Henryk Anglart

CFD application to prediction of void distribution in two-phase bubbly flows in rod bundles

Abstract This paper is concerned with the prediction of the void fraction distribution in two-phase bubbly flows in fuel rod bundles. Special attention has been devoted to the phenomena which govern the void fraction distribution in the lateral direction of a channel. A two-fluid model of two-phase flow has been formulated and implemented into a commercial computational fluid dynamics (CFD) code. The model has been used for the prediction of the void distribution in three different channels: a circular channel (inside diameter (ID), 34.5 mm) with a single heated rod of 13.9 mm outside diameter (OD), and circular channels (ID, 71 mm) with six heated rods (13.8 and 13.9 mm OD each). The predicted axial and lateral avoid fraction distributions in subcooled and bulk boiling regions have been area averaged in three lateral zones and compared with experimental data: in all cases, satisfactory agreement between the predictions and measurements has been obtained.

Theoretical and Numerical Study of Heat Transfer Deterioration in High Performance Light Water Reactor

A numerical investigation of the heat transfer deterioration (HTD) phenomena is performed using the low-Re k- turbulence model. Steady-state Reynolds-averaged Navier-Stokes equations are solved together with equations for the transport of enthalpy and turbulence. Equations are solved for the supercritical water flow at different pressures, using water properties from the standard IAPWS (International Association for the Properties of Water and Steam) tables. All cases are extensively validated against experimental data. The influence of buoyancy on the HTD is demonstrated for different mass flow rates in the heated pipes. Numerical results prove that the RANS low-Re turbulence modeling approach is fully capable of simulating the heat transfer in pipes with the water flow at supercritical pressures. A study of buoyancy influence shows that for the low-mass flow rates of coolant, the influence of buoyancy forces on the heat transfer in heated pipes is significant. For the high flow rates, buoyancy influence could be neglected and there are clearly other mechanisms causing the decrease in heat transfer at high coolant flow rates.

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.

Fluid mechanics of Taylor bubbles and slug flows in vertical channels

Abstract Fluid mechanics of Taylor bubbles and slug flows is investigated in vertical, circular channels using detailed, three-dimensional computational fluid dynamics simulations. The Volume of Fluid model with the interface-sharpening algorithm, implemented in the commercial CFX4 code, is used to predict the shape and velocity of Taylor bubbles moving along a vertical channel. Several cases are investigated, including both a single Taylor bubble and a train of bubbles rising in water. It is shown that the potential flow solution underpredicts the water film thickness around Taylor bubbles. Furthermore, the computer simulations that are performed reveal the importance of properly modeling the three-dimensional nature of phenomena governing the motion of Taylor bubbles. Based on the present results, a new formula for the evaluation of bubble shape is derived. Both the shape of Taylor bubbles and the bubble rise velocity predicted by the proposed model agree well with experimental observations. Furthermore, the present model shows good promise in predicting the coalescence of Taylor bubbles.

Phase Distribution in a BWR Fuel Assembly and Evaluation of a Multidimensional Multifield Model

Abstract The phase and mass flux distribution is analyzed in the fuel bundle of a boiling water reactor (BWR). The numerical predictions of phase distribution, obtained with the multifield two-phase flow model implemented in a computational fluid dynamics code, are compared with detailed void measurements. The present model takes into account the detailed geometry of the assembly and the spatial distribution of heat sources. The influence of spacers is modeled by introducing pressure loss and turbulence sources in the momentum and turbulence equations, respectively. The model has been applied for simulation of bubbly two-phase flow for both subcooled and saturated nucleate boiling in a seven-rod bundle and a typical BWR fuel assembly. The predictions are in good agreement with tomographic measurements performed in the FRIGG loop at Westinghouse Atom.

Numerical Predictions of Bubbly Two-Phase Flows with OpenFOAM

A new model for simulation of bubbly two-phase flows has been developed and implemented into an open-source Computational Fluid Dynamics (CFD) code OpenFOAM. The model employs the two-fluid framework with closure relationships for the interfacial momentum transfer. The bubble size is calculated based on the solution of the interfacial area concentration equations. The predictions are validated against a wide range of experimental data containing measured void fraction, the phasic velocity and the interfacial area concentration. The new model demonstrates the ability to capture the wall peaking of void fraction for small bubbles. The predicted levels of void fraction and phasic velocities are in good agreement with measured data.

Simulation on the Flow and Heat Transfer Characteristics of Confined Bubbles in Micro-Channels

3D simulations on confined bubbles in micro-channels with diameter of 1.24 mm were conducted. The working fluid is R134a with a mass flux range from 125kg/m2s to 375kg/m2s. The VOF model is chosen to capture the 2 phase interface while the geo-construction method was used to re-construct the 2-phase interface. A heated boundary wall with heat flux varying from 15kW/m2 to 102kW/m2 is supplied. The wall temperature was calculated. The effects of mass flux and heat flux are studied. The shape of the bubble was predicted by the simulation successfully and the results show that they are independent of the initial shape. Both thin film evaporation and micro convection enhance the heat transfer. However, the micro convection which is caused by bubble motion has greater contribution to the total heat transfer at the stage of bubble growth studied.Copyright

Study of Post Dryout Heat Transfer in Annulus With Flow Obstacles

The purpose of this paper is to present the experimental setup, experimental method and results of the recent post-dryout heat transfer investigations in an annulus with pin spacers. The experiments were performed in the thermal-hydraulic laboratory at the Royal Institute of Technology (KTH), Stockholm, Sweden. The experimental facility has an annular test section which consists of two double-heated concentric tubes manufactured of Inconel 600. Five levels of pin spacers were installed along the test section to keep the rod and the tube equidistant during experiments. The experimental matrix includes measurements of wall temperature distributions for single phase and two-phase flows, for both convective boiling and post-dryout heat transfer regimes. The influence of variations in mass flux (500–1500) kg/(m2 s) and inlet subcooling 10 and 40 K at system pressure of 7 MPa were investigated. The experimental results indicate that post dryout heat transfer is influenced by the pin spacers. In particular it has been observed that the dry patch appearing in the test section can be quenched downstream of the pins-spacer. The current results provide additional experimental database which can be used for validation of post-dryout heat transfer models including the flow obstacle effects.Copyright

BWR steam line and turbine model with multiple piping capability

Abstract A new steam line and turbine model for the prediction of flow transients in main steam supply lines has been developed. The major objectives of the new model are to predict transients in an arbitrary steam line and turbine configuration and to calculate with good accuracy and speed. The model is incorporated in the BISON BWR dynamics program, but it can also be executed as a stand-alone computer code. The steam flow model is based on one-dimensional adiabatic (and optionally isentropic) flow, including spatial acceleration and compressibility effects. In the finite difference form we use the nonconservative form for momentum and energy equation. An implicit time integration scheme is used to eliminate a time-step limitation. The main features of the model are: • - ability to analyse steam single-phase flow transients in any piping network, - models for valves, tees, turbine assembly • - control system interface • - consistent steady-state initialization technique, - restart facility • - time step control algorithm for transient and steady-state solutions. The model has been assessed by comparing numerical results with both analytical solution and experimental data.

Modeling of Spacer Influence on Post-Dryout Heat Transfer in Heated Channels

Post-dryout heat transfer plays an important role in safe and economical operations of Light Water Reactors (LWR). This type of heat transfer is avoided under normal operational conditions of nucle ...