Numerical simulation of horizontal stratified flows
Slug flow as a multiphase flow regime can occur in the cold legs of pressurized water reactors, for instance after a small break Loss of Coolant Accident (SBLOCA). Slug flow is potentially hazardous to the structure of the system due to the strong oscillating pressure levels formed behind the liquid slugs. It is usually characterized by an acceleration of the gaseous phase and by the transition of fast liquid slugs, which carry out a significant amount of liquid with high kinetic energy. For the experimental investigation of air/water flows, a horizontal channel with rectangular crosssection was build at HelmholtzZentrum DresdenRossendorf (HZDR). Experimental data were used to check the feasibility to predict the slugging phenomenon with the existing multiphase flow models build in ANSYS CFX. Further it is of interest to prove the understanding of the general fluid dynamic mechanism leading to slug flow and to identify the critical parameters affecting the main slug flow parameters (like e.g. slug length, frequency and propagation velocity; pressure drop).
For free surface simulations, the inhomogeneous multiphase model was used, where the gaseous and liquid phases can be partially mixed in certain areas of the flow domain. In this case the local phase demixing after a gas entrainment is controlled by buoyancy and interphase drag and is not hindered by the phase interface separating the two fluids.
The fluiddependent shear stress transport (SST) turbulence models were selected for each phase. Damping of turbulent diffusion at the interface has been considered.
The picture sequence (see Fig. 1) shows comparatively the channel flow of the experiment and the corresponding CFD calculation. In both cases, a slug is developing. The tail of the calculated slug and the flow behind it is in good agreement with the experiment. The entrainment of small bubbles in front of the slug could not be observed in the calculation. However, the front wave rolls over and breaks. This characteristic of the slug front is clearly to be seen in Fig. 1. It is created due to the high air velocity.
Fig. 1: Comparative picture sequence of the recalculated slug
Furthermore pretest calculations CFD were designed to simulate a slug current in a real geometry and under parameters relevant for the reactor safety. These calculations occurred for a flat model of the hot leg which copies the geometry of a 1:3 scaled Konvoi reactor. Steam and water were taken as a model fluid with a pressure of 50 bar and the accompanying saturation temperature of 264 C. The pretest calculations began with a partial waterfull channel and quiescent gas phase. At the beginning of the steam supply the surface of the still standing water phase rises in the direction of the steam generator simulator. This effect is caused by the momentum exchange between flowing out steam and quiescent water. The calculation shows spontaneous waves which grow in the elbow to slugs originate in the horizontal part of the hot leg model. The Fig. 2 shows this state as a snapshot of the results of the calculations.
Fig. 2 Snapshot of the results of the calculations
Exact description of stratified flow morphologies – the AIAD model
CFD simulations of stratified flows are often conducted on the basis of the homogeneous modelbased 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.
To overcome this problem HZDR uses the EulerEuler twofluid approach for industrial macroscale 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 twofluid 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 twophase 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 twoequation turbulence models (ke; kw), 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 kw 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 subgrid wave turbulence (SWT) that means waves created by KelvinHelmholtz 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 countercurrent flow experiments at the hot leg model and for slugs and the stationary hydraulic jump in the channel HAWAC. They showed consistently good results.
References
Mehlhoop, J.P.; Höhne, T.
Validation of closure models for interfacial drag and turbulence in numerical simulations of horizontal stratified gasliquid 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), 253342
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, 271286
Montoya, G.; Deendarlianto; Lucas, D.; Höhne, T.; Vallee, C.
ImageProcessingBased Study of the Interfacial Behavior of the Countercurrent GasLiquid TwoPhase Flow in a Hot Leg of a PWR
Science and Technology of Nuclear Installations 2012(2012), ID 209542
Prayitno, S.; Santoso, R.; Darlianto, D.; Höhne, T.; Lucas, D.
Countercurrent Flow Limitation of GasLiquid Two phase Flow in Nearly Horizontal Pipe
Science and Technology of Nuclear Installations (2012), 513809
Deendarlianto; Höhne, T.; Murase, M.
Countercurrent flow limitations in a pressurized water reactor
Science and Technology of Nuclear Installations (2012), 608678
Deendarlianto,; Höhne, T.; Apanasevich, P.; Lucas, D.; Vallée, C.; Beyer, M.
Application of a new drag coefficient model at CFDsimulations on free surface flows relevant for the nuclear reactor safety analysis
Annals of Nuclear Energy 39(2012), 7082
Deendarlianto,; Höhne, T.; Lucas, D.; Vallée, C.; Montoya, G.
CFD studies on the phenomena around countercurrent flow limitations of gas/liquid twophase flow in a model of a PWR hot leg
Nuclear Engineering and Design 241(2011), 51385148
Deendarlianto,; Höhne, T.; Lucas, D.; Vierow, K.
Gasliquid countercurrent twophase flow in a PWR hot leg: a comprehensive research review
Nuclear Engineering and Design 243(2012), 214233
Höhne, T.; Deen, D.; Lucas, D.
Numerical simulations of countercurrent twophase flow experiments in a PWR hot leg model using an interfacial area density model
International Journal of Heat and Fluid Flow 32(2011), 10471056
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, 131143
Höhne, T.; Vallee, C.
Modelling of stratified two phase flows using an interfacial area density model
WIT Transactions on Engineering Sciences Volume 63(2009), 123135
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 CFDcodes  Comments on the CFDexperiment integration and best practice guideline
Nuclear Engineering and Design 240(2010), 23752381
Vallee, C.; Höhne, T.; Prasser, H.M.; Sühnel, T.
Experimental investigation and CFD simulation of horizontal stratified twophase flow phenomena
Nuclear Engineering and Design 238(2008), 637646
Vallee, C.; Höhne, T.; Prasser, H.M.; Sühnel, T.
Experimental investigation and CFD simulation of horizontal air/water slug flow
Kerntechnik 71(2006)3, 95103
Acknowledgement
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.
Cooperations

stratetic partnership ANSYS CFX, Otterfing

OECD GAMA CFD WG2 Benchmark Test Facility
Links
 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