## Advancements, Developments and Validations of CFD Theoretical Modeling for High Void Fraction Flows in the Nuclear and Petrochemical Industries - Proposal Accepted

Invited Book Chapter for the American Society of Mechanical Engineering (ASME) Book Series: Advances in Computers and Information in Engineering Research (ACIER) (2017).

Montoya, G.

## CFD-Simulations of boiling in a heated pipe including flow pattern transitions using the GENTOP approach - UNDER REVIEW

Journal of Nuclear Engineering and Design (2017).

Hoehne, T.; Krepper, E.; Montoya, G.; Lucas, D.

Boiling flow inside a wall heated vertical pipe is simulated by a multi-field CFD approach. Sub-cooled water enters the pipe from the lower end and heats up first in the near wall region leading to the generation of small bubbles. Further along the pipe larger and larger bubbles are generated by coalescence and evaporation. This leads to transitions of the two-phase flow patterns from bubbly to churn-turbulent and annular flow. The CFD simulation bases on the recently developed GEneralized TwO Phase flow (GENTOP) concept. It is a multi-field model using the Euler-Euler approach. It allows the consideration of different local flow morphologies including transitions between them. Small steam bubbles are handled as dispersed phases while the interface of large gas structures is statistically resolved. The paper presents the extension of the GENTOP model for phase transfer and discusses the sub-models used. Finally the above mentioned boiling pipe is considered as demonstration case.

## Development, Analysis, and Validation of a Surface Tension and Wall Adhesion Model for a Generalized Two-Phase Flow Approach - UNDER REVIEW

Chemical Engineering and Science (2017).

Montoya, G.; Baglietto, E.; Lucas, D.; Gauss, F.

Accurate modeling of complex multiphase flow, including detailed void fracture distribution, is of utmost importance for the safe and economical operation of nuclear reactors under steady, transient, and accidental conditions. Strong coalescence at high void fraction leads to the formation of large deformable bubbles. A suitable multiphase CFD model should be able to account for both large and small interfacial structures, and their effect on the average flow distribution through adequate closure modeling. A concept known as GEneralized TwO Phase flow or GENTOP has been developed at the Helmholtz-Zentrum Dresden-Rossendorf to address such flow configurations, by separately treating a resolved potentially continuous gas field, one or more poly-dispersed gas fields, and a continuous liquid phase. Application of the model to churn-turbulent and slug flow in vertical pipes has evidenced significant limitations related to the lack of surface tension modeling at the interface, which leads to unphysical accumulation of void near the pipe wall. This work discusses the implementation of a surface tension model, in combination with contact angle effects, within the GENTOP approach. The surface tension model is first validated against an analytical solution for oscillating periods of different shapes of ethanol droplets suspended in air. The effectiveness of the contact angle implementation is then verified for the case of a water droplet on a smooth surface. Finally, rising velocities and shape for single bubbles rising in a vertical pipe are validated against experimental data for both large and small cases. The implementation of the surface tension model in the GENTOP approach demonstrates significant improvements in the resolution of the bubble shapes, with considerable reduction of the numerical diffusion of the interface.

## Analysis, Application, and Validation of a Generalized Multi-Field Two-Fluid Approach for Treatment of Multi-Scale Interfacial Structures in High Void-Fraction Regimes - UNDER REVIEW

Annals of Nuclear Energy (2017).

Montoya, G.; Lucas, D.; Baglietto, E.; Hoehne, T.

This paper presents the application, analysis, and improvement of a recently developed concept for the treatment of multiphase flows, where different scales in terms of interfacial structures can be found. This approach, known as Generalized TwO Phase flow (GENTOP), considers the definition of a fully-resolved continuous gas phase where the continuous gas summarizes all gas structures which are large enough to be resolved within the computed mesh. The concept works as part of an extension of the bubble population balance approach known as the inhomogeneous MUltiple SIze Group (iMUSIG), which allows the treatment of different bubble size fractions, each with its own velocity field inside the dispersed phase. Within the poly-dispersed gas, bubble coalescence and breakup allow the transfer between different size structures, while the modeling of mass transfer between the poly-dispersed and continuous gas, allows considering transitions between the various gas morphologies depending on the flow situations. Within the concept, different parametric studies have been made for co-current vertical gas-water pipe flow, and comparisons against experimental data for all the current calculations are shown. The experiments have been conducted in the TOPFLOW and the MT-Loop facilities at the Helmholtz-Zentrum Dresden-Rossendorf in Germany. The limitations of adopting the original concept of GENTOP for high gas volume fraction were discussed, and improvements, in order to eliminate assumptions and advance the physics behind the model, were implemented. These include improvements on closure laws formulations inside the concept, implementation of a surface tension model and analysis of the bubble size distribution for the continuous gas. Furthermore, the capability of modeling annular flow regime using the assembled approach is presented.

## Analysis, Application, and Validation of a Two-Fluid Multi-Field Hydrodynamic Model for High Void Fraction Regimes - UNDER REVIEW

Journal of Nuclear Engineering and Design (2017).

Montoya, G.; Lucas, D.; Krepper, E.; Baglietto, E.

A two-fluid multi-field hydrodynamic model was developed based on the Eulerian-Eulerian framework approach. The central emphasis of this work is to study modifications of interfacial forces for large deformable churn-like and slug bubbles. Critically complex high void fraction flow regimes typically encountered in the boiling phenomena, have been taken into account including bubbly-to-churn transitional flow, churn-turbulent flow, and developing slug flow. To start, closure models that are proposed for bubbly flows are used for drag force, wall force, lift force, turbulent dispersion, and bubble-induced turbulence. The approach includes bubble breakup and coalescence, by implementing the inhomogeneous multiple size group (iMUSIG) method. A series of calculations and improvements on these models are made using ANSYS CFX 14.5. Furthermore, the simulations are validated against experimental data extracted from the TOPFLOW and MT-Loop facilities at the Helmholtz-Zentrum Dresden-Rossendorf. With some modifications of the closures for the large bubbles, the calculated cross-sectional time averaged radial profiles for void fraction and gas velocities, as well as bubble size distribution, shows a promising agreement with the experimental data. Nevertheless, there are also apparent deviations which indicate shortcomings of the present modeling approach. To further improve the modeling of these flow regimes, a discussion on current limitations will be followed by a proposal of realistic solutions.

## A multi-scale approach simulating boiling in a heated pipe including flow pattern transition

17th International Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH-17). Xi’an, Shaanxi, China.

September 2017.

Hoehne, T.; Krepper, E.; Lucas, D.; Montoya, G.

The paper presents the extension of the GENTOP model for phase transfer and discusses the sub-models used. Boiling flow inside a wall heated vertical pipe is simulated by a multi-field CFD approach. Sub-cooled water enters the pipe from the lower end and heats up first in the near wall region leading to the generation of small bubbles. Further along the pipe larger and larger bubbles are generated by coalescence and evaporation. This leads to transitions of the two-phase flow patterns from bubbly to churn-turbulent and annular flow. The CFD simulation bases on the recently developed GEneralized TwO Phase flow (GENTOP) concept. It is a multi-field model using the Euler-Euler approach. It allows the consideration of different local flow morphologies including transitions between them. Small steam bubbles are handled as dispersed phases while the interface of large gas structures is statistically resolved. The GENTOP sub-models and the Wall Boiling Model need a constant improvement and separate, intensive validation effort using CFD grade experiments.

## Validation of a CFD-PBM model for the simulation of boiling flows

14th International Conference Multiphase Flow in Industrial Plant. AIDIC and Universita’ Degli Studi di Brescia Dipartimento di Ingegneria Meccanica e Industriale. Desenzano del Garda, BS, Italy.

September 2017.

Buffo, A.; Vanni, M.; Marchisio, D.; Montoya, G.; Baglietto, E.

Boiling flows are often conducted in vertical turbulent pipes, with the water liquid moving upwards and the steam gas bubbles formed at the walls. Many different phenomena play a major role in determining the final fluid dynamics of the system, including bubble nucleation, growth, condensation, coalescence, and breakage. The development of a fully predictive computational fluid dynamics (CFD) model is still challenging, and research efforts have focused on addressing only some of the phenomena mentioned above (i.e. coalescence and breakage) by using population balance models (PBM). In this work a PBM is solved with the quadrature method of moments (QMOM) implemented in the open-source CFD code openFOAM, by using an in-house solver, based on compressibleTwoPhaseEulerFoam, implementing, in turn, the two-fluid model. Simulation predictions are validated against the so-called TOPFLOW experiments (on simpler cold systems that mimic the complexity of real boiling flows). Comparison of the bubble size distribution (BSD), gas and liquid velocities and volume fraction show that great care must be paid on the inlet BSD at the sparger, at the coalescence and breakage kernels and the lift coefficient correlation.

## Towards a CFD Model for Boiling Flows: Validation of QMOM Predictions with TOPFLOW Experiments

12th International Conference on Computational Fluid Dynamics in the Oil and Gas, Metallurgical and Process Industries.

Trondheim, Norway. May 2017.

Buffo, A.; Vanni, M.; Marchisio, D.; Montoya, G.; Baglietto, E.

Boiling flows are very complex systems, usually confined in vertical pipes, where the liquid water moving upwards and the steam gas bubbles generated at the walls. The fluid dynamics of such systems is determined by the interplay of many different phenomena, including bubble nucleation, growth, condensation, coalescence, and breakage. For this reason, the development of a fully predictive computational fluid dynamics (CFD) model is very challenging, therefore we focus here only on some of the phenomena mentioned above (i.e. coalescence and breakage) by using population balance models (PBM). In this work, a coupled CFD-PBM model based on the two-fluid model and the quadrature method of moments (QMOM) was implemented in the open-source CFD code openFOAM. Simulation predictions are validated against the socalled TOPFLOW experiments, where simpler air-water cold systems that mimic the complexity of real boiling flows were investigated. Comparison between the available experimental data and the results show that great care must be paid on some modeling details, such as the inlet bubble size distribution (BSD) at the sparger and the coalescence and breakage rates modeling.

## Towards a Better Computational Fluid Dynamics Modeling of Multiphase Flow Systems with Direct Applicability to Light Water Reactors

MIT Energy Night 2016.

Cambridge, Massachusetts, USA. October 2016.

Montoya, G.; Magolan, B.; Demarly, E.; Agostinelli, G.; Lubchenko, N.; Kommajosyula, R.; Sugrue, R.; Baglietto, E.

Multiphase flows are usually encountered in a large range of energy-related industrial applications. In the nuclear sector, for example, Light Water Reactors (LWR) require accurate knowledge of the correct distribution for the void fraction, which allows the prediction of moderator density curves, which strongly influence the neutronics performance and local power production, as well as the heat transfer within the reactor core. Extensive experimental studies have been conducted in support of the understanding of two-phase flow, and although large advancements have been made in theoretical and computational methods, the modeling is still limited by the incomplete knowledge of the closure relations necessary to describe the interfacial transfer terms. In Prof. Baglietto’s group in MIT, we have been thrusting to attack this issue in a more systematic manner, studying the local phenomena independently, and then bringing the different models together with the goal of producing a fully predictive Multiphase Computational Fluid Dynamics (M-CFD) framework. Etienne Demarly and Ravikishore Kommajosyula are working on improving the flow boiling models in CFD. Predicting the occurrence of the boiling crisis is very challenging and current best practices demonstrate no predictive capabilities. The new boiling models greatly benefit from recent experimental advancements at MIT which capture new insights into the physics of flow boiling, which are being included in the modeling framework. Once bubbles depart from the wall, bubbly flow starts occurring. Here Rosemary Sugrue works on developing a general description of the forces acting on the bubbles, with particular focus on the lift force closure, which represents the lateral forces acting on a group of bubbles and determining their distribution in the flow; the current state of the art still relies on approximate formulation based on single bubble behavior, and soon fails as the void fraction increases in the flow. Nazar Lubchenko is working on the understanding and development of a new “law of the wall” for bubbly flows, which is a necessary approach to support the industrial applicability of M-CFD. While in the past, these wall functions for single phase flow were made consistent with the turbulence closures in the bulk, such consistency is lost for the two-phase formulation. At the same time, as these bubbles start growing in size, their lift sign will change, and they will start moving toward the bulk of the flow. These bubbles passing throughout the liquid flow introduced a non-negligible effect on the turbulence of the continuous phase (namely, our liquid) which is currently not well described by any of the available models. Ben Magolan is tackling this issue leveraging both experimental and Direct Numerical Simulation (DNS) data to understand the mechanisms and construct a model to properly modify the turbulence equations in the multiphase flow. As the void fraction increases, a higher number coalescence events to breakup occurs, producing significantly larger bubbles. As these gas structures increase in size, their interfaces become complex and wavy, entering what are known as churn and slug flow regimes. If further gas is introduced in the system, these large churn or Taylor bubbles will become unstable, until eventually the liquid phase will accumulate on the walls of the pipe, while the vapor phase and any entrained liquid will gather in the center of the channel with higher velocity. For this high void fraction cases, the closure relations which are being develop for bubbly flow regimes, are no longer applicable. Furthermore, both detailed experimental data and DNS calculations are lacking at these flow regimes. For that matter, Giulia Agostinelli and Gustavo Montoya, have been working on using a novel strategy to developed closures that could be utilized in such chaotic flows under the same CFD numerical approach. A first attempt at these new closures has leveraged validated Volume of Fluid (VOF) calculations, which resolve the instantaneous interface distribution, and demonstrated that for single Taylor bubble simulations at different gas flow rates, the method produces highly valuable data, which allow proposing a first order correlation for the interfacial area density in slug flow: an invaluable step towards the development of new high void fraction closures.

## Resolved Interface Taylor Bubble Simulations to Support Eulerian Multiphase Closures Derivation

Computational Fluid Dynamics for Nuclear Reactor Safety Applications – CFD4NRS-6.

Boston, USA. September 2016.

Montoya, G.; Baglietto, E.

Safety analysis of nuclear power plants requires accurate predictions of steam-water flows in case of postulated accident scenarios. Examples are the regimes observed in reactor cores during a Loss of Coolant Accident (LOCA), where the transition throughout different flow regimes, including slug and annular flow, plays an important role. Functional forms of the closure models for slug and even annular flow may be constructed using a simplified analysis, but detailed experimental or even DNS data is required to develop meaningful models of this interfacial closures (Lahey, 2009; Rodriguez, 2013; Behafarid, 2015). The research in this paper takes advantage of such principle and deals first with the challenge of identify an applicable functional model and further quantifying the values or functional dependence of coefficients in the individual closure laws. Numerical simulations of simple flow configurations that isolate specific closure models (such as rising Taylor bubble for different Re, Mo, and Eo numbers) are performed to simplify the closure model development. This allows capturing the relevant physical mechanisms of the interfacial forces by averaging the time resolved simulations.

## Development and Validation of Advanced Theoretical Modeling for Churn-Turbulent Flows and Subsequent Transitions

Doctoral Dissertation. Technischen Universität Berlin (2015).

Montoya, G.

The applicability of CFD codes for two-phase flows has always been limited to special cases due to the very complex nature of its interface. Due to its tremendous computational cost, methods based on direct resolution of the interface are not applicable to most problems of practical relevance. Instead, averaging procedures are commonly used for these applications, such as the Eulerian-Eulerian approach, which necessarily means losing detailed information on the interfacial structure. In order to allow widespread application of the two-fluid approach, closure models are required to reintroduce in the simulations the correct interfacial mass, momentum, and heat transfer. It is evident that such closure models will strongly depend on the specific flow pattern. When considering vertical pipe flow 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. The purpose of this dissertation is the development, testing, and validation of a multifield model for adiabatic gas-liquid flows at high gas volume fractions, for which a multiple-size bubble approach has been implemented by separating the gas structures into a specified number of groups, each of which represents a prescribed range of sizes. A fully-resolved continuous gas phase is also computed, and represents all the gas structures which are large enough to be resolved within the computational mesh. The concept, known as GENeralized TwO Phase flow or GENTOP, is formulated as an extension to the bubble population balance approach known as the inhomogeneous MUltiple SIze Group (iMUSIG). Within the polydispersed gas, bubble coalescence and breakup allow the transfer between different size structures, while the modeling of mass transfer between the polydispersed and continuous gas allows including transitions between different gas morphologies depending on the flow situations. The calculations were performed using the computational fluid dynamic code from ANSYS, CFX 14.5, with the support of STAR-CCM+ v8.06 and v9.02. A complete three-field and four-field model, including a continuous liquid field and two to three gas fields representing bubbles of different sizes, were first tested for numerical convergence and then validated against experimental data from the TOPFLOW and MT-Loop facilities.

## Implementation and Validation of a Surface Tension Model for the Multi-Scale Approach GENTOP

16th International Topical Meeting on nuclear Reactor Thermalhydraulics - NURETH-16.

Chicago, USA. September 2015.

Montoya, G.; Baglietto, E.; Lucas, D.

Multiphase flows encountered in the nuclear industry are largely of a complex nature, and knowledge of the accurate distribution of the void fraction is of utmost importance for operation of the reactor under steady, transient, and accident conditions. At high void fractions, strong coalescence leads to the formation of large deformable bubbles. An appropriate multiphase CFD modeling of these flow regimes should be able to account for both, large and small interfacial structures, also including the effect on closure modeling of the large structures. A concept known as GEneralized TwO Phase flow or GENTOP, has been developed at the Helmholtz-Zentrum Dresden-Rossendorf in order to address such flow configurations, by dealing with a resolved potentially-continuous gas field, one or more polydispersed gas fields, and a continuous liquid phase. Application of the model to churn-turbulent and slug flow in vertical pipes [1], have evidenced an important limitation related to the lack of a surface tension modeling within the free surface, which leads to an unphysical accumulation of voids near the pipe wall. This work discusses the implementation of surface tension and contact angle within the GENTOP approach, as well as the validation of these models against analytical and experimental results. The validation of the surface tension has been performed against analytically calculated oscillating periods of different shapes of ethanol droplets suspended in air. Furthermore, different contact angles are analyzed for a drop of water residing on a smooth surface. Rising velocities and deformation of a single large bubble rising in a vertical pipe were finally validated against analytical solutions. The implementation of the surface tension model in the GENTOP approach demonstrated improvements on the resolution of the bubble and stability of the interface, with considerable reduction of the numerical diffusion.

## A review on mechanisms and models for the churn-turbulent flow regime

Journal of Chemical Engineering and Science (2015).

Montoya, G.; Lucas, D.; Baglietto, E.; Liao, Y.

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 (OD, ID, 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.

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 (OD, ID, 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.

## Analysis and Applications of a Generalized Multi-Field Two-Fluid Approach for Treatment of Multi-Scale Interfacial Structures in High Void-Fraction Regimes

2014 International Congress on Advances in Nuclear Power Plants (ICAPP 2014).

Charlotte, North Carolina, USA. April 2014.

Montoya, G.; Lucas, D.; Krepper, E.; Haensch, S.; Baglietto, E.

This paper presents the application of a recently developed concept for the treatment of multiphase flows, where different scales in terms of interfacial structures can be found. This approach, known as Generalized TwO Phase flow or GENTOP, considers the definition of a fully-resolved continuous gas phase where the continuous gas summarizes all gas structures which are large enough to be resolved within the computed mesh. The concept works as part of an extension of the bubble population balance approach known as the inhomogeneous MUltiple SIze Group (MUSIG), which allows the consideration of different bubble size groups, each with its own velocity field inside the dispersed phase. Within the polydispersed gas, bubble coalescence and breakup allow the transfer between different size structures, while the modeling of mass transfer between the polydispersed and continuous gas, allows considering transitions between different gas morphologies depending on the flow situations. Within the concept, different parametric studies have been made for co-current vertical gas-water pipe flow, and comparisons against experimental data for all the current calculations are shown. The experiments have been conducted in the TOPFLOW and the MT-Loop facilities at the Helmholtz-Zentrum Dresden-Rossendorf. It was shown that to consider a concept for treatment of high void fraction regimes multiphase flow as a multi-scale problem by fully resolving large, deformable, and semi-continuous gaseous structures, when considering the right physics behind the phenomena, it is able to reproduce the flow behavior from experimental data both, in a quantitative and qualitative way.

## A Generalized Multi-Field Two-Fluid Approach for Treatment of Multi-Scale Interfacial Structures in High Void-Fraction Regimes

MIT Energy Night 2013.

Cambridge, Massachusetts, USA. October 2013.

Montoya, G.; Baglietto, E.; Lucas, D.; Krepper, E.

High void fraction multiphase-flow regimes are commonly encountered in the nuclear industry where safety analysis of nuclear power plants requires reliable predictions on steam-water flows in case of different accident scenarios. Within the boiling phenomena in pipes, a transition throughout different flow patterns from bubbly to churn to annular flow is expected to occur. Those flow regimes, characterized by very high void fractions, are represented by different scales in terms of their gas structures. A concept has been recently developed for the treatment of multiphase flows where different scales in terms of interfacial structures can be found. This

approach, known as Generalized TwO Phase flow or GENTOP, considers the definition of a fully-resolved continuous gas phase where the continuous gas summarizes all gas structures which are large enough to be resolved within the computed mesh. The concept works as part of an extension of the bubble population balance approach known as the inhomogeneous MUSIG, which allows the consideration of different bubble size groups, each with its own velocity field. Within the polydispersed gas, bubble coalescence and breakup allow the transfer between different size structures, while the modeling of mass transfer between the polydispersed and continuous gas, allows considering transitions between different gas morphologies depending of the flow situations.

approach, known as Generalized TwO Phase flow or GENTOP, considers the definition of a fully-resolved continuous gas phase where the continuous gas summarizes all gas structures which are large enough to be resolved within the computed mesh. The concept works as part of an extension of the bubble population balance approach known as the inhomogeneous MUSIG, which allows the consideration of different bubble size groups, each with its own velocity field. Within the polydispersed gas, bubble coalescence and breakup allow the transfer between different size structures, while the modeling of mass transfer between the polydispersed and continuous gas, allows considering transitions between different gas morphologies depending of the flow situations.

## Analysis and Applications of a Two-Fluid Multi-Field Hydrodynamic Model for Churn-Turbulent Flows

21st International Conference on Nuclear Engineering (ICONE 21).

Chengdu, China. July 2013.

Montoya, G.; Liao, Y.; Lucas, D.; Krepper, E.

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.

## Comparative Simulations of Free Surface Flows Using VOF-Methods and a New Approach for Multi-Scale Interfacial Structures

ASME 2013 Fluids Engineering Summer Meeting - FEDSM2013.

Nevada, USA. July 2013.

Haensch, S.; Lucas, D.; Hoehne, T.; Krepper, E.; Montoya, G.

This paper presents free surface flow simulations using different VOF-like interface capturing methods. Both the interFoam solver available in OpenFOAM and the Free Surface Model implemented in ANSYS CFX are applied for the collapse of a water column hitting an obstacle. The computational results of these established methods are compared to a new multi-field concept which is developed for flow situations with multi-scale interfacial structures. The new concept extends the inhomogeneous MUltiple SIze Group (MUSIG)-Model for polydispersed flows by adding a large-scale continuous gas phase. It represents the largest gas structures whose filtered gas-liquid interfaces are captured within the computational domain. Adequate interfacial transfer formulations are introduced for area density and drag and allow the use of different closure models depending on the local morphology. By including appropriate models for the mass transfer, transitions between dispersed and continuous gas morphologies can be described. Thus not only gas-liquid interfaces for large gas structures are detected, but also smallscale bubbles that are entrained under the free surface can be described properly taking into account coalescence- and breakup processes. The concept further improves free surface simulations by including sub-grid information about small waves and instabilities at the free surface. Therefore a new treatment of turbulent kinetic energy is applied via source terms at the free surface. The application of this concept to the dambreak-case with an obstacle demonstrates the breakup of a continuous gas phase and the appearance of polydispersed gas. The collapse of the water column is accompanied by trapping of gas which breaks up to smaller structures. The quality of interface detection during the Address all correspondence to this author. Email: s.haensch@hzdr.de simulation is compared to the above mentioned VOF-methods. Furthermore the formation of a bubble size distribution underneath the surface serves as a demonstration of the possible benefit using such an averaged multi-field approach.

## Application of a multi-field concept to the dam-break case with an obstacle

The 15th International Topical Meeting on Nuclear Reactor Thermalhydraulics (NURETH-15).

Pisa, Italy. May 2013.

Haensch, S.; Lucas, D.; Hoehne, T.; Krepper, E.; Montoya, G.

This paper presents new results of a generalized approach developed for the simulation of two-phase flows with multi-scale interfacial structures. By extending the inhomogeneus Multiple Size Group-model the approach enables transitions between dispersed and continuous gas morphologies, including the evanescence and appearance of a particular phase. Adequate interfacial transfer formulations, which are consistent with such an approach, are introduced for interfacial area density and drag. A new drag-formulation considers shear stresses within the free surface area. The application of the concept to a collapsing water column demonstrates the breakup of continuous gas into a polydispersed phase consisting of different bubble sizes. Both resolved free surface structures, as well as the entrainment of bubbles and their coalescence and breakup underneath the surface can be described. The simulations have been performed with the CFD-code CFX 14.0 and will be compared with experimental images. The paper will further investigate the possible improvement of such free surface simulations by including sub-grid information about small waves and instabilities at the free surface. A comparison of the results will be used for a discussion of possible new mass transfer models between filtered free surface areas and dispersed bubble size groups as part of the future work.

## Image-Processing-Based Study of the Interfacial Behavior of the Countercurrent Gas-Liquid Two-Phase Flow in a Hot Leg of a PWR

Journal of Science and Technology in Nuclear Installation. Volume 2012 (2012), Article ID 209542 (Special Issue named Countercurrent Flow Limitations in a Pressurized Water Reactor).

Montoya, G.; Deendarlianto; Lucas, D.; Hoehne, T.; Valle, C.

The interfacial behavior during countercurrent two-phase flow of air-water and steam-water in a model of a PWR hot leg was studied quantitatively using digital image processing of a subsequent recorded video images of the experimental series obtained from the TOPFLOW facility, Helmholtz-Zentrum Dresden-Rossendorf e.V. (HZDR), Dresden, Germany. The developed image processing technique provides the transient data of water level inside the hot leg channel up to flooding condition. In this technique, the filters such as median and Gaussian were used to eliminate the drops and the bubbles from the interface and the wall of the test section. A Statistical treatment (average, standard deviation, and probability distribution function (PDF)) of the obtained water level data was carried out also to identify the flow behaviors. The obtained data are characterized by a high resolution in space and time, which makes them suitable for the development and validation of CFD-grade closure models, for example, for two-fluid model. This information is essential also for the development of mechanistic modeling on the relating phenomenon. It was clarified that the local water level at the crest of the hydraulic jump is strongly affected by the liquid properties.

## CFD studies on the phenomena around counter-current flow limitations of gas/liquid two-phase flow in a model of a PWR hot leg

Journal of Nuclear Engineering and Design. Volume 241, Issue 12, Pages 5138–5148.

December 2011.

Deendarlianto; Hoehne, T.; Lucas, D.; Valle, C.; Montoya, G.

In order to improve the understanding of counter-current two-phase flow and to validate new physical models, CFD simulations of a 1/3rd scale model of the hot leg of a GermanKonvoi pressurized water reactor (PWR) with rectangular cross section were performed. Selected counter-current flow limitation (CCFL) experiments conducted at Helmholtz-Zentrum Dresden-Rossendorf (HZDR) were calculated with ANSYS CFX using the multi-fluid Euler–Euler modelling approach. The transient calculations were carried out using a gas/liquid inhomogeneous multiphase flow model coupled with a shear stress transport (SST) turbulence model. In the simulation, the drag law was approached by a newly developed correlation of the drag coefficient (Höhne and Vallée, 2010) in the Algebraic Interfacial Area Density (AIAD) model. The model can distinguish the bubbles, droplets and the free surface using the local liquid phase volume fraction value. A comparison with the high-speed video observations shows a good qualitative agreement. The results indicate also a quantitative agreement between calculations and experimental data for the CCFL characteristics and the water level inside the hot leg channel.

## Determination and study of hold up and flow patterns in two-phase flow liquid-liquid systems for horizontal and inclined pipes using image processing techniques Computational Techniques for Multiphase Flows

Presented Poster: ICMF 2010 P1.53

International Conference on Multiphase Flow 2010 (ICMF-2010).

Tampa, USA. June 2010.

Montoya, G.; Valecillos, M.; Garcia, J.; Gonzales, D.

Several flow patterns for horizontal and inclined pipes were visualized, in order to calculate hydrodynamics parameters using a computational algorithm that was generated for this investigation. In order to accomplish this, the images were acquired using a high speed camera in a tube bank equipment of the Transport Phenomena Laboratory of the University Simon Bolivar for the horizontal configuration, and in different tube bank equipment in the Unitary Operations Laboratory for 45 degrees of inclination. Diverse flow patterns for the horizontal pipes were characterized following the classification used by Trallero (1996) [9] and Flores [10], then the images were processed in order to obtain the height’s phase and hold up. The resulting patterns that were obtain for 0 degrees consist in four models: two segregates (ST and ST&MI) and two disperse (o/w and Do/w&w). For 45 degrees, three dominate by water: Oil in water dispersion-pseudo pattern (PS), oil in water dispersion-cocurrent (CC), very fine dispersion of oil in water (VDF o/w), and one dominates both by water and oil: transition (TF). were the pattern that was observed. Finally, a flow pattern map that which depends of the superficial velocities was elaborated for the horizontal pipes in order to relate the hydrodynamics parameters behavior using the presented parameters. This data was compared with correlations and previous experimental results.

## Determination of hydrodynamic parameters on two-phase flow gas-liquid in pipes with different inclination’s angles using image processing algorithm

International Conference on Multiphase Flow 2010 (ICMF-2010).

Tampa, USA. June 2010.

Montoya, G.; Valecillos, M.; Romero, C.; Gonzales, D.

In the present research a digital image processing-based automated algorithm was developed in order to determine the phase's height, hold up, and statistical distribution of the drop size in a two-phase system water-air using pipes with 0º, 10º, and 90º of inclination. Digital images were acquired with a high speed camera (up to 4500fps), using an equipment that consist of a system with three acrylic pipes with diameters of 1.905, 3.175, and 4.445 cm. Each pipe is arranged in two sections of 8 m of length. Various flow patterns were visualized for different superficial velocities of water and air. Finally, using the image processing program designed in Matlab/Simulink®, the captured images were processed to established the parameters previously mentioned. The image processing algorithm is based in the frequency domain analysis of the source pictures, which allows to find the phase as the edge between the water and air, through a Canny filter that extracts the high frequency components of the image. The drop size was found using the calculation of the Feret diameter. Five predominant flow patterns were used to subdivided the Taylor’s bubble in order to analyzed it, corresponding with Chen’s patterns (2001): slug flow(SG), dispersed bubble flow (DB), Stratified smooth flow (SS), Stratified wavy flow (SW), Annular flow (AD), Annular wavy flow (AW).

## Determinación de altura de fase y hold up para flujo bifásico liquido-liquido en tuberías horizontales por medio de procesamiento de imágenes

American Society of Mechanical Engineers (ASME) Congress “Ideas Practicas…Soluciones Eficientes”.

University Simon Bolivar (USB), Caracas-Venezuela. November 2009.

Montoya, G.; Garcia, K.; Valecillos, M.; Garcia, J.; Romero, C.; Gonzales, D.

Se visualizaron patrones de flujo para el sistema agua-aceite en tuberías horizontales y se relacionaron con parámetros hidrodinámicos calculados a partir de un algoritmo computacional generado para este trabajo. Para ello, se adquirieron imágenes con una cámara de alta velocidad en el equipo del Laboratorio de Fenómenos de Transporte de la Universidad Simón Bolívar. Para diferentes juegos de caudales se caracterizaron los patrones de flujo de acuerdo con la clasificación propuesta por Trallero et al. [1] y se procesaron las fotografías tomadas, determinándose la altura de la fase y el hold-up. Como resultado, se registraron cuatro patrones: dos segregados (ST y ST&MI) y dos dispersos (o/w y Do/w&w). Finalmente, se elaboró un mapa de patrones de flujo en función de las velocidades superficiales y se relacionó el comportamiento de los parámetros hidrodinámicos en función de los patrones presentados.

## Determination of Hydrodynamic Parameters on Two-Phase Flow Gas - Liquid in Pipes with Different Inclination Angles Using Image Processing Algorithm

62 Annual Meeting of the American Physical Society’s Division of Fluid Dynamics (DFD).

Minneapolis, Minnesota. November 2009.

Montoya, G.; Valecillos, M.; Romero, C.; Gonzales, D.

In the present research a digital image processing-based automated algorithm was developed in order to determine the phase's height, hold up, and statistical distribution of the drop size in a two-phase system water-air using pipes with 0º, 10º, and 90º of inclination. Digital images were acquired with a high speed camera (up to 4500fps), using an equipment that consist of a system with three acrylic pipes with diameters of 1.905, 3.175, and 4.445 cm. Each pipe is arranged in two sections of 8 m of length. Various flow patterns were visualized for different superficial velocities of water and air. Finally, using the image processing program designed in Matlab/Simulink, the captured images were processed to establish the parameters previously mentioned. The image processing algorithm is based in the frequency domain analysis of the source pictures, which allows to find the phase as the edge between the water and air, through a Sobel filter that extracts the high frequency components of the image. The drop size was found using the calculation of the Feret diameter. Three flow patterns were observed: Annular, ST, and ST&MI.