Application of Discrete Element Method and Computational Fluid Dynamics to Selected Dispersed Phase Flow Problems

A parallel discrete particle modelling framework (PAR_DPM3D) is applied to study three fundamental multiphase flow problems: The sedimentation of a cluster of particles in a viscous ambient fluid, multiphase flow in a bench-scale fluidized bed and granular segregation and mixing dynamics in a rotating drum. Various phenomena including torus formation and particle cluster breakup are reproduced. We provide new insights into the volume fraction dependence of the dynamic characteristics of a settling particle cluster and find a similar dependence in the simulations as in the theoretical predictions of Nitsche and Batchelor 1. Similarities in the interaction between a system of two particle clouds and a system of two immiscible droplets was established with an observed increase in the velocity of the trailing cloud due to drag reduction in the wake of the leading cloud. Second, we show how existing drag models may be inadequate to predicting the macroscale properties of a gas-solid fluidized bed. Using an energy and force balance approach we provide new closures that account for some inhomogeneous flow structures and implement these closures within the PAR_DPM3D framework to predicting the axial pressure drop and transverse particle velocity profiles Finally, we present results from particle dynamics simulation of “S+D” granular systems (where size and density drive segregation simultaneously) in various irregular shaped tumblers in the rolling regime (10-4 \u3c Fr \u3c 10-2). We develop a new way of quantifying the state of mixing or segregation has been developed. Using this new measure of segregation (or entropy of mixing) we compare segregation dynamics for different shapes of tumblers

IMPACT OF COHESION FORCES ON PARTICLE MIXING AND SEGREGATION

The objective of this work is to advance the fundamental understanding of mixingand segregation of cohesive granular materials. Cohesion can arise from a variety ofsources: van der Waals forces, electrostatic forces, liquid bridging (capillary) forces.These forces may play a significant role in the processing of fine and/or moist powdersin many industries, from pharmaceuticals to materials synthesis; however, despite itsprevalence, there is only limited information available in the literature on processingof cohesive materials. Instead, the vast majority of work has been directed at thestudy of non-cohesive (i.e., free-flowing) particles, and a wealth of information hasbeen learned about the behavior of cohesionless materials. With growing emphasis oncontrolling the structure of materials at increasingly small length-scales (even tending toward the nano-scale), understanding the effects of particle interactions - which tendto dominate at smaller length-scales - on processing operations has become moreimportant than ever.This project focuses on the effects of cohesion on mixing and segregation in simple,industrially-relevant, granular flows. In particular, the paradigm cases of a slowlyrotated tumbler and the flow in a simple shear cell are examined. We take a novel approach to this problem, placing emphasis on microscopic (particle-level), discrete modeling so as to take as its staring point the well understood interaction laws governing cohesion (capillary, van der Waals, etc.), and build to the view of the macroscopic flow via experiment and Particle Dynamics Simulation. We develop and use discretecharacterization tools of cohesive behavior in order to construct a simple theoryregarding the mixing and segregation tendency of cohesive granular matter. This theory allows us to analytically determine a phase diagram, showing both mixed and segregated phases, and agrees both quantitatively and qualitatively with experiment. These results have implications for industrial mixing/separation processes as well as novel particle production methods (e.g., engineered agglomerates with preciselyprescribed compositions)

Preparation and characterization of metal decorated metal oxide materials and study of porous structures

Today’s energy needs are presenting challenges which have to be addressed with novel, as well as sustainable and efficient solutions. Within this context, the family of metal oxide (MO) materials offers a broad diversity of both structural and physicochemical properties, along with great versatility of synthesis. Nanoscale MO particles can be decorated with other nanoparticles, particularly of metals, and new properties may arise from such combinations. The present dissertation work reports the details of the development of an experimental method to accomplish the decoration of MO materials with metal species originated by an effusive source, along with the characterization of the decorated materials’ properties. A custom-made vacuum chamber was optimized with several components (e.g. a calibration sensor to measure the effusive beam’s flux; a conical vessel to mix and expose the MO powders to such beam) and utilized to produce different decorated materials. The pure MO materials were synthesized as high-quality, size-selected particles with a patented method.Due to their different structure and optoelectronic character, magnesium (MgO) and zinc (ZnO) oxide were chosen as substrates to explore this novel decoration process; while copper, nickel and cobalt were selected as the decorating metal species, offering the opportunity to study not yet fully understood transition metal electronic properties. The several examples of newly formed materials were characterized using different techniques, in particular X-ray diffraction, photoluminescence and diffuse reflectance spectroscopy (reflectance was then converted into absorbance). These techniques revealed trends in some properties of the novel materials, and were paired with more microscopic probes, namely magnetization and electron microscopy measurements. Great emphasis was put on the Cu/MgO system. In particular, Z-contrast scanning transmission electron microscopy has shown clear presence of metal deposits of copper on the MgO surfaces, confirming the results gained with the previous techniques. Furthermore, a computer code was developed to facilitate the pore size distribution analysis obtained on mesoporous substrates. Several different materials were studied, in order to show the viability of the automated process, and the potential applicability of this method to studying porous structures for the same decoration process described earlier

Granular flow modelling of rotating drum flows using positron emission particle tracking

Tumbling mills are characterized by a flowing granular mixture comprising slurry, ore and grinding media. Akin to fluid flow, a rheological description underpinning granular flow has long been expected and pursued by many researchers. Unfortunately, no single theory has hitherto been able to successfully describe all the peculiar features and flow phases of granular systems. Tumbling mills exhibit a rich coexistence of all known flow phases and is arguably the most complicated of the granular flow geometries. Not surprisingly, current comminution models are almost entirely empirical with limited predictive capability beyond their window of design. Using Positron Emission Particle Tracking (PEPT) data we recover the key ingredients (velocity, shear rate, volume concentration, bed depth) for developing, testing and calibrating granular flow models. In this regard, 5 mm and 8 mm glass beads are rotated within a 476 mm diameter mill, fitted with angled lifter bars along the inner azimuthal walls and operated in batch mode across a range of drum rotation speeds that span cascading and cataracting Froude regimes. After averaging the PEPT outputs into representative volume elements, subsequent continuum analysis of the flowing layer revealed a rich coexistence of flow regimes - (i) quasi-static, (ii) dense (liquid-like), and (iii) inertial - that are consistent with the measured volume concentrations spanning these regimes in rotating drums. Appropriately matched constitutive choices for the shear stresses then facilitated the derivation of a new granular rheology that is able to (smoothly) capture all phases of the tumbling mill flow at transition points that match leading experimental findings reported in the literature. Limiting our models to athermal boundary conditions, we then derive the power density for better understanding of flow dissipation that ultimately drives the comminution purpose of tumbling mills. The rheology and power density models were then applied to the 5 mm and 8 mm glass bead data to reveal that shear power density is an order of magnitude larger than the normal component. Notwithstanding, the effective friction coefficient - which is akin to viscosity in typical fluids - remains relatively constant across most of the flowing layer with notable exponential growth across the interface from dense-to-inertial that continued into the inertial regime

Discrete Element Modeling of the Shape- and History-Dependent Behavior of Granular Materials

Granular materials, such as biomass feedstocks, agricultural grains, pharmaceutical pills, and geomaterials, are widespread in nature, industrial systems, and everyday life. Fundamentally, the bulk mechanical behavior of granular materials is governed by particle-level attributes such as particle morphology, surface roughness, and contact behavior. Among various numerical methods developed for modeling granular materials, the particle-based discrete element method (DEM) is particularly suited and effective in modeling the mechanical, flow, and failure behavior of granular materials. Focusing on one specific type of granular material (i.e., biomass feedstocks), the main objective of this dissertation is to develop and validate novel DEM models that can effectively capture complex particle shapes and the history-dependent contact behavior of biomass particles. Revolving around the main objective, four studies have been conducted: In the first study, the computed tomography-informed DEM models are proposed for modeling complex-shaped biomass particles, in which particle surface geometries are approximated by a polyhedral model and a sphero-polyhedral model. These models are applied to simulate compressibility tests of biomass particles, where the polyhedral model demonstrates convincingly better suitability than the sphero-polyhedron model. The polyhedral model is then applied in the simulation of the friction test. Remarkably, the polyhedral model is capable of predicting both the compressive and frictional behavior of the pine particles when evaluated against experimental data. In the second study, a set of hysteretic nonlinear DEM contact models are developed and calibrated to capture the history-dependent and the strain-hardening behavior of granular biomass feedstocks. The developed models are applied to simulate axial compression tests of biomass pine particles. Results show that the proposed models can reproduce the bulk stress-strain profiles of the physical samples and that the predicted bulk compressibility and constrained modulus under repeated compression agree reasonably with the experimental data. In the third study, the exponential form of the proposed hysteretic models is applied to granular hopper flow simulations. Numerical studies are conducted to predict the potential processing upsets and their relationships to hopper design parameters. A detailed analysis of the granular hopper flow has been provided in cross-validation of the experimental flow tests over wide ranges of the processing parameters of the hopper and material attributes of pine particles. In the fourth study, the exponential form of the proposed hysteretic models is applied to simulate the quasi-static and dense flow along an inclined plane. The effect of irregular shapes is approximated by a motion (rolling) resistance model, and the impact of particle shapes on bulk flowability is then investigated. DEM studies have verified the strong influence of inter-particle motion resistance (equivalent to particle interlocking) as critical material attributes on determining the flowability in the dense flow regime

In-situ Internal Study of Liquid Binder Penetration and Nucleation Dynamics in Wet Granulation of Pharmaceutical Powders using Synchrotron X-ray Imaging

Wet granulation is a common form of granulation with a liquid binder, having broad applications in the chemical and pharmaceutical industries. In order to produce high-quality granules, it is essential to continuously monitor the granule's microstructure during the granulation process. Wet granulation is a fast process and pharmaceutical powders are typically opaque in nature. So conventional methods cannot capture wet granulation internally. In contrast, synchrotron X-ray imaging techniques allow for visualization of the internal fast process due to higher photon flux, compared to lab-based X-ray imaging. This study employed the synchrotron X-ray imaging technique to examine the internal characteristics of powder beds and how they influence the dynamics of single droplet penetration and the microstructure of the dry granule. The single-drop impact method was used to obtain in-depth knowledge of the process of wet granulation. In this study, the liquid binders were deionized water and isopropanol, and the powders were binary mixtures of acetaminophen (APAP) as the active pharmaceutical component, with lactose monohydrate (LMH) and microcrystalline cellulose (MCC), two common excipients. An internal analysis of powders revealed that, for particles of various sizes, increasing the excipient led to the presence of more void spaces and thereby increased the porosity. It is essential to understand the powder's mixing quality before granulation. A higher mixing quality was obtained by increasing the APAP component. In general, MCC mixtures exhibited fewer aggregations and more uniform pore distribution than those from LMH ones. The spreading and vertical imbibition of an isopropanol droplet during penetration exhibited competing behaviors, demonstrating that penetration in coarse MCC powders followed a more linear vertical movement, mostly because of aligned pore distribution. For the first time, the internal rapid nucleation with liquid binders was studied. Granules in more uniformly distributed, coarser, and homogeneous powders experienced a faster rate of pore evolution during the nucleation. Wetting investigations revealed that the non-uniform pore distribution in powder beds was responsible for the Crater mechanism for the majority of the 50% of excipients. For MCC with the largest droplet diameter growth, the Spreading mechanism was observed, and for 90% of fine LMH with the longest penetration length, the Tunneling occurred. The spreading and Tunneling mechanisms produced final granules with the highest and lowest porosity, respectively. In-situ monitoring of the wet granulation process using synchrotron X-ray imaging was demonstrated in this work, and for the first time, data on pore studies throughout the nucleation and growth stages were provided. This study revealed how wet granulation and the resulting granules were affected by liquid-powder interactions. The new information gained from this research will be highly helpful for choosing the desirable powders and process conditions for granulation processes in the chemical and pharmaceutical industries

NASA patent abstracts bibliography: A continuing bibliography. Section 2: Indexes (supplement 23)

Entries for 4000 patent and patent applications citations for the period May 1969 through June 1983 are listed. Subject, invention, source, number, and accession number indexes are included

NASA patent abstracts bibliography: A continuing bibliography. Section 1: Abstracts (supplement 22)

Entries for over 4000 patents and patent applications citations for the period May 1969 through December 1982 are listed. Subject, invention, source, number, and accession number indexes are included

Research and technology

Significant research and technology activities at the Johnson Space Center (JSC) during Fiscal Year 1990 are reviewed. Research in human factors engineering, the Space Shuttle, the Space Station Freedom, space exploration and related topics are covered

NASA patent abstracts bibliography: A continuing bibliography. Section 2: Indexes (supplement 04)

For abstract, see N74-20609