Martin Obligado is a professor at Centrale Lille Institute since 2023. Previously, he was an associate professor (maître de conférences) at LEGI lab (Grenoble, France). He has defended his PhD (also in LEGI) in 2013 and done a three-year postdoc at Imperial College London. He is an expert on the experimental characterisation and theoretical modelling of single and two-phase non-canonical turbulent flows. His current research topics include the study of gravitational settling of particles in turbulence, their clustering and the hydrodynamics of bubble column reactors. He also works on the modelling and generation of free-shear turbulent flows, jets, fundamental turbulence theory and environmental flows.
Title of presentation: Hydrodynamics of bubble columns
Abstract:
Bubble column reactors, where gas is injected at the bottom of an initially stagnant liquid, are widely used in chemical engineering. These reactors often operate in the heterogeneous regime, characterized by a strongly polydisperse distribution of bubble sizes, high gas concentrations (15 to 40% by volume), and strong velocity fluctuations (up to 50% of the mean). Owing to such flow complexity, no reliable scaling rules are available for reactor designers. This problem dramatically hampers optimization and process control. In particular, extrapolation between laboratory-scale and industrial-sized columns (with diameters ranging from 5 to 10 m and heights between 20 and 40 m) is not controlled, and therefore simulations yield realistic results only through ad hoc adjustments depending on the size of the column.
In this work, the hydrodynamics of bubble columns are revisited with a focus on the increase in the apparent relative velocity between phases observed in the heterogeneous regime. We report experiments in an air-water bubble column of diameter 0.4 m using a new Doppler probe that provides access to bubble velocities conditioned by the void fraction. In the first part of the seminar, we will derive and test a new scaling for the liquid velocity. Using our data and results from the literature, we will show that such scaling appears to hold for a wide range of operating conditions and bubble columns of varying sizes.
Later, we focus on the characterization of meso-scale structures corresponding to high (clusters), low (voids), and intermediate gas concentrations relative to the mean gas holdup. These structures are identified using Voronoï tessellations built from the phase indicator function. The resulting conditional bubble velocity data demonstrate that meso-scale structures drive the transport of both phases in the heterogeneous regime. The unconditional relative velocity is recovered from conditional relative velocities weighted by the probability of bubble presence in a given structure. These results provide a physical basis for the swarm factors introduced in simulations. They also offer a way to connect relative velocity enhancement with the characteristics of meso-scale structures, advancing our understanding of scaling rules.
Finally, some preliminary work on the unsteady properties of these flows and their numerical modeling will be briefly discussed. Overall, this work aims to present an overview of the current state of the art in upscaling bubble columns in the heterogeneous regime.
