Accurate description of stratified flow morphologies – the AIAD model
CFD simulations of stratified flows are often conducted on the basis of the homogeneous model-based approach in which only one set of balance equations is considered. The different phases are taken into account by different material properties. Since only a velocity field is calculated leads to problems if the phases are not clearly separated, for example, if there is bubble entrainment in a wave or slug flow regime. The separation of the phases can then not be described properly.
|Fig. 1 Different flow morphologies in horizontal channels at slug flow regime|
To overcome this problem HZDR uses the Euler-Euler two-fluid approach for industrial macro-scale multiphase flow modeling. On both sides of the free surface, shear layers are expected which require a specific attention since complex phenomena with turbulent transfers coupled to possible interfacial waves take place. It was found necessary to be able to track the interface position in order to treat this zone in a similar way as a wall boundary layer using wall functions. When trying to use a two-fluid approach, the development of a morphology detection method was found necessary.
Therefore the Algebraic Interfacial Area Density (AIAD) model has been developed in close cooperation with ANSYS CFX. The model detects the local flow pattern, such as the local phase content and switches to a transition algorithm to the valid correlations for example for the interfacial area density, the drag coefficient, or the local characteristic length scale. Now it can be distinguished between areas where droplets or bubbles are present and the region of the free surface. An important goal of the description of two-phase flows is the correct determination of turbulence parameters. These have for example a decisive influence on the generation of surface instabilities. Without special treatment of the free surface resulting from the use of two-equation turbulence models (k-e; k-w), the high velocity of the gas phase, particularly leads to high turbulence at the phase interface. In the new approach, the calculation of the turbulence now is done separately for each phase of the k-w turbulence model. An additional function similar to the wall damping function for the turbulent diffusion is introduced on the free surface.
A further step of improvement of modeling the turbulence at the free surface is the consideration of sub-grid wave turbulence (SWT) that means waves created by Kelvin-Helmholtz instabilities that are smaller than the grid size. In fact, the influence on the turbulence kinetic energy of the liquid side can be significantly large.
Also an improved description of the drag coefficient on the free surface on the basis of the surface shear stress is built into the model.
CFD calculations were validated for co- and counter-current flow experiments at the hot leg model (Fig. 2) and WENKA test facility (Fig. 3) and for slugs and the stationary hydraulic jump in the channel HAWAC. They showed consistently good results.
|Fig. 2 HOTLEG cocurrent flow CFD vs. Experiment|
Fig. 3 Countercurrent flow WENKA test facility,
Code Standard vs. AIAD Model
Porombka, P.; Höhne, T.
Drag and turbulence modelling for free surface flows within the two-fluid Euler-Euler framework
Chemical Engineering Science 134(2015), 348-359
Höhne, T.; Hänsch, S.
A droplet entrainment model for horizontal segregated flows
Nuclear Engineering and Design 286(2015), 18-26
Mehlhoop, J.-P.; Höhne, T.
Validation of closure models for interfacial drag and turbulence in numerical simulations of horizontal stratified gas-liquid flows
International Journal of Multiphase Flow (2013)
Lucas, D.; Coste, P.; Höhne, T.; Lakehal, D.; Bartosiewicz, Y.; Bestion, D.; Scheuerer, M.; Galassi, M. C.
CFD modeling of free surface flow with and without condensation
Multiphase Science and Technology 23(2011), 253-342
Höhne, T.; Darlianto, D.; Lucas, D.
Numerical simulations of CCFL experiments using a morphology detection algorithm
The Journal of Computational Multiphase Flows 4(2012)3, 271-286
Höhne, T.; Deen, D.; Lucas, D.
Numerical simulations of counter-current two-phase flow experiments in a PWR hot leg model using an interfacial area density model
International Journal of Heat and Fluid Flow 32(2011), 1047-1056
Höhne, T.; Vallée, C.
Experiments and numerical simulations of horizontal two phase flow regimes using an interfacial area density model
The Journal of Computational Multiphase Flows 2(2010)3, 131-143
Bartosiewicz, Y.; Seynhaeve, J.-M.; Vallee, C.; Höhne, T.; Laviéville, J.
Modeling free surface flows relevant to a PTS scenario: comparison between experimental data and three RANS based CFD-codes - Comments on the CFD-experiment integration and best practice guideline
Nuclear Engineering and Design 240(2010), 2375-2381
The work is carried out in the frame of a current research project funded by the German Federal Ministry of Economics and Labour, project number 150 1265.
- Overview TOPFLOW
- CFD simulation of fibre material transport in a PWR core under loss of coolant conditions
- CFD calculations of the coolant mixing in Pressurised Water Reactors
- CFD development group