Mikio Sakai

Three-dimensional simulation of a solid–liquid flow by the DEM–SPH method

Abstract In this paper, we describe a new Lagrangian–Lagrangian algorithm, which is referred to be the DEM–SPH method, for solid–liquid flows involving free surfaces. The DEM solid phase and the SPH liquid phase are coupled using the local averaging technique described by Lagrangian approaches, where both the continuity equation and the interaction force, i.e. drag force, are connected with the local mean voidage. Conservative forms of momentum transformation are derived for the DEM–SPH interaction via a variational approach. By introducing a correction to the SPH approximation with explicit inclusion of boundary information, arbitrary boundaries can be modeled without any extra wall particles, where the boundary is used commonly for both DEM and SPH phases. We deploy level-set distance functions to efficiently construct and evaluate this boundary model. To examine the validity of the present method, we perform three-dimensional simulations of a dynamic flow in a solid–liquid dam break and a quasi-steady flow in a rotating cylindrical tank; and we conduct validation experiments to justify the simulation results. In the dam-break problem, positions of wave fronts during the collapse are computed and compared with experimental measurements; for the circulating tank, some macroscopic aspects of the steady flow, e.g. the shape, dimension and velocity profile of the solid bed, are obtained for validation data. In both cases, the simulation results are in good agreement with those of the experiment. Consequently, the DEM–SPH method is proved to be adequate in modeling solid–liquid flows through this study.

Effect of Nuclear Fuel Particle Movement on Nuclear Criticality in a Rotating Cylindrical Vessel

In conventional criticality safety evaluations of nuclear facilities, nuclear fuel particle behavior could not be considered because any appropriate computational tools did not be developed. A new criticality evaluation tool, which can consider the particle behavior, is developed by coupling a Discrete Element Method with a continuous energy Monte Carlo type code. Criticality evaluations considering the nuclear particle movement in a rotating cylindrical tank are performed by applying this tool. Effects of the powder-filling ratio and the rotating speed on nuclear criticality are investigated in the present study. The nuclear powder free surface is fluctuated more remarkably as the filling ratio and the rotating speed are higher. The fluctuation makes the effective multiplication factor lower because of neutron leakage from the system by the increase of the superficial area and reduction of the atomic number densities due to the flowability.

Numerical Analysis of Droplet Impingement Using the Moving Particle Semi-implicit Method

Droplet impingement onto a rigid wall is simulated in two and three dimensions using the moving particle semi-implicit method. In two-dimensional calculations, the convergence is achieved and the propagation of a shockwave in a droplet is captured. The average pressure on the contact area decreases gradually after the maximum value. The numerically obtained maximum average impact pressure agrees with the Heymann correlation. A large shear stress appears at the contact edge due to jetting. A parametric study shows that the droplet diameter has only a minor effect on the pressure load dueto droplet impingement. When the impingement takes place from an impact angle of π/4 rad, the pressure load and shear stress show a dependence only on the normal velocity to the wall. A comparison between the three-dimensional and two-dimensional results shows that consideration of the three-dimensional effect can decrease the average impact pressure by about 12%.

Investigation of Droplet Impingement onto Wet Walls Based on Simulation Using Particle Method

In order to investigate the mitigation effect brought by the water film, liquid droplet impingement (LDI) on a wet rigid wall has been simulated using the moving particle semi-implicit (MPS) method. Propagation of the shock waves has been well resolved. High peak pressures that appear at the contact edge in the dry-wall impingement are mitigated or even eliminated by the water film. Furthermore, the magnitude of the average pressure load is reduced by the water film. Effects of the flow conditions, including the thickness of the water film, droplet velocity, droplet diameter, and impinging angle, have been investigated through a parametric study. Based on the parametric study, a correlation for the maximum average pressure is devised as [Ptilde] = f1 + f2 Ṽ, where f1 = a + be−pδ + (1 - a - b)e−qδ and f 2 = ce−pδ + (κ - c)e−qδ . The coefficients are a = 0:240, b = 0:213, c = 3:50, p = 3:70, and q = 36:0. The valid parameter ranges of the correlation are 0 ≤ δ ≤ 1:0 and 1/15 ≤ Ṽ ≤ 1/5. In the oblique impingement, the normal velocity to the wall determines the pressure load. The shear stress is found playing a minor role in the overall mechanical load in the wet-wall impingement.

Development of a criticality evaluation method involving the granular flow of the nuclear fuel in a rotating drum

Abstract Particle dynamics of nuclear fuel material has not been considered in conventional nuclear criticality evaluations. However, the particle motion influences nuclear criticality significantly. In the present study, the criticality calculation is combined with the discrete element method (DEM) to investigate the effects of the particle macroscopic behavior on nuclear criticality. Particle motion is analyzed in a rotating drum by the DEM, and then, the nuclear calculation is carried out. This paper focuses on particle size distribution, size segregation, and change of surface area of the particle bed. The particle size distribution has an important influence on the nuclear criticality evaluation because it affects not only the particle movement but also the atomic number densities in the bed. The surface area of the particle bed shows a close correlation with the multiplication factor. On the other hand, the size segregation does not have a significant effect on nuclear criticality.

Direct numerical simulation of gas-solid-liquid flows with capillary effects: An application to liquid bridge forces between spherical particles

In this study, a numerical method is developed to perform the direct numerical simulation (DNS) of gas-solid-liquid flows involving capillary effects. The volume-of-fluid method employed to track the free surface and the immersed boundary method is adopted for the fluid-particle coupling in three-phase flows. This numerical method is able to fully resolve the hydrodynamic force and capillary force as well as the particle motions arising from complicated gas-solid-liquid interactions. We present its application to liquid bridges among spherical particles in this paper. By using the DNS method, we obtain the static bridge force as a function of the liquid volume, contact angle, and separation distance. The results from the DNS are compared with theoretical equations and other solutions to examine its validity and suitability for modeling capillary bridges. Particularly, the nontrivial liquid bridges formed in triangular and tetrahedral particle clusters are calculated and some preliminary results are reported. We also perform dynamic simulations of liquid bridge ruptures subject to axial stretching and particle motions driven by liquid bridge action, for which accurate predictions are obtained with respect to the critical rupture distance and the equilibrium particle position, respectively. As shown through the simulations, the strength of the present method is the ability to predict the liquid bridge problem under general conditions, from which models of liquid bridge actions may be constructed without limitations. Therefore, it is believed that this DNS method can be a useful tool to improve the understanding and modeling of liquid bridges formed in complex gas-solid-liquid flows.

Rigid body simulation using a particle method

Rigid body simulations have been developed by many researchers of computer graphics recently [E. Guendelman et al. 2003]. The main purpose of rigid body simulations in the computer graphics field is not to analyze the physics accurately but to make motion based on physical modeling. Although a rigid body does not exist in this real world, it is useful to treat a very rigid and little deformable object as a rigid body because the calculation cost is reduced. MPS (Moving Particle Semi-implicit) method [S. Koshizuka et al. 1996] is one of the particle methods to simulate incompressible fluids. Although a rigid body is represented by polygons traditionally, we represent it by particles because all material is represented by particles in the MPS method. Indeed polygons represent the rigid body shape precisely but collision detection is very complicated and time consuming. Particle representation of a rigid body is not very precise but it is easy to detect collisions between rigid bodies. The quantity of data using particles is less than that using polygons. Besides, interaction with fluids is easy to calculate, for example, by coupling with a particle method for fluids. In this research, the method to calculate rigid bodies which consist of particles is developed and some examples are calculated.

An efficient multi-dimensional implementation of VSIAM3 and its applications to free surface flows

We propose an efficient multidimensional implementation of VSIAM3 (volume/surface integrated average-based multi-moment method). Although VSIAM3 is a highly capable fluid solver based on a multi-moment concept and has been used for a wide variety of fluid problems, VSIAM3 could not simulate some simple benchmark problems well (for instance, lid-driven cavity flows) due to relatively high numerical viscosity. In this paper, we resolve the issue by using the efficient multidimensional approach. The proposed VSIAM3 is shown to capture lid-driven cavity flows of the Reynolds number up to Re = 7500 with a Cartesian grid of 128 × 128, which was not capable for the original VSIAM3. We also tested the proposed framework in free surface flow problems (droplet collision and separation of We = 40 and droplet splashing on a superhydrophobic substrate). The numerical results by the proposed VSIAM3 showed reasonable agreements with these experiments. The proposed VSIAM3 could capture droplet collision and separation of...

Development of the Concrete Cask Horizontal Transfer System

Concrete cask system is focused as the candidate one for spent fuel dry storage facilities from economic potential in Japan. Concrete cask consists of a concrete storage cask and a steel canister. A canister containing nuclear spent fuel is shipped by a transportation cask from a nuclear power plant to an interim storage facility. The canister is transferred from the transportation cask to a storage cask by a transfer cask in the storage facility. IHI has developed a concrete cask horizontal transfer system. This transfer system indicates that a canister is transferred to a storage cask horizontally. This transfer system has a merit against canister drop accident in transfer operation, i.e. spent fuel assemblies can be kept safe during the transfer operation. There are guide rails inside of the concrete cask, and the canister is installed into the storage cask with sliding on the rails. To develop the horizontal transfer system, IHI carried out a heat load test and numerical analyses by CFD. Heat load experiment was carried out by using a full-scale prototype canister, storage cask and transfer vessel. The decay heat was simulated by an electric heater installed in the canister. Assuming high burn-up spent fuel storage, heat generation was set between 20.0kW and 25.0kW. This experiment was focused on the concrete temperature distribution. We confirmed that the maximum concrete temperature in transfer operation period was lower than 40 °C (Heat generation 22.5kW). Moreover we confirmed the maximum concrete temperature passed 24 hours with horizontal orientation was below 90 °C (Heat generation 22.5kW). We analyzed the thermal performance of the concrete cask with horizontal transfer condition and normal storage condition. Thermal analyses for horizontal transfer operation were carried out based on the experimental conditions. The tendency of the analytical results was in good agreement with experimental results. The purpose of vertical thermal analysis was to estimate the concrete temperature increase in the case a canister contacts with guide rails in normal storage. It has a possibility that a canister contacts with guide rails during storage period after concrete cask is upended from transfer operation. The temperature increase due to this contact was calculated 5 °C at small local area. This result implies that the affect of the contact is very small. This paper addresses that the storage cask concrete is kept its integrity in transfer operation period and normal storage period.Copyright

Numerical Study on Coarse Grain Modeling of the DEM for Industrial Scale Gas-Solid Flow Systems

The Discrete Element Method (DEM) is widely used in various numerical simulations related to granular media. The DEM is a Lagrangian approach where individual particle is calculated based on the Newton’s second law of motion. Therefore, the DEM enables us to investigate the granular flow characteristics at the particle level. On the other side, the DEM has a difficulty to be applied in large-scale powder systems because the calculation cost becomes too expensive when the number of particles is huge. To solve this issue, we have developed a coarse grain modeling as a large scale model of the DEM. The coarse grain particle represents a group of original particles. The coarse grain model was used in typical gas-solid and solid-liquid two phase flows so far, where the particle size was relatively large, namely, cohesive force did not act between the solid particles. In the present study, the coarse grain model is evolved to simulate fine particles by considering the interparticle van der Waals force. The adequacy of the coarse grain model is proved by comparing the simulation results of original particle system. Through this study, the coarse grain model is shown to simulate the cohesive particle behavior precisely.Copyright