Mirka Deza

CFD Modeling and X-Ray Imaging of Biomass in a Fluidized Bed

Computational modeling of fluidized beds can be used to predict the operation of biomass gasifiers after extensive validation with experimental data. The present work focused on validating computational simulations of a fluidized bed using a multifluid Eulerian― Eulerian model to represent the gas and solid phases as interpenetrating continua. Simulations of a cold-flow glass bead fluidized bed, using two different drag models, were compared with experimental results for model validation. The validated numerical model was then used to complete a parametric study for the coefficient of restitution and particle sphericity, which are unknown properties of biomass. Biomass is not well characterized, and so this study attempts to demonstrate how particle properties affect the hydrodynamics of a fluidized bed. Hydrodynamic results from the simulations were compared with X-ray flow visualization computed tomography studies of a similar bed. It was found that the Gidaspow (blending) model can accurately predict the hydrodynamics of a biomass fluidized bed. The coefficient of restitution of biomass did not affect the hydrodynamics of the bed for the conditions of this study; however, the bed hydrodynamics were more sensitive to particle sphericity variation.

Computational Modeling of Biomass in a Fluidized Bed Gasifier

Computational modeling of fluidized beds can be used to predict operation of biomass gasifiers after extensive validation with experimental data. The present work will focus on computational simulations of a fluidized bed gasifier with a multifluid Eulerian-Eulerian model to represent the gas and solid phases as interpenetrating continua. The simulations described in this paper will model cold-flow fluidized bed experiments, and consider factors such as particle sphericity, coefficient of restitution, and drag coefficient calibration. Hydrodynamic results from the simulations will be qualitatively compared with X-ray flow visualization studies of a similar bed.Copyright

Effects of Mixing Using Side Port Air Injection on a Biomass Fluidized Bed

Fluidized beds are being used in practice to gasify biomass to create producer gas, a flammable gas that can be used for process heating. However, recent literature has identified the need to better understand and characterize biomass fluidization hydrodynamics, and has motivated the combined experimental-numerical effort in this work. A cylindrical reactor is considered and a side port is introduced to inject air and promote mixing within the bed. Comparisons between the computational fluid dynamics (CFD) simulations with experiments indicate that three-dimensional simulations are necessary to capture the fluidization behavior of the more complex geometry. This paper considers the effects of increasing side port air flow on the homogeneity of the bed material in a 10.2 cm diameter fluidized bed filled with 500-600 μmground walnut shell particles. The use of two air injection ports diametrically opposed to each other is also modeled using CFD to determine their effects on fluidization hydrodynamics. Whenever possible, the simulations are compared to experimental data of time-average local gas holdup obtained using X-ray computed tomography. This study will show that increasing the fluidization and side port air flows contribute to a more homogeneous bed. Furthermore, the introduction of two side ports results in a more symmetric gas-solid distribution. Disciplines Chemical Engineering | Mechanical Engineering Comments This article is from Journal of Fluids Engineering 133 (2011): 111302, doi:10.1115/1.4005136. Posted with permission. This article is available at Iowa State University Digital Repository: http://lib.dr.iastate.edu/me_pubs/8

Approximating a Three-Dimensional Fluidized Bed With Two-Dimensional Simulations

Fluidized beds can be used to gasify biomass in the production of producer gas, a flammable gas that can replace natural gas in process heating. Modeling these reactors with computational fluid dynamics (CFD) simulations is advantageous when performing parametric studies for design and scale-up. From a computational resource point of view, two-dimensional simulations are easier to perform than three-dimensional simulations, but they may not capture the proper physics. This paper will compare two- and three-dimensional simulations in a 10.2 cm diameter fluidized bed with side air injection to determine when two-dimensional simulations are adequate to capture the bed hydrodynamics. Simulations will be completed in a glass bead fluidized bed operating at 1.5Umf and 3Umf , where Umf is the minimum fluidization velocity. Side air injection is also simulated to model biomass injection for gasification applications. The simulations are compared to experimentally obtained time-averaged local gas holdup values using X-ray computed tomography. Results indicate that for the conditions of this study, two-dimensional simulations qualitatively predict the correct hydrodynamics and gas holdup trends that are observed experimentally for a limited range of fluidization conditions.Copyright

On the Modeling of Gas-Solid Fluidization: Which Physics Are Most Important to Capture?

Recent studies to predict biomass fluidization hydrodynamics motivated a new study to reassess how to model gas-solid characteristics that capture the same physics as that measured in experiments. An Eulerian-Eulerian multifluid model was used to simulate and analyze gas-solid hydrodynamic behavior of the fluidized beds. The relations for the pressure drop measured at fluidization were used to correct for the bed mass by either adjusting the initial solids packing fraction or initial bed height, two parameters that must be specified in a CFD model. Simulations using sand as the bed medium were compared with experiments and it was found that adjusting the bulk density, or in other words, the initial solids volume packing, correctly predicted the pressure drop measured experimentally, but significantly under-predicted the minimum fluidization velocity. By adjusting the initial bed height to correct for the mass, both the pressure drop and minimum fluidization velocity were successfully predicted. Ground walnut shell and ground corncob were used as biomass media and simulations were performed for two reactor bed diameters by simply adjusting the initial bed height to match the measured pressure drop. All of the simulations correctly predicted the pressure drop curves of the experimental data. Further examination of the simulations and experimental data for walnut shell confirmed that adjusting the bed height was the best approach to model fluidization without artificially altering the physics and retaining the known characteristics of the bed material.Copyright

A Validation Study for the Hydrodynamics of Biomass in a Fluidized Bed

Computational modeling of fluidized beds can be used to predict operation of biomass gasifiers after extensive validation with experimental data. The present work focused on computational simulations of a fluidized bed using a multifluid Eulerian-Eulerian model to represent the gas and solid phases as interpenetrating continua. Hydrodynamic results from the simulations were quantitatively compared with X-ray flow visualization studies of a similar bed. It was found that the Gidaspow model can accurately predict the hydrodynamics of the biomass in a fluidized bed. The coefficient of restitution of biomass was fairly high and did not affect the hydrodynamics of the bed; however, the model was more sensitive to particle sphericity variation.Copyright

Modeling a Biomass Fluidizing Bed With Side Port Air Injection

Fluidized beds are used to gasify materials such as coal or biomass in the production of producer gas. Modeling these reactors using computational fluid dynamics is advantageous when performing parametric studies for design and scale-up. While two-dimensional simulations are easier to perform than three-dimensional simulations, they may not capture the proper physics. This paper compares two- and three-dimensional simulations with experiments for a reactor geometry with side port air injection. The side port is located within the bed region so that the injected air can help promote mixing. Of interest in this study is validating the hydrodynamics of fluidizing biomass. Two operating conditions of the fluidized bed are studied for superficial gas velocities of 1.5Umf and 3.0Umf , where Umf is the minimum fluidization velocity. The material used to represent biomass is ground walnut shell because it tends to fluidize uniformly and falls within the Geldart type B classification. The simulations are compared to experimental data of time-averaged local gas holdup values using X-ray computed tomography. Results indicate that for the conditions of this study, two-dimensional simulations overpredict the gas holdup trends when compared to the experiments. However, the three-dimensional simulations compare exceptionally well with the experiments, thus predicting the fluidization hydrodynamics, irrespective of flowrate or complexity due to the side air port. Furthermore, the study demonstrates the importance of using a three-dimensional model for bubbling fluidized beds with complex physics.Copyright

High Fidelity CFD Modeling of Natural Ventilation in a Solar House

Natural ventilation is an important factor in the design of sustainable buildings; it has the potential to improve air quality, while providing thermal comfort at reduced energy costs. Computational fluid dynamics (CFD) simulations provide comprehensive information on the internal flow pattern and can be used as a design tool. The present work offers insight of natural ventilation in a fully functional building, namely, the solar facility Interlock House in Iowa. Ventilation in the house is studied during summer months with some of its furniture included. The results show quantitative agreement between numerical simulations and experiments of vertical temperature profiles for each room. The temperature profile of the room with the inlet opening shows a more pronounced temperature variation. Flow patterns show higher velocities near the walls and marked flow circulation towards the opposite side of the building. The purpose of this work is to validate the numerical model that predicts airflow distribution for different configurations.Copyright

Pressure Fluctuation Analysis in Gas-Solid Fluidized Beds Using CFD Simulations

Reliable computational methods provide valuable insight into gas-solid flow processes and can be used as a design tool. Of particular interest is the hydrodynamics of a binary mixture of sand-biomass in a fluidized bed. Our study interprets the hydrodynamic states of a fluidized bed by analyzing the local pressure fluctuations of beds of sand and a mixture of cotton stalks and sand over long time periods. Standard deviation of pressure drop will determine different fluidization regimes at inlet gas velocities ranging from 2 to 9 times the minimum fluidization velocity. Bode plots will present the pressure spectra and reveal characteristic frequencies that describe the bed hydrodynamics for different fluidization regimes. This works contribution will present CFD as a useful tool to perform pressure fluctuation analysis, the study of pressure fluctuations in the turbulent regime and the analysis of a binary mixture using CFD.Copyright

Effects of Increasing Inlet Velocities and Side Port Air Injection on a Biomass Fluidizing Bed

Fluidized beds are being used in practice to gasify biomass to create producer gas, a flammable gas that can be used for process heating. However, recent literature has identified the need to better understand and characterize biomass fluidization hydrodynamics, and computational fluid dynamics (CFD) is one approach in this effort. Previous work by the authors considered the validity of using two-dimensional versus three-dimensional simulations to model a cold-flow fluidizing biomass bed configured with a single side port air injection. The side port is introduced to inject air and promote mixing within the bed. Comparisons with experiments indicated that three-dimensional simulations were necessary to capture the fluidization behavior for the more complex geometry. This paper considers the effects of increasing fluidization air flow and side port air flow on the homogeneity of the bed material in a 10.2 cm diameter fluidized bed. Two air injection ports diametrically opposed to each other are also considered to determine their effects on fluidization hydrodynamics. Whenever possible, the simulations are compared to experimental data of time-averaged local gas holdup obtained using X-ray computed tomography. This study will show that increasing the fluidization and side port air flows contribute to a more homogeneous bed. Furthermore, the introduction of two side ports results in a more symmetric gas-solid distribution.Copyright