A Morphology-adaptive Multifield Two-fluid Model

Multiphase flows are highly dynamic and characterized by very complex physics and a large range of length scales. The interfaces between the liquids, their morphology and the processes in their vicinity play an important role for many industrial applications. Hence, there is high interest in research to understand and simulate such complex multiphase flows. However, the selection of a suitable simulation method depends mainly on the morphology of the interfaces. So far methods have been developed to simulate either

• dispersed structures, e.g., Euler-Euler, Euler-Lagrange, or
• resolved interfaces, e.g., Volume-of-Fluid, Level-Set.

Unfortunately, for many technical applications like flotation cells, refrigerating machines, biological and chemical reactors, distillation columns, cyclone separators or centrifugal pumps, the flow morphology is not known in advance. Hence, selecting an appropriate simulation method is difficult.

Demonstration of the morphology-adaptive multifield two-fluid for a plunging jet into a water basin.

Model Approach

Since a few years a morphology-adaptive multifield two-fluid model for multiphase flows is developed at HZDR for the open source CFD software provided by the OpenFOAM Foundation. The source code is available in the Rossendorfer Data Repository (RODARE) under GNU/GPL v3 license. The model is based on a 4-field approach and distinguishes into continuous phases that share a resolved interface and dispersed phases with statistically modelled interfaces.

The main design criteria are:

• robust model for design processes of engineering applications, e.g. chemical and process engineering,
• equations of the two-fluid model all morphologies,
• in the limit of highly resolved interfaces, the volume-of-fluid method is recovered,
• in the limit of poorly resolved dispersed interfaces, the Euler-Euler method is recovered,
• each phase has its fixed morphology (disperse, continuous) and forms an independent numerical phase,
• mass transfer models account for morphology transition between resolved and disperse phases,
• disperse phases interact with resolved interfaces (crossing interface, burst at interface, formation of foam).

Development Status

Introduction to the current development status of the morphology-adaptive multifield two-fluid model .

Applications

Plunging Jet

A liquid jet plunging into a free surface is a situation that is easy to imagine. This example is a prototype for a number of different situations of technical relevance, such as filling of bottles or other containers. However, the involved fluid mechanical processes are complex and hard to predict. Besides entrapment of large gas bubbles, also their breakup into smaller bubbles as well as direct entrainment of disperse gas play an important role. The aim is to simulate plunging jets with different levels of spatial resolution.

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Cyclone Separator

Solid or fluid particles are usually removed from the carrier fluid by means of cyclone separators. If dispersed bubbles are present, they can merge to a continuous gaseous vortex core due to centrifugal forces. Since this changes the flow conditions, the numerical model must be able to reproduce this transition from a disperse to a continuous representation. This should only apply if the grid is fine enough to resolve the continuous core. Otherwise it is approximated simply in a disperse manner. The method can be used effectively on both coarse and fine grids, making the best possible use of the given mesh.

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Distillation Column

Distillation is a physical process, which is frequently used in processing industry, while requiring a large amount of energy. Hence, there is a lot of potential in optimisation of efficiency of distillation processes in technical applications. Distillation columns are build for continuous operation and typically consist of multiple trays, through which gas is injected into a flowing liquid from the bottom. The aim is to simulate such processes with different spatial resolutions.

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Modelling of Turbulent Stratified Gas-liquid Flows

The goals is to advance the capabilities of current two-fluid (Euler-Euler) based modelling tools towards simulation of industrially relevant turbulent two-phase flows. Present work is focused on the development, implementation and validation of advanced morphology adaptive models for turbulent stratified flows. Recent advancement include improvements concerning the combined resolved and under-resolved gas-liquid interfaces, treatment of turbulence near the interface, and heat and mass transfer models.

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Publications

Meller, R., Tekavčič, M., Krull B., and Schlegel, F. (2023). Momentum exchange modeling for coarsely resolved interfaces in a multifield two‐fluid model. International Journal for Numerical Methods in Fluids, in press, 1‐25. 10.1002/fld.5215.

Wiedemann, P., Meller, R., Schubert, M., and Hampel, U. (2023). Application of a hybrid multiphase CFD approach to the simulation of gas--liquid flow at a trapezoid fixed valve for distillation trays. Chemical Engineering Research and Design. 10.1016/j.cherd.2023.04.016.

Yin, J., Zhang, T., Krull, B., Meller, R., Schlegel, F., Lucas, D., Wang, D. and Liao, Y. (2023). A CFD approach for the flow regime transition in a vane-type gas-liquid separator. International Journal of Multiphase Flow, 159, 104320. 10.1016/j.ijmultiphaseflow.2022.104320.

Schlegel, F., Meller, R., Krull, B., Lehnigk, R., and Tekavčič, M. (2022). OpenFOAM-Hybrid - A Morphology Adaptive Multifield Two-Fluid Model. Nuclear Science and Engineering, 1-14. 10.1080/00295639.2022.2120316.

Meller, R., Schlegel, F., and Klein, M. (2022). Sub-grid Scale Modelling and a-Posteriori Tests with a Morphology Adaptive Multifield Two-Fluid Model Considering Rising Gas Bubbles. Flow, Turbulence and Combustion, 108(3), 895-922. 10.1007/s10494-021-00293-8.

Meller, R., Schlegel, F., and Lucas, D. (2021). Basic verification of a numerical framework applied to a morphology adaptive multifield two‐fluid model considering bubble motions. International Journal for Numerical Methods in Fluids, 93(3), 748-773. 10.1002/fld.4907.

Tekavčič, M., Meller, R., and Schlegel, F. (2021). Validation of a morphology adaptive multi-field two-fluid model considering counter-current stratified flow with interfacial turbulence damping. Nuclear Engineering and Design, 379, 111223. 10.1016/j.nucengdes.2021.111223.