Eckart Laurien

Three-dimensional numerical simulation of flow and heat transport in high-temperature nuclear reactors

In next generation nuclear High-Temperature Reactors an annular nuclear core consisting of a central column of graphite spheres and a surrounding ring of fuel pebbles is employed. Due to the complex feeding and shutdown mechanisms three-dimensional effects of heat production, gas flow and heat transport may become important for safety analysis. To simulate flow and heat transport in the core and the surrounding graphite reflector a new code system based on CFX-4 has been developed and run on the NEC-SX4 and NEC-SX5 supercomputers. The simulations are performed with the Heterogeneous Model of porous media. The program has been verified by comparison with two-dimensional simulations of the HTR-MODUL using the well established thermal analysis code THERMIX. A sensitivity study of several models for pressure drop and heat transfer on a simplified model of an HTR-MODUL is performed. Additionally the influence of the variation of the volume porosity near walls on flow and heat transport is analysed. In order to demonstrate the simulation of three-dimensional effects the influence of a package of fuel pebbles located asymmetrically in the central column is investigated. A significant influence on the temperature distribution and the maximum temperature core is found.

Entropy Generation in the Viscous Parts of Turbulent Boundary Layers

The local (pointwise) entropy generation rate per unit volume S is a key to improving many energy processes and applications. Consequently, in the present study, the objectives are to examine the effects of Reynolds number and favorable streamwise pressure gradients on entropy generation rates across turbulent boundary layers on flat plates and—secondarily—to assess a popular approximate technique for their evaluation. About two-thirds or more of the entropy generation occurs in the viscous part, known as the viscous layer. Fundamental new results for entropy generation in turbulent boundary layers are provided by extending available direct numerical simulations. It was found that, with negligible pressure gradients, results presented in wall coordinates are predicted to be near “universal” in the viscous layer. This apparent universality disappears when a significant pressure gradient is applied; increasing the pressure gradient decreases the entropy generation rate. Within the viscous layer, the approximate evaluation of S differs significantly from the “proper” value but its integral, the entropy generation rate per unit surface area S, agrees within 5% at its edge.

Quenching Experiments: Coolability of Debris Bed

Abstract In the case of a severe accident, continuous unavailability of cooling water to the core will result in overheating and subsequent meltdown of the fuel elements that would eventually result in the loss of fuel integrity. Under such conditions a porous structure, which is made of heat-generating particles of different sizes and shapes, may be formed. The presence of decay heat in such a debris bed poses a critical threat to the reactor pressure vessel (RPV). To avoid any damage to the RPV, the removal of decay heat from the debris bed is of great importance. The debris bed needs to be quenched by water either flooding from the top or flooding from the bottom until continuous cooling is established. To investigate the quenching behavior of the debris bed by means of experiments, the nonnuclear test facility “DEBRIS” has been established at Institut für Kernenergetik und Energiesysteme (IKE). Experimental investigations of quenching behavior for a preheated debris bed, at various initial bed temperatures, are carried out at IKE. In the new quenching tests, the cooling-down behavior of a superheated polydispersed particle bed from stainless steel spheres at different thermohydraulic conditions has been investigated. Numerical investigation with IKE’s MEWA-2D code has also been carried out for the quenching experiments in order to promote better understanding of the experimental results as well as to verify the code’s applicability to the quenching process.

An Aspect-Oriented Approach for the Development of Complex Simulation Software.

We propose an aspect-oriented approach for the development of simulation software aiming at increasing the exibility, the rapidity of development, and maintainability of simulation software. The horizontal decomposition method is used to separate the core functionality of the simulation application from simulation-specic cross-cutting concerns like distribution, tool integration, persistence, and fault tolerance. We analyze an existing dispersion simulation application to demonstrate the applicability of our approach and provide a proof of concept in form of the aspect-oriented implementation of two cross-cutting concerns, namely distribution and tool integration.

An Aspect-Oriented Approach for Disaster Prevention Simulation Workflows on Supercomputers, Clusters, and Grids

Computer simulation is an important factor intoday’s disaster prevention procedures. Simulation codes assessthe evolution and impact of various physical phenomena indomains such as nuclear and environmental sciences, andultimately help saving lives. However, new and more computationallydemanding models, and new regulations for personneltraining have increased the demand for computational power.While the existing simulation codes can be ported to computingenvironments that can meet the new demand, suchas supercomputers, clusters, and grids, it is too expensiveand time-consuming to rewrite and re-certify them. Instead,in this work we propose an aspect-oriented approach thattakes existing simulation functionality and combines it withfunctionality required to run the simulation on different computingenvironments transparently to the simulation developer.Through experiments in the DAS-3 multi-cluster grid we showthat our approach increases the reusability, the maintainability,the scalability, and the robustness of a real disaster preventionsimulation, while incurring a low performance overhead.

Experimental and Numerical Investigation of Gravity-Driven Pipe Flow With Cavitation

Gravity driven pipe flows contain no risk of pump failure and are considered to be reliable even under accident conditions. However, accurate prediction methods are only available for single phase flow. In case of the occurrence of two-phase flow (caused e.g. by boiling or cavitation), a considerable reduction in mass flux can be observed. In this study, an experimental and numerical investigation of gravity driven two-phase pipe flow was performed in order to understand and model such flows. An experiment was conducted to analyse gravity driven flow of water near saturation temperature in a complex pipe consisting of several vertical and horizontal sections. The diameter was 100 mm with a driving height of 13 m between an elevated tank and the pipe outlet. The experiment shows that cavitation leads to formation of steam. The two-phase character of the flow causes a significant reduction of mass flux in comparison to a single phase flow case. The experimental flow rate was reproduced by one dimensional single and two phase flow analysis based on standard one dimensional methods including models for steam formation. The main part of this study consists of a three dimensional CFD analysis of the two phase flow. A three dimensional model for cavitation and recondensation phenomena based on thermal transport processes was developed, implemented and validated against our experimental data. Due to the fact that beside bubbly flow, also the stratified and droplet flow regimes occur, a new approach to model phase interaction terms of the Two-Fluid Model for mass, momentum and energy is presented. Thereby, the transition from one flow regime to another is taken into account. The experimental mass flow rate can be predicted with an accuracy of 10%. The three dimensional analysis of the flow situation demonstrates the influence of pipe elements such as horizontal and vertical sections, bends and valves of the pipe on the mass flux and the steam distribution. The analysis of secondary flows in bends emphases their importance for the steam distribution within the pipe, for the pressure loss and the average mass flux.Copyright

Direct Numerical Simulation of Heated Turbulent Pipe Flow at Supercritical Pressure

For fluids at supercritical pressure, the phase change from liquid to gas does not exist. In the meanwhile, the fluid properties change drastically in a narrow temperature range. With supercritical fluid as working fluid in a heated pipe, heat transfer deterioration and recovery has been observed, which correspond to the turbulent flow relaminarization and recovery. Direct Numerical Simulation (DNS) of supercritical carbon dioxide flow in a heated vertical circular pipe at a pressure of 8 MPa is developed with the open-source code OpenFOAM in the present study. Forced convection cases and mixed convection including upward and downward flow have been considered in the simulation. The change of the mean flow and turbulence statistics has been analysed in detail. In the forced convection, flow turbulence is attenuated due to acceleration from thermal expansion, which leads to a peak of the wall temperature. Buoyancy has a stronger impact to the flow. In the upward flow, the average streamwise velocity distribution turns into an M-shape profile because of the ‘external’ effect of buoyancy. Besides that, negative buoyancy production caused by the density variation reduces the Reynolds shear stress to almost zero, which means that the flow is relaminarized. Further downstream turbulence is recovered. This behaviour of flow turbulence is confirmed by visualization of turbulent streaks and vortex structures. The observation of the flow turbulence of this can help to develop advanced turbulence models for applications in nuclear or conventional energy generation technologies.

Investigation of Convective Heat Transfer to Supercritical Carbon Dioxide with Direct Numerical Simulation

For fluids at supercritical pressure, the phase change from liquid to gas does not exist. In the meanwhile, the fluid properties change drastically in a narrow temperature range. With supercritical fluid as working fluid in a heated pipe, heat transfer deterioration and recovery have been observed, which correspond to the turbulent flow relaminarization and recovery. Direct Numerical Simulation (DNS) of supercritical carbon dioxide flow in a heated vertical circular pipe at a pressure of 8 MPa is developed with the open-source code OpenFOAM in the present study. Forced convection cases and mixed convection including upward and downward flow have been considered in the simulation. The change of the mean flow and turbulence statistics has been analysed in detail. In the forced convection, flow turbulence is attenuated due to acceleration from thermal expansion, which leads to a peak of the wall temperature. Buoyancy has a stronger impact to the flow. In the upward flow, the average streamwise velocity distribution turns into an M-shape profile because of the “external” effect of buoyancy. Besides that, negative buoyancy production caused by the density variation reduces the Reynolds shear stress to almost zero, which means that the flow is relaminarized. Further downstream turbulence is recovered. This behaviour of flow turbulence is confirmed by visualization of turbulent streaks and vortex structures. The observation of the flow turbulence of this can help to develop advanced turbulence models for applications in nuclear or conventional energy generation technologies.

3D Numerical Simulation of Flow with Volume Condensation in Presence of Non-condensable Gases Inside a PWR Containment

One severe accident scenario in a Pressurized Water Reactor (PWR) is a leak in the primary circuit of a reactor resulting in hydrogen and steam injection into the containment. The steam-air-hydrogen mixture could reach the conditions for deflagration of combustion or local detonations. Because of the influence of steam condensation on the gas mixing and hydrogen stratification in the containment, the wall and volume condensation phenomena are of interest for the safety considerations. The wall condensation model is available in the CFD code ANSYS CFX. This paper presents a newly developed volume condensation model in the presence of non-condensable gases with two-phase flow for the ANSYS CFX code. The two-fluid model is applied with a continuous gas phase consisting of a steam-air-light gas mixture, and a dispersed liquid phase composed of water droplets. Both phases are modeled with separate temperatures and velocities. The motion of the droplets due to gravitational force is considered. Volume condensation is modeled as a sink of mass and source of energy at the droplet interfaces. The newly developed volume condensation model is validated with a condensation experiment TH13, which was performed in the German THAI facility within the OECD/NEA International Standard Problem (ISP-47). Finally, in order to predict the local hydrogen behavior within a real containment during a severe accident, the containment flow was simulated at the time of accident in a ‘Generic Containment’, which was developed based on a German PWR.

Large-Eddy Simulations of Stratified and Non-stratified T-junction Mixing Flows

Thermal mixing of coolants with large temperature differences in cooling circuits of power plants may lead to high cycle thermal fatigue in the pipe material. In these mixing regions cracks are often observed in the vicinity of weld seams. In this study the influence of a weld seam in straight pipe flows, isothermal T-junction mixing flows and stratified T-junction mixing flows are investigated with Large-Eddy Simulations (LES). The results are compared to experimental data. Furthermore, T-junction flows with large temperature differences, which are based on an experimental setup at the Institute of Nuclear Energy and Energy Systems (IKE), are numerically investigated and characterized.