Eckhard Krepper

Evolution of the two-phase flow in a vertical tube: decomposition of gas fraction profiles according to bubble size classes using wire-mesh sensors

The wire-mesh sensor developed by the Forschungszentrum Rossendorf produces sequences of instantaneous gas fraction distributions in a cross section with a time resolution of 1200 frames per second and a spatial resolution of about 2–3 mm. At moderate flow velocities (up to 1–2 m·s−1), bubble size distributions can be obtained, since each individual bubble is mapped in several successive distributions. The method was used to study the evolution of the bubble size distribution in a vertical two-phase flow. For this purpose, the sensor was placed downstream of an air injector, the distance between air injection and sensor was varied. The bubble identification algorithm allows to select bubbles of a given range of the effective diameter and to calculate partial gas fraction profiles for this diameter range. In this way, the different behaviour of small and large bubbles in respect to the action of the lift force was observed in a mixture of small and large bubbles.

Prediction of radial gas profiles in vertical pipe flow on the basis of bubble size distribution

A method for the prediction of the radial gas profile for a given bubble size distribution is presented. It is based on the assumption of the equilibrium of the forces acting on a bubble perpendicularly to the flow direction. These forces strongly depend on the bubble size [14, 18]. For the simulation of transient flow regime effects, the modelling of several bubble classes in a 1D model and consideration of their radial profiles seems to be more promising than a detailed 3D modelling. The radial profile of the liquid velocity is calculated by the model of Sato [21, 22]. On the basis of this velocity profile, radial distributions are calculated separately for all bubble classes according to the given bubble size distribution. The sum of these distributions is the radial profile of the gas fraction. It is used in an iteration process to calculate a new velocity profile. There is a strong interaction between the profiles of liquid velocity and gas volume fraction. The model is the basis of a fast running one-dimensional steady state computer code. The results are compared with experimental data obtained for a number of gas and liquid volume flow rates. There is a good agreement between experimental and calculated data. In particular, the change from wall peaking to centre peaking gas fraction distribution is well predicted.

Review of Available Data for Validation of Nuresim Two-Phase CFD Software Applied to CHF Investigations

The NURESIM Project of the 6th European Framework Program initiated the development of a new-generation common European Standard Software Platform for nuclear reactor simulation. The thermal-hydraulic subproject aims at improving the understanding and the predictive capabilities of the simulation tools for key two-phase flow thermal-hydraulic processes such as the critical heat flux (CHF). As part of a multi-scale analysis of reactor thermal-hydraulics, a two-phase CFD tool is developed to allow zooming on local processes. Current industrial methods for CHF mainly use the sub-channel analysis and empirical CHF correlations based on large scale experiments having the real geometry of a reactor assembly. Two-phase CFD is used here for understanding some boiling flow processes, for helping new fuel assembly design, and for developing better CHF predictions in both PWR and BWR. This paper presents a review of experimental data which can be used for validation of the two-phase CFD application to CHF investigations. The phenomenology of DNB and Dry-Out are detailed identifying all basic flow processes which require a specific modeling in CFD tool. The resulting modeling program of work is given and the current state-of-the-art of the modeling within the NURESIM project is presented.

Application of CFD Codes in Nuclear Reactor Safety Analysis

Computational Fluid Dynamics (CFD) is increasingly being used in nuclear reactor safety (NRS) analyses as a tool that enables safety relevant phenomena occurring in the reactor coolant system to be described in more detail. Numerical investigations on single phase coolant mixing in Pressurised Water Reactors (PWR) have been performed at the FZD for almost a decade. The work is aimed at describing the mixing phenomena relevant for both safety analysis, particularly in steam line break and boron dilution scenarios, and mixing phenomena of interest for economical operation and the structural integrity. For the experimental investigation of horizontal two phase flows, different non pressurized channels and the TOPFLOW Hot Leg model in a pressure chamber was build and simulated with ANSYS CFX. In a common project between the University of Applied Sciences Zittau/Gorlitz and FZD the behaviour of insulation material released by a LOCA released into the containment and might compromise the long term emergency cooling systems is investigated. Moreover, the actual capability of CFD is shown to contribute to fuel rod bundle design with a good CHF performance.

CFD Simulation of Polydispersed Bubbly Two-Phase Flow around an Obstacle

This paper concerns the model of a polydispersed bubble population in the frame of an ensemble averaged two-phase flow formulation. The ability of the moment density approach to represent bubble population size distribution within a multi-dimensional CFD code based on the two-fluid model is studied. Two different methods describing the polydispersion are presented: (i) a moment density method, developed at IRSN, to model the bubble size distribution function and (ii) a population balance method considering several different velocity fields of the gaseous phase. The first method is implemented in the Neptune_CFD code, whereas the second method is implemented in the CFD code ANSYS/CFX. Both methods consider coalescence and breakup phenomena and momentum interphase transfers related to drag and lift forces. Air-water bubbly flows in a vertical pipe with obstacle of the TOPFLOW experiments series performed at FZD are then used as simulations test cases. The numerical results, obtained with Neptune_CFD and with ANSYS/CFX, allow attesting the validity of the approaches. Perspectives concerning the improvement of the models, their validation, as well as the extension of their applicability range are discussed.

COMPARATIVE SIMULATIONS OF FREE SURFACE FLOWS USING VOF-METHODS AND A NEW APPROACH FOR MULTI-SCALE INTERFACIAL STRUCTURES

This paper presents free surface flow simulations using different VOF-like interface capturing methods. Both the interFoam solver available in OpenFOAM and the Free Surface Model implemented in ANSYS CFX are applied for the collapse of a water column hitting an obstacle. The computational results of these established methods are compared to a new multi-field concept which is developed for flow situations with multi-scale interfacial structures. The new concept extends the inhomogeneous MUltiple SIze Group (MUSIG)-Model for polydispersed flows by adding a large-scale continuous gas phase. It represents the largest gas structures whose filtered gas-liquid interfaces are captured within the computational domain. Adequate interfacial transfer formulations are introduced for area density and drag and allow the use of different closure models depending on the local morphology. By including appropriate models for the mass transfer, transitions between dispersed and continuous gas morphologies can be described. Thus not only gas-liquid interfaces for large gas structures are detected, but also smallscale bubbles that are entrained under the free surface can be described properly taking into account coalescence- and breakup processes. The concept further improves free surface simulations by including sub-grid information about small waves and instabilities at the free surface. Therefore a new treatment of turbulent kinetic energy is applied via source terms at the free surface. The application of this concept to the dambreak-case with an obstacle demonstrates the breakup of a continuous gas phase and the appearance of polydispersed gas. The collapse of the water column is accompanied by trapping of gas which breaks up to smaller structures. The quality of interface detection during the

Implementation of a Pressure Drop Model for the CFD Simulation of Clogged Containment Sump Strainers

The present study aims at modeling the pressure drop of flows through growing cakes of compressible fibrous materials, which may form on the upstream side of containment sump strainers after a loss-of-coolant accident. The model developed is based on the coupled solution of a differential equation for the change of the pressure drop in terms of superficial liquid velocity and local porosity of the fiber cake and a material equation that accounts for the compaction pressure dependent cake porosity. Details of its implementation into a general-purpose three-dimensional computational fluid dynamics code are given. An extension to this basic model is presented, which simulates the time dependent clogging of the fiber cake due to capturing of suspended particles as they pass trough the cake. The extended model relies on empirical relations, which model the change of pressure drop and removal efficiency in terms of particle deposit in the fiber cake.

CFD for Subcooled Flow Boiling: Parametric Variations

We investigate the present capabilities of CFD for wall boiling. The computational model used combines the Euler/Euler two-phase flow description with heat flux partitioning. Very similar modeling was previously applied to boiling water under high pressure conditions relevant to nuclear power systems. Similar conditions in terms of the relevant nondimensional numbers have been realized in the DEBORA tests using dichlorodifluoromethane (R12) as the working fluid. This facilitated measurements of radial profiles for gas volume fraction, gas velocity, liquid temperature, and bubble size. Robust predictive capabilities of the modeling require that it is validated for a wide range of parameters. It is known that a careful calibration of correlations used in the wall boiling model is necessary to obtain agreement with the measured data. We here consider tests under a variety of conditions concerning liquid subcooling, flow rate, and heat flux. It is investigated to which extent a set of calibrated model parameters suffices to cover at least a certain parameter range.

Modeling the evolution of bubbly flow along a large vertical pipe

A detailed experimental database, obtained for a 195-mm inner diameter, 9-m-long pipe was used for the validation of models applied in computational fluid dynamics codes for the simulation of bubbly flow. Since the bubbles were injected via holes at the pipe wall, very useful information on the bubble migration from the pipe wall toward the pipe’s center was obtained by measurements at different distances between gas injection and measuring plane. The bubble migration is determined by the forces acting on the bubbles. The multibubble-size group test solver, introduced earlier but with some new extensions, was used to analyze the data. A comparison of results from a simulation and the experimental findings indicate that the turbulent dispersion force according to the Favre averaged drag model is too strong compared with the drag in the radial direction. No appropriate models for bubble coalescence and breakup, which can be applied for a wide range of gas and liquid volume flow rates, are available as yet. Nevertheless, for selected combinations of volume flow rates, the calculated bubble size distributions and radial gas volume fraction profiles show an acceptable agreement with the experimental data.

Pre- and post-test calculations to natural circulation experiments at the integral test facility ISB-VVER using the thermalhydraulic code ATHLET

Abstract In 1995 at the integral test facility ISB-VVER in Elektrogorsk near Moscow natural circulation experiments were performed, which were scientifically accompanied by the Forschungszentrum Rossendorf. These experiments were the first of this kind at a test facility, which models VVER-1000 thermalhydraulics. Using the code ATHLET which is being developed by ‘Gesellschaft fur Anlagen und Reaktorsicherheit’, pre- and post-test calculations were done to determine the thermalhydraulic events to be expected and to define and tune the boundary conditions of the test. The conditions found for natural circulation instabilities and cold leg loop seal clearing could be confirmed by the tests. Besides the thermalhydraulic standard measuring system, the facility was equipped with needle shaped conductivity probes for measuring the local void fractions.