The modeling of two-phase flows has always been limited to special cases due to the very complex nature of its interface. When considering vertical pipe flows with low gas volume flow rates, bubbly flow occurs. With increasing gas volume flow rates larger bubbles are generated by bubble coalescence, which further leads to transition to slug, churn-turbulent, and annular flow. Considering, as an example, a heated tube producing steam by evaporation, as in the case of a vertical steam generator, all these flow patterns including transitions are expected to occur in the system. Despite extensive attempts, robust and accurate simulations approaches for such conditions are still lacking. This paper summarizes the state-of-the-art on the understanding of the physics behind churn-turbulent flow, and transitions to and from this flow pattern. Both, benefits and limitations of the existent experimental approaches and their usefulness for model development and validation at these high void fraction conditions are discussed. Limitation of both, low-dimensional approaches (0D, 1D, and 2D), and high resolution approaches such as Direct Numerical Simulations (DNS) are analyzed. Averaging procedures, such as the Eulerian-Eulerian approach including the interfacial momentum closures which has been used in the past for simulating churn flow, are review thoroughly. Finally, possible improvements are proposed.

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Complex multiphase gas-liquid flows, including boiling, are usually encountered in safety related nuclear applications. For CFD purposes, modeling the transition from low to high void fraction regimes represents a non-trivial challenge due to the increasing complexity of its interface. For example, churn-turbulent and slug flows, which are typically encountered for these gas volume fraction ranges, are dominated by highly deformable bubbles. Multiphase CFD has been so far relying on an averaged Euler-Euler simulation approach to model a wide range of two-phase applications. While this methodology has shown to date demonstrated reasonable results (Montoya et al., 2013), it is evidently highly dependable on the accuracy and validity of the mechanistic models for interfacial forces, which are necessary to recover information lost during the averaging process. Unfortunately existing closures, which have been derived from experimental as well as DNS data, are hardly applicable to high void fraction highly-deformable gas structures. An alternative approach for representing the physics behind the high void fraction phenomena, is to consider a multi-scale method. Based on the structure of the gas-liquid interfaces, different gaseous morphologies should be described by different CFD approaches, such as interface tracking methods for larger than the grid size interfacial-scales, or the averaged Euler-Euler approach for smaller than grid size scales, such as bubbly or droplet flow. A novel concept for considering flow regimes where both, dispersed and continuous interfacial structures, could occur has been developed in the past (Hänsch et al., 2012), and has been further advanced and validated for pipe flows under high void fraction regimes (Montoya et al., 2014) and other relevant cases, such as the dam-break with an obstacle (Hänsch et al., 2013). Still, various short-comings have been shown in this approach associated mostly to the descriptive models utilized to obtain the continuous gas morphology from within the averaged Eulerian simulations. This paper presents improvements on both concepts as well as direct comparison between the two approaches, based on newly obtained experimental data. Both models are based on the bubble populations balance approach known as the inhomogeneous MUltiple SIze Group or MUSIG (Krepper et al., 2008) in order to define an adequate number of bubble size groups with its own velocity fields. The numerical calculations have been performed with the commercially available ANSYS CFX 14.5 software, and the results have been validated using experimental data from the MT-Loop and TOPFLOW facilities from the Helmholtz-Zentrum Dresden-Rossendorf in Germany (Prasser et al., 2007).

The purpose of this work is modeling of a horizontal oil–water flow with and without the Algebraic Interfacial Area Density (AIAD) model. Software and hardware developments in the past years have significantly increased and improved the accuracy, flexibility, and performance of simulations for large and complex problems typically encountered in industrial applications. At Helmholtz-Zentrum Dresden-Rossendorf (HZDR), the focus has been concentrated on the R&D of new modeling capabilities for Euler–Euler approach where interfaces exist. In this research paper, the applicability of the AIAD model for a horizontal oil–water flow is investigated. The comparison between the standard ANSYS Fluent Eulerian Interface Capabilities (namely Multi-Fluid VOF) without AIAD and ANSYS CFX with AIAD implemented via user functions for the oil–water flow was performed. Thereafter, the obtained results were compared with existing experimental data produced by the Department of Thermodynamics and Transport Phenomena of the University Simon Bolivar (USB) in Caracas, Venezuela. The results of the simulations show that horizontal oil–water flow can be modelled with rather acceptable accuracy when using regime transition capabilities as those offered by the AIAD model.