Validation of AIAD Sub-Models for Advanced Numerical Modelling of Horizontal Two-Phase Flows

In the last decade, applications of Computational Fluid Dynamic (CFD) methods for industrial applications received more and more attention, as they proved to be a valuable complementary tool for design and optimization. The main interest towards CFD consists in fact in the possibility of obtaining detailed 3D complete flow-field information on relevant physical phenomena at lower cost than experiments. Typically free surfaces manifest as stratified and wavy flows in horizontal flow domain where gas and liquid are separated by gravity.Stratified two-phase flows are relevant in many industrial applications, e.g. pipelines, horizontal heat exchangers and storage tanks. This paper presents different CFD-simulations on flows using a new modeling approach for the interfacial drag at free surfaces. The developed drag coefficient model was implemented together with the Algebraic Interfacial Area Density (AIAD) model into the three-dimensional (3-D) computational fluid dynamics (CFD) code ANSYS-CFX. The applications considered include the prediction of countercurrent flow limitations (CCFL) in a pressurized water reactor (PWR) hot leg, the development of hydraulic jump during the air-water co-current flow in a horizontal channel, and pressurized thermal shock (PTS) phenomena in a PWR cold leg and downcomer. For the modeling of these tasks, an Euler–Euler approach was used. This approach allows the use of different models depending on the local morphology. In the frame of an Euler-Euler simulation, the local morphology of the phases has to be considered in the drag model.To demonstrate the feasibility of the present approach, the computed main parameters of each case were compared with experimental data. It is shown that the CFD calculations agree well with the experimental data. This indicates that the AIAD model combined with new drag force modeling is a promising way to simulate the phenomena in frame of the Euler-Euler approach. Moreover the further validation of the model by including mass transfer effects should be carried out.© 2013 ASME

Numerical simulations of counter-current two-phase flow experiments in a PWR hot leg model using an interfacial area density model

This paper presents the results from numerical simulation of jet flow emanating from a cruciform orifice. OpenFOAM is used for the simulations with RANS standard k-e turbulence model. The results are validated with the experimental data of Quinn [1]. Mean flow quantities and turbulent quantities are discussed in detail. The evolution of cruciform shape to circular shape is described with various contours of mean flow and turbulent quantities. Possible reasons for the evolution of cross sectional shapes are discussed.

One of the fundamental problems in simulating the motion of sharp interfaces between immiscible fluids is a description of the transition that occurs when the interfaces merge and reconnect. It is well known that classical methods involving sharp interfaces fail to describe this type of phenomena. Following some previous work in this area, we suggest a physically motivated regularization of the Euler equations which allows topological transitions to occur smoothly. In this model, the sharp interface is replaced by a narrow transition layer across which the fluids may mix. The model describes a flow of a binary mixture, and the internal structure of the interface is determined by both diffusion and motion. An advantage of our regularization is that it automatically yields a continuous description of surface tension, which can play an important role in topological transitions. An additional scalar field is introduced to describe the concentration of one of the fluid components and the resulting system of equations couples the Euler (or Navier–Stokes) and the Cahn–Hilliard equations. The model takes into account weak non–locality (dispersion) associated with an internal length scale and localized dissipation due to mixing. The non–locality introduces a dimensional surface energy; dissipation is added to handle the loss of regularity of solutions to the sharp interface equations and to provide a mechanism for topological changes. In particular, we study a non–trivial limit when both components are incompressible, the pressure is kinematic but the velocity field is non–solenoidal (quasi–incompressibility). To demonstrate the effects of quasi–incompressibility, we analyse the linear stage of spinodal decomposition in one dimension. We show that when the densities of the fluids are not perfectly matched, the evolution of the concentration field causes fluid motion even if the fluids are inviscid. In the limit of infinitely thin and well–separated interfacial layers, an appropriately scaled quasi–incompressible Euler–Cahn–Hilliard system converges to the classical sharp interface model. In order to investigate the behaviour of the model outside the range of parameters where the sharp interface approximation is sufficient, we consider a simple example of a change of topology and show that the model permits the transition to occur without an associated singularity.

Advances in direct numerical simulation for MHD modeling of free surface flows

The results from analytical, numerical and experimental modelling of free-surface vortex flows are presented. Vortex flow is induced in a gravity-driven, open-channel flow chamber with a subcritical approach flow and is simulated using the ANSYS CFX steady Eulerian multiphase flow model in order to determine water surface- and velocity-field characteristics. Solution sensitivity to mesh type, density and various turbulence closure methods is considered. The water surface and tangential velocity profile are also modelled using the Vatistas (n = 2) analytical model. The numerical solution is validated using experiments conducted in a scaled physical model of the chamber which permits the investigation of the air/water interface and determination of the velocity fields using particle tracking velocimetry. The sensitivity analysis carried out presents a case for mesh independence and gives evidence that the baseline Reynolds stress model is most suited in simulating free-surface vortex flows. The predicted shape of the air core is in agreement with the physical model but the location of the resolved free-surface interface is under predicted. Concerning the velocity field, the Reynolds stress model makes a fair to moderate prediction of the tangential velocity field; however, the radial velocity field is typically underpredicted. It is concluded that unsteady flow features inherent in the vortex, namely, free-surface instabilities, are preventing the steady-state model from achieving the required accuracy, thus requiring further transient analysis.

One-dimensional unsteady Reynolds-averaged Navier–Stokes computations were performed for oscillatory transitional and turbulent pipe flows and the results were validated against existing experimental data for a wide variety of oscillatory Reynolds and Womersley numbers. An unsteady version of the Johnson–King model was implemented with optional near-wall modification to account for temporal pressure gradient variations, and the predictions were compared with those of the Spalart–Allmaras and k–ε turbulence models. Transition and relaminarization were based on empirical Womersley number correlations and assumed to occur instantaneously: in the former case, this assumption was valid, but in the latter case, deviations between data and predictions were observed. In flows where the oscillatory Reynolds numbers are substantially higher than the commonly accepted steady critical value (~2000), fully or continuously turbulent models produced the best correspondence with experimental data. Critically and conditionally turbulent models produced slightly inferior correspondence, and no significant benefit was observed when near-wall pressure gradient effects were implemented or when common one- and two-equation turbulence models were employed. The turbulent velocity profiles were mainly unaffected by the oscillations and this was explained by noting that the turbulent viscosity is significantly higher than its laminar counterpart. Thus, a turbulent Womersley number was proposed for the analysis and categorization of oscillatory pipe flows.

Physical and numerical modelling of air-water flows: An Introductory Overview

A gas-kinetic scheme coupled with SST model for turbulent flows

Numerical modeling of wavy stratified two-phase flow in pipes

A three-dimensional non-hydrostatic coupled model for free surface – Subsurface variable – Density flows

Experimental investigation and numerical modelling of transient two-phase flow in a geysering geothermal well

Reviewing the literature of CFD-based numerical wave tanks for wave energy applications, it can be observed that different flow conditions and different turbulence models are applied during numerical wave energy converter (WEC) experiments. No single turbulence model can be identified as an `industry standard’ for WEC modeling. The complexity of the flow field around a WEC, together with the strong dependency of turbulence effects on the shape, operational conditions, and external forces, hampers the formulation of such an `industry standard’. Furthermore, the conceptually different flow characteristics (i.e., oscillating, free surface flows), compared to the design cases of most turbulence models (i.e., continuous single-phase flow), can be identified as a source for the potential lack of accuracy of turbulence models for WEC applications. This communication performs a first step towards analyzing the accuracy and necessity of modeling turbulence effects, by means of turbulence models, within CFD-based NWTs for WEC applications. To that end, the influence of turbulence models and, in addition, the influence of the initial turbulence intensity is investigated based on different wave–structure interaction cases considering two separately validated WEC models. The results highlight the complexity of such a `turbulence analysis’ and the study suggests specific future work to get a better understanding of the model requirements for the flow field around WECs.

Up to now, no a real liquid metal (LM) free surface flow have been successfully used in magnetic fusion devices as MHD instability and unavoidable rivulet flow. Recently, after we carried out a guidable free curve-surface flow on theoretically and experimentally, seeking for other way to get a full-cover free surface flow is also in implementing. The superficial layer MHD effect in free surface flow is experimentally observed. After compared and analyzed the characteristic parameters of the free surface flow, the new two parameters of surface cover ratio and rivulet flow index are introduced to characterize the flowing characteristic of the full-cover free surface flow under magnetic field. According to the analysis rule, for different liquid metal, there are the different unique conditions to meet full-cover free surface flow under magnetic field.

A two-phase flow model coupling with volume of fluid and immersed boundary methods for free surface and moving structure problems