Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session M34: Thermo-Acoustics and Flame Instabilities |
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Chair: Tim Lieuwen, Georgia Institute of Technology Room: 2024 |
Tuesday, November 25, 2014 8:00AM - 8:13AM |
M34.00001: Linear stability analysis of a premixed flame with lateral shear Carlos Pantano, Xiaoyi Lu The hydrodynamic instability analysis of one-dimensional steady premixed planar flames, known as Darrieus-Landau, is extended to a lateral (side) uniform shear configuration. Here, in the steady planar flame, there is a transverse pressure gradient orthogonal to the density gradient and it is a situation of interest when a turbulent flame travels into a region of free-shear turbulence (such as a jet or shear layer). It is shown that the problem can be formulated analytically and a new dispersion relation can be determined. We were able to analytically solve Euler's equation (with a constant shear parameter) and obtain the growth rate of flame front perturbation. The study of the dispersion relation shows that perturbations have two types of behavior as wavenumber increases. First, for negligible shear, we recover Darrieus-Landau result. Second, as the nondimensional shear parameter increases the flame becomes more unstable initially but eventually it completely stabilizes. There is a finite range of values of shear for which the flame remains stable. Finally, for sufficiently high shear, the flame becomes unstable again. Further details will be discussed at the talk. [Preview Abstract] |
Tuesday, November 25, 2014 8:13AM - 8:26AM |
M34.00002: Hydrodynamic instabilities in swirl-stabilized combustion: experimental assessment and theoretical modelling Kilian Oberleithner, Michael St\"ohr, Steffen Terhaar, Oliver Paschereit In gas turbine industry, it is common practice to implement swirling jets and associated vortex breakdown to stabilize the flame and to enhance turbulent mixing. The flow field of such swirl-stabilized combustors features a wide range of flow instabilities that promote the formation of large-scale flow structure. This talk presents recent experimental studies at the Technical University Berlin and the German Aerospace Center (DLR) targeting the impact of these instabilities on the combustion performance. Particular focus is placed on two types of instability: (i) a self-excited helical instability, typically known as the precessing vortex core, which crucially affects mixing and flame anchoring; (ii) the axisymmetric Kelvin-Helmholtz instability, which crucially affects the flame dynamics at thermo-acoustic oscillations. All experimental observations are correlated with analytic flow models utilizing linear hydrodynamic stability theory. This mathematical framework reveals the driving mechanisms that lead to the formation, saturation, and suppression of large-scale flow structures and how these mechanisms interact with the combustion process. [Preview Abstract] |
Tuesday, November 25, 2014 8:26AM - 8:39AM |
M34.00003: Global Stability Analysis of Jet Diffusion Flames D. Moreno-Boza, W. Coenen, A. Sevilla, A.L. S\'anchez This work investigates the global stability of axisymmetric laminar jet diffusion flames at moderately large Reynolds numbers, including effects of buoyancy, temperature increase due to chemical reaction and air coflow. The ultimate objective is to clarify the two different types of instabilities observed in experiments, as well as the connection of these instabilities with the phenomenon of diffusion-flame flickering. Quasi-isobaric conditions corresponding to low-Mach-number jets are considered and stability results regarding hot and light jets are also described. The limit of infinitely fast chemical reaction is used in the development, which assumes also a unity value of the fuel Lewis number, thereby enabling a simplified description of the temperature and composition fields in terms of a single mixture-fraction variable. A finite-element method is developed to integrate the steady equations of continuity, momentum and mixture fraction, which determine the basic steady flame structure as well as the associated perturbed equations that determine its 2D global stability. [Preview Abstract] |
Tuesday, November 25, 2014 8:39AM - 8:52AM |
M34.00004: The dynamics of cellular two-dimensional flames Christophe Almarcha, Joel Quinard, Bruno Denet, Elias Al-Sarraf, Jean-Marie Laugier, Emmanuel Villermaux Premixed flames propagating in an initially quiescent medium undergo hydrodynamic instabilities that corrugate their shape, leading to non stationary cells. The shape of a flame is a critical issue as it rules its speed or the presence of incomplete reaction zones. We report here on experiments of premixed propane-air and methane-air flames freely propagating in a vertically oriented Hele-Shaw cell. In such configuration, the quasi two dimensional flames are easy to study by image analysis thanks to a high speed camera. The dynamics is favorably compared to numerical simulations of Michelson-Sivashinsky equation. The cell size distribution is analyzed and seems to be self similar whatever the gas mixture composition, provided that the dynamics is sufficiently rich, ie the flame is sufficiently unstable. We propose an explanation for this distribution. [Preview Abstract] |
Tuesday, November 25, 2014 8:52AM - 9:05AM |
M34.00005: Parametrized mode decomposition for bifurcation analysis applied to a thermo-acoustically oscillating flame Taraneh Sayadi, Peter Schmid, Franck Richecoeur, Daniel Durox Thermo-acoustic systems belong to a class of dynamical systems that are governed by multiple parameters. Changing these parameters alters the response of the dynamical system and causes it to bifurcate. Due to their many applications and potential impact on a variety of combustion systems, there is great interest in devising control strategies to weaken or suppress thermo-acoustic instabilities. However, the system dynamics have to be available in reduced-order form to allow the design of such controllers and their operation in real-time. As the dominant modes and their respective frequencies change with varying the system parameters, the dynamical system needs to be analyzed separately for a set of fixed parameter values, before the dynamics can be linked in parameter-space. This two-step process is not only cumbersome, but also ambiguous when applied to systems operating close to a bifurcation point. Here we propose a parametrized decomposition algorithm which is capable of analyzing dynamical systems as they go through a bifurcation, extracting the dominant modes of the pre- and post-bifurcation regime. The algorithm is applied to a thermo-acoustically oscillating flame and to pressure signals from experiments. A few selected mode are capable of reproducing the dynamics. [Preview Abstract] |
Tuesday, November 25, 2014 9:05AM - 9:18AM |
M34.00006: Partial Extinction and the Rayleigh Index in Acoustically Driven Fuel Droplet Combustion Dario Valentini, Phuoc Hai Tran, Brett Lopez, Ari Ekmekji, Owen Smith, Ann Karagozian This experimental study examines burning liquid fuel droplets exposed to standing acoustic waves created within an atmospheric pressure waveguide. Building on prior studies which study relatively low-level excitation conditions in which the droplet is situated in the vicinity of a pressure node (PN), \footnote{Sevilla, et al., Comb. Flame \textbf{161}, pp. 1604-1619, 2014} the present experiments focus on higher amplitude excitation which can lead to periodic flame extinction. Phase-locked OH* chemiluminescence imaging reveals temporal oscillations in flame standoff distance from the droplet as well as chemiluminescent intensity in response to the applied acoustic perturbations. Temporal variation in the chemiluminescent intensity as well as pressure in the vicinity of the burning droplet enable quantification of combustion-acoustic coupling via the Rayleigh index. While the sign of the Rayleigh index is consistent with oscillatory combustion during low-level acoustic excitation, when periodic partial extinction occurs at higher amplitude excitation, the Rayleigh index is insufficient to fully represent such coupling. Alternative metrics and methods are explored to enable a more robust study under such conditions. [Preview Abstract] |
Tuesday, November 25, 2014 9:18AM - 9:31AM |
M34.00007: Finite amplitude wave interaction with premixed laminar flames Mohamad Aslani, Jonathan D. Regele The physics underlying combustion instability is an active area of research because of its detrimental impact in many combustion devices, such as turbines, jet engines, and liquid rocket engines. Pressure waves, ranging from acoustic waves to strong shocks, are potential sources of these disturbances. Literature on flame-disturbance interactions are primarily focused on either acoustics or strong shock wave interactions, with little information about the wide spectrum of behaviors that may exist between these two extremes. For example, the interaction between a flame and a finite amplitude compression wave is not well characterized. This phenomenon is difficult to study numerically due to the wide range of scales that need to be captured, requiring powerful and efficient numerical techniques. In this work, the interaction of a perturbed laminar premixed flame with a finite amplitude compression wave is investigated using the Parallel Adaptive Wavelet Collocation Method (PAWCM). This method optimally solves the fully compressible Navier-Stokes equations while capturing the essential scales. The results show that depending on the amplitude and duration of a finite amplitude disturbance, the interaction between these waves and premixed flames can produce a broad range of responses. [Preview Abstract] |
Tuesday, November 25, 2014 9:31AM - 9:44AM |
M34.00008: Influence of flame-induced vorticity on acoustic wave generation Mathieu Blanchard, Peter Schmid, Denis Sipp, Thierry Schuller An unsteady laminar premixed M-flame is examined using a linearized direct numerical simulation of a reactive compressible flow around a steady baseflow. Its response to a periodic acoustic forcing is considered. It is shown that the flame wrinkling process is associated with the generation and convection of vorticity waves. The impact of this vorticity on the upstream flow is examined. It is shown that vorticity waves have a strong impact on the flame tip dynamics. Results from optimal forcing computations are presented to illustrate this phenomenon. A simplified model equation, capturing the essential features of sound wave generation from vorticity, is then developed and analyzed. In particular, the impact of flame-induced unsteady vorticity on the generation of acoustic radiation at flame tip is emphasized. [Preview Abstract] |
Tuesday, November 25, 2014 9:44AM - 9:57AM |
M34.00009: Blowoff characteristics of bluff-body stabilized syngas premixed flame in a meso-scale channel Bok Jik Lee, Hong G. Im Syngas has been actively studied recently for the application to Integrated Gasification Combined Cycle systems. It is also considered a candidate of fuels for combustion-based portable power-generating devices accompanied with a micro-reformer. In the present study, high-fidelity reacting flow simulations are conducted to investigate the instability near the blowoff limit of syngas premixed flames stabilized by a bluff-body in a meso-scale channel. Flames in a two-dimensional channel of 1 mm height and 10 mm length with a square box of 0.5 mm sides is considered. When the vortex shedding in a non-reacting flow at a mean inflow velocity remains symmetric as time passes, the flame at this inflow velocity tends to remain stable. By increasing the mean inflow velocity from the solution of this stable condition, the blowoff limit of a CO-to-H2 ratio is identified. At near-blowoff regime, the detail dynamics of flame instability and combustion characteristics associated to the instability are presented. The comparison with the simulations for lean hydrogen/air premixed flames is also discussed. [Preview Abstract] |
Tuesday, November 25, 2014 9:57AM - 10:10AM |
M34.00010: Forced Response of Globally Unstable Reacting Wakes Benjamin Emerson, Tim Lieuwen In many practical combustors, a flame is stabilized in the confined wake of a bluff body. In such devices, the flame's dynamics and its unsteady heat release are strongly governed by the fluid dynamics of the bluff body shear layers and wake. This unsteady heat release can couple with an acoustic mode of the combustor to cause a troublesome self-excited oscillation known as combustion instability. This coupling often occurs through the fluid dynamics, where the flame is dynamically wrinkled by acoustically excited vortical structures in the wake. This study experimentally investigates the acoustically excited hydrodynamic response of reacting bluff body wakes using time resolved PIV and chemiluminescence. The focus of the study is to understand how the flow responds to a varicose excitation on top of its globally unstable sinuous mode. In the experiment, the varicose mode is externally excited through harmonic, longitudinal acoustic forcing. The results show a varicose response. However, when forcing near the global mode frequency, the symmetrically arranged structures composing the varicose response quickly stagger to form a rapidly growing sinuous response. This resonant amplification of the sinuous mode is explained using linear spatial stability analysis and a bispectral analysis of the sinuous-varicose interaction. [Preview Abstract] |
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