Bulletin of the American Physical Society
60th Annual Meeting of the Divison of Fluid Dynamics
Volume 52, Number 12
Sunday–Tuesday, November 18–20, 2007; Salt Lake City, Utah
Session ET: Multiphase Flows III |
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Chair: Pavlos Vlachos, Virginia Tech Room: Salt Palace Convention Center Ballroom FH |
Sunday, November 18, 2007 4:10PM - 4:23PM |
ET.00001: Computational and Experimental Studies of Fluidized Beds for Biomass Gasification Francine Battaglia, Mirka Deza, Nathan Franka, Theodore Heindel Fluidized bed gasifiers can convert feedstocks with low-carbon content into valuable products such as ethanol. Understanding fluidized bed hydrodynamics is important for reactor design and avoiding issues such as agglomeration or defluidization of the bed. In particular, biomass gasification is not well characterized and is the focus of this work. Glass beads or sand particles are typically used as bed materials due to their high sphericity and uniform properties. X-ray imaging will be used to visualize these complex flows and alternative bed materials will be considered to increase X-ray penetration and resolution to enhance flow visualization. Furthermore, computational modeling of fluidized beds can be used to predict operation of biomass gasifiers after extensive validation with experimental data. The hydrodynamics will be modeled assuming each phase behaves as interpenetrating continua using an Eulerian model and each solid phase is characterized by a particle diameter and density so that segregation and elutriation can be described. The simulations will model the cold-flow fluidized bed experiment, and consider factors such as sphericity of the particles, and calibration of drag coefficients. Hydrodynamic results from the simulations will be qualitatively and quantitatively compared to X-ray flow visualization studies of a similar bed. [Preview Abstract] |
Sunday, November 18, 2007 4:23PM - 4:36PM |
ET.00002: Turbulence modeling of sediment-laden, open channel flows Sanjeev Jha, Fabian Bombardelli In spite of the knowledge already gained of multi-phase flows by employing the multi-component flow theory in fluid mechanics, there is still no consensus among researchers on the most appropriate models to use in a given case. The issue complicates even further with the modeling of turbulence in those flows. In the special case of sediment transport in natural open channels (i.e., rivers), the understanding of the interaction between the different phases (water and sediment) presents diverse challenges. First, the bottom of the channel interchanges material with the water column, creating a constant source of disperse phase; second, the turbulence is non-homogeneous and non-isotropic. Most studies in the past have focused on the mean flow characteristics and on the distribution of sediment in the vertical, but they have not dealt with the turbulence statistics. In this work, we test diverse theoretical and numerical models for one-dimensional, open-channel flow with the dataset of Muste et al. (2005), one of the few datasets reporting detailed distributions of turbulence statistics in the vertical direction in the open channel. We analyze the performance of models of different complexity, ranging from simple ``mixture'' models to complete two-fluid models. For turbulence closure, we test the standard $k-\varepsilon $, $k-\omega $ and Reynolds stress (RSM) models, and also their extended versions proposed by different authors. [Preview Abstract] |
Sunday, November 18, 2007 4:36PM - 4:49PM |
ET.00003: Numerical simulation of high shear rate two-phase flow with high density ratio Dokyun Kim, Marcus Herrmann, Parviz Moin It is a challenging problem to simulate two-phase flow in the incompressible limit with high density ratio when there is also high shear rate, since a numerical instability can develop at the phase interface. Even if a kinetic energy-conserving scheme is used for the single phase regions in the one fluid approach, kinetic energy can grow at the phase interface and overall energy is not conserved. We present a numerical method that addresses this problem. We descritize the variable density Navier-Stokes equations based on a finite volume formulation with a balanced force algorithm. A central-difference scheme is used for the convective term in the single phase regions ensuring kinetic energy conservation there. The central- difference scheme switches to an upwind-biased dissipative scheme at the phase interface to control the potential numerical instability. In order to track the phase interface, the Refined Level Set Grid (RLSG) method is used, which solves the level set equations on a separate equidistance Cartesian grid. The present numerical method is applied to representative two-phase flow problems with high density ratio and high shear rate, including liquid jet atomization. It is shown to be stable and accurate. [Preview Abstract] |
Sunday, November 18, 2007 4:49PM - 5:02PM |
ET.00004: Compositional space parameterization for general multi-component multiphase systems Denis Voskov, Hamdi Tchelepi We present a general parameterization of the thermodynamic behavior of multiphase, multi-component systems. The phase behavior in the compositional space is represented using a low dimensional tie-simplex parameterization. This parameterization improves the robustness of the phase behavior representation as well as the efficiency of various types of compositional computations. We demonstrate this Compositional Space Parameterization (CSP) framework for large-scale compositional reservoir simulation. In the standard compositional simulation approach, an Equation of State (EoS) is used to detect the phase state and calculate the phase compositions, if needed. These EoS computations can dominate the overall simulation cost. We compare our adaptive CSP approach with standard EoS based simulation for several challenging problems of practical interest. The comparisons indicate that the CSP strategy is more robust, and computational efficient. Another type of applications is an equilibrium flash calculation of systems with a large number of phases. The complexity and strong nonlinear behaviors associated with such problems pose serious difficulties for standard techniques. Here, we describe an effective tie-simplex parameterization for such systems at a fixed pressure and temperature. The preprocessed data can be used in conventional EoS based calculations as an initial guess to accelerate convergence. [Preview Abstract] |
Sunday, November 18, 2007 5:02PM - 5:15PM |
ET.00005: Modeling the dynamics of turbulent multiphase gravity currents: the importance of geologically diverse boundary conditions in volcanic flows Josef Dufek, Michael Manga, George Bergantz Pyroclastic flows produced during explosive volcanic eruptions represent a high-energy end-member for granular flows, and permit exploration of a vast parameter space of particle-particle and particle-gas interactions. The inherent difficulty of observing volcanic events as well as the scarcity of on-going eruptions has resulted in a continuing discussion about the internal particle concentration of pyroclastic flows from dilute to dense end-members. For instance, it remains unclear to what degree basally concentrated bed-load regions of the flow are responsible for mass and momentum transfer. In order to probe the internal structure of these flows an Eulerian-Eulerian-Lagrangian (\textit{EEL}) computational approach was coupled with an examination of deposits of flows that have traversed a body of water (and thereby filtering out their bed-load) versus flow that have traveled over-land. We integrate the EEL model with laboratory experiments to better understand momentum and heat transfer at multiple length and timescales. This investigation reveals that energy-dissipation at the basal boundary is one of the primary factors determining the run-out distance of pyroclastic flows and determines the emergence of concentrated and poorly-sorted bed-load regions. [Preview Abstract] |
Sunday, November 18, 2007 5:15PM - 5:28PM |
ET.00006: Resolving Discontinuous Interfaces of Multiphase Flows Using a Material Point Method Xia Ma, Duan Zhang, Qisu Zou, Paul Giguere Numerically tracking oscillating sharp interfaces in multiphase flows imposes a great challenge, especially when the phases have different constitutive relations. It is demonstrated here that the material point method (MPM) is a powerful way to tackle these kinds of problem. For instance, in the case of fluid structure interaction, the elastic stress in the solid structure and viscous stress in the fluid can be calculated accurately in the MPM method because Lagrangian points can be used to track the history of solid deformation and the fluid phase can be calculated with Eulerian method. This combination has been implemented in our code CartaBlanca. We show that the MPM method has a big advantage over a pure Eulerian method when a moving and oscillating interface is to be clearly maintained for long time. We will also show simulation of a GNEP project (Global Nuclear Energy Partnership). In this simulation, a sharp wall/fluid moving interface is tracked in order to predict a possible accident. The results will show the effects of thermal expansion and flow-structure interaction on the speculated accident. [Preview Abstract] |
Sunday, November 18, 2007 5:28PM - 5:41PM |
ET.00007: Numerical Simulation of Immiscible Multiple Fluids Flow by Diffuse Interface Model Kohei Okita, Kenji Ono A diffuse interface model for immiscible multiple fluids is developed by modifying a free energy model for binary fluid. The present free energy model, in which an interface is indicated by two different order parameters unlike the usual diffuse interface model, includes a term for immiscibility of each order parameter and interaction terms of gradient energy. Then, Cahn-Hilliard \& Navier-Stokes system with the diffuse interface model for immiscible multiple fluids are proposed. Firstly, in order to examine the immiscibility of triple fluids with the present free energy model, the time evolution of the order parameters is solved by the one-dimensional Cahn-Hilliard equation only. The immiscibility of fluids is reasonably satisfied as the order of $10^{-5}$ in an equilibrium state. Secondly, we reproduce the equilibrium angle at the triple junction by changing the combinations of surface tension. The results of the angle are good agreement with that derived from the theory of Neumann Triangle in wide range from 40 to 170 degrees. At the presentation, the equilibrium shapes of a droplet in two different fluid interfaces and the motion of two immiscible droplets with Boussinesq approximation will be demonstrated by three-dimensional calculation. [Preview Abstract] |
Sunday, November 18, 2007 5:41PM - 5:54PM |
ET.00008: The filtered description of momentum transfer in continuum type gas-solid flow models Juray De Wilde Whereas in the non-filtered continuum gas-solid flow models, momentum transfer is described by the drag force and the solid volume fraction of the gas phase pressure gradient, the description of momentum transfer in a filtered way is not yet fully understood. An apparent drag force approach is often proposed. It is shown that the unacceptable linear wave propagation speed behavior obtained with an apparent drag force approach can be corrected by an appropriate apparent distribution of the filtered gas phase pressure gradient over the phases. As coarser meshes are used, the microscopic drag force type description of gas-solid momentum transfer should be progressively replaced by a more macroscopic description that basically consists of distributing the filtered gas phase pressure gradient, the ultimate macroscopic driving force of both the phases, over the phases. The linear wave propagation speed behavior and the reformulation of the generalized added mass learn that the latter can guarantee an appropriate apparent distribution of the filtered gas phase pressure gradient over the phases for a given apparent drag force. This suggests the use of a generalized added mass closure model approach to completely describe filtered gas-solid momentum transfer, that is including both the filtered drag force and the correlation between the solid volume fraction and the gas phase pressure gradient. [Preview Abstract] |
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