Steven P. Antal

A coupled phasic exchange algorithm for three-dimensional multi-field analysis of heated flows with mass transfer

Abstract A three-dimensional multi-field coupled phasic exchange (CPE) algorithm, for the prediction of general two-phase flows is presented. The algorithm is applicable to an arbitrary number of fields, a four-field construct is adopted here. Ensemble averaged transport equations for mass, momentum, energy and turbulence transport are solved for each field (continuous liquid, continuous vapor, disperse liquid, disperse vapor). This four field structure allows for analysis of adiabatic and boiling systems which contain flow regimes from bubbly through annular. Interfacial mass, momentum, turbulence and heat transfer models provide coupling between fields. The CPE algorithm is a semi-coupled implicit method to solve the set of 25 equations which arise in the formulation. In this paper, the CPE algorithm is summarized, with emphasis on six component numerical strategies employed in the method. These are: (1) incorporation of interfacial momentum force terms in the control volume face flux reconstruction, (2) coupled solution of the discrete linearized system of four constituent field equations for each transport scalar, (3) a consistent pressure-velocity correction scheme which properly accounts for drag and mass transfer, (4) an additive correction strategy for efficient solution of the mixture continuity and coupled field continuity equations, (5) an implicit source term treatment for volume fraction equations which ensures realizability of volume fraction fields during the course of iteration, (6) coupling of the phasic continuity and compatibility equations within the framework of a pressure–volume fraction-velocity correction scheme. The necessity/effectiveness of these strategies is demonstrated in applications to several test cases. The effectiveness and accuracy of the overall method is demonstrated using results of a three-dimensional analysis of boiling SUVA flow in a vertical coolant passage element.

Three-dimensional fluid mechanics of particulate two-phase flows in U-bend and helical conduits

The results of numerous studies performed to date have shown that the performance of various hydraulic systems can be significantly improved by using curved conduit geometries instead of straight tubes. In particular, the formation of Dean vortices, which enhance the development of centrifugal instabilities, has been identified as a factor behind reducing the near-wall concentration buildup in particulate flow devices (e.g., in membrane filtration modules). Still, several issues regarding the effect of conduit curvature on local multidimensional phenomena governing fluid flow still remain open. A related issue is concerned with the impact that conduit geometry makes on the concentration distribution of a dispersed phase in two-phase flows in general, and in particulate flows (solid/liquid or solid/gas suspensions) in particular. It turns out that only very limited efforts have been made in the past to understand the fluid mechanics of such flows via advanced computer simulations. The purpose of this paper...

Coupled DNS/RANS Simulation of Fission Gas Discharge During Loss-of-Flow Accident in Generation IV Sodium Fast Reactor

Abstract The objective of this paper is to give an overview of a multiscale modeling approach to three-dimensional (3-D) two-phase transient computer simulations of the injection of a jet of gaseous fission products into a partially blocked sodium fast reactor (SFR) coolant channel following localized cladding overheat and breach. The phenomena governing accident progression have been resolved at two different spatial and temporal scales by the intercommunicating computational multiphase fluid dynamics codes PHASTA (at direct numerical simulation level) and NPHASE-CMFD (at Reynolds-averaged Navier-Stokes level). The issues discussed in the paper include an overview of the proposed 3-D two-phase-flow models of the interrelated phenomena that occur as a result of cladding failure and the subsequent injection of a jet of gaseous fission products into partially blocked SFR coolant channels and gas-molten-sodium transport along the channels. An analysis is presented on the consistency and accuracy of the models used in the simulations, and the results are shown of the predictions of gas discharge and gas-liquid-metal two-phase flow in a multichannel fuel assembly. Also, a discussion is given of the major novel aspects of the overall work.

On the Modeling of Gas Bubble Evolution and Transport Using Coupled Level-Set/CFD Method

The ability to predict the shape of gas/liquid interface is important for various multiphase flow and heat transfer applications. Specific issues of interest to nuclear reactor thermal hydraulics include the evolution of the shape of bubbles attached to solid surfaces during nucleation, bubble/surface interactions in complex geometries, etc. The development of an innovative approach to model the time-dependent shape of gas/liquid interfaces is discussed. The proposed approach combines a modified level-set method with an advanced computational fluid dynamics code, NPHASE. The coupled numerical solver can be used to simulate the evolution of gas/liquid interfaces in two-phase flows for a variety of geometries and flow conditions. The novel aspects of the work include the development of direct coupling between the level-set algorithm and the finite-volume code NPHASE, the development of a novel mass conservation algorithm for the level-set method, the analysis of the influence of fluid physical properties on the predicted bubble flow conditions, and the use of a three-dimensional model to simulate gas bubble flow in channels of various geometries and orientations.

Multidimensional Model of Fluid Flow and Heat Transfer in Generation-IV Supercritical Water Reactors

This paper is concerned with the mechanistic modeling and theoretical/computational analysis of flow and heat transfer in future Generation-IV Supercritical Water Cooled Reactors (SCWR). The issues discussed in the paper include: the development of analytical models of the properties of supercritical water, and the application of full three-dimensional computational modeling framework to simulate fluid flow and heat transfer in SCWRs. Several results of calculations are shown, including the evaluation of water properties (density, specific heat, thermal conductivity, viscosity, and Prandtl number) near the pseudo-critical temperature for various supercritical pressures, and the CFD predictions using the NPHASE computer code. It is demonstrated that the proposed approach is very promising for future mechanistic analyses of SCWR thermal-hydraulics and safety.Copyright

DEVELOPMENT AND VALIDATION OF A MULTIFIELD MODEL OF CHURN-TURBULENT GAS/LIQUID FLOWS

The accuracy of numerical predictions for gas/liquid two-phase flows using Computational Multiphase Fluid Dynamics (CMFD) methods strongly depends on the formulation of models governing the interaction between the continuous liquid field and bubbles of different sizes. The purpose of this paper is to develop, test and validate a multifield model of adiabatic gas/liquid flows at intermediate gas concentrations (e.g., churn-turbulent flow regime), in which multiple-size bubbles are divided into a specified number of groups, each representing a prescribed range of sizes. The proposed modeling concept uses transport equations for the continuous liquid field and for each bubble field. The overall model has been implemented in the NPHASE-CMFD computer code. The results of NPHASE-CMFD simulations have been validated against the experimental data from the TOPFLOW test facility. Also, a parametric analysis on the effect of various modeling assumptions has been performed.

On the modeling of dispersed particulate flows using a multified model

Publisher Summary This chapter discusses the selected computational and modeling issues associated with the usage of multifield modeling framework to perform computational fluid dynamics (CFD) simulations of multiphase flows. An approach is proposed to resolve an apparent multifield modeling inconsistency of an induced shear–stress correction term. The implementation of this correction term and other convergence improving algorithms into the NPHASE solver is also discussed. The range of applications and accuracy of predictions of the multifield model strongly depend on the degree to which the closure laws. The results of model testing are shown for dispersed particulate flows in a variety of geometries, such as straight pipes, a U-bend, and a helical tube.