Theoretical simulations of dispersed solid particle behaviour inside a scrubbing pool within the bubble rise region are presented. The goal is to evaluate the decontamination factor of the particles ...during the pool scrubbing process. The basic phenomena of pool scrubbing are described. The setup used for the simulation validation is presented. Then, the boundary and initial conditions of the PECA experiments, which were performed at CIEMAT (Madrid, Spain) and were used for simulations, are presented. The subgrid model for decontamination through transfer of particles from gas bubbles to the surrounding liquid is described. The calculation results are evaluated and compared with the part of the PECA experimental results to which the proposed modelling is applicable.
The present study investigates the behaviour of thermal streaks on a heated foil which is cooled with turbulent flow in a square duct channel. Real-time infrared thermography is used to visualize and ...measure the spacing between the thermal streaks. A stainless-steel foil with a thickness of 25 microns is cooled by water. The experiments were performed in a range of Reynolds numbers from 5000 to 20000 and Prandtl numbers from 3 to 7. The mean temperature, root-mean-square of temperature and autocorrelation function have been calculated and used to measure the average thermal streak spacing and power spectra in the spanwise and streamwise directions. The root mean square temperature was 0.3 °C to 0.5 °C which corresponds to roughly 10% of the mean temperature difference between foil and water. The uncertainty in mean temperature difference and root mean square temperature was around 5% and 10%, respectively. The measured thermal streak spacing was 100 wall unit to 180 wall unit under the present experimental range. The uncertainty in measured thermal streak spacing was around 2.5%. The effects of Reynolds number, Prandtl number and heat flux on the thermal streak spacing and also on the statistics of the temperature field have been presented and discussed in this paper. A new correlation has been proposed to predict the dimensionless thermal streak spacing. The error in the prediction is estimated within ± 15 %.
Subcooled flow boiling was experimentally investigated in a horizontal annulus with a temperature-controlled boiling surface and transparent outer pipe facilitating visualization. Boiling occurs on a ...copper tube with a diameter of 12 mm in an annulus with a 2 mm gap. Refrigerant R245fa is used as a working fluid. The focus of this study is to explore the effect of heat flux variation on the boiling flow patterns at approximately constant inlet flow conditions of the working fluid (fixed mass flux and inlet fluid temperature). Subcooled flow boiling is recorded by a high-speed camera, images are analyzed by a neural network to determine the bubble size distributions and their variation with the heat flux. The experimental setup being a part of the laboratory THELMA (Thermal Hydraulics experimental Laboratory for Multiphase Applications) at the Reactor Engineering Division of Jožef Stefan Institute, analysis methods and measurement results are presented and discussed.
► Simulating the onset and propagation of intergranular cracking. ► Model based on the as-measured geometry and crystallographic orientations. ► Feasibility, performance of the proposed computational ...approach demonstrated.
Development of advanced models at the grain size scales has so far been mostly limited to simulated geometry structures such as for example 3D Voronoi tessellations. The difficulty came from a lack of non-destructive techniques for measuring the microstructures. In this work a novel grain-size scale approach for modelling intergranular stress corrosion cracking based on as-measured 3D grain structure of a 400μm stainless steel wire is presented. Grain topologies and crystallographic orientations are obtained using a diffraction contrast tomography, reconstructed within a detailed finite element model and coupled with advanced constitutive models for grains and grain boundaries. The wire is composed of 362 grains and over 1600 grain boundaries. Grain boundary damage initialization and early development is then explored for a number of cases, ranging from isotropic elasticity up to crystal plasticity constitutive laws for the bulk grain material. In all cases the grain boundaries are modeled using the cohesive zone approach. The feasibility of the approach is explored.
•LES was conducted to obtain the mean flow characteristics of multiple impinging jets.•Results were validated on hexagonal configuration benchmark.•Good agreement with experimental data (key flow ...phenomena are predicted correctly).•Minor discrepancies due to the modelling of inlet conditions were identified.
The highly turbulent flow of 13 air impinging jets in hexagonal arrangement is analyzed numerically by the means of Large Eddy Simulation (LES). All important flow phenomena, i.e. the formation of the fountain flow as well as the negative production of normal stresses near target wall are successfully predicted by the simulation. The mean velocity field and turbulent stresses are validated against existing experimental data. Numerical results show good agreement with experiment. Minor discrepancies can be attributed to the inaccurate modeling of the inlet boundary conditions. A thorough comparison between the simulation results and experimental data is presented and discussed.
•Alternative approach to cohesive elements is proposed: cohesive-zone contact.•Applicability to measured and simulated grain structures is demonstrated.•Normal and normal/shear separation as a damage ...initialization is explored.•Normal/shear damage initialization significantly reduces ductility.•Little difference in Voronoi aggregate size on macroscopic response.
Understanding and controlling early damage initiation and evolution are amongst the most important challenges in nuclear power plants, occurring in ferritic, austenitic steels and nickel based alloys. In this work a meso-scale approach to modeling initiation and evolution of early intergranular cracking is presented. This damage mechanism is present in a number of nuclear power plant components and depends on the material (e.g. composition, heat treatment, microstructure), environment and load. Finite element modeling is used to explicitly model the microstructure – both the grains and the grain boundaries. Spatial Voronoi tessellation is used to obtain the grain topology. In addition, measured topology of a 0.4mm stainless steel wire is used. Anisotropic elasticity and crystal plasticity are used as constitutive laws for the grains. Grain boundaries are modeled using the cohesive zone approach. Different modeling assumptions/parameters are evaluated against the numerical stability criteria. The biggest positive contribution to numerical stability is the use of cohesive-type contact instead of cohesive elements. A small amount of viscous regularization should be also used along with the addition of a small amount of viscous forces to the global equilibrium equations. Two cases of grain boundary damage initiation are explored: (1) initiation due to normal separation and (2) initiation due to a combination of normal and shear separation. The second criterion significantly decreases the ductility of an aggregate and slightly improves the numerical stability.
The test and maintenance activities are conducted in the nuclear power plants in order to prevent or limit failures resulting from the ageing or deterioration. The components and systems are ...partially or fully unavailable during the maintenance activities. This is especially important for the safety systems and corresponding equipment because they are important contributors to the overall nuclear power plant safety. A novel method for optimization of the maintenance activities in the nuclear power plant considering the plant safety is developed and presented. The objective function of the optimization is the mean value of the selected risk measure. The risk measure is assessed from the minimal cut sets identified in the Probabilistic Safety Assessment. The optimal solution of the objective function is estimated with genetic algorithm. The proposed method is applied on probabilistic safety analysis model of the selected safety system of the reference nuclear power plant. Obtained results show that optimization of maintenance decreases the risk and thus improves the plant safety. The implications of the consideration of different constraints on the obtained results are investigated and presented. The future prospects for the optimization of the maintenance activities in the nuclear power plants with the presented method are discussed.
Stress tests performed in Europe after accident at Fukushima Daiichi also required evaluation of the consequences of loss of safety functions due to station blackout (SBO). Long-term SBO in a ...pressurized water reactor (PWR) leads to severe accident sequences, assuming that existing plant means (systems, equipment, and procedures) are used for accident mitigation. Therefore the main objective was to study the accident management strategies for SBO scenarios (with different reactor coolant pumps (RCPs) leaks assumed) to delay the time before core uncovers and significantly heats up. The most important strategies assumed were primary side depressurization and additional makeup water to reactor coolant system (RCS). For simulations of long term SBO scenarios, including early stages of severe accident sequences, the best estimate RELAP5/MOD3.3 and the verified input model of Krško two-loop PWR were used. The results suggest that for the expected magnitude of RCPs seal leak, the core uncovery during the first seven days could be prevented by using the turbine-driven auxiliary feedwater pump and manually depressurizing the RCS through the secondary side. For larger RCPs seal leaks, in general this is not the case. Nevertheless, the core uncovery can be significantly delayed by increasing RCS depressurization.
► DNS database for turbulent channel flow at Prandtl number 0.01 and various Reτ. ► Two ideal boundary condition analyzed: non-fluctuating and fluctuating temperature. ► DNS database with conjugate ...heat transfer for liquid sodium–steel contact. ► Penetration of the turbulent temperature fluctuations into the solid wall analyzed.
Direct Numerical Simulation (DNS) of the fully developed velocity and temperature fields in a turbulent channel flow coupled with the unsteady conduction in the heated walls was carried out. Simulations were performed with passive scalar approximation at Prandtl number 0.01, which roughly corresponds to the Prandtl number of liquid sodium. DNSs were performed at friction Reynolds numbers 180, 395 and 590. The obtained statistical quantities like mean temperatures, profiles of the root-mean-square (RMS) temperature fluctuations for various thermal properties of wall and fluid, and various wall thicknesses were obtained from a pseudo-spectral channel-flow code. Even for the highest implemented Reynolds number the temperature profile in the fluid does not exhibit log-law region and the near-wall RMS temperature fluctuations show Reynolds number dependence. Conjugate heat transfer simulations of liquid sodium–steel system point to a relatively intensive penetration of turbulent temperature fluctuations into the heated wall. Database containing the results is available in a digital form.
•The paper demonstrates the initiation and evolution of the intergranular cracking using cohesive zone in a 3D model.•Analytical expression on viscous regularization effect in cohesive element ...presented, recommendations on usage provided.•Statistical evaluation of normal grain boundary stress to applied load ratio for large polycrystalline aggregates.•The effect of increased grain boundary stiffness evaluated, resulting in improved stability.
A cohesive element approach to modelling the intergranular cracking on the grain level is presented. The cohesive elements with zero physical thickness are directly inserted between the adjacent grains. These are obtained using 3D Voronoi tessellation. The grain boundary with the highest normal stress has on average 50% higher normal stress compared to the applied external load, which in this work is equal to half yield stress. Setting the maximum traction of a cohesive element equal to yield stress therefore results in limited grain boundary damage. Most of the early damage initiates at intersections of grain boundaries with the external surfaces of the model. A smaller fraction of damage develops inside the model, mostly at the triple lines where several grain boundaries come together.
A novelty in this work is the developed analytical expression for assessing the cohesive element response when using viscous regularization for alleviating the convergence issues. It shows that the viscous regularization should not be higher than 10% of the step time. Otherwise the delay in the damage evolution and the area underneath the traction-separation response of a cohesive element increase considerably. This is also demonstrated on a model with 500 grains where the time-evolution of the displacements in the cohesive elements is significantly more complicated compared to the linear assumption in the theoretical expression. Also, the fraction of the elastic displacements compared to the total displacements at complete failure plays a role, with a higher elastic part resulting in smaller increase of the maximum traction in the cohesive element.
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