Motivation and Strategy
Two-phase flows occur in many industrial processes. Reliable predictions on flow characteristics are required for the design, process optimization and safety analysis of related apparatuses and processes. Experimental investigations are expensive and in most cases not transferable to modified geometries, different scales, flow conditions or material systems. For this reason there is a clear need for numerical tools. Due to the 3D nature of flows and the importance of turbulence in most cases this means a strong need for reliable 3D CFD-tools rather than 1D system codes or simplified correlations.
Most of the activities of the CFD department are related to CFD-model development and validation for gas-liquid flows, but some activities also concern gas-solid, liquid-solid and gas-liquid-solid flows.
The general aim is to provide reliable simulation tools for the design, optimization and safety analyses of medium and large scale applications in which multiphase flows are involved (mainly industrial applications). This contributes to the Helmholtz research programmes on “Energy efficiency, Materials and resources” (EMR) and “Nuclear safety research” (NUSAFE).
Such tools can contribute to improve the efficient use of energy and resources (e.g. in chemical engineering and oil industries) and to guarantee the safe operation (especially nuclear safety).
Since medium and large scale applications are considered such as chemical reactors or components of the cooling system of a nuclear power plant the Euler-Euler two- or multi fluid model is the basis for most activities. The corresponding balance equations are obtained by averaging procedures. In these averaging processes information on the complex interface gets lost. However the local interfacial structures determine the phase interaction and vice versa the local interfacial structures are determined by the interaction between the phases. For an appropriate modelling the information on phase interactions and local interfacial structures has to be reflected by closure models.
Problems of multiphase CFD in frame of the Euler-Euler approach
The above mentioned closure models have to reflect all the non-resolved local phenomena. However, due to limited experimental access these local phenomena are generally often not yet well understood. In the result many different closure models for special phenomena can be found in literature. During last years many investigations on the use of these closure models for different experimental data were published with contradicting results. For this reason the CFD capabilities for reliable predictions on complex two-phase flows basing on the multi-fluid approach are still limited.
General strategy of the CFD department
Accordingly there is a two-fold strategy of the CFD department:
- Consolidation of multiphase CFD basing on the multi-fluid approach by the so-called baseline model strategy.
- Establishment of innovative modelling frameworks to extend the range of applicability of multiphase CFD basing on the multi-fluid approach.
There is a need for some rather generic basic developments, but in parallel topics for specific industrial applications are in the focus of the activities.
CFD-Codes and experimental basis
Most of the models were implemented into the commercial CFD-code ANSYS-CFX. In the framework of a long-term cooperation various model were jointly implemented and validated in the standard release of the code. For specific applications also Star-CCM+ von Siemens is used. Currently the main developments for the numerical simulation of multiphase flows are implemented into OpenFOAM.
The model development as well as the validation of codes is only possible with high quality experimental data. Such data are required with high resolution in space and time. They are obtained be own dedicated experiments or in cooperation with the department Experimental Fluid Dynamics. A main topic is experimental two-phase flow investigation using innovative measuring techniques. These experiments supply information on the time-dependent shape and size of the interfacial area which is the basis for mass, momentum and heat transfer between the phases.
TOPFLOW is one of the major scientific facilities at HZDR. It is the central facility of the National Research Initiative of the Federal Ministry of Economy and Technology (BMWi) to qualify CFD codes for nuclear safety research. With the widely used wire-mesh sensor technology as well as with the new developed ultrafast X-ray electron-beam tomography HZDR has a leading position in the area of fast two-phase flow measuring technique with high spatial resolution. In addition the R&D program is closely connected to the activities in the Magnetohydrodynamics department.