Modeling of churnturbulent flows
==> Click on the pictures/videos for full view.
The computational modeling of the flow is done using an Eulerian multifluid approach with the ANSYS software package CFX, while ICEM CFD is generally utilized for generating the mesh. The starting point which is based on existing models for bubbly flows include drag force, turbulent dispersion, bubble induced turbulence, among others. Some examples of these are the particle induced turbulence model developed by Rzehak (Rzehak and Krepper, 2012), the bubblebubble interaction model established by Liao (Liao et al, 2011), and the inhomogeneous MUSIG (MUltiple SIze Group) approach developed by Krepper (Krepper et al., 2008). The last one allows to defined different bubble sizes groups with different velocity fields for both large and small bubbles. For the simulation of churnturbulent flows, the gas phase is represented by 3 different gas fields corresponding to the 3 types of bubbles mentioned above. Such a multi size group approach allows creating a more realistic approximation of the churnphenomenon (see Fig. 2a, 2b, and 2c). Experimental data for upwards vertical pipe flow obtained at the TOPFLOW facility are used for the validation of the CFD models.

The Eulerian multifluid approach is widely used to describe dispersed flows like bubbly or droplet flow since such flow patterns are characterized by scales of interfacial structures which are smaller than the grid size. For flow situations with largescale interfaces like film, annular or horizontal stratified flows usually interface tracking methods are used. In principle, the interface capturing methods should be used for interfacial length scales several times larger than the grid size, while for an averaged twofluid approach bubble sizes smaller than the grid size are required (Hänsch et al., 2012). Since in case of churnturbulent flows dispersed flows and large interfaces occur simultaneously, a combination of these modeling concepts would be needed. Such an approach, the GEneralized TwO Phase flow (GENTOP) concept, was recently developed in our CFD group. This concept which consider dispersed and continous gas phases is applied to churn turbulent flow (see Fig.3).

Each simulation is carefully validated against experimental data acquired from the TOPFLOW vertical test section facility, where water and air are use as test fluids. Experiments using large (DN 200) and small (DN 50) pipes are used and an upward flow is study. The radial profiles at different heights are measured using a twolevel wiremesh sensor in order to obtain gas velocities, bubble size distribution, total gas holdup, among other parameters (see Fig. 4 and 5).


Current studies are being devoted on the improvement of the modeling capabilities for high void fraction regimes, such as the development and modifications of different closure laws for large distorted gas structures (churn and slug bubbles), and the constant improvement of the GENTOP –concept for the physically accurate full resolution of such large structures in highly turbulent flows (see Fig. 6).


Fig. 6: Videos of full 3D transient simulation using the GENTOP –concept. (JL = 1.017 m s1; JG = 0.342 m s1) 
References

Montoya, G.; Liao, Y.; Lucas, D.; Krepper, E.
Analysis and Applications of a TwoFluid MultiField Hydrodynamic Model for ChurnTurbulent Flows
21st International Conference on Nuclear Engineering  ICONE 21. China (2013) 
Montoya, G.; Baglietto, E.; Lucas, D.; Krepper, E.
A Generalized MultiField TwoFluid Approach for Treatment of MultiScale Interfacial Structures in High VoidFraction Regimes
MIT Energy Night 2013. Cambridge, Massachusetts, USA (2013) 
Montoya, G.; Lucas, D.; Krepper, E.; Hänsch, S.; Baglietto, E.
Analysis and Applications of a Generalized MultiField TwoFluid Approach for Treatment of MultiScale Interfacial Structures in High VoidFraction Regimes
2014 International Congress on Advances in Nuclear Power Plants  ICAPP 2014. USA (2014) 
Montoya, G.; Baglietto, E.; Lucas, D.; Krepper, E.; Hoehne, T.
Comparative Analysis of High Void Fraction Regimes using an Averaging EulerEuler MultiFluid Approach and a Generalized TwoPhase Flow (GENTOP) Concept
22nd International Conference on Nuclear Engineering  ICONE 22. Czech Republic (2014) 
Montoya, G.; Baglietto, E.; Lucas, D.; Krepper, E.
Development and Analysis of a CMFD Generalized MultiField Model for Treatment of Different Interfacial Scales in ChurnTurbulent and Transitional Flows
CFD4NRS5  Application of CFD/CMFD Codes to Nuclear Reactor Safety Design and their Experimental Validation. Switzerland (2014)
Acknowledgement
This work is carried out in the frame of a current research project funded by E.ON, and in cooperation with the Nuclear Science and Engineering Department of the Massachusetts Institute of Technology (MIT). 
