Bulletin of the American Physical Society
61st Annual Meeting of the APS Division of Fluid Dynamics
Volume 53, Number 15
Sunday–Tuesday, November 23–25, 2008; San Antonio, Texas
Session LQ: Reacting Flows III: Flames and Modeling |
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Chair: Carlos Pantano, University of Illinois Room: 202B |
Monday, November 24, 2008 3:35PM - 3:48PM |
LQ.00001: Autoignition of hydrogen and air using direct numerical simulation Jeffrey Doom, Krishnan Mahesh Direct numerical simulation (DNS) is used to study to auto--ignition in laminar vortex rings and turbulent diffusion flames. A novel, all--Mach number algorithm developed by {Doom et al} ({\it J. Comput. Phys.} 2007) is used. The chemical mechanism is a nine species, nineteen reaction mechanism for $H_2$ and Air from Mueller at el ({\it Int. J. Chem. Kinet.} 1999). The vortex ring simulations inject diluted $H_2$ at ambient temperature into hot air, and study the effects of stroke ratio, air to fuel ratio and Lewis number. At smaller stroke ratios, ignition occurs in the wake of the vortex ring and propagates into the vortex core. At larger stroke ratios, ignition occurs along the edges of the trailing column before propagating towards the vortex core. The turbulent diffusion flame simulations are three--dimensional and consider the interaction of initially isotropic turbulence with an unstrained diffusion flame. The simulations examine the nature of distinct ignition kernels, the relative roles of chemical reactions, and the relation between the observed behavior and laminar flames and the perfectly stirred reactor problem. These results will be discussed. [Preview Abstract] |
Monday, November 24, 2008 3:48PM - 4:01PM |
LQ.00002: Influence of Standing Acoustic Waves on Combustion of Alternative Fuels Sophonias Teshome, Alec Pezeshkian, Owen Smith, Ann Karagozian The present experimental study focuses on exposure of single burning fuel droplets to external acoustical excitation created in a cylindrical waveguide bounded by two loudspeakers. This configuration creates a relatively symmetric acoustic field whereby standing waves may be created, forming a pressure node (PN) or antinode (PAN) within the waveguide. A range of alternative liquid fuels is considered in these experiments, including ethanol, methanol, white gasoline, JP-8, and blends of JP-8 and liquid synthetic fuel. Droplet burning rates, flame characteristics, and their dependence on the position of the droplet relative to the PN and PAN are quantified. In some cases, large scale acoustic forcing is observed to cause flame deflection so strong that the effective acoustic radiation force appears to approach the magnitude of the buoyant force acting on the flames. Flame orientation is observed to change abruptly as the droplet position is moved from one side to the other relative to the PN or PAN, consistent with theory,\footnote{Tanabe, et al., {\bf Proc. Comb. Inst.} 30, p. 1957-1964, 2005} although for very large amplitude acoustic forcing, the magnitude of the acoustic acceleration can exceed theoretical predictions. Variations in burning rates for a range of fuels and excitation conditions relevant to engine systems are quantified. [Preview Abstract] |
Monday, November 24, 2008 4:01PM - 4:14PM |
LQ.00003: Constructing Slow Invariant Manifolds for Reactive Systems with Detailed Kinetics Ashraf N. Al-Khateeb, Joseph M. Powers, Samuel Paolucci, Andrew J. Sommese, Jeffrey A. Diller, Jonathan D. Hauenstein, Joshua D. Mengers Rational reduction of reactive systems becomes possible when their Slow Invariant Manifolds (SIMs) are identified. In this work, a robust method of constructing the one-dimensional SIMs for unsteady spatially homogeneous reactive systems is presented. The method is based on global analysis of the composition phase space of the reactive system, where all critical points, finite and infinite, are identified using a projective space technique. Then by connecting these equilibria via trajectories, the SIM can be characterized. Employing the projective space technique offers the possibility to construct SIMs for detailed kinetic systems ({\em e.g.} $H_2$- $Air$). Moreover, the relation between reactive systems dynamics and the thermodynamics is examined, and it is shown that classical thermodynamics potentials cannot identify the SIM. [Preview Abstract] |
Monday, November 24, 2008 4:14PM - 4:27PM |
LQ.00004: Pinning of reaction fronts by moving vortices Justin Winokur, Garrett O'Malley, Tom Solomon We present experimental and numerical studies of the effects of moving vortices on the propagation of reaction fronts. The front is produced by the excitable Belousov-Zhabotinsky chemical reaction, and the flow is forced with a magnetohydrodynamic technique. An individual vortex or vortex pair moving in the same direction as a front often pins and drags the front, with a maximum pinning speed that depends on the strength of the vortex. In an extended system, the moving vortex leaves a wake-like structure that dramatically affects the overall front. Multiple pinning vortices leave a pattern of wakes that combine to form more complicated front structures. We extend these experiments to random patterns of vortices which produce complicated pinned fronts whose detailed structure depends on the speed of the vortices relative to the background fluid. The experiments are complemented by numerical simulations that simplifies the large-scale behavior by modelling moving vortices as pinning centers. [Preview Abstract] |
Monday, November 24, 2008 4:27PM - 4:40PM |
LQ.00005: The Effects of Time-Periodic Shear on a Diffusion Flame Anchored to a Model Propellant Amir H.G. Isfahani, Ju Zhang, Thomas L. Jackson Propellants of solid rocket motors are subject to intense time-dependent shear flows and the response of the combustion field to these flows is of fundamental interest. Erosive burning (EB), the enhanced regression rate that can arise due to these flows, affects the performance of the solid rocket motor: the specific-impulse history. It is generally agreed that EB results from an increased heat transfer to the surface. The geometry is that of two quarter-planes of ammonium perchlorate (AP) and binder (or a blend of AP/binder). Three step kinetics is considered: AP decomposition and two diffusion flames, one between the virgin AP gases and binder and one between AP decomposed gases and binder. Gas and solid phases are coupled and temperature along the surface as well as the burn rate is solved for. We present an estimation of the shear parameters as a function of the motor size using a 2D planar periodic rocket (PPR) analysis without resorting to fully time-dependent three-dimensional turbulent simulations. These parameters are then used to study the change in the heat flux to the surface and the burn rate. It is shown that the burn rate is increased by more than two folds for larger amplitudes and frequencies. [Preview Abstract] |
Monday, November 24, 2008 4:40PM - 4:53PM |
LQ.00006: Flame propagation in dusty gases Nicholas Poole, Moshe Matalon The combustion of finely atomized dust particles is of great importance to many practical technologies. While the study of flame propagation through gaseous fuel-air mixtures is relatively well developed, there currently exists a lack of fundamental understanding on the mechanisms that govern flame propagation through dust clouds. Unlike homogeneous gas flames, the study of metal particle combustion is highly complex, involving chemical and physical processes occurring in multiple phases. The mathematical description requires particular importance to be placed on the processes occurring in the condensed phase, for they present novel challenges in dust combustion modeling and ultimately provide its most distinguishing characteristics. Typical reaction zones are confined to a thin region of space and necessitate the use of perturbation methods with distinguished limits in order to capture the essential behavior of the system. Through matched asymptotic expansions we are able to arrive at an expression for the speed of the propagating flame front in terms of important combustion parameters such as mixture strength, particle loading, and particle size. [Preview Abstract] |
Monday, November 24, 2008 4:53PM - 5:06PM |
LQ.00007: The structure of partially-premixed methane/air flames under varying premixing Celine Kluzek, Adonios Karpetis The present work examines the spatial and scalar structure of laminar, partially premixed methane/air flames with the objective of developing flamelet mappings that capture the effect of varying premixture strength (air addition in fuel.) Experimental databases containing full thermochemistry measurements within laminar axisymmetric flames were obtained at Sandia National Laboratories, and the measurements of all major species and temperature are compared to opposed-jet one-dimensional flow simulation using Cantera and the full chemical kinetic mechanism of GRI 3.0. Particular emphasis is placed on the scalar structure of the laminar flames, and the formation of flamelet mappings that capture all of the salient features of thermochemistry in a conserved scalar representation. Three different premixture strengths were examined in detail: equivalence ratios of 1.8, 2.2, and 3.17 resulted in clear differences in the flame scalar structure, particularly in the position of the rich premixed flame zone and the attendant levels of major and intermediate species (carbon monoxide and hydrogen). [Preview Abstract] |
Monday, November 24, 2008 5:06PM - 5:19PM |
LQ.00008: Spectral closure for the probability density function of reactive scalars in isotropic turbulence Yanjun Xia, Lance Collins We present a spectral closure for the joint composition probability density function (PDF) for multiple scalars undergoing isothermal chemical reactions. The formulation, based on the eddy damped quasi-normal Markovian (EDQNM) theory, accounts for macroscopic mixing and scalar dissipation separately and allows for differential diffusion of reactant and/or product species due to differences in their molecular diffusivities. The EDQNM theory has been reformulated into a stochastic differential equation for the particle concentrations in a Monte Carlo scheme. We present results for non-premixed isotropic scalars and one-dimensional mixing without and with differential diffusion. Results are compared with direct numerical simulations (DNS). The model captures well the competing effects of mixing, chemical reaction and differential diffusion. [Preview Abstract] |
Monday, November 24, 2008 5:19PM - 5:32PM |
LQ.00009: Turbulence-flame interactions in type Ia supernovae Andrew Aspden, John Bell, Marc Day, Stan Woosley, Mike Zingale The small-scale dynamics of nuclear flames in the supernova environment are examined using high-resolution three-dimensional simulations in which the details of the flame structure are fully resolved. The range of densities examined, $1$ to $8 \times 10^7$~g~cm$^{-3}$, spans the transition from the laminar flamelet regime to the distributed burning regime, where small-scale turbulence disrupts the flame. The use of a low Mach number algorithm facilitates the accurate resolution of the thermal structure of the flame and the inviscid turbulent kinetic energy cascade, while implicitly incorporating kinetic energy dissipation at the grid-scale cutoff. For an assumed background of isotropic Kolmogorov turbulence with an energy characteristic of a type Ia supernova, we find a transition density between $1$ and $3 \times 10^7$~g~cm$^{-3}$, where the nature of the burning changes qualitatively. This transition from flamelet to distributed burning reveals important consequences for turbulent flame models. [Preview Abstract] |
Monday, November 24, 2008 5:32PM - 5:45PM |
LQ.00010: Front propagation in vortex-dominated flows Garrett O'Malley, Justin Winokur, Tom Solomon We present experiments that explore how the propagation of a reaction front is affected by a two-dimensional flow dominated by vortices. The reaction is the excitable Belousov-Zhabotinsky chemical reaction. The flow is driven by the interaction between an electrical current passing through the fluid and a spatially-varying magnetic field produced by an array of magnets below the fluid. For some of the experiments, the forcing is strong enough to produce a weakly turbulent flow. Measurements are made both of the enhanced diffusion coefficient $D^*$ describing transport in the flow and of the propagation speed $v$ of a reaction front in the same flow. Scaling of $v$ versus $D^*$ is compared with that for the standard Fisher-Kolmogorov-Petrovsky-Piskunov prediction $v \sim \sqrt{D}$ (with $D$ as the molecular diffusion coefficient) for the reaction-diffusion limit with no fluid advection. We also study the effects of superdiffusive transport and L\'evy flights on front propagation in a time-dependent vortex array with wavy jet regions. [Preview Abstract] |
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