Pool Convection Research Articles

Investigations on flow-regime characteristics during debris bed formation behavior in sodium-cooled fast reactor by releasing high-temperature particles

It is crucial to understand the Debris Bed Formation (DBF) behavior for the improved assessment of Core Disruptive Accident (CDA) of Sodium-cooled Fast Reactors (SFR). To elucidate the DBF flow-regime characteristics, a series of visualization simulated experiments were conducted at Sun Yat-Sen University by releasing solid particles into water pools. Because of different mechanisms of the inter-particle and fluid-particle interactions, four different flow regimes were identified along with four different final bed shapes found. Motivated to comprehensively understand the effect of coolant boiling on flow-regime transitions during the falling of high-temperature debris, new experiments are performed in present work by releasing high-temperature particles under varying parametric conditions. It is found that the boiling generated from falling high-temperature particles can effectively centralize the particle accumulation and diminish the particle-flow-induced pool convection, leading to the potential regime transitions. Based on the detailed experimental analyses and data attained, an extension function for evaluating the boiling effect caused by falling high-temperature particles is successfully developed to incorporate into our base model, whose applicability is examined to be restricted to the predictions under non-boiling condition. Knowledge from present work is useful for future development and verifications of SFR safety analysis models and codes in China.

Scalability of Nek5000 on High-Performance Computing Clusters Toward Direct Numerical Simulation of Molten Pool Convection

In a postulated severe accident, a molten pool with decay heat can form in the lower head of a reactor pressure vessel, threatening the vessel’s structural integrity. Natural convection in molten pools with extremely high Rayleigh (Ra) number is not yet fully understood as accurate simulation of the intense turbulence remains an outstanding challenge. Various models have been implemented in many studies, such as RANS (Reynolds-averaged Navier–Stokes), LES (large-eddy simulation), and DNS (direct numerical simulation). DNS can provide the most accurate results but at the expense of large computational resources. As the significant development of the HPC (high-performance computing) technology emerges, DNS becomes a more feasible method in molten pool simulations. Nek5000 is an open-source code for the simulation of incompressible flows, which is based on a high-order SEM (spectral element method) discretization strategy. Nek5000 has been performed on many supercomputing clusters, and the parallel performance of benchmarks can be useful for the estimation of computation budgets. In this work, we conducted scalability tests of Nek5000 on four different HPC clusters, namely, JUWELS (Atos Bullsquana X1000), Hawk (HPE Apollo 9000), ARCHER2 (HPE Cray EX), and Beskow (Cray XC40). The reference case is a DNS of molten pool convection in a hemispherical configuration with Ra = 1011, where the computational domain consisted of 391 million grid points. The objectives are (i) to determine if there is strong scalability of Nek5000 for the specific problem on the currently available systems and (ii) to explore the feasibility of obtaining DNS data for much higher Ra. We found super-linear speed-up up to 65536 MPI-rank on Hawk and ARCHER2 systems and around 8000 MPI-rank on JUWELS and Beskow systems. We achieved the best performance with the Hawk system with reasonably good results up to 131072 MPI-rank, which is attributed to the hypercube technique on its interconnection. Given the current HPC technology, it is feasible to obtain DNS data for Ra = 1012, but for cases higher than this, significant improvement in hardware and software HPC technology is necessary.

Analysis of the accuracy of residual heat removal and natural convection transients in reactor pools

The assessment of the inaccuracy and sensitivity of simulations of natural convection in pools is of the highest importance for the nuclear industry. Therefore, in this article we qualify the uncertainty on the calculation of three dimensional spatio-temporal evolution of Raileigh-Bènard instability.This assessment is carried out through the creation of a surrogate fast model. The model is build utilizing Proper Orthogonal Decomposition and Galerkin projection. The Proper Orthogonal Decomposition allows finding the most energetic modes. Those are derived post-processing a high fidelity CFD calculation. Those modes constitute the vectors of a new, reduced, basis. In the Galerkin Projection the governing equations are written in this reduced basis.Once the model is available, we discuss the uncertainties on the determination of the amplitudes of each mode. Concretely, we consider the hypothesis of deviations that follow the normal distribution for each mode. This normal distribution is centered in the nominal value of the amplitude and have a standard deviation which amounts 5% of the amplitude.The expected results and its uncertainty are henceforth computed by Monte-Carlo method, calculating hundreds of solutions of the Initial Value Problem with the surrogate model.

Influence of Sulphur Content on Structuring Dynamics during Nanosecond Pulsed Direct Laser Interference Patterning.

The formation of melt and its spread in materials is the focus of many high temperature processes, for example, in laser welding and cutting. Surface active elements alter the surface tension gradient and therefore influence melt penetration depth and pool width. This study describes the application of direct laser interference patterning (DLIP) for structuring steel surfaces with diverse contents of the surface active element sulphur, which affects the melt convection pattern and the pool shape during the process. The laser fluence used is varied to analyse the different topographic features that can be produced depending on the absorbed laser intensity and the sulphur concentration. The results show that single peak geometries can be produced on substrates with sulphur contents lower than 300 ppm, while structures with split peaks form on higher sulphur content steels. The peak formation is explained using related conceptions of thermocapillary convection in weld pools. Numerical simulations based on a smoothed particle hydrodynamics (SPH) model are employed to further investigate the influence of the sulphur content in steel on the melt pool convection during nanosecond single-pulsed DLIP.

Analysis of the accuracy of residual heat removal in gen-IV reactors

The assessment of the inaccuracy and sensitivity of simulations of natural convection in pools is of the highest importance for the nuclear industry and concretely for the design of the 4th generation reactors.Therefore, in this article we qualify the uncertainty on the calculation of three dimensional spatio-temporal evolution of Rayleigh-Bénard instability.This assessment is carried out through the creation of a surrogate–fast–model. The model is build utilizing Proper Orthogonal Decomposition and Galerkin projection. The most energetic modes are derived from a high fidelity CFD calculation. The governing equations are henceforth projected into the space generated.Once the model is available, we consider the uncertainties on the dimensionless numbers, Re, Pr, Ra. The expected result and its uncertainty are computed by Monte-Carlo method, calculating hundreds of solutions of the Initial Value Problem with the surrogate model. By this we have verified the applicability of the methodology to this kind of problems and the viability of the approach for uncertainty qualification.

Effect of laser power on molten pool evolution and convection

The cladding layer formed by laser cladding is formed by solidification of the molten pool. The quality of the cladding layer is directly determined by the evolution of the molten pool. The numerical simulation of the evolution of the molten pool is carried out, revealing the relationship between different powers and the evolution of the molten pool. The results show that the volume of the molten pool exhibits a nonlinear expansion of “expansion-shrinkage-re-expansion” before reaching the steady state, and the volume and expansion rate of the molten pool are determined by the laser power. In addition, the degree of shrinkage of the molten pool and the speed of convection can also be controlled by the laser power. When the laser power reaches a certain level, evaporation occurs on the surface of the molten pool. The convection pattern of the molten pool can be changed by evaporation. However, due to the impact of shielding gas and Marangoni movement, the effect of evaporation on the geometry of the cladding layer is limited.

Effect of different electrode tip angles with tilted torch in stationary gas tungsten arc welding: A 3D simulation

In this study, the effect of different tip angles (30°, 60°, 90° and 120°) on the arc and weld pool behavior is analyzed in 2 mm and 5 mm arc lengths with tilted (70°) torch. Arc temperature, velocity, current density, heat flux and gas shear are investigated in the arc region and pool convection and puddle shapes are studied in the weld pool region. The arc temperature at the tungsten electrode is found the maximum with sharp tip and decreases as the tip angle increases. The arc temperature on the anode (workpiece) surface becomes concentrated with increase in tip angle. The arc velocity and gas shear stress are observed large with sharp tip and decreasing as the tip angle increases. Current density on the anode surface does not change with tip angle and observed almost the same in all the tip angles in both 2 mm and 5 mm arc lengths. Heat flux due to conduction and convection is observed more sensitive to the tip angle and decreases as the tip angle increases. The electromagnetic force is slightly observed increasing and the buoyancy force is observed slightly decreasing with increase in tip angle. Analyzing each driving force in the weld pool individually shows that the gas drag and Marangoni forces are much stronger than the electromagnetic and buoyancy forces. The weld pool shape is observed wide and shallow in sharp and narrow and deep in large tip angle. Increasing the arc length does not change the weld pool width; however, the weld pool depth significantly changes with arc length and is observed deep in short arc length. The arc properties and weld pool shapes are observed wide ahead of the electrode tip in the weld direction due to 70° torch angle. Good agreement is observed between the numerical and experimental weld pool shapes.

Thermocapillary Convection and Buoyant-Thermocapillary Convection in the Annular Pools of Silicon Melt and Silicone Oil

The thermocapillary convection and buoyant-thermocapillary convection in the annular pool of silicon melt (Pr=0.011) and silicone oil (Pr=6.7) with depth d=10 mm differentially heated at the outer wall and cooled at the inner wall are investigated by 2-D numerical simulation. The numerical results exhibit that the thermocapillary flow is enhanced by buoyancy force for silicon melt while it is weakened for silicone oil. Linear stability analysis indicates that the buoyancy force destabilizes the thermocapillary convection, which is different from that for silicone oil. The detailed reason of different influence of buoyancy force on the thermocapillary flow with different Pr numbers is explained according to present numerical results.

Mathematical modeling of the dynamic behavior of gas tungsten arc weld pools

The dynamic behavior of stationary fully penetrated gas tungsten arc weld pools was investigated through numerical simulation. The effects of arc pressure, electromagnetic force, and surface tension gradients on surface depression, convection, and temperature distribution were calculated. The top surfaces of fully penetrated pools were easily depressed since they were only supported by surface tension. Circulatory convection patterns were generated by electromagnetic forces and surface tension gradients and were significantly affected by the vertical velocity component produced by pool oscillation. The temperature distribution in and around the pool was influenced by pool convection. During pool formation and growth, the fully penetrated molten pool sagged dramatically when the bottom pool diameter approached the top diameter. The sagged pool oscillated with higher magnitude and lower frequency than partially penetrated or fully penetrated pools before sagging occurred. The dynamic behavior and the amount of material lost during melt-through were affected by the pool size and the magnitude of arc pressure.

Thermal Analysis of Heat-Generating Pools Bounded From Below by Curved Surfaces

A computer code that features the use of a directional effective thermal conductivity in modeling natural convection in heat-generating pools has been developed to analyze heat transfer in such pools, which are bounded from below by curved surfaces. Illustrative calculations pertaining to two published experimental studies on convective heat transfer in water pools with uniformly distributed volumetric energy sources are carried out using the code. The water pools used in the two studies under consideration were cooled either from the top or from the bottom, but not from both. The utility as well as the limitations of the effective thermal conductivity approach in the context of addressing the issue of melt-pool coolability is demonstrated by comparisons of calculated results with the experimental data.

Natural convection in pools of evaporating liquids

In this exploratory study, schlieren photography has been used to reveal a variety of convection patterns in pools of evaporating liquids. Pure materials as well as binary solutions were investigated, both in the presence and absence of surface contamination, which was shown to exert a considerable influence on the convective flow. It was found that, with the exception of water, the observed flow structures were largely dependent on the depth of the evaporating pool and to a much lesser extent on the properties of the liquid. Also, it was possible to identify certain flow patterns as being induced by a surface-tension-driven instability and others as being due to buoyancy-driven convection; still other patterns seemed to be uniquely associated with the presence of surface contamination.