Direct Numerical Simulations of Reactive Transport Processes at Single Bubbles

Reactive mass transfer processes are found in many chemical engineering applications. Though these phenomena have been object of investigation for long time already, a complete understanding of the physico-chemical mechanisms involved has not been achieved yet. More detailed insights about these processes, in particular the local interplay between the two-phase hydrodynamics, mass transfer and chemical reactions, have been obtained in recent years by means of Direct Numerical Simulations. However, this type of simulation is nowadays still computationally demanding. Being the concentration boundary layers at bubble interfaces remarkably thinner than the velocity boundary layer, an accurate solution of the species concentration transport equations requires a numerical resolution significantly higher than that needed for the two-phase hydrodynamics. Two different numerical techniques have been developed in this work with the aim of alleviating the multiscale issue. In both the cases, the hydrodynamic problem is tackled by means of an Arbitrary Lagrangian-Eulerian Interface-Tracking method. Instead, they differ regarding the treatment of the mass transfer. The former technique relies on the so-called Radial Basis Functions by which a finite-difference method on unstructered sets of points is formulated. The latter employs a finite-volume discretisation on a mesh obtained from a specialised refinement of the grid used for the two-phase hydrodynamics. In the present research, a systematic analysis of the reactive mass transfer from single rising bubbles with significant interface deformations, i.e. technically relevant diameters, has been carried out by means of 3D Direct Numerical Simulations. Two distinct reaction prototypes representative of many applications have been considered. Simulations from slow to fast reaction intensities allowed to assess the influence of different chemical time scales on these processes. Additionally, the impact of low to moderate dissolving species diffusivities has been investigated. The last part of the study has been dedicated to the comparison of the simulation results with the predictions of the film theory