Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 55, Number 16
Sunday–Tuesday, November 21–23, 2010; Long Beach, California
Session EU: Multiphase Flows III |
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Chair: Federico Toschi, Technische Universiteit Eindhoven Room: Hyatt Regency Long Beach Regency A |
Sunday, November 21, 2010 4:10PM - 4:23PM |
EU.00001: Predicting Biomass Fluidization through Appropriate Modeling of Initial Conditions Francine Battaglia, Jonas England, Santhip Kanholy, Mirka Deza Fluidized bed gasifiers can be used to convert feedstock with low-carbon content into fuels, basic chemicals and hydrogen. When biomass is the feedstock, there is a difference in the fluidization behavior between the solid particles and bed media (e.g., refractory sand) due to contrasting size, shape and particle density. The differences can lead to poor solid fuel distribution and diminished gasifier performance. The present work will focus on computational simulations of a fluidized bed gasifier using an Eulerian-Eulerian model to represent the gas and solid phases as interpenetrating continua. Recent studies to predict biomass fluidization motivated this study to reassess how to best model gas-solid characteristics that capture the same physics measured experimentally. Relations for pressure drop are 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 CFD models. It was found that adjusting the initial solids volume packing correctly predicted the pressure drop measured experimentally but underpredicted 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 without artificially altering the physics and retaining the known characteristics of the bed material. [Preview Abstract] |
Sunday, November 21, 2010 4:23PM - 4:36PM |
EU.00002: Numerical study of impact of evaporation on liquid jet in cross-flow Marios Soteriou, Xiaoyi Li, Marco Arienti Atomization of a liquid fuel jet by a high speed cross-flowing gas plays a critical role in many propulsion devices. High fidelity simulation offers the potential of a better understanding and enhancement of this atomization process. In this work, a computationally efficient hybrid Eulerian-Lagrangian approach is coupled with a droplet evaporation model and is used to probe the impact of evaporation on the spray development. The Coupled Level Set and Volume of Fluid (CLSVOF) method is used to directly calculate the breakup and coalescence of the liquid-gas interface. Adaptive Mesh Refinement (AMR) is adopted to achieve high resolution at the interface. Small fuel droplets in dilute regions are removed from the Eulerian description, transformed into Lagrangian particles and tracked by a discrete phase transport model. The coupling of the spray evaporation to the gas phase is examined with respect to jet blockage, spray penetration, and overall far-field spray dispersion. The calculation is validated with flow rate, spray size distribution and velocity data acquired in a spray rig at high-Weber, high-Reynolds number injection conditions. The effect of evaporation on spray distribution is also discussed. [Preview Abstract] |
Sunday, November 21, 2010 4:36PM - 4:49PM |
EU.00003: Characterization of primary atomization mechanism of straight liquid jets Junji Shinjo, Akira Umemura Detailed numerical simulations of straight liquid jets have been carried out to elucidate the mechanism of liquid primary atomization. The impact of liquid against the gas forms the umbrella-shaped front where initial atomization occurs subsequently. At later time, surface instability also develops on the liquid core surface, leading to ligament/droplet formation from the core. As the ligament/droplet formation mechanism has been already reported, this study mainly focuses on this surface instability development. Disturbances for instability development come from the front through the recirculating gas flow and from the gas-liquid interaction of the core itself. Several cases are compared to identify the parameter dependence of instability development and the results are compared with the theoretical prediction. [Preview Abstract] |
Sunday, November 21, 2010 4:49PM - 5:02PM |
EU.00004: Modeling and simulation of primary atomization with phase transition Peng Zeng, Bernd Binninger, Norbert Peters, Heinz Pitsch, Marcus Herrmann This paper is intended to demonstrate the capability of a numerical approach to investigate the primary atomization process. The evaporation on the interface of two--phase flow is computationally studied within the context of a hydrodynamic theory. A level-set method is used to track down the phase interface, which is treated as a free boundary surface. The flow field is described by the incompressible Navier-Stokes equations, with different densities and viscosities for the liquid and gaseous phases, supplemented by singular source terms that properly account for thermal expansion effects. The numerical scheme has been tested on several benchmark problems and was shown to be stable and accurate. [Preview Abstract] |
Sunday, November 21, 2010 5:02PM - 5:15PM |
EU.00005: Analysis of Column Instability Modes in Liquid Jet in Crossflow Atomization Sina Ghods, Marco Arienti, Marios Soteriou, Marcus Herrmann Atomizing liquids by injecting them into crossflows is a common approach to generate fuel sprays in gas turbines and augmentors. The mechanisms by which the liquid jet initially breaks up, however, are not well understood. To analyze the instability mechanism of the liquid column, we perform proper orthogonal decomposition of side view images extracted from detailed simulations of the near injector primary atomization region. This analysis shows a single dominant wavelength with the associated interface corrugation traveling downstream with the jet. Using consistent temporal averaging of the simulation data we extract mean interface geometries and boundary layer velocity profiles. These are used to calculate the most unstable wavelength of the shear layer instability following the procedure of Boeck \& Zaleski (2005). The theoretical wavelengths are comparable to those extracted from the simulation data. In addition to shear layer instability we analyze Rayleigh-Taylor as a potential instability mechanism of the liquid column. [Preview Abstract] |
Sunday, November 21, 2010 5:15PM - 5:28PM |
EU.00006: Droplets dynamics and breakup in turbulent flows Federico Toschi, Luca Biferale, Prasad Perlekar, Mauro Sbragaglia Turbulent emulsions are of relevance to many Natural and industrial flows alike. In order to study the statistical properties of droplets deformation and breakup in turbulence we perform high resolution numerical simulations of a multicomponent flow composed by two fluid with equal density. We aim at investigating the interplay between turbulent fluctuations and surface tension. The flow is solved in a cubic periodic box with a stirring at the largest scales in order to realize an homogeneous and isotropic turbulent flow field. The numerical simulations are performed by means of a fully-parallel Lattice Boltzmann code where the two fluid components are described by means of a Shan-Chen model without need for explicit interface tracking. Our numerical experiment allow to investigate e.g. the probability distribution function of droplet radii and the physics of the exchange of energy between surface and fluid fluctuations. We present preliminary results for a selected number of problem parameters. [Preview Abstract] |
Sunday, November 21, 2010 5:28PM - 5:41PM |
EU.00007: Simulations of bubble coalescence and breaking-up using connectivity-free point-set front tracking method with finite element Chu Wang, Lucy Zhang The capability of handling constant and multi-scale bubble topological changes is essential in modeling and simulating bubble coalescence and breaking up. The traditional front tracking method relies on the connectivity of the interfacial points to calculate the normal and curvature in order to evaluate surface tension. In bubble coalescence and breaking up, such connectivity reconstruction can be quite expensive. In this work, we adopt the point-set method [1] to construct each individual interfacial point without any connectivity. This approach combined with the original front tracking concept allow us to model bubble topological changes automatically. By letting the interface to be at a constant level, the indicator field is smeared out using the quintic B-Spline function. A regeneration method adopting one-dimensional Newton iteration can update the interfacial points in order to cope with the topology change. The interface points are then coupled with a finite element fluid solver to study bubble rising in a channel testing case. The coalescence and breaking up are also simulated to show the advantage of using the point-set method. \\[4pt] [1] D. J. Torres, J. U. Brackbill, The Point-Set Method: Front-Tracking without Connectivity, J. Comupt. Phys, 2000,165(2):620-644 [Preview Abstract] |
Sunday, November 21, 2010 5:41PM - 5:54PM |
EU.00008: Direct Numerical Simulation of Air Layer Drag Reduction over a Backward-facing Step Dokyun Kim, Parviz Moin Direct Numerical Simulation (DNS) of two-phase flow is performed to investigate the air layer drag reduction (ALDR) phenomenon in turbulent flow over a backward-facing step. In their experimental study, Elbing et al. (JFM, 2008) have observed a stable air layer on an entire flat plate if air is injected beyond the critical air-flow rate. In the present study, air is injected at the step on the wall into turbulent water flow for ALDR. The Reynolds and Weber numbers based on the water properties and step height are 22,800 and 560, respectively. An inlet section length before the step is 3h and the post expansion length is 30h, where h is the step height. The total number of grid points is about 271 million for DNS. The level set method is used to track the phase interface and the structured-mesh finite volume solver is used with an efficient algorithm for two-phase DNS. Two cases with different air-flow rates are performed to investigate the mechanism and stability of air layer. For high air-flow rate, the stable air layer is formed on the plate and more than 90\% drag reduction is obtained. In the case of low air-flow rate, the air layer breaks up and ALDR is not achieved. The parameters governing the stability of air layer from the numerical simulations is also consistent with the results of stability analysis. [Preview Abstract] |
Sunday, November 21, 2010 5:54PM - 6:07PM |
EU.00009: DNS of turbulent two-layer flows Roman Zhvansky, Peter Spelt Results will be presented for pressure-driven turbulent gas flow over a liquid layer in a 3D channel. These have been obtained with a DNS code that resolves all discontinuities across the gas/liquid interface in a sharp manner. In the simulations considered here, the interface is forced to remain flat; the corresponding kinematic condition is implemented exactly, as is the continuity in tangential stress. The wall-type region near the interface is therefore resolved. Mean profiles near the interface obtained with this method will be used to assess to what extent the turbulence can be represented by near-wall turbulence. The results for the distribution of shear stress exerted by the gas on the liquid layer have implications on large-scale modeling of turbulent two-phase flows. [Preview Abstract] |
Sunday, November 21, 2010 6:07PM - 6:20PM |
EU.00010: Interfacial flows in micro-channels: flow regimes and transitions Majid Ahmadlouydarab, Peng Gao, James J. Feng We report simulations of gas-liquid flows in periodically patterned micro-channels with grooves and ridges. A diffuse-interface model is used to handle the interfacial motion and the three-phase contact line. A constant body force applies on both components to simulate a pressure-driven flow. Depending on the competition between the driving force and capillary force and the level of liquid saturation, several flow regimes have been observed in the micro-channel, including slug flows with air bubbles, slug flows with water drops, water rivulets alongside air flow and driven sessile drops. We investigate the critical conditions for the transition among the regimes as affected by substrate wettability, initial morphology of the interface, geometry of the micro-channel and viscosity ratio. [Preview Abstract] |
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