Discrete Particle Simulation of the Gas-Solid Flow in a Circulating Fluidized Bed

This paper presents a numerical study of the gas-solid flow in a three-dimensional Circulating Fluidized Bed (CFB) by means of Combined Continuum and Discrete Method (CCDM) in which the motion of discrete particles is described by Discrete Particle Method (DPM) on the basis of Newton’s laws of motion applied to individual particles and the flow of continuum fluid by the traditional Computational Fluid Dynamics (CFD) based on the local averaged Navier-Stokes equations. The simulation is achieved by incorporating DPM codes into the commercial CFD software package Fluent. It is shown that the discrete particle simulation can capture the key flow features in CFB such as core-annulus structure, axial solid segregation and S-shaped axial solid concentration. The numerical results also show the effect of the pulsation arising from the expansion of the fluidized bed on the performance of the cyclone separator. The gas-solid, particle-wall and particle-particle interactions are analysed to understand the underlying mechanisms of CFB systems

CFD Modeling of Hydrodynamics of Fluidized Bed

The objective of this project is to simulate a gas-solid fluidized by applying CFD techniques in order to investigate hydrodynamics and heat transfer phenomena. Reactor model predictions will be compared with the corresponding experimental data reported in the literature to validate the model . To simulate a gas-solid fluidized bed we need to use the multiphase flow approach . First we have to write the equations for the different flow regimes and then different CFD techniques are applied for discretization of those equations. After that a code is written for calculating the values of volume fraction , velocity and temperature

Sand-assisted fluidization of large cylindrical and spherical biomass particles: Experiments and simulation

In this study, bubbling fluidization of a sand fluidized bed with different biomass loadings are investigated by means of the experiments and numerical simulation. The radioactive particle tracking (RPT) technique is employed to explore the impact of the particle shape factor on the biomass distribution and velocity profiles when it is fluidized in a 152 mm diameter bed with a 228 mm static height. Using a pair of fiber optic sensors, the bubbling characteristics of these mixtures at the upper half of the dense bed are determined at superficial gas velocities ranging from U=0.2 m/s to U=1.0 m/s. The experimental results show that despite cycling with a similar frequency, spherical biomass particles rise faster and sink slower than the cylindrical biomass particles. Furthermore, bubbles are more prone to break in the presence of biomass particles with lower sphericity. In the separate series of experiments, the reliability of the “frozen bed” technique to quantify the axial distribution of biomass particles is assessed by the RPT results. Using NEPTUNE_CFD software, three-dimensional numerical simulations are carried out via an Eulerian n-fluid approach. Validation of the simulation results with the experiments demonstrates that, in general, simulation satisfactorily reproduces the key fluidization and mixing features of biomass particles such as the global and local time-average distribution and velocity profiles

Experimental and cfd simulation study of binary solid-liquid fluidized bed

Solid liquid fluidization experiments were carried out for binary mixture of iron ore-quartz and chromite-quartz.All the experiments were performed with close size range particles to reduce the size and shape effect of the particles. It has been found that weight ratio of flotsam and jetsam affect the expansion of the fluidized bed. It was also found that particle density affect the bed expansion and other hydrodynamic characteristics. Also a feed for liquid/solid fluidization observed three types of binary system, (a) easily separable (b) difficult to separate (c) non separable. For an unstable binary fluidized bed system corresponding to a pure heavier particle bottom of the bed, lighter particles segregate at the top, and some particles neither segregate nor sink, and they missed place at the middle. Therefore knowing the physical properties of the mineral particles, an appropriate fluidization can be chosen. The objective of the CFD analysis in this study is to investigate numerically the hydrodynamic behaviour of a liquid- solid fluidized bed. The methodology used in CFD to solve problems relating mass, momentum and heat transfer and the details about problem description and approach used in ANSYS FLEUNT 13.0 to get the solution. Finally results of simulation and comparison with experimental results are shown. CFD predicts the flow characteristics, bed hydrodynamics etc. The simulation is done for a column of 150cm height and 10cm diameter filled with 125µm iron ore, chromite and quartz mixture till a certain height. It is observed that the bed expands considerably with increase in water velocity. Key Word: Liquid-solid fluidization, segregation, bed expansion, flotsam, jetsam, CF

Particle Attrition with Supersonic Nozzles in a High Temperature Fluidized Bed

Fluidized beds are widely used for a variety of processes such as food, pharmaceutical, petrochemical and energy production. As a typical application of fluidized beds, the fluid coking process uses thermal cracking reactions to upgrade heavy oils and bitumen from oil sands. In order to maintain a well fluidized bed and a satisfactory operation, a series of supersonic nozzles are used to inject high pressure steam in the bed to maintain the coke particle within an optimal range. Currently, the attrition nozzles consume a large florwrate of high pressure and superheated steam, which accounts for about 40 % of the total energy consumption in fluid coking reactors. Improving the efficiency of the attrition process would increase energy efficiency and reduce sour waste water production, reducing the environmental impact of heavy oil upgrading. Therefore, the main objective of the present thesis is an experimental and numerical study of particle attrition with supersonic nozzles in high temperature fluidized beds. The specific objective is to improve particle grinding efficiency and reduce the steam consumption in the fluid coking process. To achieve the research objective, the primary investigations focused on the solids entrainment and penetration of jets issuing from supersonic nozzles, which have significant effects on particle attrition. Novel measuring techniques, therefore, were developed to accurately measure the flowrate of solids entrained into the jet and its penetration length. The numerical and experimental studies reveal that the jet penetration lengths are related to the two-phase Froude number. A new correlation was developed to predict the penetration length of jets issuing from supersonic nozzles in high temperature fluidized beds, based on Benjelloun’s correlation and the Froude number. The attrition experimental results demonstrate that larger scale nozzles, operating with a high flowrate of a low molecular weight gas at high temperature provide the highest grinding efficiency. A jet-induced attrition model in fluidized beds at high temperature has been proposed and developed. The model is a coupled Eulerian-Eulerian multiphase model with a population balance method. The particle-particle interactions are described with the kinetic theory of granular flow. Experimental results were used to determine and modify the critical parameters of the model. The best prediction was obtained using the Ghadiri breakage kernel, generalized daughter size distribution function, and discrete solution method. Finally, the research focused on the enhancement of jet-induced attrition in fluidized bed. A twin-jet nozzle gave a grinding efficiency that is about 35% higher than with a single nozzle. The benefits of the twin-jet nozzle seem stronger at higher nozzle pressures and high temperature. It is likely that the twin-jet nozzle entrains more solids into the jets when compared with a single nozzle with the same gas flowrate

Pressure waves oscillation levels: the influence of scale effects in solid rocket motors

The main objective of the present research is the characterisation of the unsteady internal fluid dynamics occurring in solid rocket motors (SRM). In particular, the attention is focused on the evaluation and the analysis of pressure waves oscillation (PWO) available in such type of motors and the manner through which such waves modify the amplitude and the frequency as the motor characteristic dimensions vary

Transferência de calor em leitos fluidizados: influência dos parâmetros da superfície de troca térmica

Dissertação (mestrado) - Universidade Federal de Santa Catarina, Centro Tecnológico, Programa de Pós-Graduação em Engenharia Química, Florianópolis, 2015.Leitos fluidizados são equipamentos versáteis cujo número de aplicações experiencia um crescimento constante. Apesar disso, os fenômenos que ocorrem dentro destes equipamentos ainda não são perfeitamente compreendidos, sobretudo no que se refere à transferência de calor. A fluidodinâmica computacional tem desempenhado um importante papel na compreensão, design e scale up de operações de engenharia, porém para isso é preciso que se disponha de modelos computacionais confiáveis e validados frente a dados experimentais. Neste trabalho foram estudados vários parâmetros da simulação de leitos fluidizados, como modelo de arraste e coeficiente de especularidade, a fim de obter uma configuração que melhor descrevesse este processo. Após a conclusão desta etapa, o trabalho seguiu com o estudo de diferentes superfícies de troca térmica submersas no leito fluidizado, a fim de cumprir o objetivo proposto: verificar a influência dos parâmetros destas superfícies no fenômeno de transferência de calor entre leito e superfície. Para isso foi utilizado o software ANSYS® CFD (FLUENT®), com uma abordagem Euleriana-Euleriana, que descreve ambas as fases como fluidos interpenetrantes, sendo que as propriedades da fase particulada foram descritas através da teoria cinética dos escoamentos granulares. O leito fluidizado simulado foi baseado no trabalho experimental de Di Natale, Lancia e Nigro (2007) e utilizava ar como fase gasosa e partículas de vidro, classe B de Geldart (1973) como fase particulada. Os resultados de Di Natale, Lancia e Nigro (2007) foram utilizados para validação. Todas as simulações foram realizadas num domínio bidimensional, utilizando axissimetria. Após vários testes definiu-se que o modelo que melhor representa os dados experimentais utiliza o modelo de Gidaspow (1986) para o arraste, coeficiente de especularidade igual a 0,1 e modelo ? - e disperso para turbulência. Após validação do modelo foram construídas diferentes geometrias, com diversas superfícies de troca térmica, incluindo diferentes cilindros, uma esfera e um cone. O aumento das dimensões do cilindro, principalmente do diâmetro, gerou uma diminuição do coeficiente de transferência de calor. A superfície esférica produziu o maior valor de coeficiente de transferência de calor, o que está de acordo com o trabalho experimental de Di Natale, Lancia e Nigro (2007). Outras velocidades de entrada do gás foram testadas, porém o modelo falhou em descrever o comportamento fluidodinâmico a uma velocidade mais baixa, provando a importância de se verificar para quais condições um modelo se aplica, antes de usá-lo indiscriminadamente

Numerical modeling of gas-particle flows inside fluidized beds

Fluidized beds have widespread application in industry due to their increased rate of heat, mass, and momentum transfer. In order to effectively design fluidized beds at the industrial scale, it is essential to have an understanding of the complex hydrodynamic behavior of the dense gas-particle flows inside them. This thesis is focused on the bubbling fluidization of Geldart B particles. The Eulerian–Eulerian “Two-fluid model” (TFM) approach was used to simulate dense gas-particle flows inside two different three-dimensional (3D) bubbling beds. The numerical code Multiphase Flow with Interphase eXchanges (MFIX) was used to perform all the 3D simulations. The results were validated against published experimental data. This manuscript-based thesis documents four different studies. The first study, Chapter 2, reports an in-depth investigation of two different models for the particle stress tensor in the elastic-inertial regime and assesses their ability to predict the hydrodynamics of a 3D cylindrical fluidized bed. Contours of inertial number, defined as the ratio of the inertial forces to the frictional forces, were used to visualize the flow properties. Analysis of the flow properties for a range of gas-particle regimes based on the inertial number enhances our insight into the flow behavior in such a complex system. Chapter 3 reports a comprehensive study to assess the effect of three different particle-wall boundary conditions (BCs) on the structural features of a dense gas-particle flow inside a 3D thin bubbling bed. Accordingly, the effect of each wall model on the velocity field, 3D bubble statistics, gas-pressure fluctuations, and particle resolved-scale Reynolds stress were investigated. Also, the dominant mixing regions inside the bed were identified in order to quantitatively describe the bed performance. Chapter 4 performs an in-depth systematic study that uses a particle energy budget analysis to investigate the dynamics of the bubbling bed discussed in Chapter 3. The budget analysis helps not only to quantify the relative importance of various terms contributing to the energy cascade, but also to identify the regions in the bed where most of the energy transfer takes place. Chapter 5 applies state-of-the-art post-processing methodologies, namely, the Proper Orthogonal Decom- position (POD) and the swirling strength criterion to the fluctuating particle flow fields predicted by the TFM of a bubbling bed to identify and analyze the dominant spatio-temporal patterns of the particulate phase. The variation of the POD temporal coefficients associated with the particle volume fraction fluctu- ation field suggested the existence of a low-dimensional attractor and irregular periodicity in the flow. The particle vortical motions were characterized by their flat structure. POD was used to obtain a reduced-order reconstruction of the particle velocity and volume fraction fields using a subset of eigenmodes. In summary, this thesis attempts to quantitatively describe some important features of bubbling beds dynamics that have received relatively little attention in the literature. To this end, it was observed that the use of inertial number, investigation of the energy cascade process, and studying particle vortical structures were helpful to quantitatively explore the underlying physics of bubbling beds. A major objective was also to identify a set of proper TFM parameters and particle-wall BC for high-fidelity simulation of bubbling beds

Investigation of packed bed and moving bed reactors with benchmarking using advanced measurement and computational techniques

Trickle bed reactors (TBR), as typical packed bed reactors (PBR), are widely used in various fields. Very limited information regarding the flow behaviors, hydrodynamic, and mathematical models in extrudate catalyst shapes, such as cylinders, trilobes, and quadrilobes, can be found in literatures because the major focus was on spherical shape. Therefore, a hybrid pressure drops and liquid holdup phenomenological model for extrudate catalyst shapes was developed based on two-phase volume averaged equations, which showed high accuracy against experimental data. The maldistribution and dynamic liquid holdup were investigated in quadrilobe catalyst using gamma-ray computed tomography. A pseudo-3D empirical model was developed and compared with deep neural network predictions. Both models were in good agreement with experimental data. The accretion locations of heavy metal contaminants entrained with flow were tracked by the dynamic radioactive particle tracking technique in the packed beds of sphere, cylinder, trilobe, and quadrilobe, respectively. Kernel density estimator was used to indicate the accretion probability distribution, showing that pressure drop played an important role in heavy metal accretions. CFD simulations of random packed trilobe catalyst bed were conducted to obtain the local information and were validated by experimental data. Moving bed reactors (MBR), as a relatively new type of reactor, encounter many challenges due to the bed expansion because of the concurrent gas-liquid upflow. DEM simulation was used to generate expanded bed. A porosity distribution correlation was developed and implemented in CFD simulations to investigate the hydrodynamics --Abstract, page iv