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Open-source environments such as the Computational Fluid Dynamics software OpenFOAM are very appealing for research groups since they allow for an efficient prototyping of new models or concepts. However, for downstream developments to be sustainable, i.e. reproduci-ble and reusable in the long term, a significant amount of maintenance work must be account-ed for. To allow for growth and extensibility, the maintenance work should be underpinned by a high degree of automation for repetitive tasks such as build tests, code deployment and validation runs, in order to keep the focus on scientific work. Here, an information technology environment referred to as OpenFOAM_RCS is presented that aids the centralized mainte-nance of simulation code and setup files for OpenFOAM developments concerned with reac-tor coolant system safety research. It fosters collaborative developments and review processes. State-of-the-art tools for managing software developments are adapted to meet the require-ments of OpenFOAM. A flexible approach for upgrading the underlying installation is pro-posed, based on snapshots of the OpenFOAM development line rather than yearly version releases, to make new functionality available when needed by associated research projects. The process of upgrading within so-called sprint cycles is accompanied by several checks to ensure compatibility of downstream code and simulation setups. Furthermore, the foundation for building a validation data base from contributed simulation setups is laid, creating a basis for continuous quality assurance. The content of this article was initially presented at the 33rd German CFD Network of Com-petence Meeting, held on March 22-23 2022 at GRS in Garching, Germany.

The Euler-Euler model is widely used in bubbly flow simulations up to industrial dimensions. The standard Euler-Euler model is based on the phase-averaging method. After averaging, the bubble forces in the field equations are functions of the local gas volume fraction. In simulations, when the bubble diameter is larger than the computational cell spacing, the forces can transport the gas belonging to the same bubble in different directions. By contrast, a closure model for the bubble force is typically developed based on the assumption that the force is a resultant force that acts on the bubble's center-of-mass. This inconsistency can lead to a nonphysical gas concentration in the center of a channel or near the channel wall if the bubble diameter is larger than the cell spacing. The purpose of the present contribution is to developed an Euler-Euler model where the bubble force consistency is recovered for two-phase flow simulations where the diameter of the disperse phase can be larger than the cell spacing. Such an Euler-Euler model is developed by combining an existing particle-center-averaged Euler-Euler framework with a Gaussian convolution method. To validate this Euler-Euler approach, a comparison is made with experimental data for the bubbly flows in two different vertical pipes. The results show that the proposed Euler-Euler model recovers the bubble force consistency and alleviates the over-prediction of the gas volume fraction peak near the wall, while its simulation results in the axial gas and liquid velocity and the liquid turbulence kinetic energy are similar to the results of the standard Euler-Euler model.

Bubble size and deformation are important factors for the closure models required in Euler-Euler simulations of bubbly flows. To properly simulate polydisperse bubbly flows where the bubble size spectrum may cover a range of several millimeters, several velocity groups with different sizes have to be considered. To this end, the theory for the particlecenter-averaged Euler-Euler model is generalized for the simulation with multiple bubble velocity groups. Furthermore, bubble deformation effects have been included in appropriate bubble force models. The particle-center-averaged Euler-Euler model provides additional freedom to consider the bubble shape during the conversion between the bubble number density and the gas volume fraction. Therefore, the theory is also generalized to consider an oblate ellipsoidal bubble shape in simulations. A bubbly pipe flow is used to validate the theory and to demonstrate the improvements of the proposed generalizations.

Effects of surfactants on lift coefficients, CL, of single ellipsoidal bubbles rising through linear shear flows were investigated. Two types of surface-active agents, Triton X-100 and 1-octanol, were used. The liquid properties except for the surface tension were identical to those in a clean system of logM = -5.5, where M is the Morton number. The range of the bubble Reynolds number was 0.1 < Re < 70. Bubble shapes were either spherical or ellipsoidal. Comparing with clean bubbles, less deformation of contaminated bubbles was confirmed. A shape correlation without taking the dimensionless shear rate into account gave good evaluations of the bubble aspect ratio, which means that the shear rate is not a dominant factor causing the change of shape deformation. However, drag coefficients were affected by the shear rate. A new correlation of drag coefficients was deduced, which agreed well with the experimental data. Both clean and contaminated CL data showed a similar tendency: after a drastic decrease in CL, CL slightly increases and then decreases to a negative value while increasing the bubble Reynolds number, Re. A difference in concentration of Trion X-100 resulted in only a slight change in CL at high Re regime. Different types of surfactant resulted in noticeably different values of CL especially at low Re. The CL of small bubbles in contaminated systems could be reproduced by a correlation for solid particles, supporting that fully contaminated spherical bubbles behave like solid spheres. For deformed bubbles, CL can be expressed by relating the negative lift force due to shape deformation with the drag force.

Pressure relief by blowdown is one of the most important measures to prevent excessive pressures in the primary circuit or containment in severe nuclear accidents. Pool scrubbing can significantly reduce the release of radioactive materials, e.g., aerosols, to the environment during the pressure relief. The decontamination factor indicating the particle retention efficiency depends, among other factors, on the hydrodynamic conditions of the gas-liquid two-phase flow inside the pool. In the present work, the hydrodynamics in two typical pool scrubbing experiments is investigated with the two-fluid model, and the influence of some key factors including bubble diameter, nozzle submergence as well as interaction models are analysed. One case is a rectangular pool and the other is a cylindrical column, and their injection Weber number is around 2×103 and 4×105, respectively. The numerical results show that as the distance from the nozzle exit increases, the void fraction and velocity field expand from the central region, where the nozzle is located, to the whole cross section. The profile and its development depends largely on the bubble size and the interaction force model. It reveals that in the monodisperse simulation, the tuning of bubble diameter is necessary for achieving good agreement, although it is difficult for high velocity gas injection. More information is required to properly describe the bubble size distribution as well as its evolution in pool scrubbing conditions. Furthermore, the experimental data show clear drag reduction in the bubble swarm generated by the gas jet, and the mechanism and model improvement possibilities need to be investigated.

The lift coefficient CL of a bubble in a shear flow is known to change its sign depending on the bubble shape, i.e., CL of spherical bubbles are positive for any bubble Reynolds numbers, while those of ellipsoidal bubbles can be negative due to the deformation-induced negative lift. The critical bubble diameter dC for the lift reversal was discussed in this study by making use of available CL correlations for clean bubbles in linear shear flows. As a result, the following conclusions were obtained: (1) the CL correlations well describe the complex characteristics of dC, and the lift reversal criterion in terms of ReC is well reproduced with the correlations; (2) in the surface tension and inertial force dominant regime, the critical Eötvös and Weber numbers can be used to develop a simple criterion of lift reversal, i.e., they are almost constant, and the capillary number for dC can be expressed in terms of the Morton number only, M; these dimensionless groups in the viscous force dominant regime however show more complex dependence on M; and (3) the critical Ohnesorge number OhC monotonically increases with increasing M even in the viscous force dominant regime. Therefore a tentative dC correlation based on OhC was developed.

Polydispersity is a challenging feature of many industrial and environmental multiphase flows, influencing all related transfer and transport processes. Besides their size, the fluid or solid particles may be distributed with respect to other properties such as their velocity or shape. Here, a population balance model based on the method of classes is combined with a multifluid solver within the open source computational fluid dynamics library OpenFOAM. The model allows for tracking the evolution of one or more size-conditioned secondary properties. It is applied to two different problems, the first being bubbly flow of air and water in a vertical pipe, where considering the velocity as a secondary property allows to resolve the size-dependent radial segregation. The second application is the gas phase synthesis of titania powder, where non-spherical particle aggregates appear whose shape is modeled through a collision diameter, leading to an improved prediction of the size distribution.

The properties of bubble-laden turbulent flows at different scales are investigated experimentally, focusing on the flow kinetic energy, energy transfer and extreme events. The experiments employed particle shadow velocimetry measurements to measure the flow in a column generated by a homogeneous bubble swarm rising in water, for two different bubble diameters ( $2.7$ mm and $3.9$ mm) and moderate gas volume fractions ( $0.26\,\%\sim 1.31\,\%$ ). The two velocity components were measured at high resolution, and used to construct structure functions up to twelfth order for separations spanning the small to large scales in the flow. Concerning the flow anisotropy, the velocity structure functions are found to differ for separations in the vertical and horizontal directions of the flow, and the cases with smaller bubbles are the most anisotropic, with a dependence on void fraction. The degree of anisotropy is shown to increase as the order of the structure functions is increased, showing that extreme events in the flow are the most anisotropic. Our results show that the average energy transfer with the horizontal velocity component is downscale, just as for the three-dimensional single-phase turbulence. However, the energy transfer associated with the vertical component of the fluid velocity is upscale. The probability density functions of the velocity increments reveal that extreme values become more probable with decreasing Reynolds number, the opposite of the behaviour in single-phase turbulence. We visualize those extreme events and find that regions of intense small-scale velocity increments occur near the turbulent/non-turbulent interface at the boundary of the bubble wake.

Flow boiling occurs when a subcooled liquid enters a heating pipe and its temperature near the heating wall exceeds the boiling onset temperature. Bubbles are generated on the heating wall and the more downstream the larger the average bubble size due to progressing evaporation and coalescence. Further downstream, the two-phase flow morphologies may change from bubbly to slug, plug, and annular flow. Since these flow patterns have a great impact on the heat and mass transfer rates, an accurate prediction of them becomes critical. In this work, the recently developed GEneralized-TwO Phase concept (GENTOP) was used for flow patterns transition modelling and their effects on the wall heat transfer during the upward subcooled flow boiling inside a vertical heating pipe. Furthermore, a previously developed mechanistic bubble dynamics model was implemented in the GENTOP framework as a sub-model. This model is based on the force balance on a single growing bubble considering evaporation of the microlayer underneath the bubble, thermal diffusion and condensation around the bubble as well as the dynamic inclination and contact angles. It does not require a recalibration of parameters to predict the bubble dynamics. For implementing this model in the Euler-Euler (E-E) framework an extension of the current nucleation site density and heat partitioning model was required. Eventually, for a generic test case, flow boiling regimes of water in a vertical heating pipe were simulated using ANSYS CFX 18.2.

The standard Euler-Euler model is based on the phase-averaging method. Each bubble force is a function of the local gas volume fraction. As a result, the coherent motion of each bubble as a whole is not enforced when the bubble diameter is larger than the mesh size. However, the bubble force models are typically developed by tracking the bubbles' centers of mass and assuming that the forces act on these locations. In simulations, this inconsistency can lead to a nonphysical gas concentration in the center or near the wall of a pipe when the bubble diameter is larger than the mesh size. Besides, a mesh independent solution may not exist in such cases. In the present contribution, a particle-center-averaging method is used to average the solution variables for the disperse phase, which allows to represent the bubble forces as forces that act on the bubbles' centers of mass. An approach to simulate bubbly flows is formed by combining the Euler-Euler model framework using the particle-center-averaging method and a diffusion-based method that relates phase-averaged and particle-center-averaged quantities. The remediation of the inconsistency with the standard Euler-Euler model based on phase-averaging method is demonstrated using a simplified two-dimensional test case. The test results illustrate that the particle-center-averaging method can alleviate the over-prediction of the gas volume fraction peak in the channel center and provide mesh independent solutions. Furthermore, a comparison of both approaches is shown for several bubbly pipe flow cases where experimental data are available. The results show that the particle-center-averaging method can alleviate the over-prediction of the gas volume fraction peaks in the wall peaking cases as well.

Heat transfer on a vapor bubble rising in superheated liquid is investigated by direct numerical simulation. The vapor–liquid system is described by the one-fluid formulation with the level set method capturing the interface. The proportional-integral-derivative controller is employed to keep the bubble's location fixed and evaluate interfacial forces. The heat transfer performance featured by the Nusselt number is evaluated based on the energy balance. Simulations are carried out for the bubble Reynolds number ranging from 20 to 500 and Morton number from 1.10 × 10−10 to 3.80 × 10−4. The aim of this paper is to shed some light on the effect of bubble deformation and oscillation on interfacial heat transfer. The results show that the front part of the bubble contributes to the majority of the interfacial heat transfer, while the rear part mainly affects the oscillation amplitude of the total heat transfer. The interface stretch during bubble oscillation is considered as a key mechanism in enhancing the instantaneous Nusselt number. The potential flow solution of the averaged Nusselt number is corrected by considering the influence of the aspect ratio. This research provides additional insights into the mechanism of interfacial heat transfer, and the results apply equally to interfacial mass transfer.

We report on a series of fully resolved simulations of the flow around a rigid sphere translating steadily near a wall, either in a fluid at rest or in the presence of a uniform shear. Non-rotating and freely rotating spheres subject to a torque-free condition are both considered to evaluate the importance of spin-induced effects. The separation distance between the sphere and wall is varied from values at which the wall influence is weak down to gaps of half the sphere radius. The Reynolds number based on the sphere diameter and relative velocity with respect to the ambient fluid spans the range $0.1-250$, and the relative shear rate defined as the ratio of the shear-induced velocity variation across the sphere to the relative velocity is varied from $-0.5$ to $+0.5$, so that the sphere either leads the fluid or lags behind it. The wall-induced interaction mechanisms at play in the various flow regimes are analyzed qualitatively by examining the flow structure, especially the spanwise and streamwise vorticity distributions. Variations of the drag and lift forces at low-but-finite and moderate Reynolds number are compared with available analytical and semiempirical expressions, respectively. In more inertial regimes, empirical expressions for the two force components are derived based on the numerical data, yielding accurate fits valid over a wide range of Reynolds number and wall-sphere separations for both non-rotating and torque-free spheres., Comment: 27 pages, 25 figures

Unsteady bubble plume oscillations significantly influence the characteristics of the flow and especially the mixing occurring in bubble column reactors. In the present work, the frequencies of oscillations of centrally aerated bubble plumes in rectangular bubble columns are experimentally studied in water and aqueous glycerol solutions. The effects of the superficial gas velocity, aspect ratio, aerated width, needle size and the liquid viscosity on the low-frequency oscillations are investigated. The performance of the dimensionless numbers characterizing the plume oscillation published in the literature is assessed with the present experimental data. Dimensionless analysis is performed and a new empirical correlation is proposed based on the present experiments, and then validated through a large number of available experimental data in the literature. The experimental data and dimensionless analysis presented here can help to optimize the flow characteristics for specific processes in bubble column reactors. Further, the results can be used for validating corresponding computational models.

The baseline model for the CFD-simulation of turbulent bubbly flows in the Euler-Euler framework was proposed several years ago. It defines a fixed set of closures including all constants to avoid case by case tuning and to have a basis for sustainable model development. The present paper extends the baseline model by improving the modelling of bubble-induced turbulence. The closure terms in the transport equations of the k-ω SST model are revisited and replaced with the model recently proposed by Ma et al. (Ma et al., 2017. Physical Review Fluids 2, 034301) which is based on an analysis of the turbulent kinetic energy budget obtained from direct numerical simulation data. Detailed validation results for various flow configurations with a wide range of gas and liquid volumetric fluxes are presented. In case of vertical pipe flow significant improvements in the predicted gas volume fraction and velocity profiles are obtained, especially in high gas volume fraction cases where bubble-induced turbulence is dominant. Simulations of other configurations, such as uniform and non-uniform bubble columns, show that the model of Ma et al. (2017) results in an also for these cases overall improvement. Therefore, the baseline model is now updated to include the new model for bubble-induced turbulence.

In line with the best practice guidelines for computational fluid dynamics in nuclear reactor safety, Helmholtz-Zentrum Dresden – Rossendorf proposed an Euler-Euler baseline closure concept some years ago. The consideration is that simulations with a fixed set of closures instead of case by case tuning may help to identify model inadequacy and facilitate further improvement. Currently, the baseline model concerns interphase momentum and turbulence exchange as well as bubble coalescence and breakup. It has been tested for a wide range of isothermal applications with different geometrical configurations and material systems. In the present work, the baseline model is extended to non-isothermal flows by including a mechanistic model for the computation of overall interphase heat transfer coefficient. The extended baseline model is validated for both bubble-growing in superheated liquid and -condensing in sub-cooled liquid. The baseline model proposal is independent on the use of a certain CFD code. The presented simulation is carried out with the commercial software ANSYS CFX by employing the best practice guidelines. The simulated liquid temperature, gas volume fraction, vapor bubble size and velocity are compared with the measured ones. The effectivity of the model is demonstrated by a general good agreement.

The lift force acting on particles, drops, and bubbles in a shear field is a well know effect that was extensively investigated since the 1980s. Experiments with single bubbles in a linear shear field reveal that the lift force coefficient has at a specific bubble size a zero where the coefficient switches its sign from positive to negative. Solving the lateral force balance for a polydisperse bubbly flow with the liquid flow field usually found in a bubble column, the lift force causes a spatial separation of small, which tend to the wall, and big bubbles, which tend to the center. Bubbles with a zero lift force coefficient on the other hand are equally distributed over the cross section. In order to investigate the influence of the flow field on the force balance, simulations with the Euler-Euler two-fluid model were conducted. The simulations are in good agreement with experiments and showed that the liquid flow field found in the used bubble column has a minor influence on the steady state distribution of the bubbles. From evaluating six different bubble column experiments conducted in air/water, the critical diameter at which the lift force coefficient is zero is determined between 5.1 and 5.2 mm. This result is very close to the critical diameter that we were able to determine in previous, single-bubble experiments. Therefore, the conclusion is possible that the lift force coefficients that are determined with single bubble experiments are applicable to the complex flow field found in polydisperse bubbly flows at low void fractions and low turbulence levels. Moreover, a very simple experimental setup is described to determine the critical diameter in complex flow situations, which is the most important point when it comes to modelling the lift force. Such a setup is therefore beneficial when complex substance mixtures used in a specific industrial or environmental application needs to be investigated since lift force correlations are only available for clean, ideal substances.

The phenomena of initially subcooled liquid flashing into vapour due to depressurization frequently occur in the nature and technology. They are very complex systems, and the fluid dynamics is affected by the interplay of many different sub phenomena, including bubble nucleation, evaporation, condensation, coalescence, and breakup. Research on the flashing flows has received much attention, but CFD modelling and simulation of such scenarios is still challenging, because knowledge is often insufficient for a precise mathematical description of the physical problems. Attempts of numerical analysis having been made before are all based on the assumption of mono-disperse bubbles by prescribing either the size or number density or using a mixture model, which deviates largely from the physical picture. In the present work, a CFD-PBM coupled approach is extended for the investigation of bubble dynamics in a flashing pipe flow. The comparison with experimental data demonstrates that the model is effective in capturing the physics of vapour bubbles’ generation and growth as well as their spatial motion and distribution during the decompression. Although further polishing of the bubble coalescence and breakup as well as interphase momentum and turbulence transfer models is necessary, the agreement between measured and simulated cross-section averaged flow parameters such as void fraction, liquid temperature and bubble size distribution is satisfying. The wide range of bubble size changing confirms the necessity of using a poly-disperse approach instead of mono-disperse assumptions.

An X-ray microtomography (µCT) system was adapted so that 3D scans of fixed horizontal or vertical test sections can be performed. The mobile µCT system has been applied to measure the local, time-averaged volume fraction distribution of developing annular air-water flow in a horizontal pipe with µm spatial resolution. Based on the volume fraction data the liquid film thickness profile is computed and the accumulation, stripping and renewal of the annular liquid film at a circular orifice is studied. The development length of the annular flow downstream of the orifice is evaluated based on the integral volume fraction and the change of the film thickness profile along the pipe axis. Both parameters give a consistent result, indicating that liquid film renewal can be judged based on integral measurement techniques in this case. Further, the detailed 3D data enables the validation of computational fluid dynamics codes based on phase-averaged variables such as the Euler-Euler approach. Graphic abstract

The bubble shape affects the gas–liquid interface momentum, heat and mass transfer, as well as the flow field around the bubble. Correctly predicting the bubble shape is challenging but indispensable under bubble swarm conditions. In this work, the bubble aspect ratio, which is adopted for the characterization of the bubble shape, is obtained through high-speed photography combined with an image processing algorithm from several experiments with an oscillating bubble plume. The results show that the bubble aspect ratio tends to be a constant value with increasing the bubble diameter, rather than decreasing as predicted by various empirical correlations developed in single bubble experiments. None of the available empirical correlations can accurately correlate the bubble aspect ratio with Weber number or Tadaki number. Among the available correlations in literature, Eo based correlation proposed by Besagni and Inzoli (2016) and Eo-Re based correlation proposed by Besagni and Deen (2020) show a better performance. The predicted values of Eo-Re based correlation developed in this work are the closest to the experimental data compared with others.

Eulerian-Eulerian computational fluid dynamic (CFD) models allow the prediction of complex and large-scale industrial multiphase gas–liquid bubbly flows with a relatively limited computational load. However, the interfacial transfer processes are entirely modelled, with closure relations that often dictate the accuracy of the entire model. Numerous sets of closures have been developed, often optimized over few experimental data sets and achieving remarkable accuracy that, however, becomes difficult to replicate outside of the range of the selected data. This makes a reliable comparison of available model capabilities difficult and obstructs their further development. In this paper, the CFD models developed at the University of Leeds and the Helmholtz-Zentrum Dresden-Rossendorf are benchmarked against a large database of bubbly flows in vertical pipes. The research groups adopt a similar modelling strategy, aimed at identifying a single universal set of widely applicable closures. The main focus of the paper is interfacial momentum transfer, which essentially governs the void fraction distribution in the flow, and turbulence modelling closures. To focus on these aspects, the validation database is limited to experiments with a monodispersed bubble diameter distribution. Overall, the models prove to be reliable and robust and can be applied with confidence over the range of parameters tested. Areas are identified where further development is needed, such as the modelling of bubble-induced turbulence and the near-wall region, as well as the best features of both models to be combined in a future harmonized model. A benchmark is also established and is available for the testing of other models. Similar exercises are encouraged to support the confident application of multiphase CFD models, together with the definition of a set of experiments accepted community-wide for model benchmarking.

International audience; The three-dimensional flow around a spherical clean bubble translating steadily in a wall-bounded linear shear flow is studied numerically. The present work is concerned with the drag and lift forces experienced by the bubble over a wide range of Reynolds number (0.1 ≤ Re ≤ 103, Re being based on the bubble diameter and relative velocity with respect to the ambient fluid), wall distance (1.5 ≤ LR ≤ 8, LR being the distance from the bubble center to the wall normalized by the bubble radius), and relative shear rate (−0.5 ≤ Sr ≤ 0.5, Sr being the ratio between the velocity difference across the bubble and the relative velocity). Based on the above range of parameters, situations where the bubble is repelled from or attracted to the wall are both covered. The flow structure and vorticity field are analyzed to obtain qualitative insight into the interaction mechanisms at work. The drag and lift forces are computed as well. Their variations agree well with theoretical predictions available in the limit of low-but-finite Reynolds number and, when the fluid is at rest, in the potential flow limit. Numerical results and analytical expressions are combined to provide accurate semi-empirical expressions for the drag and lift forces at arbitrary Reynolds number and separation distance.

The thermohydraulic conditions in a storage pool for spent nuclear fuel are studied for a loss of cooling accident leading to partially uncovered fuel racks. While under normal operating conditions, the fuel is cooled by means of single-phase natural convection in the water of the pool, the heat transfer rate into the gaseous pool atmosphere for the above scenario involves several mechanisms to a similar amount which are interacting in a complicated manner, such as heat convection, thermal radiation and heat conduction. These mechanisms are analyzed with the aid of Computational Fluid Dynamics using two complementary models that address different length scales, one for the large scales of the pool and the containment geometry, another one for the small scales of the fuel rods. The models emulate the conditions at reactor unit 4 of the former Fukushima Daiichi nuclear power plant at the time of the accident, because this is a case example for a loss of cooling scenario and sufficient material for adequate modeling is available in the literature. The pool scale model serves to analyze the developing flow patterns. It includes the fuel represented as porous medium as well as the pool and reactor building atmosphere. The results show that a characteristic flow field forms in the pool atmosphere that prevails for all studied water levels and decay heat rate distributions. The temperature level, on the other hand, can be reduced significantly by means of a checkerboard storage. However, the boundary conditions in the head region of a fuel assembly are clearly a function of its storage location in the pool. Representative conditions are extracted and applied to the second model that represents a single geometry-resolved fuel assembly with a portion of the atmosphere above it. The corresponding simulations show that the heat exchange rate between the fuel assembly and the atmosphere above is higher for the conditions near the pool wall compared to the conditions in the center of the pool. The study provides important information on how to best arrange the fuel assemblies in a spent fuel pool for optimal heat exchange in case of an accident involving loss of cooling water. Checkerboarding should be combined with a strategy to place fuel with higher decay heat rate near the pool walls.

An Euler–Euler two-fluid approach was used to simulate the behavior of gas bubbles rising in a stagnant liquid metal. A single point injection in the range of moderate gas flow rates results in the formation of bubble chains undergoing distinct oscillations of the bubble trajectories. A set of interfacial closures and a shear stress transport k-ω (SST) turbulence model, namely the baseline model for bubbly flow (Rzehak and Krepper, 2013b) was applied for simulating the transient behavior of the bubble chain. X-ray radiography measurements were conducted to establish an experimental data base for validating the numerical results. The experiments provide a visualization of the two-phase flow in a flat container and allow for determining essential bubble quantities such as the size, shape, trajectory and velocity. The comparison between numerical simulations and experimental data showed a very good qualitative and quantitative agreement with respect to the distribution of the void fraction and the dynamics of the bubble chain. Wrong results were obtained by simulations where the effect of the bubble induced turbulence (BIT) was neglected. Two BIT models were applied within this study, the baseline BIT model and the Sato BIT model. Both models showed a good agreement with the experimental observations, while the results of the baseline model were even closer to the measurements. Thus, the baseline model originally developed for the air-water system has proved to be capable of reproducing the complex transient behavior of oscillating bubble chains in liquid metals.

The recently developed GENTOP (Generalized Two Phase Flow) concept, which is based on the multifield Euler‒Euler approach, was applied to model a free-surface vortex—a flow situation that is relevant for hydraulic intake. A new bubble entrainment model has been developed and implemented in the concept. In general, satisfactory agreement with the experimental data can be achieved. However, the gas entrainment can be significantly affected by several parameters or models used in the CFD (Computational Fluid Dynamics) simulation. The scale of curvature correction Cscale in the turbulence model, the coefficient in the entrainment model Cent, and the assigned bubble size to be entrained have a significant influence on the gas entrainment rate. The gas entrainment increases with higher Cscale values, which can be attributed to the stronger rotation captured by the simulation. A smaller bubble size gives higher gas entrainment, while a larger bubble size leads to a smaller entrainment. The results also show that the gas entrainment can be controlled by adjusting the entrainment coefficient Cent. Based on the modeling framework presented in this paper, further improvement of the physical modeling of the entrainment process should be done.

An Euler-Euler two-fluid approach was used to simulate the behavior of gas bubbles rising in a stagnant liquid metal. A single point injection with four gas flow rates resulted in the formation of bubble chains undergoing either slight or distinct oscillations of the bubble trajectories. A set of interfacial closures with a shear stress transport (SST) k-ω turbulence models was applied for simulating the transient behavior of the bubble chain. X-ray radiography measurements were conducted to establish an experimental data base for validating the numerical results. The experiments provide a visualization of the bubble chain in a flat container and allow determining the bubble size and integral void fraction. Two bubble induced turbulence (BIT) models (Rzehak and Krepper, 2013a, Sato et al., 1981) and a modified turbulent viscosity approach (Johansen et al., 2004) were applied within this study. For all gas flow rates, the Rzehak and Sato BIT model alone predicted a steady bubble chain in contrast to the oscillating bubble plume observed in the experiments. Without a BIT model the oscillating bubble chain can be predicted but the oscillation frequency is underestimated especially for high gas flow rates. In addition, calculations without a BIT model predicted over-dispersion of the averaged gas fraction through the whole fluid container for the high gas flow rates. The best results in terms of a satisfying agreement with the experimental data were achieved by adopting a modified turbulent viscosity approach proposed by Johansen together with the Rzehak and Krepper BIT model. These findings demonstrate the significance of the turbulence model.

A unified set of closures have been applied to simulating different configurations and fluids, i.e. pipe flow and bubble column, air/water and air/liquid metal. The simulated velocity, void fraction and turbulence profiles were compared with the measured ones. Starting from the baseline model for poly-disperse flows the present work is intended to prove the performance of a recently published model for bubble-induced turbulence, which was established on the basis of physical analyses and direct numerical simulation data. The model is shown to work well under various conditions without any need of tuning, and significant improvement in the prediction of turbulence parameters in comparison to other models is demonstrated. This is a great step towards developing the baseline closure concept. Finally, a brief discussion on the further development and future work regarding Eulerian closure models was given.

We report on generic experimental studies dealing with the direct contact condensation of steam at a falling sub-cooled water jet at high pressure and temperature. This generic problem concerns some safety-relevant thermal hydraulic scenarios in light water reactors. One of such is the question for heat transfer and mixing when a sub-cooled jet of water in an emergency core-cooling scenario enters a hot pressurized component of the primary reactor circuit. The involved phenomena of heat transfer are complex in this case, reaching from direct condensation of steam via steam bubble entrainment to single phase mixing of hot and cold water. Experiments were performed in the TOPFLOW pressure tank, which is an experimental facility for high-pressure thermal hydraulics experiments in pressure equilibrium. The facility has been designed for studying steam-water two-phase flows at pressures of up to 50 bar. It enables to run experiments in flow domains of complex shape without high difference pressures across the wall. The concept therefore allows us to use thin metal walls and even glass windows to observe flows in complex geometry domains with the help of video camera and to considerably reduce cost and complexity of experimental installations. The experimental results show, that high ambient pressure promotes the direct contact condensation heat transfer of subcooled jet flow heavily.

In this paper, we report on experimental investigations of air/water two-phase flow in a vertical pipe, which is a benchmark experiment for the development and validation of two-phase CFD models and codes. We conducted experiments for co-current upward and downward flow as well as counter-current flow at ambient temperature and pressure conditions. As measurement technique, we employed ultrafast X-ray tomography, which gives two-dimensional cross-sectional images of the gas-liquid distribution in two planes up to 2500 Hz scanning speed. In this paper, we briefly explain this technique, introduce essential data processing steps and discuss the findings of the study.

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.