Hydrodynamics of Gas-Liquid Bubble Columns and Air-Lift Loop Reactors: Experiments and Numerical Simulations

摘要

It is generally accepted that a key to improvements in the modeling of gas-liquid reactors is a realistic description of two-phase flow. One of the main unresolved problems is the modeling of turbulence in the continuous phase. In the majority of publications on numerical simulations of stationary two-phase flows, the standard k-epsilon model developed for single-phase flows has been used. The general applicability of this model to instationary two-phase flows is however disputed. In our last contribution on the ISCRE-15, we discussed the above question with the example of a locally aerated, flat bubble column. The results showed that it is possible to obtain a realistic dynamic solution with the k-epsilon turbulence model. It was however obvious that the general applicability of the 3D k-epsilon turbulence model to the dynamic simulation of bubbly flows needed further validation for a large spectrum of test cases of turbulent gas-liquid flows. A series of new test cases for symmetrically aerated, empty bubble column with different aspect ratios will be presented in this contribution. The comparison of detailed experiments and flow simulation results will be given. Another important unresolved problem is the modeling of the turbulence induced by the gas phase. In the test cases of locally aerated, flat bubble colums mentioned above, the gas phase occupied only a very small part of the reactor, and the influence of the bubble-induced turbulence could be fully neglected. In this contribution some new simulation results for air-lift loop reactors of different geometries will be presented. In this new test cases, the gas phase occupies the whole cross-section of the riser, so that the bubble-induced turbulence is of the same order of magnitude as the shear-induced one. The neglection of this effect in the model leads to very poor agreement between the simulation results and experiments. The most popular modifications of the standard k-epsilon model, which try to describe the influence of the rising gas bubbles on the turbulence in liquid phase will be discussed. Unfortunately, in some test cases, none of them can lead to a better agreement with experiments, so that further investigations in this area are necessary.