Bojan Niceno

A sharp-interface phase change model for a mass-conservative interface tracking method

A new phase-change model has been developed for a mass-conservative interface tracking method. The mass transfer rate is directly calculated from the heat flux at the liquid-vapor interface, and the phase change takes place only in the cells which include this interface. As a consequence of the sharpness of the mass transfer rate distribution, the velocity jump across the interface can be captured, and high accuracy can be maintained. The method has been implemented in an incompressible Navier-Stokes equations solver employing a projection method based on a staggered finite-volume algorithm on Cartesian grids. The model has been verified for one-dimensional phase-change problems and a three-dimensional simulation of a growing vapor bubble in a superheated liquid under zero gravity condition. The computed results agree with theoretical solutions, and the accuracy of the model is confirmed to be of second-order in space using a grid refinement study. A three-dimensional simulation of a rising vapor bubble in a superheated liquid under gravity has been performed as a validation case, and good agreement with experimental data is obtained for the bubble growth rate. As a demonstration of the applicability of the method to engineering problems, a nucleate boiling simulation is presented with a comparison to experimental data. Good agreement is obtained for the bubble shapes and the bubble departure period. In all the simulation cases, strict mass conservation is satisfied.

A depletable micro-layer model for nucleate pool boiling

A depletable micro-layer model has been developed for the simulation of nucleate pool boiling within the framework of Computational Fluid Dynamics (CFD) modeling using an interface-tracking method. A micro-layer model is required for the CFD simulation to take into account vaporization from the thin liquid film - called the micro-layer - existing beneath a growing vapor bubble on a hot surface. In our model, the thickness of the micro-layer is a variable defined at each discretized fluid cell adjacent to the heat-transfer surface; the layer decreases due to vaporization, and can finally disappear. Compared to existing micro-region models, most of them based on the concept of contact-line evaporation, as originally proposed by Stephan and Busse, and by Lay and Dhir, our model incorporates simplified modeling ideas, but can nonetheless predict the temperature field beneath the growing bubble accurately. The model proposed in this paper has been validated against measurements of pool boiling in water at atmospheric pressure. Specifically, the bubble principal dimensions and the temperature distribution over the heat-transfer surface are in good agreement with experimental data.

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.

Simulation of single-phase mixing in fuel rod bundles, using an immersed boundary method

Over recent decades, the study of turbulent flow structures and the mixing behavior in fuel rod bundles has been an active research topic. In this paper, the (isothermal) turbulent mixing in rod bundles is simulated for two different cases using a finite-volume/immersed boundary method with a staggered velocity discretization. The solution procedure is based on an efficient multigrid algorithm, and simulations are performed on state-of-the-art massively parallel computer architectures. As the first case, the cross-flow through a staggered tube bundle under fully turbulent conditions is computed using large eddy simulation. The computed velocities were compared with the experimental data from the literature, and the agreement between the experimental and the computational results was found to be rather good. As a second case, we considered an experiment performed in the SUBFLOW test facility. The SUBFLOW facility models a 4???4 vertical rod bundle and uses water in atmospheric conditions as the working fluid. Experiments are based on the wire-mesh sensor technique, which measures the conductivity of the working fluid, and report the distribution of the tracer inserted at various heights of the bundle. This case is even more challenging for large eddy simulations, since it does not feature large structures present in the flow crossing the bundles. However, large eddy simulations of the SUBFLOW facility reproduced the concentration of the tracer liquid and its fluctuations well. The good agreement obtained for the two cases of flows in rod bundles serves as a motivation to focus future research on the modeling of bubbly flows in the same arrangements.

MULTI-SCALE MODELING AND ANALYSIS OF CONVECTIVE BOILING

In this paper we describe current activities on the project Multi-Scale Modeling and Analysis of convective boiling (MSMA), conducted jointly by the Paul Scherrer Institute (PSI) and the Swiss Nuclear Utilities (Swissnuclear). The long-term aim of the MSMA project is to formulate improved closure laws for Computational Fluid Dynamics (CFD) simulations for prediction of convective boiling and eventually of the Critical Heat Flux (CHF). As boiling is controlled by the competition of numerous phenomena at various length and time scales, a multi-scale approach is employed to tackle the problem at different scales. In the MSMA project, the scales on which we focus range from the CFD scale (macro-scale), bubble size scale (meso-scale), liquid micro-layer and triple interline scale (micro-scale), and molecular scale (nano-scale). The current focus of the project is on micro- and meso- scales modeling. The numerical framework comprises a highly efficient, parallel DNS solver, the PSI-BOIL code. The code has incorporated an Immersed Boundary Method (IBM) to tackle complex geometries. For simulation of meso-scales (bubbles), we use the Constrained Interpolation Profile method: Conservative Semi-Lagrangian 2nd order (CIP-CSL2). The phase change is described either by applying conventional jump conditions at the interface, or by using the Phase Field (PF) approach. In this work, we present selected results for flows in complex geometry using the IBM, selected bubbly flow simulations using the CIP-CSL2 method and results for phase change using the PF approach. In the subsequent stage of the project, the importance of effects of nano-scale processes on the global boiling heat transfer will be evaluated. To validate the models, more experimental information will be needed in the future, so it is expected that the MSMA project will become the seed for a long-term, combined theoretical and experimental program.

Evaluation of the PAR Mitigation System in Swiss PWR Containment Using the GOTHIC Code

Abstract The installation of passive autocatalytic recombiners (PARs) in the containment of operating nuclear power plants (NPPs) is increasingly based on three-dimensional studies of severe accidents that accurately predict the hydrogen pathways and local accumulation regions in containment and examine the mitigation effects of the PARs on the hydrogen risk. The GOTHIC (Generation Of Thermal-Hydraulic Information for Containments) code is applied in this paper to study the effectiveness of the PARs installed in the Gösgen NPP in Switzerland. A fast release of a mixture of hydrogen and steam from the hot leg during a total station blackout is chosen as the limiting scenario. The PAR modeling approach is qualified simulating two experiments performed in the frame of the OECD/NEA (Organisation for Economic Co-operation and Development/Nuclear Energy Agency) THAI (Thermal-hydraulics, Hydrogen, Aerosols and Iodine) project. The results of the plant analyses show that the recombiners cannot prevent the formation of a stratified cloud of hydrogen (10% molar concentration), but they can mitigate the hydrogen accumulation once formed. In the case of the analyzed fast release scenario, which is characterized by increasing loads with large initial flow rate and high hydrogen concentration values, it is shown that, when a large number of recombiners are installed, the global outcome in relation to the combustion risk does not depend on the details of the single PAR behavior. The hydrogen ignition risk can be fully mitigated in a timeframe ranging from 15 to 30 min after the fast release, according to the dependence of the PAR efficiency model on the adopted parameters.

A Three-Dimensional, Immersed Boundary, Finite Volume Method for the Simulation of Incompressible Heat Transfer Flows around Complex Geometries

The current work focuses on the development and application of a new finite volume immersed boundary method (IBM) to simulate three-dimensional fluid flows and heat transfer around complex geometries. First, the discretization of the governing equations based on the second-order finite volume method on Cartesian, structured, staggered grid is outlined, followed by the description of modifications which have to be applied to the discretized system once a body is immersed into the grid. To validate the new approach, the heat conduction equation with a source term is solved inside a cavity with an immersed body. The approach is then tested for a natural convection flow in a square cavity with and without circular cylinder for different Rayleigh numbers. The results computed with the present approach compare very well with the benchmark solutions. As a next step in the validation procedure, the method is tested for Direct Numerical Simulation (DNS) of a turbulent flow around a surface-mounted matrix of cubes. The results computed with the present method compare very well with Laser Doppler Anemometry (LDA) measurements of the same case, showing that the method can be used for scale-resolving simulations of turbulence as well.

Development of Mass-Conservative Phase-Change Model for Convective Boiling Simulations

A numerical method based on Computational Fluid Dynamics (CFD) has been developed to simulate convective nucleate boiling flows in laminar and turbulent flow regimes. A single set of Navier-Stokes equations is solved based on a staggered finite-volume algorithm on Cartesian grids using Smagorinsky model for sub-grid scale turbulence. A color function is used for the two-phase flow model with a local sharpening scheme [1] to prevent the smearing of the color function, and Brackbill’s Continuum Surface Force (CSF) model [2] is used for the surface tension and the wall adhesion force. A sharp-interface mass-conservative phase change model [3], which can capture the velocity jump accurately, is used for the mass transfer model. A micro-region model developed by Stephan [4] is used to model the mass transfer at the vapor-liquid-solid triple point, and the mass transfer rate and the surface tension force in the micro-region is introduced to the Navier-Stokes solver. The developed scheme is validated against the experiments of convective boiling flows in the horizontal and vertical flow directions [5]. The validation cases considered in this paper are based on a single nucleation site and single bubble growth, in the same approach as Li and Dhir [6], and several assumptions are used for the initial and boundary conditions to complement the limited measured data. Thus, the comparison between the simulation and the experiment may not have the true meaning of validation, but the computed bubble liftoff diameter and time show agreement with experiments under the given conditions, and the applicability of the developed method to the simulation of convective boiling flows is demonstrated.Copyright

Computational Study of Conjugate Heat Transfer in T-Junctions

Thermal fatigue is a relevant problem in the context of life-time extension of nuclear power plants (NPP). In many piping systems in NPPs hot and cold water is mixed, which leads to high temperature fluctuations in the region close to the solid wall and resulting thermal loads on the pipe walls that can cause fatigue. One of the relevant geometric test cases for thermal fatigue is the mixing in T-junctions. In this study we apply large–eddy simulations (LES) to the mixing of hot and cold water in a T-junction. We perform a set of simulations by using different formulations of the LES subgrid scale model, i.e. standard Smagorinsky and dynamic procedure, to identify the influence of the modelled subgrid scales on the simulation results. The results exhibit a large difference between the models, which is caused by the use of turbulent viscosity wall–damping functions when applying the standard model.Copyright