Today Computational Fluid Dynamic (CFD) codes are widely used for industrial applications, mostly in the case of single phase flows in automotive or aircraft engineering, but multiphase flow modeling had gain an increasing importance in the last years. Safety analyses on nuclear power plants require reliable prediction on steam-water flows in case of different accident scenarios. This is particularly true for passive safety systems such as the GEKO component of the KERENA reactor. Here flashing may occur in the riser (Leyer and Wich, 2012). In such case, high gas volume fractions and the churn-turbulent flow regime may ensue. In the past, the codes for the prediction of churn-regime have not shown a very promising behavior. In this paper, a two-fluid multi-field hydrodynamic model has been developed based in the Euler-Euler framework. The main emphasis of this work has been on the modeling and applicability of various interfacial forces between dispersed gaseous phases and the continuous liquid, as well as bubble-bubble interactions, and the evolution of different bubble sizes in an adiabatic vertical pipe inside the churn-turbulent flow regime. All the expected mechanistic models that intervene in this flow pattern have been taken into account including drag force, wall force, lift force, turbulent dispersion, and bubble induced turbulence. Bubble breakup and coalescence has been defined (Liao et al., 2011), and in order to design a polydispersed model related to reality, the inhomogeneous MUSIG approach (Krepper et al., 2008) has been used to defined an adequate number of bubble size fractions which are arranged into different groups with their own velocity field. Based on these models, a series of simulations were made on the framework of ANSYS CFX 14.0, and all of the calculations were further validated with experimental data extracted from the TOPFLOW facility at the Helmholtz-Zentrum Dresden-Rossendorf. Different water and gas flow rates were used inside the churn-turbulent flow regime, as well as for the transition from bubbly to churn flow. The calculated cross-section averaged bubble size distributions, gas velocities, and time averaged radial profile for the gas fraction have shown a promising agreement with the experimental data. Nevertheless there are also clear deviations which indicate shortcomings of the present modelling. In order to further improve the modeling of this flow regime, a discussion based on the results will be used to shown a series of limitations of the actual modeling and possible solutions to be implemented in future works.

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