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Evolution of the Rayleigh Taylor instability obtained with various models, (a) VOF with geometrical interface reconstruction in FLUENT (Štrubelj, 2009), (b) VOF with interface sharpening in CFX (Štrubelj, 2009), (c) single-fluid model with interface sharpening with Štrubelj's in-house code (Štrubelj, 2009), (d) two-fluid model with interface sharpening with Štrubelj's in-house code (Štrubelj, 2009), (e) two-fluid model with interface sharpening in the NEPTUNE_CFD code, left to right: t = 2 s, t = 2.5 s and t = 4 s.
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Safety issues in nuclear power plant involve complex bubbly flows. To predict the behavior of these flows, the two-fluid approach is often used. Nevertheless, this model induces a numerical diffusion of interfaces, which results in a poor accuracy in the calculation of the local parameters. Therefore, to simulate large interfaces such as slugs or f...
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Citations
... Multiphase flows with separated phases are ubiquitous in nature and in industrial applications. Energy [2,3], environment [4], chemical engineering [5,6], hydrology, material processes [7,8], petroleum engineering [9], vehicle design [10] or civil engineering, are examples of applications where the conditions and two-phase flow regimes encountered spread wide and far in terms of density and viscosity ratios, inertial, viscous, gravity or capillary effects. Because of the complexity of the underlying physics, the understanding of these flows is of high interest in order to control or prevent malfunctions such as for boiling in nuclear power plants, overflow in dams or cavitation in turbo-machinery, to mention just a few. ...
In the framework of the in-house code Fugu, a fully-coupled solver is developed for massively parallel simulations of three-dimensional incompressible multiphase flows. The linearized momentum and continuity equations arising from the implicit solution of the fluid velocities and pressure are solved simultaneously. The method uses a BiCGStab(2) (Dongarra et al., 1998) iterative solver with an original preconditioner for the velocity block and an approximation of the inverse of the Schur complement. This is achieved by using PFMG or SMG from HYPRE and an efficient sparse matrix–vector multiplication using the CSR storage format. The construction and the tracking of the interface separating the different involved phases is based on a conservative VOF method. Test cases, such as a spherical bubble rising in quiescent liquid and the free fall of a dense sphere, are performed to validate the models, especially in the presence of strong density and viscosity ratios between fluids. Other cases, such as the phase inversion, demonstrate the ability of the new fully-coupled solver to solve two-phase problems with more than 1 billion degrees of freedom with excellent scalability.
... Multiphase flows with separated phases are ubiquitous in nature and in industrial applications. Energy [40,70], environment [120], chemical engineering [2,136], hydrology, material processes [111,184], petroleum engineering [35], vehicle design [187] or civil engineering, are examples of applications where the conditions and two-phase flow regimes encountered spread wide and far in terms of density and viscosity ratios, inertial, viscous, gravity or capillary effects. ...
The two-phase Navier-Stokes equations are involved in several industrial and environmental applications. However, their resolution poses many problems and becomes a major issue for mathematicians and engineers. In the context of incompressible two-phase flows, the challenges are two-fold : on the one hand, the velocity-pressure coupling induced by the incompressibility constraint and the high density and viscosity ratios lead to very ill-conditioned linear systems and, on the other hand, an inconsistent discretization between momentum transport and mass
transport lead to a divergence of the computational code in some situations. This work is devoted to the development and implementation of a direct simulation tool for two-phase flows taking into account all interfacial scales in order to meet our needs. In this context, an original 3D parallel fully-coupled solver allowing the direct coupling between velocity and pressure has been developed. This solver has been validated on problems using several analytical solutions, numerical benchmarks and experiments, regardless of the ratios of density and viscosity discontinuities across the interface. Good scalability associated with the fully-coupled solver has been
observed, for a number of processors ranging from 40 to 12800. A conservative scheme AMP (Algebraic Momentum Preserving) based on a consistent discretization of density and momentum has been constructed and validated. Quantitative and qualitative comparisons have shown the robustness of our fully-coupled solver combined with the new AMP scheme when facing complex problems.
... A specific drag force law has been developed for the simulation of large interfaces within the two-fluid model. These two elements have been previously detailed and validated in Mimouni et al. [2014] and Fleau et al. [2015]. ...
Bubbly flows occurring in nuclear power plants remain a major limiting phenomenon for the analysis of operation and safety. Therefore, the choice was made to investigate these complex flows with a multifield approach, considering the gas phase as two separated fields. In so doing, the small and spherical bubbles are considered as parts of the dispersed field whereas the distorted bubbles are simulated with an interface locating method. The flow motion is followed using the two-fluid model of Ishii [1975, Thermo-fluid dynamic, theory of two-phase flow, Eyrolles] extended to n-phases. Nevertheless, this model is known to spread numerically large interfaces, which results in a poor accuracy in the calculation of the local flow parameters such as curvature. Therefore, this paper is focused on the accurate simulation of the large scale interfaces. The implementation of an artificial interface sharpening equation is detailed to limit the interface smearing. The activation criteria are also described. Special attention is given to mass conservation. All these steps are illustrated with test cases of isothermal, incompressible and laminar separated two-phase flows. A final validation is proposed with the simulation of the Bhaga's rising bubble problem [Bhaga, D. 2 and Weber, M.E., 1981, Bubbles in viscous liquids: shape, wakes and velocities, J. of Fluid Mech., Vol. 105, pp. 61-85] and the Kelvin-Helmhotz instability in the Thorpe's experiment configuration [Thorpe, S., 1969, Experiments on the instability of stratified shear flows: immiscible fluids, Highlights:-A special treatment including capillary forces is proposed to resolve large interfaces within a multifield approach using a computational multi-fluid dynamics code based on a finite volume discretization.-An interface sharpening equation is implemented and adapted to the two-fluid formulation.-The large interface locating approach is validated with a large range of isothermal, incompressible and laminar test cases from bubbly flows to interfacial liquid/liquid test cases, including convergence studies. 3
... Specific treatments are required to simulate these interfaces, which are referred as the Large Bubble Model (LBMo). This model has been extensively validated on laminar isothermal flows in previous papers [e.g., Denèfle et al. 2015 andFleau et al. 2015]. The criteria to switch from the large interfaces treated with the LBMo to the dispersed field and the opposite are not given in this paper since we will focus on the large interfaces. ...
... The Large Bubble Model [e.g., Denèfle et al. 2015 andFleau et al. 2015] has been developed to simulate large interfaces with a two-fluid model. This model consists in three elements necessary for an accurate simulation of large and deformable inclusions. ...
... Surface tension. The surface tension model used in the code NEPTUNE_CFD for the simulation of the large interfaces corresponds to the volumetric formulation [e.g., Bartosiewicz et al. 2008, Denèfle et al. 2015and Fleau et al. 2015 of the Continuum Surface Force (CSF) model proposed by Brackbill et al. [1992]: ...
Safety issues in nuclear power plant involve complex turbulent bubbly flows. To predict the behavior of these flows, the two-fluid approach is often used. Nevertheless, this model has been developed for the simulation of small spherical bubbles, considered as a dispersed field. To deal with bubbles with a large range of sizes, a multifield approach based on this two-fluid model has been proposed. A special treatment, called the Large Bubble Model (LBMo), has been implemented and coupled to the dispersed model. However, only laminar and isothermal flows were considered in previous papers. Thus, here, Large Eddy Simulations (LES) are investigated to model turbulence effects. For this purpose, the two-fluid model equations are filtered to highlight the specific subgrid terms. Then, an a priori LES study using filtered Direct Numerical Simulation (DNS) results is detailed. This analysis allows classifying these terms according to their relative weight and then concentrating the modelling efforts on the predominant ones. Five different turbulence models are compared. These results are finally used to perform true LES on a turbulent two-phase flow. Moreover, in order to tackle non-isothermal flows occurring in industrial studies, a new heat transfer model is implemented and validated to deal with phase change at large interfaces using the Large Bubble Model.
... Specific treatments are required to simulate these interfaces, which are referred as the Large Bubble Model (LBMo). This model has been extensively validated on laminar isothermal flows in previous papers [e.g., Denèfle et al. 2015 andFleau et al. 2015]. The criteria to switch from the large interfaces treated with the LBMo to the dispersed field and the opposite are not given in this paper since we will focus on the large interfaces. ...
... The Large Bubble Model [e.g., Denèfle et al. 2015 andFleau et al. 2015] has been developed to simulate large interfaces with a two-fluid model. This model consists in three elements necessary for an accurate simulation of large and deformable inclusions. ...
... Surface tension. The surface tension model used in the code NEPTUNE_CFD for the simulation of the large interfaces corresponds to the volumetric formulation [e.g., Bartosiewicz et al. 2008, Denèfle et al. 2015and Fleau et al. 2015 of the Continuum Surface Force (CSF) model proposed by Brackbill et al. [1992]: ...
... Various scenarios involve interfaces between liquid and vapor or gas which are generally much larger than the computational cells size: separated flow or large interface. Specific models to deal with them were developed such as in NEPTUNE_CFD: the Large Interface Model (LIM) Coste (2013), and the Large Bubble Model Denèfle et al. (2014); Fleau et al. (2015) (LBM). Large interface flow are located thanks to interface tracking method. ...
... Large Bubble Model developed in Denèfle et al. (2014); Fleau et al. (2015) is similar to the Large Interface Model in term of concept but different in closures laws. The interface recognition method is exactly the same as for the LIM. ...
In nuclear power plants, steam generator tubes vibrate because of steam/water cross-flows. In order to understant this phenomenon, reduced-scale experiments are performed. Numerical simulations have shown their ability to accurately reproduce the vibration induced by a single phase flow in a tube bundle. The aim of the present work is to do the same with two-phase flow and to characterize the effect of the mixture physical properties on vibration.To do so, a CFD code based on a two-fluid approach is used. A "discrete forcing" method is implemented in order to allow solid body motion in a two-phase flow. The validation is performed with simple and industrial cases using experimental and theoretical results.Using an existing implicit algorithm, a fluid-structure coupling based on the developed interface tracking method is implemented. Validated for single and two-phase flows, it is now possible to have solid motion induced by fluid forces.The different numerical models dedicated to two-phase flows are then evaluated on a freon/freon flow across an inclined tube bundle. The use of a multi-regime model is required. In order to investigate the role of the different physical properties on the vibration, three simple studies are performed.Finally, the industrial application, a freon/water flow across a square pitch tube bundle, is performed. First, it is compared to a steam/water flow in order to characterize the discrepancies when we are using a modeling mixture. Then, the vibration induced by single- and two-phase flows is reproduced by the developed method on feasibility test cases.
... Such an approach has been implemented both in the aforementioned GENTOP formulation ( Montoya et al., 2015 ), and in the NEP-TUNE_CFD code where it is termed the Large Bubble Model (LBM, Denèfle et al., 2015;Mimouni et al., 2017 ). Preliminary assessment of this approach in canonical configurations, such as the Kelvin-Helmholtz and Rayleigh-Taylor instabilities, was reported by Fleau et al. (2015Fleau et al. ( , 2016 . ...
Simulating gas-liquid flows involving a wide range of spatial and temporal scales and multiple topological changes remains a major challenge nowadays, as the computational cost associated with direct numerical simulation still makes this approach unaffordable. A common alternative is the two-fluid Euler-Euler formulation that avoids solving all scales at the price of semi-empirical closures of mass, momentum and energy exchanges between the two fluids. Many of such closures are available but their performances in complex flows are still in debate. Closures considering separately large gas structures and smaller bubbles and making these two populations evolve and possibly exchange mass according to their interactions with the surrounding liquid have recently been proposed. In order to assess the validity of some of these closures, we carry out an original experiment in a simple configuration exhibiting a rich succession of hydrodynamic events, namely the emptying of a water bottle. We simulate this experiment with the NEPTUNE_CFD code, using three different closure approaches aimed at modelling interfacial momentum exchanges with various degrees of complexity. Based on experimental results, we perform a detailed analysis of global and local flow characteristics predicted by each approach to unveil its potentialities and shortcomings. Although all of them are found to predict correctly the overall features of the emptying process, striking differences are observed regarding the distribution of the dispersed phase and its consequences in terms of liquid entrainment.
... The interface is tracked accurately thanks to the Large Interface Model [10]. In order to avoid diffused interfaces, an interface sharpening equation is solved [11]. The bubble rise is governed by three forces : drag, surface tension and gravity. ...
Interactions between a cylindrical structure and two-
phase flows are a major concern for industrial applica-
tions especially in heat exchangers where cross-flows may
occur. In order to understand the vibrations induced by a
two-phase flow, experiments were conducted in the past
with a single tube for various two-phase flow regimes.
Since numerical simulation allows one to handle two-
phase flows, it is possible to carry out complex investi-
gations at the finest scale: the bubble. Using a finite-
volume CFD software dedicated to multi-phase flow and
based on a two-fluid approach, the authors study the im-
pact of a single bubble in a fluid at rest on a cylinder with
fully-refined meshes. Flow being laminar, no turbulence
model is applied. Firstly, the rise of single bubbles in
the fluid at rest is correctly reproduced according to the
experiments of Bhaga&Weber [1]. Then, we study the in-
teraction between the bubbles and a cylindrical structure
with two mesh-refinements. Since the gas-liquid interface
is tracked, the break-up of different bubbles and its influ-
ence on the cylinder are studied, in order to obtain a more
microscopic information to characterize two-phase flows
across tube arrays. Finally, the authors propose some
conclusions on the importance of small-scale analysis to
understand macroscopic industrial phenomena.
... As a first step towards this new approach, we have simplified the general concept by considering the liquid phase as continuous in our simulations. The main results obtained with NEPTUNE CFD are presented in [61][62][63][64]. ...
The objective of this paper is to give an overview of the capabilities of Eulerian bifluid approach to meet the needs of studies for nuclear safety regarding hydrogen risk, boiling crisis, and pipes and valves maintenance. The Eulerian bifluid approach has been implemented in a CFD code named NEPTUNE_CFD. NEPTUNE_CFD is a three-dimensional multifluid code developed especially for nuclear reactor applications by EDF, CEA, AREVA, and IRSN. The first set of models is dedicated to wall vapor condensation and spray modelling. Moreover, boiling crisis remains a major limiting phenomenon for the analysis of operation and safety of both nuclear reactors and conventional thermal power systems. The paper aims at presenting the generalization of the previous DNB model and its validation against 1500 validation cases. The modelling and the numerical simulation of cavitation phenomena are of relevant interest in many industrial applications, especially regarding pipes and valves maintenance where cavitating flows are responsible for harmful acoustics effects. In the last section, models are validated against experimental data of pressure profiles and void fraction visualisations obtained downstream of an orifice with the EPOCA facility (EDF R&D). Finally, a multifield approach is presented as an efficient tool to run all models together.
... The diverse range of structures occurring in churn flow, namely thin liquid film, large liquid waves, liquid droplets and entrained gas bubbles and so on, pose a significant challenge in the modelling efforts which require mixed or hybrid approach by combining several modelling methodologies, that are otherwise specialised for a particular flow morphology (Štrubelj and Tiselj, 2011;Hänsch et al., 2012;Coste, 2013;Wardle and Weller, 2013;Höhne and Mehlhoop, 2014;Fleau et al., 2015). For example, the two-fluid approach can be used for dispersed droplets in the gas flow or entrained bubbles in the liquid flow. ...
In upward air-water churn flow in a vertical tube, the liquid fed at the inlet
is entrained upwards by the gas flow in the core, resulting in a wavy film
flow. The present work considers the influence of the modelling of the liquid
inlet on the wave frequencies, with the final purpose to understand the causes
and effects in an actual flow.
The perforated wall liquid inlet section, commonly used in experiments,
is modelled in simulations as a simple inlet boundary condition on a
short section of a pipe wall. At a given liquid mass flow rate in the
experiment, the magnitude of the wall normal velocity component is
controlled by the inlet area, which is used as a modelling parameter in the
present study. The study shows that the calculated wave frequency is
proportional to the imposed wall normal velocity at the liquid inlet.
The velocity
profile at the perforated wall liquid inlet section was not measured in
available experiments and presents a major source of uncertainty in
simulations.
The parametric study revealed that a suitable value for the magnitude of the
wall normal velocity can be determined over a range of flow conditions,
leading to good agreement of simulated and measured wave frequencies.
This finding suggests, that the wave frequency in actual churn flow is not a
property depending only on geometric and flow conditions, but on the liquid
inlet flow as well.
Simulations were performed in three-dimensional tube section using the
multiphase solver interFoam from the open-source code OpenFOAM.
