Liancheng Guo

Numerical Simulation of Rheological Behavior in Melting Metal Using Finite Volume Particle Method

In multiphase flow analyses, rheological behavior has a significant influence on not only the heat and mass transfer but also the dynamics of the solid and fluid during melting and solidification. Based on previous work, it is possible to consider rheological behavior by estimating the viscosity of the liquid phase with its compositional development. The present study investigates this rheological behavior through simulations of multiphase heat transfer problems using the moving finite volume particle (FVP) method, by introducing a viscosity model that takes into account viscosity changes due to phase changes. To validate the applicability of this viscosity model, a series of melting experiments using Woods metal are conducted, and the observed melting characteristics form the basis for computer simulations and 3D numerical analysis using the FVP method. Good agreement between simulation and experiment indicates that the proposed viscosity model reproduces well the rheological behavior during melting.

Numerical simulations of gas-liquid-particle three-phase flows using a hybrid method

For the analysis of debris behavior in core disruptive accidents of liquid metal fast reactors, a hybrid computational tool was developed using the discrete element method (DEM) for calculation of solid particle dynamics and a multi-fluid model of a reactor safety analysis code, SIMMER-III, to reasonably simulate transient behavior of three-phase flows of gas–liquid–particle mixtures. A coupling numerical algorithm was developed to combine the DEM and fluid-dynamic calculations, which are based on an explicit and a semi-implicit method, respectively. The developed method was validated based on experiments of water–particle dam break and fluidized bed in systems of gas–liquid–particle flows. Reasonable agreements between the simulation results and experimental data demonstrate the validity of the present method for complicated three-phase flows with large amounts of solid particles.

3D simulation of solid-melt mixture flow with melt solidification using a finite volume particle method

Relocation and freezing of molten core materials mixed with solid phases are among the important thermal-hydraulic phenomena in core disruptive accidents of a liquid-metal-cooled reactor (LMR). To simulate such behavior of molten metal mixed with solid particles flowing onto cold structures, a computational framework was investigated using two moving particle methods, namely, the finite volume particle (FVP) method and the distinct element method (DEM). In FVP, the fluid movement and phase changes are modeled through neighboring fluid particle interactions. For mixed-flow calculations, FVP was coupled with DEM to represent interactions between solid particles and between solid particles and the wall. A 3D computer code developed for solid-liquid mixture flows was validated by a series of pure-and mixed-melt freezing experiments using a low-melting-point alloy. A comparison between the results of experiments and simulations demonstrates that the present computational framework based on FVP and DEM is applicable to numerical simulations of solid-liquid mixture flows with freezing process under solid particle influences.

Development of the evaluation methodology for the material relocation behavior in the core disruptive accident of sodium-cooled fast reactors

The in-vessel retention (IVR) of core disruptive accident (CDA) is of prime importance in enhancing safety characteristics of sodium-cooled fast reactors (SFRs). In the CDA of SFRs, molten core material relocates to the lower plenum of reactor vessel and may impose significant thermal load on the structures, resulting in the melt-through of the reactor vessel. In order to enable the assessment of this relocation process and prove that IVR of core material is the most probable consequence of the CDA in SFRs, a research program to develop the evaluation methodology for the material relocation behavior in the CDA of SFRs has been conducted. This program consists of three developmental studies, namely the development of the analysis method of molten material discharge from the core region, the development of evaluation methodology of molten material penetration into sodium pool, and the development of the simulation tool of debris bed behavior. The analysis method of molten material discharge was developed based on the computer code SIMMER-III since this code is designed to simulate the multi-phase, multi-component fluid dynamics with phase changes involved in the discharge process. Several experiments simulating the molten material discharge through duct using simulant materials were utilized as the basis of validation study of the physical models in this code. It was shown that SIMMER-III with improved physical models could simulate the molten material discharge behavior, including the momentum exchange with duct wall and thermal interaction with coolant. In order to develop an evaluation methodology of molten material penetration into sodium pool, a series of experiments simulating jet penetration behavior into sodium pool in SFR thermal condition were performed. These experiments revealed that the molten jet was fragmented in significantly shorter penetration length than the prediction by existing correlation for light water reactor conditions, due to the direct contact and thermal interaction of molten materials with coolant. The fragmented core materials form a sediment debris bed in the lower plenum. It is necessary to remove decay heat safely from this debris bed to achieve IVR. A simulation code to analyze the behavior of debris bed with decay heat was developed based on SIMMER-III code by implementing physical models, which simulate the interaction among solid particles in the bed. The code was validated by several experiments on the fluidization of particle bed by two-phase flow. These evaluation methodologies will serve as a basis for advanced safety assessment technology of SFRs in the future.

Numerical simulation of gas–liquid–solid three-phase flow using particle methods

We want to simulate, based on particle methods, the dynamic behavior of multi-phase flows in a gas–solid–liquid mixture system. With the governing equations discretized within the finite volume particle method, the effects of contact and collision between solid particles were modeled by the distinct element method. Applicability of the viscosity model and an empirical drag force model were confirmed for the hydrodynamic interactions between solid particles and fluid. Simulations were performed of a single bubble rising in a tank of stagnant solid particle–liquid. The results for the dynamic behavior indicate that the present computational framework of particle-based simulation method may be useful for numerical simulations of multi-phase flow behavior in a solid particle–fluid mixture system.

Numerical Simulation of Self-Leveling Behavior in Debris Bed by a Hybrid Method

The postulated core disruptive accidents (CDAs) are regarded as particular difficulties in the safety analysis of liquid-metal fast reactors (LMFRs). In the CDAs, the self-leveling behavior of debris bed is a crucial issue to the relocation of molten core and heat-removal capability of the debris bed. The fast reactor safety analysis code, SIMMER-III, which is a 2D, multi-velocity-field, multiphase, multicomponent, Eulerian, fluid dynamics code coupled with a fuel-pin model and a space- and energy-dependent neutron kinetics model, was successfully applied to a series of CDA assessments. However, strong interactions among rich solid particles as well as particle characteristics in multiphase flows were not taken into consideration for fluid-dynamics models of SIMMER-III. In this article, a developed hybrid method, by coupling the discrete element method (DEM) with the multi-fluid model of SIMMER-III, is applied in the numerical simulation of self-leveling behavior in debris bed. In the coupling algorithm the motions of gas and liquid phases are solved by a time-factorization (time-splitting) method. For particles, contact forces among particles and interactions between particles and fluid phases are considered through DEM. The applicability of the method in such complicate three phase flow is validated by taking the simulation of a simplified self-leveling experiment in literature. Reasonable agreement between simulation results and corresponding experimental data shows that the present method could provide a promising means for the analysis of self-leveling behavior of debris bed in CDAs.Copyright

Validation of a 3D Hybrid CFD-DEM Method Based on a Self-Leveling Experiment

The postulated core disruptive accidents (CDAs) are regarded as particular difficulties in the safety analysis of liquid-metal fast reactors (LMFRs). In the CDAs, core debris may settle on the core-support structure and form conic bed mounds. Heat convection and vaporization of coolant sodium will level the debris bed, which is named “self-leveling behavior” of debris bed. To reasonably simulate such transient behavior, as well as thermal-hydraulic phenomena occurring during a CDA, a comprehensive computational tool is needed. The SIMMER code is a successful computer code developed as an advanced tool for CDA analysis of LMFRs. It is a multi-velocity-field, multiphase, multicomponent, Eulerian, fluid dynamics code coupled with a fuel-pin model and a space- and energy-dependent neutron kinetics model. Until now, the code has been successfully applied to simulations of key thermal-hydraulic phenomena involved in CDAs as well as reactor safety assessment. However, strong interactions among rich solid particles as well as particle characteristics in multiphase flows were not taken into consideration for its fluid-dynamics models. Therefore, a hybrid computational method was developed by combining the discrete element method (DEM) with the multi-fluid models to reasonably simulate the particle behaviors, as well as the thermal-hydraulic phenomena of multiphase fluid flows. In this study, 3D numerical simulation of a simplified self-leveling experiment is performed using the hybrid method. Reasonable agreement between simulation results and corresponding experimental data demonstrated the validity of the present method in simulating the self-leveling behavior of debris bed.© 2014 ASME

Development of assessment method for a self-leveling behavior of debris bed and analyses of experiments

When core melt occurs in severe accident in Sodium Cooled Fast Reactor (SFR), molten core material moves to the lower plenum in reactor vessel and fragmented by fuel coolant interaction. These fragmented particles, so called debris, accumulate on the structure surface to form debris bed. If the thickness of the debris bed exceeds the coolable thickness of the decay heat, boiling of sodium occurs inside the debris bed. It is found from past in-pile experiments that the sodium flow and boiling inside the debris bed caused by a decay heat planarize the debris bed to lower the debris bed thickness. This mechanism is called self-leveling of debris bed. In the accident sequence of SFR, when fuel debris locally accumulates beyond the coolable thickness, fuel debris remelts with decay heat and they cannot be retained in-vessel. However, it is expected that the debris bed thickness lowers the coolable thickness with self-leveling phenomenon and they can be safely retained in-vessel. This is why an appropriate assessment for self-leveling behavior is important for safety analysis of SFR with the object of safety cooling of fuel debris. Therefore, the object of this study is to develop new analytical methods to simulate unique phenomena in self-leveling behavior and implement it to SFR safety analysis code. The characteristic of self-leveling is that when the larger external forces caused by environmental fluids are larger than a threshold value, the debris bed is fluidized. The new methods are developed with assuming that the debris bed behaves as Bingham fluid from this feature. They are categorized into two main parts. The first part is particle interaction models to model the effect of particle-particle contacts and collisions. Particle pressure and particle viscosity related to particle-particle collisions and contacts, respectively, are applied to pressure and viscosity term in the particle momentum equation. The second part is a large deformation method, which simulates Bingham fluid characteristic of debris bed. This method numerically judges a onset of debris bed fluidization which depends on a shear stress strength. An experimental study of self-leveling behavior, in which the particle bed behavior driven by bubbles inflow from the bottom of bed in gas-solid-liquid three-phase flow was observed, is analyzed to validate the new methods. Simulation results well reproduced the transient changes of particle bed, whose elevation angle and form deformation becomes gradually small and obscure, respectively. Their dependencies on particle size and density are also well simulated with new methods. The assessment results show that these methods provide a basis to develop analytical methods of self-leveling behavior of debris bed in the safety assessment of SFRs.Copyright

Numerical Simulation of Three-Phase Flows With Rich Solid Particles by Coupling Multi-Fluid Model With Discrete Element Method

The postulated core disruptive accidents (CDAs) are regarded as particular difficulties in the safety analysis of liquid-metal fast reactors (LMFRs). In CDAs, the motions and interactions of solid particles, such as refrozen fuels, disrupted pellets, etc., not only dominate fundamental behaviors of multiphase flows, but also drastically influence the process of CDAs. The fast reactor safety analysis code, SIMMER-III, which is a 2D, multi-velocity-field, multiphase, multicomponent, Eulerian, fluid dynamics code coupled with a fuel-pin model and a space- and energy-dependent neutron kinetics model, was successfully applied to a series of CDA assessments. However, strong interactions among solid particles as well as particle characteristics in multiphase flows with rich solid particles were not taken into consideration for fluid-dynamics models of SIMMER-III. In this article, a hybrid method for multiphase flows analysis is developed by coupling the discrete element method (DEM) with the multi-fluid model of SIMMER-III. In the coupling algorithm, motions of liquid and gas phases are solved by a time-factorization (time-splitting) method. For the solid phases, contacts among particles and interactions with fluid phases are considered through DEM. Numerical simulations of dam-break behavior with rich solid particles show reasonable agreements with corresponding experimental results. It is expected that SIMMER-III coupled with DEM could provide a promising and useful computational tool for complicated multiphase-flow phenomena with high concentration of solid particles.Copyright