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October 14, 2015
Pedro Maximiano Raimundo, Ph.D. student from LEGI and LGP2, defended his thesis: "Analysis and modeling of the local hydrodynamics in bubble columns".
This Université Grenoble Alpes doctoral thesis was prepared under the supervision of Alain Cartellier, CNRS Research Director (LEGI) and the co-supervision of Davide Beneventi, CNRS Researcher (Grenoble INP-Pagora / LGP2).
Bubble columns reactors are widely used in chemical and biological engineering due to their simple configuration with no mobile parts. However, the scale-up rules for such bubble column reactors is still a quite challenging process. In particular, using two-fluid approaches, the current practice relies on an ad-hoc fitting of the bubble size with the column dimension.
To progress in the up-scaling of bubble columns hydrodynamics, experiments have been achieved over a wide range of parameters, with columns diameters from 0.15 to 3m and with gas superficial velocities from 3 to 35cm/s, yielding void fractions up to 35%. To ensure comparable hydrodynamic conditions, almost identical bubble size distributions were produced for all these conditions. In the same spirit, coalescence was blocked. A battery of measuring techniques has been exploited including phase detection optical probes, endoscopic imaging and Pavlov tubes. A new technique has also been developed that provides the mean horizontal diameter of bubbles. That method, which is based on the spatial correlation of the signals from two optical probes located side by side, has been validated in strongly agitated, unsteady bubbly flows at high void fractions.
The database collected on the radial and axial evolutions of local hydrodynamics properties (gas hold-up, bubble size, phasic velocities and their fluctuations…) has led to a clarification of the scaling laws for such systems. In particular, we have shown that the auto-similarity of the flow structure in heterogeneous conditions leads to an entrained liquid flow rate that growths with the column diameter as D2 (gD)1/2. In other words, the entrainment capability of a bubble column is only set by the column size and does not depend on the injected gas superficial velocity. Further more, the heterogeneous character of the flow has been shown to originate from strong concentration gradients that define meso-scale structures: the resulting collective dynamics has a profound impact on the mean relative velocity between phases. Inspired by Simonnet et al. (2008), that dynamics can be well represented by introducing a swarm factor in the drag law. With such an approach, 3D URANS two-fluid simulations become able to reproduce without any ad-hoc adjustment the scale effect observed over the whole range of flow conditions considered here.