Andreas G. Class

CtFD Analysis of HTS Current Lead Fin-Type Heat Exchanger for Fusion Applications

We apply a recently proposed Computational thermal Fluid Dynamics (CtFD) strategy to the analysis of a meander-flow path (MF) fin-type heat exchanger (HX), to be used in the HTS current leads for the LTS coils of both the W7-X stellarator and the JT-60SA tokamak. A mock-up of the HX was tested at the Karlsruhe Institute of Technology providing the database for the validation of the computational model. The hydraulic characterization of the mock-up is considered first, and then the heat transfer characteristic is analyzed.

Investigation of a Heavy Liquid Metal Free Surface Target Experiment

Several distinct reactor strategies are proposed within the context of the IP Eurotrans framework programme for the transmutation of nuclear waste. A pool type reactor filled with liquid heavy metal and containing a subcritical core is one of the promising designs. Additional neutrons required for the nuclear reaction are generated by a spallation reaction inside the core. A high power proton beam is guided through a vacuum tube from an accelerator into the liquid heavy metal pool i.e. into the reactor core. At the point where the beam hits the metal surface special construction effort is indispensable to handle the high heat production.A specific target design is used to ensure a high fluid velocity and a stable surface at the beam entry. This design employs a concentric vertical feeder establishing a free conical surface with velocity up to 2.5 m/s ensuring stable surface flow and appropriate heat removal. The proposed target geometry has been constructed using underlying rules developed by the MYRRHA design group for the free surface target in the MYRRHA research reactor.A full scale model of this design using lead bismuth eutectic (LBE) has been set up and experimentally investigated at the of the Karlsruhe Institute of Technology (KIT). Measurements taken by high speed digital imaging visualize both conical inner and outer jet free surface. They show a stable surface in a wide range of operating conditions starting from 35% of the nominal flow rate and agree well with numerical investigations using commercial CFD code Star-CD and Star-CCM+. Previous concerns related to splashing or cavitation during the start-up or shut-down procedure proofed unjustified.Copyright

Anisotropic Porosity Formulation of the Coarse-Grid-CFD (CGCFD)

Computational fluid dynamics (CFD) simulations for large complex geometries such as a complete reactor core of a nuclear power plant requires exceedingly huge computational resources. State of the art computational power and CFD software is restricted to simulations of representative sections of these geometries. The conventional approach to simulate such complex geometries is 1D subchannel analysis employing experimental correlations in the transport models. With the development of the Coarse-Grid-CFD (CGCFD) ( [1], [4], [7]), an alternative to the traditional 1D subchannel analysis becomes available which does not need empirical correlations nor specific model constants. The CGCFD approach is based on strongly under-resolved CFD and the inviscid Euler equations. Although the use of the Euler equations and coarse grids does not resolve the subgrid physics like viscous dissipation or turbulence, the subgrid physical information is taken into account by volumetric source terms derived from fully resolved representative CFD simulations. Non-resolved geometrical information due to the use of very coarse meshes is taken into account by volume porousi-ties and directional surface permeabilities in the finite volume scheme. The volume porosity is defined as the ratio of the control volume that is occupied by the fluid compared to the complete control volume. The surface permeability is defined as the ratio of the individual control surface that is unobstructed to fluid flow compared to the corresponding complete control surface. Due to the use of the volumetric source terms that are derived from fully resolved CFD simulations, distributed resistance used in standart porous media approaches may be omitted and instead be resolved through the volumetric source terms. This is advantageous because the friction factor normally is not very well known in thermal-hydraulic problems and must be derived from comprehensive experiments. Such an anisotropic porosity formulation was originally used in the COMMIX [6] code which was designed to compute complex flow applications in a time when computational resources were limited. The benefits and limitations of our technique are explored by simulating a section of a water rod bundle containing a spacer. General recommendations for the proper application of our technique are presented in this work.© 2012 ASME

Tomographische Laser-Doppler-Anemometrie (TLDA) – ein neues Verfahren zur Steigerung der Ortsauflösung (Tomographic Laser Doppler Anemometry (TLDA) – a New Method to increase Spatial Resolution)

Bei der Geschwindigkeitsmessung mittels Laser-Doppler-Anemometrie (LDA) kann das Messvolumen meist als punktförmig betrachtet werden. Einige Anwendungen erfordern eine noch höhere örtliche Auflösung, die mit konventionellen Methoden nicht verbessert werden kann. In dieser Arbeit wird ein Algorithmus vorgestellt, mit dem die örtliche Auflösung des Messvolumens durch ein tomographisches Verfahren deutlich gesteigert werden kann. Anhand künstlich generierter Geschwindigkeitsfelder und Partikelverteilungen werden Messwerte für die Verifikation generiert. Das Verfahren eignet sich für optische Messtechniken, bei denen die Messgröße diskret verteilt vorliegt. In some applications the finite size of the probe volume of a Laser Doppler Anemometry (LDA) system limits its spatial resolution. Conventional methods to increase resolution often fail because of geometrical constraints. In this work a tomographic algorithm is presented to increase the spatial resolution of the LDA. The integral information on velocity and particle density within the probe volume is analyzed to obtain sub-volume resolution, which is limited by the increment of traversing the probe volume through the flow field. Employing an artificial velocity field the algorithm is verified and demonstrates its applicability to other measurement techniques with discrete measurand distribution.

Extension of the MELCOR code for analysis of late in-vessel phase of a severe accident

The simulation of severe accidents in nuclear power plants with system codes is a powerful tool to improve the safety measures to prevent severe accidents. The further development of severe accident codes is part of current research. MELCOR, as the leading nuclear safety code, provides the possibility to be coupled to other codes. A detailed knowledge of this coupling interface is necessary to use this possibility. Therefore, the software tool DINAMO, which contains the coupling routines and an interface to communicate with other programs, was developed. Using DINAMO it is possible to utilize new models for specific phenomena in MELCOR. In the present work the Phase-Change Effective Convectivity Model was coupled using the CFD-software OpenFOAM and DINAMO to MELCOR to improve the prediction of molten core material in the lower plenum of a reactor pressure vessel. The simulation results were compared to the experimental findings of the LIVE-facility.

A hard optimisation test function with symbolic solution visualisation for fast interpretation by the human eye

We propose a class of test problems for evaluating the performance of global function optimisers based on finding an optimal spatial distribution of nonidentical particles interacting with two different potential fields. Because of the possibility of intuitive solution visualisation it can be of particular benefit during development of optimisation algorithms. An ensemble of N particles is constrained to a low-dimensional space and each particle contributes in two ways to the total potential energy: by its position on a hilly track and through repulsive neighbour potentials. The task of minimising the ensembles total potential energy corresponds to searching an N-dimensional space with many local minima separated through higher and lower barriers; hence, it can serve as a performance measure for evolutionary algorithms (EA). The search difficulty is scalable through the number of particles and the hilliness of the track. In particular, if the particles are made nonidentical by giving them different masses or charges, the search will become very challenging because of the introduced combinatorial aspect and the “curse of dimensionality”. Among many similarly challenging optimisation problems this test function class has the advantage that solution candidates can be plotted in ways which allow humans to estimate not only relative objective function values but also DNA vector relations upon a quick glance. For the EA developer this allows a fast feedback cycle between a modification to the EA and the observed change in optimisation history behaviour. This makes experimentation with EA elements at a fundamental level easier. Furthermore, this class of real-domain search offers a wide range of difficulty and complexity levels and can be split up into a two-objective optimisation.

Verification of Nonlinear Proper Orthogonal Decomposition Reduced Order Modeling for BWR Fuel Assemblies

Thermal-hydraulic coupling between power, flow rate and density, intensified by neutronics feedback are the main drivers of boiling water reactor (BWR) stability behavior. Studying potential power oscillations require focusing on BWR operation at high-power low-flow conditions interacting with unfavorable power distribution. Current design rules assure admissible operation conditions by exclusion regions determined by numerical calculations and analytical methods.Analyzing an exhaustive parameter space of the non-linear BWR system becomes feasible with methodologies based on reduced order models (ROMs) saving computational cost and improving the physical understanding. A general reduction technique is given by the proper orthogonal decomposition (POD).Model-specific options and aspects of the POD-ROM-methodology are considered. A first verification is illustrated by means of a chemical tubular reactor (TR) setup. Experimental and analytical results for natural convection in a closed circuit (NCC) [1, 2] serve as a second verification example. This setup shows a strongly non-linear character. The implemented model is validated by means of a linear stability map. Transient behavior of the NCC-POD-ROM can not only reproduce the input data but rather predict different states.Copyright