Presentation at the 12th International Conference on Multiphase Flows (ICMF) 2025.

The occurrence of extremely thin concentration boundary layers at fluid interfaces for high local Peclet numbers is a severe obstacle for efficient and accurate numerical simulation of mass transfer processes in two-phase fluid systems. Especially challenging are liquid-liquid systems, in which thin concentration boundary layers can appear on both sides of the fluid interface under convection- dominated conditions. In those cases, the one-sided species concentrations at the interface are not known a-priori and are determined by a conjugate mass transfer problem governed by interfacial conditions.

We present a two-sided subgrid-scale (SGS) boundary layer model for conjugate mass transfer at fluid interfaces that accurately computes the local mass transfer rates even for very high concentration gradients on moderate or coarse mesh resolutions. For this purpose, SGS modeling is applied on both sides of a boundary transmissive to passive scalars, such as the fluid interface, enabling the accurate capture of the conjugate effects of a thin boundary layer on one or both sides of said interface.

We implement our approach in the unstructured Finite-Volume Arbitrary Lagrangian / Eulerian Interface-Tracking (ALE-IT) OpenFOAM module twoPhaseInterTrackFoam. We made twoPhaseInterTrackFoam publicly available in our previous publication.