Analysis of Heat Transfer between Two Particles for DEM simulations

The purpose of the present study was to develop a model of heat transfer between particles that can be incorporated into the discrete element method (DEM). The flow around a particle was measured by particle imaging velocimetry (PIV) and the temperature of the particle was measured using a thermocouple and an infrared camera. The experimental data of heat transfer were classified according to the heat transfer mechanism, namely convection, conduction and contact. These values for heat transfer were compared with those calculated using previously derived estimation equations. From these results, we adopted the thermal contact resistance model, which is related to the surface roughness and contact force. Experiments were also carried out to examine the validity of the model. The heat transfer increased with as the surface roughness increased. This is a not general trend because a large surface roughness causes a large thermal resistance, resulting in a small heat transfer. This trend is considered to be due to the increase in the contact area that accompanies an increase in surface roughness. The contact heat transfer calculated by considering the effect of the surface roughness on the contact area was found to show better agreement with the experimentally obtained values

DIRECT NUMERICAL SIMULATION OF FLUIDIZED BED WITH IMMERSED BOUNDARY METHOD

The applicability of the immersed boundary (IB) method, which is one of direct numerical simulations (DNS) for multiphase flow analyses, has been examined to simulate a fluidized bed. The volumetric-force type IB method developed by Kajishima et al. (2001) has been applied in the present work. While particle-fluid interaction force is calculated with the surface integral of fluid stress at the interface between particle and fluid in the standard IB method, the volume integral of interaction force is used in the volumetric-force type IB method. In order to validate the present simulation code, drag force and lift force firstly were calculated with IB method. Then calculated drag coefficients were compared with values estimated with Schiller-Nauman and Ergun equations, while calculated lift coefficients were compared with the previous simulated results. The difference of drag was within approximately 1% except in the range of low Reynolds number. Thus, the accuracy of the present simulation code was confirmed. Next, simulation of fluidized bed was carried out. Since DNS requires a large computer capacity, only 400 particles were used. The particle is 1.0mm in diameter and 2650kg/m3 in density. From the simulated results, concentrated upward stream lines from the bottom wall were observed in some regions. This inhomogeneous flow would be attributed to particulate structure

DEM-CFD Simulations for Various Fluidizations

Geldart has classified the powders characteristics into 4 groups according to the particles sizes and density. This is generally known as Geldart’s powder classification chart. In the classification chart, there are fluidization behaviors that still cannot be clarified yet such as the homogeneous and also bubbling fluidizations. In the bubbling fluidization, bubble behavior is also varies, i.e. fast and slow bubble. Numbers of study have been conducted in order to clarify the constitutive mechanism of fluidization behind the differences between the homogeneous and bubbling fluidizations. However, the basic fundamental mechanisms have not been sufficiently clarified yet. In this study, we adopted DEM-CFD coupling model to utilize the analysis of the fluidization in fast and slow bubbling, homogeneous and liquid fluidization

Non-dimensionalization and Three-dimensional Flow Regime Map for Fluidization Analyses

This article is on the dimensional analysis and the classification of fluidization from the viewpoint of numerical analysis. At first, the governing equations used in the DEM (Discrete Element Method) and CFD (Computational Fluid Dynamics) coupling model was non-dimensionalized with the method of Hellums and Churchill (1964). From the resulting dimensionless equations, it was concluded that the five dimensionless numbers, i.e. Re: Reynolds number, Ar: Archimedes number, Ga: Galilei number, Fr: Froude number and �*: ratio of particle density divided by fluid density, can be derived and hydrodynamically dominant on the fluid behaviors. Further, these can illustrate the dimensionless numbers proposed in the previous studies. Secondary, a three-dimensional flow regime map of homogeneous, bubbling and turbulent fluidizations was proposed with these dimensionless numbers using the DEM-CFD simulations. Finally, the plane of the minimum bubbling fluidization velocity umb in the map can be proposed and expressed as,�Re�_mb=0.263�^(*-0.553) �Ar�^0.612. umb can be estimated using this equation for various conditions