A CFD Study of Existing Drag Models for Geldart A Particles in Bubbling Fluidized Beds

In this study, seven drag models are examined to determine how they affect fluidization behavior of Geldart A particles of biomass and coal. Notwithstanding the notable number of numerical studies to find the best drag model for larger particles, there is a dearth of information related to drag models for finer Geldart A particles. Additionally, to our knowledge, these drag models have not been tested with a binary mixture of Geldart A particles. Computational fluid dynamics was used to model the gas and solid phases in an Eulerian-Eulerain approach to simulate the particle-particle interactions of coal-biomass mixtures and compare the predictions with experimental data. In spite of the previous findings that bode badly for using predominately Geldart B drag models for fine particles, the results of our study reveal that if static regions of mass in the fluidized beds are considered, these drag models work well with Geldart A particles. It was found that the seven drag models could be divided into two categories based on their performance. One category included the Gidaspow family of drag models (Gidaspow, Gidaspow-Blend, and Wen-Yu) and the Syamlal-O’Brien drag model; these models closely predicted the experiments for single solids phase fluidization. For binary mixtures, however, the other drag model group (BVK, HYS, Koch and Hill) yielded better predictions.

Invariant Forms of Conservation Equations and Some Examples of Their Exact Solutions

A scale-invariant model of statistical mechanics is described leading to invariant Boltzmann equation and the corresponding invariant Enskog equation of change. A modified form of Cauchy stress tensor for fluid is presented such that in the limit of vanishing intermolecular spacing all tangential forces vanish in accordance with perceptions of Cauchy and Poisson. The invariant forms of mass, thermal energy, linear momentum, and angular momentum conservation equations derived from invariant Enskog equation of change are described. Also, some exact solution of the conservation equations for the problems of normal shock, flow over a flat plate, and flow within a spherical droplet located at the stagnation point of opposed cylindrically-symmetric gaseous jets are presented.

Particle-Induced Flow Reattachment in a Diffuser

In this work, a numerical investigation on the gas-particle flow in a vertical diffuser is carried out. This study was motivated by the experimental work of Kale and Eaton [1], who noticed that the fully attached flow in a diffuser in the freeboard region of a particle bed would become detached if no particles were present. It was concluded at the time that this effect was not caused by the high inlet turbulence levels, but rather by the particles. With the goal to better understand the interactions between the particles and the fluid in a diffuser, simulations of a dilute particle-laden gas flow in a vertical diffuser are run using the Euler/Lagrange approach. The model, which includes interparticle collisions, the particle influence on the gas phase and wall roughness effects, is first validated based on experimental results from a horizontal channel and a vertical diffuser for both the continuous and dispersed phases at different mass loadings. Investigations on the effects of particles at different mass loadings and wall roughness on the diffuser flow are then carried out. It has been found that, even at moderate mass loadings, particles can significantly affect the diffuser flow pattern, and actually reattach the otherwise separated flow under some conditions. It has also been found that wall roughness plays a very important role in homogenizing the particle distribution at the diffuser section. The resulting more uniform concentration and velocity profiles can then reenergize the otherwise separated boundary layer and reattach it to the wall. The mechanism for the flow reattachment owing to the particle flow and the high wall roughness is investigated and an explanation is proposed.

Effect of Schmidt Number on Dynamics of Particle-Driven Gravity Currents

Direct numerical simulations (DNS) of particulate gravity currents in a lock-exchange set-up are presented. The effect of the Schmidt number Sc on the current dynamics is analyzed by means of Eulerian-Eulerian simulations. Eulerian-Lagrangian simulations are used as a benchmark for assessing the results of Eulerian-Eulerian simulations. The Schmidt number Sc, the particle properties and the Reynolds number Re are varied. A significant influence of Sc was found, whose magnitude depends on the particle properties (being highest for fine particles) and on Re. For the finest particles used, the deposited particle mass was found to be different by almost 25% when comparing Lagrangian and Eulerian simulations with Sc = 1. The instantaneous flow features like the vorticity field are affected as well. When doubling Re, the effect was still found to be significant for finer particles, though less than that for low Re.

Spray-Quenching for Process-Integrated Heat Treatment of Construction Components

Spray quenching processes in heat treatment processes of specimen and components demonstrated its efficiency and potential of direct in-process integration in applications as forging or sheet metal forming. Ensuring local quenching homogeneity and offering a range of quenching intensities, spray-quenching provides optimal solutions to carry out energy-efficient, homogeneous and intensive heat treatment processes. In this contribution, spray quenching of specimen using twin-fluid, flat-spray nozzles is evaluated in order to integrate the heat treatment process within an automated production line of forging or forming components. The quenching strategies firstly aim at providing appropriate microstructures (homogeneous and bainitic) to forged metallic parts of various geometries. Experiments and simulations are carried out to derive optimal cooling strategies. Temperature-dependent heat transfer coefficient (HTC) distributions based on a parameter study of the spray nozzle involving thermography have been evaluated. Transient heat transport simulations where the components are quenched according to local and dynamic HTC distribution using various arrangements of the nozzle field were performed. It is shown that the a priori simulated process parameters provided homogeneous microstructures in the components. The characteristic specimen geometries under investigation range from flat plates to cylindrical parts as e.g. stepped shafts. The possibility to extend spray quenching to more complex-shaped specimen geometries is outlined.

The Structure of Forward Facing Step Flows in Adverse Pressure Gradient

This paper reports an investigation of the effects of adverse pressure gradient on turbulent flows over forward facing step. Three adverse pressure gradients were created for this study using diverging channels. A particle image velocimetry technique was used to conduct measurements in the streamwise-wall-normal (x-y) planes at the mid-plane of test section at several locations downstream to 68 step heights. A Reynolds number of Reh = 4800 and δ/h = 4.7 were employed, where h is the mean step height and δ is the approach boundary layer thickness. The results include the mean flow and turbulence quantities as well as proper orthogonal decomposition analysis. The mean reattachment length obtained indicates that the adverse pressure gradient created in this study does not have significant effects on the reattachment length. The triple velocity correlations imply that there is negative transport of turbulence kinetic energy close to the wall and positive transport away from the wall. In addition to the physical insight, the high quality data reported are useful for assessing the ability of turbulence models to reproduce the behaviour of complex flows.

Direct Numerical Simulation of Particle Heat and Mass Transfer in a Fluidized Bed

We have developed a Direct Numerical Simulation combined with the Immersed Boundary method (DNS-IB) to study heat transfer in particulate flows. In this method, fluid velocity and temperature fields are obtained by solving the modified momentum and heat transfer equations, which result from the presence of heated particles in the fluid; particles are tracked individually and their velocities and positions are solved based on the equations of linear and angular motions; particle temperature is assumed to be a constant. The momentum and heat exchanges between a particle and the surrounding fluid at its surface are resolved using the immersed boundary method with the direct forcing scheme. The DNS-IB method has been used to study heat transfer of 1024 of heated spheres in a fluidized bed. By exploring the rich data generated from the DNS-IB simulations, we are able to obtain statistically averaged fluid and particle velocity as well as overall heat transfer rate in a fluidized bed.

CFD Study of Hydrodynamics and Separation Performance of a Novel Crossflow Filtration Hydrocyclone (CFFH)

This research addresses various hydrodynamic aspects and the separation performance of a novel cross-flow filtration hydrocyclone (CFFH) using computational fluid dynamics. A CFFH is a device that combines the desirable attributes of a cross-flow filter and a vortex separator into one unit to separate oil from water. The velocity and pressure fields within the CFFH are estimated by numerically solving the filtered Navier-Stokes equations (by using a Large Eddy Simulation (LES) approach). The Lagrangian approach is employed for investigating the trajectories of dispersed droplets based on a stochastic tracking method called the Discrete Phase Model (DPM). The mixture theory with the Algebraic Slip Model (ASM) is also used to compute the dispersed phase fluid mechanics and for comparing with results obtained from the DPM. In addition, a comparison between the statistically steady state results obtained by the LES with the Wall Adaptive Local Eddy-Viscosity (WALE) subgrid scale model and the Reynolds Average Navier-Stokes (RANS) closed with the Reynolds Stress Model (RSM) is performed for evaluating their capabilities with regards to the flow field within the CFFH and the impact of the filter medium. Effects of the Reynolds number, the permeability of the porous filter, and droplet size on the internal hydrodynamics and separation performance of the CFFH are investigated. Results indicate that for low feed concentration of the dispersed phase, separation efficiency obtained based on multiphase and discrete phase simulations is almost the same. Higher Reynolds number flow simulations exhibit an unstable core and thereby numerous recirculation zones in the flow field are observed. Improved separation efficiency is observed at a lower Reynolds number and for a lower permeability of the porous filter.

DEM Predictions of NETL Small Scale Challenge Problem

The Discrete Element Method (DEM) coupled to Computational Fluid Dynamics (CFD) is used to predict dense fluid-particle system in a blind study using the in-house code GenIDLEST (Generalized Incompressible Direct and Large Eddy Simulation of Turbulence). The experimental measurements were performed at the Department of Energy’s (DOE’s) National Energy Technology Laboratory (NETL) at three different superficial velocities in a 0.076m×0.23m×1.22m bubbling fluidized bed with 3.26 mm Nylon beads. Experimental measurements include the mean and rms of pressure drop between specific locations, the first four moments of solid velocity components, and time series of pressure drop at selected locations in the bed. The predictions capture the trends in the change in bed hydrodynamics with an increase in the superficial velocity. While good qualitative agreement is found with experiments, quantitative agreement is fair. Factors that might cause deviations in predictions are discussed.

Experimental Investigation of Single Roughness Element Effects on Supersonic Boundary Layer Transition

Predicting the characteristics of a transitional boundary layer remains an open challenge in supersonic flow fields. An experimental campaign to understand the effects of a single roughness element on a supersonic laminar boundary layer was designed. Two Mach numbers were tested, 1.6 and 2.3, including two roughness heights, 0.1 mm and 1 mm, over a flat plate. Steady and unsteady wall temperature and pressure levels were recorded to interpret the influence of the wake of the roughness. Heat flux and adiabatic wall temperature trends, temperature and pressure fluctuations RMS trends and time evolution of spectral content were reported. The initial wall temperature was varied during the wall temperature measurements and the resulting steady and unsteady effects on the roughness wake were investigated.