A modelling approach for the numerical simulation of disperse and resolved multiphase structures including morphology transfer

In the past decades, numerical simulation tools have developed much, becoming a reliable instrument for design and failure analysis of components in the field of aero- and hydrodynamics. The focus of research continuously shifts towards challenges that are more complex, such as multiphase flow problems. These flows typically involve strong dynamics, a number of complex physical phenomena as well as a large range of length and time scales, which are mainly connected to interfacial and turbulence structures. Such problems are particularly prevalent in the chemical and process industries. Typically, the development of tools for the numerical simulation focuses on a single morphology: a) disperse interfacial structures, which are not captured by the computational grid (Euler-Euler or Euler-Lagrange) or b) approaches resolving the interface between two different phases (volume-of-fluid or level-set). In many practical applications, the occurring flow morphology is unknown in advance and therefore, the most appropriate numerical model cannot be easily identified a-priori. Corresponding industrial facilities are flotation cells, refrigeration systems, biological and chemical reactors, distillation columns, swirl separators or centrifugal pumps, to name a few examples. In the past years, special hybrid simulation approaches have been developed to describe the aforementioned problems: 2-field methods considering blending between different morphologies, 4-field methods, where two phases represent the same physical phase, but with different morphology, drift-flux models based on the volume-of-fluid approach or methods blending between Euler-Euler and level-set models.
We present a 4-field approach, which is developed and validated at Helmholtz-Zentrum Dresden – Rossendorf as an add-on to the public domain library OpenFOAM. Central development criteria and goals are:
- Robust applicability to engineering problems at the industrial scale
- A unified set of conservation equations uniting different simulation methods without blending between them
- If the spatial resolution of the computational grid is high enough, the hybrid model should behave like a algebraic volume-of-fluid model
- In the case of coarse computational grids, disperse interfacial structures are modelled using the Euler-Euler method
- Phases forming large scale (resolved) interfaces and such that are dispersed within another phase, are treated as individual numerical phases

• Special mass transfer model formulations allow a phase to change morphology via transfer between different numerical phases
• Disperse phases can interact with large-scale interfaces by, e.g. bursting of bubbles or formation of foam

The current state of this development considers modelling of resolved interfaces in the two-fluid model including successful validation against well accepted volume-of-fluid simulations and description of disperse gas structures by means of a phase averaged treatment (Euler-Euler model). Furthermore, modelling of resolved interfaces on coarse computational grids is realised via appropriate closure models, the Euler-Euler “mode” is stabilised in the limit of high grid resolutions. Additionally, morphology transfer models allow for transition between different flow regimes. The latter is demonstrated for the case of a distillation column and a swirl separator for gas bubbles.
The main intention of this contribution is to present the simulation software described above to industry representatives in order to establish cooperations and find more real-world applications for further development regarding robustness and functionality. Cooperations between industry and research centres should help to overcome shortcomings in currently available simulation methods, in order to allow for more reliability and safety in the future design and operation of facilities.