Elsevier

Nuclear Engineering and Design

Volume 240, Issue 9, September 2010, Pages 2096-2106
Nuclear Engineering and Design

LEIS for the prediction of turbulent multifluid flows applied to thermal-hydraulics applications

https://doi.org/10.1016/j.nucengdes.2009.11.030Get rights and content

Abstract

The paper centres on the use of the so-defined LEIS approach (Large-Eddy & Interface Simulation) for turbulent multifluid flows present in thermal-hydraulics applications. Interfacial flows involving deformable, sheared fronts separating immiscible fluids are shown to be within reach of this new approach, featuring direct resolution of turbulence and sheared interface deformations within the interface tracking (ITM) framework, such as level sets and VOF. In this technique supergrid turbulence and interfacial scales are directly solved whereas the sub-grid (SGS) parts are modelled, at least the turbulence part of it. First results are shown (feasibility), and difficulties and open issues are discussed. The connection between these two particular scales will also be discussed, and potential modelling routes evoked, including combining two-fluid and ITM, local grid refinement, or combing particle tracking and ITM for sub-grid inclusions smaller than the grid size.

Introduction

The computational thermal-hydraulics scene has gone through successive transitions, motivated by new needs and developments. The first real transition triggered in the 1980s focused on removing gradually the limitations of lumped-parameter 1D modelling by further developing the two-fluid model for 3D turbulent flow problems (Ishii, 1975). This is now the state-of-the-art. The advent of the so-called interface tracking methods (ITM) in the late 80s (Kataoka, 1986), which permit to better predict the shape of interfaces while minimizing the modelling assumptions for momentum interaction mechanisms, has somewhat shifted the interest towards a new transition. The most recent transition is now underway: it specifically centres on the use of these new simulation techniques (ITM) for practical thermal-hydraulics cases, after it has been validated for canonical two-phase flow problems, including bubble rise, drop splashing or bouncing, stratified air–water flows, and many other examples (Lemonnier et al., 2005). But this latest transition poses interesting challenges to the computational thermal-hydraulics community, and raises some specific questions: how to migrate from averaged two-fluid formulation modelling to more refined interface tracking prediction (or combine them when necessary), and from steady-state Reynolds averaged modelling to unsteady large-scale turbulence simulation. The transition is not a matter of availability of computational power and resources only, but a question of adequacy of code algorithmic (precision), complex meshing, and proper modelling of the underlying flow physics, both in the core and near the interfaces. Some recent developments in this area were reviewed by Yadigaroglu and Lakehal (2005) with emphasis on both single- and multiphase thermal-hydraulics.

This paper is written in that spirit, focusing on a new concept that combines the strength of these ITM methods with the advantage of unsteady, large-scale prediction of turbulence, known as the Large-Eddy Simulation (LES). The outcome of this combination is a better way to capture transients of interfaces and associated turbulence, while minimizing modelling (of both: turbulence and interface dynamics). This is the reason why we refer to this approach as LEIS: solving in time as much interface and turbulence scales as possible. We will here limit the mathematical derivations; instead, we will focus on premises, difficulties and required future developments.

Section snippets

Nature and forms

Multiphase flows appear under various forms depending on the nature of the involved fluids and their rate of presence in the system. A fluid–fluid system may be defined as a “diffuse mixed flow” if the transported phase is rather dilute in the carrier phase, the density ratio between the phases is rather small (<10–15%), e.g. meaning that the fluids do not exert substantial momentum exchange on each other, allowing the transported phase to diffuse into the carrier media by molecular and

Dealing with interfacial scales (IS)

Interfacial flows refer to two-phase flow problems involving two or more immiscible fluids separated by sharp interfaces which evolve in time. Typically, when the fluid on one side of the interface is a gas that exerts shear (tangential) stress upon the interface, the latter is referred to as a free surface. Interface tracking/capturing schemes are methods that are able to locate the interface, not by following the interface in a Lagrangian sense (e.g. by following marker points residing on the

TransAT© multiphase flow software

The CMFD code TransAT© developed at ASCOMP is a multi-physics, finite-volume code based on solving multifluid Navier–Stokes equations. The code uses structured meshes, though allowing for multiple blocks to be set together. MPI parallel based algorithm is used in connection with multi-blocking. The grid arrangement is collocated and can thus handle more easily curvilinear skewed grids. The solver is pressure based (Projection Type), corrected using the Karki–Patankar technique for low-Mach

Validation

The stratified air–water flow of Fulgosi et al. (2003) is the appropriate validation case for SGS modelling, since a complete DNS database is available for comparison, and because the flow does not feature interface fragmentation that would have complicated the analysis. Here we compare model (9–10) with the DNS data, the under-resolved DNS without SGS modelling. The comparison includes the results of the VMS approach of Hughes et al. (2001), in which in contrast to filtering the equations are

Slug formation in circular pipes

This test case considered here consists of a co-current stratified two-phase flow containing gas and liquid ammonia (Martin, 2005) injected at various gas and liquid superficial velocities and inflow void fractions. Specifically we discuss here the results of the ‘turbulent case’ with superficial gas velocity of 14 m/s, superficial liquid velocity of 0.5 m/s; and void fraction 50% (i.e. where the pipe is simply half filled). The pipe length is 6.3 m, and the diameter is 0.14 m. The ‘laminar case’

Concluding remarks and future developments

The paper aimed at describing the way computational thermal-hydraulics is migrating to more sophisticated modelling techniques, transcending the two-fluid formulation and steady-state RANS equations for turbulence by integrating ITM's within the LES framework, defined here as “LEIS”. The case studies outlined in this paper illustrate what can be done with LEIS for a class of turbulent interfacial two-phase flows. The method can be successfully applied to generate realistic transient simulations

Acknowledgements

The theoretical part of the work mentioned here was initiated at ETH-Zurich under the supervision of Prof. George Yadigaroglu, more precisely at the CMFD group (Dr. P. Liovic, Dr. M. Fulgosi, Dr. C. Narayanan, Dr. S. Reboux). The new simulations shown here were conducted using the TransAT code of ASCOMP by Daniel Caviezel and “Chidu” Narayanan, partially within NURESIM, An EC-funded project in the framework of the Sixth EURATOM Framework Program (2004–2006). CEA has kindly provided the COSI

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