad/s. However, in the lower circulation, the gas holdup maintains a low level due to the existence of a “death zone”. The simulations indicate that the bubble breakage mainly occurs in the impeller outflow region and the coalescence occurs in the upper circulation region. For bubble behaviors being in the lower circulation region, bubble coalescence or breakage is determined by the flow field pattern in the vessel. Moreover, it is difficult for the simulation process to converge when the virtual mass force is considered and there is a lack of effective technology to measure it, which has led researchers to generally ignore this force. In order to analyze the virtual mass force, the present study utilizes computer programming which is based on the virtual mass force formula to obtain the force. The results show that at positions close to the impeller tip and the center of the upper region, the virtual mass force value is 0.015 and 0.017, respectively, but the value is approximately zero in the bulk of the tank. Hence, the bubble has two significant stages of acceleration process in the vessel.