Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session S17: Reacting Flows V: Laminar Flames & Instabilities |
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Chair: Moshe Matalon, University of Illinois at Urbana-Champaign Room: 320 |
Tuesday, November 22, 2011 3:05PM - 3:18PM |
S17.00001: A computational method for the solution of the edge flame velocity eigenvalue as a two-dimensional boundary value problem with applications to realistic heat release Kai-Pin Liao, Moshe Matalon, Carlos Pantano We present a new numerical method to determine the edge flame velocity in a counterflow as an eigenvalue of the two-dimensional boundary-value problem for the variable density equations in the zero Mach number limit. The method utilizes a collocated arrangement of all variables in space and relies on discrete mass conservation using centered second-order accurate finite-differences. The finite element method approach, weak form, is adopted to determine the discretization near boundary and ensure well-posedness of the equations. Pressure and velocities are coupled and solved iteratively, while energy and species equations are segregated and solved sequentially. The method is coupled with pseudo-arc length continuation to explore the full parametric dependence of the solution. The edge-flame velocity and structure under the combined effect of strain and heat release will be presented. [Preview Abstract] |
Tuesday, November 22, 2011 3:18PM - 3:31PM |
S17.00002: Slow invariant manifolds for reaction-diffusion systems Joshua Mengers, Joseph Powers Slow Invariant Manifolds (SIMs) are calculated as a reduction technique for isothermal closed reaction-diffusion systems. Reaction-diffusion systems are stiff infinite dimensional dynamical systems that are costly to simulate. These systems' dynamics are reduced by using a Galerkin projection onto a finite-dimensional approximate inertial manifold, and then further reduced by projection onto a low-dimensional SIM. The Galerkin projection rigorously accounts for the coupling of the reaction and diffusion processes. The SIM accurately isolates the system's long time dynamics. A robust method of constructing one-dimensional SIMs by calculating equilibria and then integrating to find heteroclinic orbits is extended to systems with weak spatial heterogeneity. Examples are demonstrated on simple physical reaction mechanisms. [Preview Abstract] |
Tuesday, November 22, 2011 3:31PM - 3:44PM |
S17.00003: Transient OH* Chemiluminescence Imaging of Acoustically Coupled Fuel Droplet Combustion Jeffrey Wegener, Cristhian Sevilla, Jennifer Smolke, Aaron Sung, Kelvin Chen, Owen Smith, Ann Karagozian This study focuses on combustion of liquid fuel droplets during exposure to external acoustic disturbances generated as standing waves within a closed acoustic waveguide. During such acoustic excitation, the mean flame orientation is observed to be dependent on the droplet's location relative to the pressure node (PN), and is consistent with the sign of a theoretical acoustic acceleration acting on the burning system. Yet experimentally estimated acoustic accelerations, measured from the degree of mean flame deflection, differ substantially in a quantitative sense from those predicted by theory.\footnote{Tanabe, et al., {\bf Proc. Comb. Inst.}, 2000} Phase-locked OH* chemiluminescence imaging reveals a deflected flame which oscillates in position relative to the droplet, with the largest degree of oscillation near the PN. A range of acoustic forcing frequencies and droplet locations are used to investigate flame movement over multiple acoustic cycles. The degree of flame oscillation, mean flame deflection angle, and fuel droplet burning rate all correlate with one another for different relative positions of the droplet. [Preview Abstract] |
Tuesday, November 22, 2011 3:44PM - 3:57PM |
S17.00004: Puffing flame instability - Part I: Numerical investigation and analysis Shyam Menon, Guillaume Blanquart, Philipp Boettcher, Joseph Shepherd Experimental and computational work has been carried out to understand hot-surface ignition and the subsequent flame propagation in low pressure environments. In the experiments, a ``puffing'' instability is observed at rich hydrocarbon fuel-air mixture ratios. The generation and transport of these flame ``puffs,'' characterized by vortex rings, is investigated in this work by combining results from numerical simulations and experiments. Analysis of the sources of vorticity shows that baroclinic torque is the major contributor to vorticity generation. Misalignment of buoyancy induced pressure gradients in the flow field with density gradients at the flame front leads to the formation of vortex rings. Circulation of these individual vortex rings is calculated and correlated to the formation time scale of a flame ``puff.'' The effect of different parameters such as gravity, flame speed, and density ratio on the ``puffing'' frequency is investigated and a correlation of the puffing frequency with these parameters is proposed. [Preview Abstract] |
Tuesday, November 22, 2011 3:57PM - 4:10PM |
S17.00005: Puffing flame instability - Part II: Predicting the onset and frequency Philipp Boettcher, Joseph Shepherd, Shyam Menon, Guillaume Blanquart Experiments and simulations have been performed on fuel rich n- hexane air mixtures in a closed vessel. Both experiments and simulations show a distinct cyclic combustion or ``puffing'' mode. The misalignment of buoyancy induced pressure gradients and density gradients across the flame front is responsible for the generation of vorticity and its subsequent roll-up into vortex rings. In the present work, a simplified model is proposed based on the fundamental interactions between fluid mechanical and chemical parameters. This simplified fluid mechanics model is based on dimensional analysis and is used to predict the onset and frequency of the puffing behavior. [Preview Abstract] |
Tuesday, November 22, 2011 4:10PM - 4:23PM |
S17.00006: Comparing a Rayleigh-Taylor Unstable Flame to a Circular Cylinder Elizabeth Hicks Rayleigh-Taylor unstable, premixed flames show different types of behavior depending on the value of the gravitational force. We present the results of two-dimensional numerical simulations of the flame for various values of gravity. If gravity is relatively small, the flame forms a cusped shape. The cusped flame produces stable shear layers, or rolls, behind the flame. For slightly higher values of gravity, the rolls become unstable and vortex shedding begins far behind the flame front. We show that the development of this vortex shedding behavior can be described by the Landau equation and can be represented dynamically by a Hopf bifurcation. This behavior is analogous to the initial instability behind a circular cylinder, which leads to the von Karman vortex street for large enough values of the Reynolds number. The applicability of the Landau equation allows the apparently complex spatio-temporal characteristics of the vortex shedding to be simply described temporally with only a secondary spatial dependence. [Preview Abstract] |
Tuesday, November 22, 2011 4:23PM - 4:36PM |
S17.00007: Magnetic field effects on convective instability of autocatalytic fronts Manoranjan Mishra, Andre Thess, Anne De Wit The coupling between autocatalytic chemical reactions and diffusion can lead to traveling fronts whereby products invade fresh reactants at a constant speed. The properties of these reaction-diffusion fronts have been studied in details experimentally in gels used to avoid any convective motions. In absence of gels, the dynamics can be perturbed by buoyancy-driven convective flows related to density changes across the front. Experiments have shown that magnetic fields can modify such convective dynamics of traveling fronts. In this context, we study theoretically the influence of a magnetic field on the density fingering pattern of such a reaction-diffusion-convection system. The magnetization is oriented perpendicularly to the plane in which the front travels. The solutions of both products and reactants are assumed diamagnetic (i.e. negative magnetic susceptibility). The influence of the magnetic field is analyzed by numerical simulations of the reaction-diffusion-convection equations for the concentration of the autocatalytic product coupled to Darcy's law for the flow velocity through a concentration dependent magnetic force. We show that the magnetic field is able to suppress or enhance the convective instability depending on the value of the magnetic Rayleigh number of the problem. [Preview Abstract] |
Tuesday, November 22, 2011 4:36PM - 4:49PM |
S17.00008: ABSTRACT WITHDRAWN |
Tuesday, November 22, 2011 4:49PM - 5:02PM |
S17.00009: ABSTRACT WITHDRAWN |
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