Modelagem de dois fluidos é largamente aplicada para simular escoamentos gássólido em risers de leitos fluidizados circulantes. As atuais simulações são de grandes clusters (SGC), executadas em domínios reais com malhas numéricas grosseiras, ou simulações sub-malha, executadas em malhas numéricas refinadas em domínios reduzidos. O propósito das simulações sub-malha é principalmente produzir dados de meso-escala a serem aplicados em simulações de grandes clusters. A atual literatura apresenta apenas umas poucas simulações sub -malha de escoamentos gás-sólido em risers aplicando modelagem de dois fluidos, todas para condições típicas de reatores de leito fluidizado circulante de craqueamento catalítico. Neste trabalho realizou-se uma simulação sub-malha para esta condição, e também para uma outra condição típica de reatores de leito circulante para combustão/gaseificação. Correlações teóricas derivadas da teoria cinética dos escoamentos granulares (TCEG) foram aplicadas para determinar a pressão e as viscosidades da fase sólida. Considerou-se um domínio de pequenas dimensões sob condições de contorno periódicas, aplicando-se malhas numéricas refinadas. Os resultados das simulações foram comparados entre si, com outros resultados de simulação de literatura, e com dados experimentais. Então, a correção das simulações foi abordada em vista dos dados empíricos disponíveis. / Two-fluid modeling is widely applied to simulate gas-solid flows in risers of circulating fluidized beds. Current simulations are either large cluster simulations (LCS), performed in real domains under coarse numerical meshes, or sub-grid simulations, performed in reduced domains under refined numerical meshes. The purpose of subgrid simulation is mostly to provide meso-scale data to be applied in large cluster simulations. The up to date literature presents only a few sub -grid simulations of gassolid flows in risers applying two-fluid modeling, all of them for conditions typical of catalytic cracking circulating fluidized bed reactors. In the present work a sub-grid simulation was performed for this condition, as well as for a condition typical of circulating fluidized bed coal combustion/gasification reactors. Theoretical correlations derived from the kinetic theory of granular flows (KTGF) were applied to determine pressure and viscosities of the solid phase. A small size domain was considered under periodic boundary conditions, and a refined numerical mesh was applied. The results of the simulations were compared to each other, to other literature results of simulation, and to experimental data. Then, the accuracy of the simulations was addressed in view of the available empirical data.

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The entrainment rate of Ferrosilicone (FeSi) particles was measured in a 140 mm perspex column with air as the fluidising medium. Two different types of FeSi were used, namely atomised FeSi, which is mostly spherical in shape with smooth surfaces, and milled FeSi, which is irregular with rough surfaces. Both the FeSi mixtures had the same solid density and the similar average particle diameters ranging from 38 µm to 50 µm. The size and density of these particles put them on the border between Geldart A and Geldart B powders, similar to the high temperature Fischer-Tropsch catalyst. The atomised FeSi had a slightly higher concentration in fines (8.6% vs 1.8%), but except for the difference in particle shape, the two mixtures had otherwise very similar physical properties. A substantial difference in entrainment rate was measured between the atomised and milled FeSi, where the atomised had an entrainment rate of about six times higher than the milled FeSi throughout the range of superficial velocities tested. It was shown that the higher entrainment rate cannot be attributed only to the higher fines concentration, but that the difference in particle shape had a significant effect on the entrainment rate. Several two dimensional shape characterisation techniques were used in attempt to quantify the difference between the atomised and the milled FeSi. Of these the particle circularity managed to differentiate the best between the two particle mixtures. The circularities of the atomised and the milled FeSi were found to be 0.782 and 0.711 respectively. The measured circularity was used instead of a sphericity to adjust for the effect of particle shape on the terminal velocity of the particles. The adjusted terminal velocity was then used in the elutriation rate constant correlations to see which of the popular correlations in literature predicts the entrainment rate of the FeSi the best. All of the correlations gave a poor performance in predicting the measured entrainment rates. The two correlations that performed the best were that of Choi et al. (1999) (AARE = 72.6%) and Geldart et al. (1979) (AARE = 79%). It was concluded that single particle drag and single particle terminal velocities are not adequate to incorporate the effect of particle shape on entrainment rate. The method i by which shape affects entrainment rate therefore deserves further investigation. Further studies should also be done to develop a three dimensional shape descriptor that predicts bulk behaviour better. / Dissertation (MEng)--University of Pretoria, 2008. / Chemical Engineering / unrestricted

This work aims on the investigation of factors influencing the discharge characteristicsof a heat storage system, which is based on the reversible reaction system of Ca(OH)2and CaO. As storage, a packed bed reactor with embedded plate heat exchanger forindirect heat transfer is considered. The storage system was studied theoretically bymeans of finite element analysis of a corresponding mathematical model. Parametricstudies were carried out to determine the influence of reactor design and operationalmode on storage discharge. Analysis showed that heat and gas transport throughthe reaction bed as well as the heat capacity rate of the heat transfer fluid affect thedischarge characteristics to a great extent. To obtain favourable characteristics interms of the fraction of energy which can be extracted at rated power, a reaction frontperpendicular to the flow direction of the heat transfer fluid has to develop. Such afront arises for small bed dimensions in the main direction of heat transport withinthe bed and for low heat capacity rates of the heat transfer fluid. Depending on thedesign parameters, volumetric energy densities of up to 309 kWh/m3 were calculatedfor a storage system with 10 kW rated power output and a temperature increase ofthe heat transfer fluid of 100 K. Given these findings, this study is the basis for thedimensioning and design of a pilot scale heat exchanger reactor and will help toevaluate the technical feasibility of thermo-chemical heat storage systems.

Electrostatic charge generation in gas-solid fluidized beds is a significant industrial problem. Associated problems include particle agglomeration and particle wall fouling. In the polymerization industry this may result in "sheets" of fused polymer, due to exothermic reaction causing the melting of the polymer, which can fall off and block the distributor plate disrupting fluidizing gas flow. Additionally, blockage of the catalyst feed or the polymer removal system can take place or the product can become non-uniform. All of these problems require shut-down of the reactor which results in lost production time. While this phenomena has been identified for many years, the mechanisms involved are not well understood, especially wall fouling and the distribution of charge within the bed. Isolation of individual parameters such as hydrodynamics, operating conditions, and material involved is necessary to evaluate how each parameter impacts charge generation during fluidization. In this thesis, the fluidization system consisted of a stainless steel column, two online Faraday cups, and a retractable distributor plate. This system allowed for the simultaneous measurement of charge within different regions of the bed: the entrained fine particles, the particles adhered to the column wall, and the bulk of the bed. Additionally, mass and particle size distributions were measured and images of the layer of particles adhered to the column wall were taken for comparison. This allowed for a charge distribution comparison and evaluation of wall fouling. Three different parameters were investigated: duration of fluidization, column wall material, and relative humidity of fluidizing gas. Fluidization time was studied for 15, 30, 60, 120, 180, and 360 min; relative humidity was investigated for 0%, 20%, 40%, 60%, and 80% relative humidity. Both fluidization time and relative humidity were evaluated at four different fluidization gas velocities, two each in the bubbling and slugging flow regimes. Column wall material was evaluated for a stainless steel and carbon steel column at two gas velocities, one each in the bubbling and slugging flow regimes. Fluidization time was found to influence wall fouling in the bubbling flow regime as the particle layer continued to build as fluidization progressed. In the slugging flow regime, the particle layer developed within 15 minutes of the onset of fluidization. The bubbling flow regime was shown to have a greater capacity for charge generation than the slugging flow regime. This was due to the vigorous mixing in the bubbling flow regime resulting in more particle-particle interactions. Column wall material was shown to influence wall fouling in the slugging flow regime due to the differences in surface roughness of the columns. This was due to the particle-wall contacts resulting in frictional charging which is the predominant charging mechanism in this flow regime. Charge was also impacted in the bubbling flow regime in those particles that were adhered to the column wall. Relative humidity was found to influence wall fouling at the lowest gas velocity tested. However, variations in generation of charge occurred at all fluidization gas velocities tested; the charge-to-mass ratios for the particles adhered to the column wall in the slugging flow regime decreased with high relative humidities. This was due to either the formation of a water film layer on the column wall or instantaneous surface water films on the particles throughout fluidization.

Aircraft propulsion systems can be exposed to a variety of solid particulates while operating in either arid or other hazardous environments. For conventional takeoff and landing aircraft, debris can be ingested directly into the gas turbine powerplant which is exposed to the ambient environment. For helicopters and other vertical takeoff and landing (VTOL) aircraft, rotor down wash presents a particular threat during takeoff and landing operations as significant amounts of groundlevel particles can be entrained in the surrounding air and subsequently ingested into the engine. Prolonged exposure to particle ingestion events leads to premature engine wear and, in extreme cases, rapid engine failure. Expanding our current understanding of these events is the first step to enabling engine manufacturers to mitigate these damage mechanisms through novel engine designs. The work described in this dissertation is aimed at increasing the scientific understanding of these ingestion events through the development of two distinct diagnostic instruments. First, an anisokinetic particle sampling probe is designed to be used for in-situ particle sampling inside of a gas turbine engine compressor. Offtake of particles during engine operation in dusty conditions will provide researchers with an improved understanding of particle breakage tendency and component erosion susceptibility. Both experimental and numerical investigations of the probe present a comprehensive realization of probe performance characteristics. Secondly, a novel particle visualization technique is developed to provide users with particle distribution and particle mass flow estimates at the inlet of a gas turbine engine. This technique yields both time-resolved and time-averaged quantities, allowing users to have a comprehensive account of particles entering the engine. / Doctor of Philosophy / Foreign debris ingested into aircraft engines can cause serious damage and degrade their performance. The source of these ingested particles may be from atmospherically suspended ash due to volcanic eruption, high altitude ice crystals, or ground-level sand and dust. Both conventional takeoff and landing aircraft and vertical takeoff and landing (VTOL) aircraft are at risk. In extreme cases, exposure to a particle-laden atmosphere has resulted in catastrophic engine failure and loss of life. For this reason, researchers are intensely focused on mitigating the effects of these harmful particulates. The work described in this dissertation establishes two novel diagnostic capabilities. These are aimed at providing the research community with an increased understanding of how particles enter an aircraft powerplant as well as describe the behavior of these particles as they traverse the initial stages of an engine. The first instrument described is a particle sampling probe which is meant to be inserted into the compressor section of a gas turbine engine. This probe will offtake particles as they enter the engine after they have had an opportunity to interact with the rotating components of the compressor. In doing so, researchers gain an improved understanding of particle breakage tendency and component erosion susceptibility. The second instrument provides a snapshot of particle distribution at the inlet of the engine as well as estimates of total particle mass flow. This capability allows researchers to have a precise understanding of the quantity of ingested material as well as a qualitative understanding of how the inflow distribution of particles looks. Each of the developed tools represent a first step to enabling engine manufacturers to mitigate these damage mechanisms through novel engine designs.

Granular flows are ubiquitous both in natural and industrial processes such as pharmaceuticals production, mining and food–processing. Within a gas-solid fluidized bed, fundamental phenomena such as heat and mass transfer are impacted by particle convection, mixing and segregation due to rising gas bubbles. The free-bubbling regime is promising in enhancing the gas-solid contact but its mathematically chaotic bubble motion make system design challenging. Furthermore, a better understanding of rheological characteristics of these gas-solid flows is pertinent to developing accurate mathematical descriptions of granular flows. Despite their decades of usage and significance in all walks of technology, gas-solid fluidized beds remain poorly understood. In the first section of this dissertation, we investigate the combined effects of vibration and gas flow on a binary gas-solid fluidized bed. At resonant conditions, this external excitation generates a periodic, triangular, equisized, structured array of rising bubbles, reducing the chaos. Through a combination of experiments, Computational Fluid Dynamics-Discrete Element Method (CFD-DEM), and Multi Fluid Model (MFM) simulations, we demonstrate that the structured bubbling facilitates particle mixing whereas the gas flow alone (no vibration conditions) results in smaller, unstructured bubbles that promote segregation. The CFD-DEM simulations accurately capture the bubble structuring and somewhat, qualitatively and quantitively, match with the optically imaged experimental data. These investigations provide valuable insights into the dynamics of particle mixing and segregation under complex fluidization conditions. The MFM simulations failed to predict the mixing observed, indicating a need for further refinement of these models. In the second section of this dissertation, we explore the complex rheological behavior of the dry, dense particulate flows using the Discrete Element Method (DEM) simulations. In these simulations, we choose 3−D Couette cell geometry to visualize Granular Taylor–Vortex flow for various particle diameters and densities. The simulations examined the effects of varying particle volume fractions, from 0.45 to 0.60, under different shear rates, revealing a distinct rheological transition from shear-thickening to Newtonian and finally to shear-thinning behavior. The formation and nature of these vortices were compared with those observed in continuum simulations of Newtonian and shear-thinning fluids, to elucidate the unique aspects of granular flow dynamics. The third section of this thesis investigated the rheology of granular particles subjected to 1−D shear combined with sinusoidal vibration, aiming to understand the effects on pressure, shear stress, and coordination number across a range of shear rates and particle fractions. A novel vibrational regime was observed at low shear rates, below a critical threshold, characterized by a rate-independent pressure intermediate between the inertial and quasi-static regimes. These findings provide significant insights while advancing our understanding of granular material dynamics.

Master of Science / Department of Mechanical and Nuclear Engineering / Steven J. Eckels / NanoActiveTM metal oxide particles have the ability to destructively adsorb organophosphorus compounds and chlorocarbons. These nanomaterials with unique surface morphologies are subjected to separate, low concentrations of gaseous ammonia in air. NanoActiveTM materials based on magnesium oxide have large specific surface areas and defective sites that enhance surface reactivity and consequently improved adsorptivity. In gas contaminant removal by adsorption, presence of vast specific surface area is essential for effective gas-solid interaction to take place. This is also the case in many industrial and chemical applications such as purification of gases, separation and recovery of gases, catalysis etc,. Typically carbonaceous compounds are utilized and engineered in toxic gas control systems. The purpose of this study was to compare NanoActiveTM materials with carbon based compounds in the effectivity of toxic gas adsorption at low concentrations. A test facility was designed to investigate the adsorption properties of novel materials such as adorption capacity and adsorption rate. Adsorption capacity along with adsorption kinetics is a function of properties of the adsorbent and the adsorbate as well as experimental conditions. Nanomaterials were placed on a silica matrix and tested with increasing flow rates. Electrochemical sensing devices were placed at inlet and outlet of the facility to monitor real time continuous concentration profiles. Breakthrough curves were obtained from the packed bed column experiments and saturation limits of adsorbents were measured. Adsorption rates were obtained from the breakthrough curves using modified Wheeler-Jonas equation. The NanoActiveTM materials adsorbed ammonia though to a lesser extent than the Norit® compounds. This study also included measurement of pressure drop in packed beds. This information is useful in estimating energy losses in packed bed reactors. Brauner Emmet Teller tests were carried out for the calculation of surface area, pore volume and pore size of materials. These calculations suggest surface area alone had no notable influence on adsorption capacity and adsorption rates. This lead to the conclusion that adsorption was insignificant cause of absence of functional groups with affinity towards ammonia. In brief, adsorption of ammonia is possible on NanoActiveTM materials. However functional groups such as oxy-flouro compounds should be doped with novel materials to enhance the surface interactions.

Orientador: Hélio Aparecido Navarro / Banca: Paulo Cesar Razuk / Banca: Luben Cabezas Gomez / Resumo: No presente trabalho desenvolve-se um estudo de modelagem matemática e simulação numérica do escoamento gás-sólido em um leito fuidizado borbulhante. É apresentado o modelo hidrodinâmico, A, para escoamentos bifásicos gás-solido considerando a Teoria Cinética dos Escoamentos Granulares. É usado o modelo Euleriano de duas fases separadas considerando a modelagem do tensor das tensões da fase sólida através do atrito entre as partículas e da teoria cinética dos escoamentos granulares. O código fonte MFIX (Multiphase Flow with Interphase eXchanges) desenvolvido no NETL (National Energy Technology Laboratory) é utilizado para as simulações numéricas. Os resultados de simulação são obtidos resolvendo a temperatura granular algebricamente ou através de uma equação diferencial parcial. Obtêm-se resultados mais realísticos no uso da EDP com condição de contorno de deslizamento parcial na parede. Uma variação no diâmetro das partículas (partículas do grupo B e do grupo A/B) é investigada, concluindo-se que deve ser acrescentado ao código MFIX outros parâmetros físico para simulações com partículas do grupo A/B. / Abstract: In the present work is described a mathematical model and numerical simulation of gas-solid flow in the bubbling fluidized bed. It is presented the hydrodynamic model, A, for gas-solid flow considering the Kinetic Theory of Granular Flows. It is used the two fluids Eulerian model where the solid phase stress tensor is modeled considering the friction between the particles and the kinetic theory of granular flows. The code MFIX (Multiphase Flow with Interphase eXchanges) developed in NETL (National Energy Tecnology Laboratory) is used for numerical simulations. The results are obtained with the compute of the granular temperature using a partial differential equation or an algebraic expression. It was obtained more realistic results when is used a PDE with boundary conditions of the partial slip. A variation in the diameter of the particles (particles in Group B and Group A/B) it is analyzed. It is also concluded that should be added to the code MFIX other physical parameters for simulations with particles of group A/B. / Mestre

The air seeding cart is an important piece of farming equipment used in the seeding process. Three factors which are necessary to monitor during the seeding process are the seeding rate (material mass flow rate), air flow rate, and blockages. In current practice, there are systems that monitor and report air flow and blockages but not the actual seeding rate. Presently, the seeding rate is based on the metering calibration before the seeding process starts, which requires a lot of time and energy from the operator. If that goes wrong, it not only takes longer, but also costs more money and increases the already significant stress and fatigue which farmers and operators have during the seeding period. Therefore, the development of reliable, and easily calibrated, on-line sensors for flow monitoring would be beneficial. Further, such sensors would facilitate closed-loop control of the flow rate itself. In order to develop a laboratory prototype for mass flow measurement, a model for mass flow estimation was established. This was accomplished by using pressure transducers to determine the pressure drop across an elevation in the primary air cart run (between the air seeding cart and the air hoe drill). An air seeding test station was designed and developed for the study. Three different types of seeds and a granular fertilizer were chosen and tested. These tested materials were canola, wheat, chickpea and urea fertilizer (46-0-0). The general form of the model was developed using data from the canola tests. The input parameters for this mass flow estimation model were pressure drop and air flow information. The average percent error of the material mass flow rate’s full range was under 10%, except for the highest rate which tested up to 20%. Overall, more than 75% of the estimations had percent errors being less than 5%. The form of the model was also applicable to other individual tested materials with the percent error of their full ranges up to 20%. However, their average of their median error was around 5% of their full ranges. The general model was also applied to the combined data from all tested materials. The results were not as accurate as when the model was applied to the individual tested material. The median of the percent error (of material mass flow rate full range) varied from as low as 1% to as high as 30%, depending on the tested materials. Nevertheless, it demonstrated that there were consistencies between the behaviour of the four tested materials.