Direct numerical simulation of multi-physics reactive mass transfer at single and multiple bubbles
Reactive mass transfer from rising gas bubbles to the ambient liquid is the basis for many chemical processes of industrial importance. The necessary process intensification leads to faster process steps and highly concentrated systems. In such two-phase flows with complex local interaction between mass transfer, transport and chemical transformations, additional multi-physics becomes relevant which can often be ignored for dilute and clean systems. This includes: volume effects for dissolving bubbles; partly immobilized bubble surfaces due to contamination, additives, or other surface active mixture components; variable distribution coefficients; ionic species with strong coupling of diffusive fluxes by the intrinsic electrical field, typically leading to local electro-neutrality of the mixture away from the interface; cross-diffusion effects as well as non-idealities in systems of higher concentrations. These complexities add to the multi-scale nature of reactive mass transfer with extremely thin concentration boundary layers due to convection-dominated transport.Besides experimental investigations, a thorough understanding of the local interplay of elementary sub-processes requires numerical simulations based on rigorous mathematical modeling. Our approach employs continuum physics based on the two-phase balances of mass, momentum and species mass. Based on the Volume of Fluid (VOF)-method, we built on our two-scalar approach for 3D Direct Numerical Simulations of mass transfer at gas bubbles which allows for variable Henry coefficients. The method will be extended to cover the above multi-scale multi-physics: a reactive subgrid-scale model accounting for flexible chemical reactions will enhance the accuracy at the interface; the combination with an appropriately modified momentum jump condition allows to model partially immobilized surface; extension of the VOF-method to systems with phase transfer allows to account for the volume effects accompanying mass transfer in a local manner, thus allowing the rigorous simulation of dissolving pure gas bubbles; a transformed formulation of the species equations in terms of the chemical potentials mitigates the boundary layer problem.Within the network of the priority program, these techniques allow to simulate the mass transfer in bubbly flows with different chemistry according to concrete applications. In particular, reactive transfer from rising form-dynamic bubbles, at interacting bubble pairs and in small pseudo-swarms will be studies. Moreover, the project will contribute to the guiding measure of dissolving Taylor bubbles with simulations including bubble shrinkage, and a theoretical-numerical benchmark for mass transfer from rising form stable bubbles is to be developed in a team. All simulation results for experimental setups will be closely examined and discussed together with the cooperating colleagues.