Francine Battaglia

Bifurcation of Low Reynolds Number Flows in Symmetric Channels

The flowfields in two-dimensional channels with discontinuous expansions are studied numerically to understand how the channel expansion ratio influences the symmetric and nonsymmetric solutions that are known to occur. For improved confidence and understanding, two distinct numerical techniques are used. The general flowfield characteristics in both symmetric and asymmetric regimes are ascertained by a time-marching finite difference procedure. The flowfields and the bifurcation structure of the steady solutions of the Navier-Stokes equations are determined independently using the finite element technique. The two procedures are then compared both as to their predicted critical Reynolds numbers and the resulting flowfield characteristics. Following this, both numerical procedures are compared with experiments.

Simulating fire whirls

A numerical investigation of swirling fire plumes is pursued to understand how swirl alters the plume dynamics and combustion. One example is the ‘fire whirl’ which is known to arise naturally during forest fires. This buoyancy-driven fire plume entrains ambient fluid as heated gases rise. Vorticity associated with a mechanism such as wind shear can be concentrated by the fire, creating a vortex core along the axis of the plume. The result is a whirling fire. The current approach considers the relationship between buoyancy and swirl using a configuration based on fixing the heat release rate of the fire and imposing circulation. Large-eddy methodologies are used in the numerical analyses. Results indicate that the structure of the fire plume is significantly altered when angular momentum is imparted to the ambient fluid. The vertical acceleration induced by buoyancy generates strain fields which stretch out the flames as they wrap around the nominal plume centreline. The whirling fire constricts radially and stretches the plume vertically.(Some figures in this article are in colour only in the electronic version; see www.iop.org)

Theoretical study of the pyrolysis of methyltrichlorosilane in the gas phase. 3. Reaction rate constant calculations.

The rate constants for the gas-phase reactions in the silicon carbide chemical vapor deposition of methyltrichlorosilane (Ge, Y. B.; Gordon, M. S.; Battaglia, F.; Fox, R. O. J. Phys. Chem. A 2007, 111, 1462.) were calculated. Transition state theory was applied to the reactions with a well-defined transition state; canonical variational transition state theory was applied to the barrierless reactions by finding the generalized transition state with the maximum Gibbs free energy along the reaction path. Geometry optimizations were carried out with second-order perturbation theory (MP2) and the cc-pVDZ basis set. The partition functions were calculated within the harmonic oscillator and rigid rotor approximations. The final potential energy surfaces were obtained using the left-eigenstate coupled-cluster theory, CR-CC(2,3) with the cc-pVTZ basis set. The high-pressure approximation was applied to the unimolecular reactions. The predicted rate constants for more than 50 reactions were compared with the experimental ones at various temperatures and pressures; the deviations are generally less than 1 order of magnitude. Theory is found to be in reasonable agreement with the experiments.

Theoretical Study of the Pyrolysis of Methyltrichlorosilane in the Gas Phase. 2. Reaction Paths and Transition States

The kinetics for the previously proposed 114-reaction mechanism for the chemical vapor deposition (CVD) process that leads from methyltrichlorosilane (MTS) to silicon carbide (SiC) are examined. Among the 114 reactions, 41 are predicted to proceed with no intervening barrier. For the remaining 73 reactions, transition states and their corresponding barrier heights have been explored using second-order perturbation theory (MP2) with the aug-cc-pVDZ basis set. Final energies for the reaction barriers were obtained using both MP2 with the aug-cc-pVTZ basis set and coupled cluster theory (CCSD(T)) with the aug-cc-pVDZ basis set. CCSD(T)/aug-cc-pVTZ energies were estimated by assuming additivity of basis set and correlation effects. Partition functions for the computation of thermodynamic properties of the transition states were calculated with MP2/aug-cc-pVDZ. Forward and reverse Gibbs free energy barriers were obtained at 11 temperatures ranging from 0 to 2000 K. Important reaction pathways are illustrated at 0 and 1400 K.

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.

Simulations of multiphase reactive flows in fluidized beds using in situ adaptive tabulation

A novel algorithm, in situ adaptive tabulation (ISAT), has been implemented into a multiphase computational fluid dynamics (CFD) code to treat complex chemistry calculations. In this work, isothermal silane pyrolysis in a fluidized bed reactor is presented to test the feasibility and explore the capabilities of ISAT with a finite-volume two-fluid code (MFIX). Based on the results of simulations, an error tolerance of 10−5 is found to be satisfactory for maintaining the accuracy for the examples investigated. Due to the rapidly changing time step used in the CFD code, the performance enhancement was found initially to be minimal. However, the performance is significantly improved when ISAT is called using a fixed time step.

Bifurcation Characteristics of Flows in Rectangular Sudden Expansion Channels

The effect of three dimensionality on low Reynolds number flows past a symmetric sudden expansion in a channel was investigated. The geometric expansion ratio in the current study was 2:1 and the aspect ratio was 6:1. Both experimental velocity measurements and two- and three-dimensional simulations for the flow along the centerplane of the rectangular duct are presented for Reynolds numbers in the range of 150 to 600. Comparison of the two-dimensional simulations with the experiments revealed that the simulations failed to capture completely the total expansion effect on the flow, which is influenced by both geometric and hydrodynamic effects. To properly do so requires the definition of an effective expansion ratio, which is the ratio of the downstream and upstream hydraulic diameters and is therefore a function of both the expansion and aspect ratios

Visualizing Cold-Flow Fluidized Beds With X-Rays

Glass beads or sand particles are typically used as bed materials in fluidized beds due to their high sphericity, uniform properties, and resistance to breaking. X-ray imaging can be used to visualize these complex flows. Glass attenuates X-rays much more than the surrounding air and, consequently, the images may be nearly saturated in order to resolve the internal flow of a large diameter bed. This paper focuses on the use of alternative bed materials to increase X-ray penetration and resolution to enhance flow visualization in a 9.5 cm diameter fluidized bed. Melamine plastic, ground walnut shell, and ground corncob particles are qualitatively compared to glass beads using X-ray computer tomography (CT) imaging and X-ray radiography. The various beds are compared at three different flow rates and the ratio of superficial gas velocity to minimum fluidization velocity is constant for each bed material. X-ray CT imaging is used to provide a qualitative view of the local time-averaged solids concentration, and clearly shows differences in fluidization between the materials. Channeling is shown in melamine, walnut shells and corncob at low flow rates, however, the beds fluidize more uniformly as gas flow rate increases. In all cases, glass beads fluidize most uniformly and flow rate does not significantly affect fluidization uniformity. Radiographic movies confirm that visualizing internal flow structures of the glass bed is much more difficult than for other materials.Copyright

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

Assessment of Drag Models for Geldart A Particles in Bubbling Fluidized Beds

In order to accurately predict the hydrodynamic behavior of gas and solid phases using an Eulerian–Eulerian approach, it is crucial to use appropriate drag models to capture the correct physics. In this study, the performance of seven drag models for fluidization of Geldart A particles of coal, poplar wood, and their mixtures was assessed. In spite of the previous findings that bode badly for using predominately Geldart B drag models for fine particles, the results of our study revealed 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 drag models derived from empirical relationships adopt better with Geldart A particles compared to drag models that were numerically developed. Overall, the Huilin–Gidaspow drag model showed the best performance for both single solid phases and binary mixtures, however, for binary mixtures, Wen–Yu model predictions were also accurate.