Bulletin of the American Physical Society
2005 58th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 20–22, 2005; Chicago, IL
Session AH: Reacting Flows I |
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Chair: Ann Karagozian, University of California, Los Angeles Room: Hilton Chicago Williford B |
Sunday, November 20, 2005 8:00AM - 8:13AM |
AH.00001: Instabilities of Reverse Smolder Waves John Buckmaster, Zhanbin Lu, Minyong Chen, Luca Massa We use numerical strategies to examine the stability of reverse smolder waves in the context of a model that can permit both fuel-rich and fuel-lean waves. The steady-state response for such waves, maximum temperature versus blowing rate, is characterized, for increasing blowing rate, by a fuel-rich branch of rising temperature followed by a fuel-lean branch of falling temperature, followed by quenching. The propagation speed at the quenching point is nonzero. For the parameters that we consider, the entire fuel-rich branch is unstable to 2-dimensional disturbances, but the dynamic consequences are modest. An interval of the fuel-lean branch whose left boundary is at the point of stoichiometry is stable, but the remainder of the branch, all the way to the quenching point, is unstable. These instabilities are destructive, and the contiguous smolder wave becomes fragmented. Tribrachial fragments can emerge, analagous to the tribrachial or triple flames familiar from gaseous edge-flame studies. Their emergence is characterized by a sharp rise in the maximum temperature, a rise that could lead to a transition to flaming (gas-phase) combustion. [Preview Abstract] |
Sunday, November 20, 2005 8:13AM - 8:26AM |
AH.00002: Infleunce of External Noise on Flame Dynamics Yevgenii Rastigejev, Moshe Matalon The evolution of hydrodynamically unstable flames has been studied within a hydrodynamic theory, where the flame is confined to a surface separating the fresh mixture from the hot products. The present study is based on the weakly-nonlinear Michelson-Sivashinsky (MS) equation, obtained in the limit of small values of thermal expansion. With periodic boundary conditions the MS equation possesses exact solutions, known as ``pole solutions,'' which are cusp-like structures that propagate with a constant speed. A stable pole solution exists for any domain size, which must therefore be obtained as the long-time behavior of the equation when integrated numerically for arbitrary initial conditions. Computations show that this is indeed the case in domains of moderate size. In large domains, although a cusp-like structure develops on the average, small random-like subwrinkles appear sporadically on the flame propagating along its surface and affecting its speed. The intensity of this phenomenon is found highly sensitive to the level of numerical noise: the appearance of wrinkles is reduced or even eliminated with lower noise, and is increased with increasing the level of noise. It is concluded that the secondary structures that appear on the flame surface during its propagation are a peculiar response to background noise, numerical or physical, amplified by the hydrodynamic instability. This behavior is not only peculiar to the MS equation, but also exists when no restriction is placed on the density contrast. [Preview Abstract] |
Sunday, November 20, 2005 8:26AM - 8:39AM |
AH.00003: Curvature effects on detonations with mole decrement reactions Viktor Gorchkov, Mark Short We analyze the structure and stability of weakly curved, quasi- steady, self-sustaining detonations having a one-step, non mole- reserving chemical reaction. For a steady planar detonation, a sufficiently large mole decrease during the reaction causes the rate of heat release by chemical reaction to reach a maximum at a point of incomplete reaction. It is this feature which allows the construction of quasi- steady, weakly curved, converging detonation solutions, in addition to the previously identified diverging wave solutions. We construct the quasi-steady detonation velocity against front curvature relationships for converging waves with mole-decrement reactions. The stability of such solutions is investigated by direct numerical simulation of the imploding detonations [Preview Abstract] |
Sunday, November 20, 2005 8:39AM - 8:52AM |
AH.00004: Detonation Initiation on the Microsecond Time Scale: DDT's David R. Kassoy, Matthew Nabity Numerical solutions to the planar, reactive Euler equations, with one-step exothermic kinetics, describe deflagration-to-detonation transition (DDT) subsequent to thermal power deposition into a finite volume of perfect gas mixture. A parametric study is carried out for deposition location relative to the bounding wall, for power deposition level and for activation energy values. Results are given for the transient, spatial distributions of temperature, pressure, reactant concentration and local chemical heat release as well as for the time variation of the global heat release and the maximum pressure (lead shock) in the flow field. All solutions are characterized by a complex sequence of reactive gasdynamic events, featuring the spontaneous generation of numerous, isolated reaction centers. Compression waves generated by sequential explosions of reaction centers strengthen the lead shock sufficiently to facilitate the sudden appearance of a coupled reaction zone. The distance from the confining boundary to the initial location of the overdriven detonation varies with the magnitude and location of the power deposition and the value of the activation energy. Subsequent reactive gasdynamic transients enable detonation relaxation to a CJ wave. [Preview Abstract] |
Sunday, November 20, 2005 8:52AM - 9:05AM |
AH.00005: The Fluid Dynamics of a Pulse Detonation Engine-VI K. Kailasanath, Sally Cheatham Pulsed Detonation Engines (PDEs) have received considerable attention recently because they have the potential to make a major impact in aerospace propulsion. Previously, several aspects of the fluid dynamics of an idealized PDE, consisting of a tube closed at one end and open at the other have been presented. Typically, gaseous fuels are used in both experiments and simulations. However, for most practical applications, liquid fuels will have to be considered. Previously, we have presented some preliminary results on multiphase detonations in tubes. In this talk, we will present simulations of the single-cycle performance of PDEs operating on JP10-Oxygen and JP10-Air mixtures. Results for a range of fuel droplet sizes, as well as results when some of the fuel is prevaporized will be presented. The implications of these results on the development and potential application of the PDE will also be discussed. [Preview Abstract] |
Sunday, November 20, 2005 9:05AM - 9:18AM |
AH.00006: On the ignition of turbulent liquid fuel spray jets using direct numerical simulation Yunliang Wang, Christopher Rutland Direct numerical simulation is used to simulate the ignition of the two-dimensional liquid fuel spray jets. The carrier fluid is solved using a compressible code with a fourth-order explicit Runge-Kutta scheme. The Lagrangian method is used to track the liquid fuel spray droplets. A chemistry mechanism for n-heptane fuel with 33 species and 65 reactions is adopted to describe the chemical reactions. The objective is to investigate the effect of mixing and evaporation cooling on the ignition and combustion processes. Some important parameters such as temperature, equivalence rate, heat release rate and the scalar dissipation rate are examined. From some samples of the results, we found that ignition first occurs at the edge of the liquid spray jet where the equivalence ratio is about 0.5 and the scalar dissipation rate is less 1.0 (1/s). In addition, for two initial spray droplet radiuses of 10 and 20 macron, the 20 macron case ignites earlier since the evaporation cooling is less than in the 10 macron case. [Preview Abstract] |
Sunday, November 20, 2005 9:18AM - 9:31AM |
AH.00007: Acoustical Excitation of Burning Droplets in Microgravity and Normal Gravity Srinivasan Dattarajan, Owen Smith, Ann Karagozian This experimental study focused on methanol droplet combustion characteristics during exposure to external acoustical perturbations in both normal gravity and microgravity. Emphasis was placed on examination of excitation conditions in which the droplet was situated at or near a pressure node or antinode. Acoustic excitation had a significantly greater influence on droplet burning rates and flame structures in microgravity as compared with those in normal gravity. In normal gravity, acoustic excitation of droplets situated near a pressure node produced only very moderate increases in burning rate (about 11-15\% higher than for non-acoustically excited, burning droplets) and produced no significant change in burning rate near a pressure antinode in normal gravity. In microgravity, for the same range in sound pressure level, droplet burning rates increased by over 75\% and 200\% for droplets situated at or near pressure antinode and pressure node locations, respectively. Observed flame deformations for droplets situated near pressure nodes or antinodes were generally consistent with the notion of acoustic radiation forces arising in connection with acoustic streaming, yet both velocity and pressure perturbations were seen to affect flame behavior, even when the droplet was situated precisely at or extremely close to node or antinode locations. [Preview Abstract] |
Sunday, November 20, 2005 9:31AM - 9:44AM |
AH.00008: Assessment of flame/kinetic models through detailed comparisons with experiment Jeffrey Bergthorson, Paul Dimotakis Planar premixed flames are stabilized in the stagnation flow field of an impinging jet. Methane, ethane, and ethylene premixed flames are studied experimentally as a function of stoichiometry and imposed strain rate. Simultaneous measurements of axial velocity and CH radical concentration profiles are made using Particle Streak Velocimetry (PSV) and Planar Laser Induced Fluorescence (PLIF). Stagnation-wall temperature and inlet mixture-composition data are acquired concurrently and permit a full specification of the wall and inflow boundary conditions. Experimental results are simulated numerically using the Cantera reacting-flow package in terms of a one-dimensional formulation and a multi-component transport model. Simulated velocity profiles are corrected for particle inertia, thermophoretic, and finite particle-track time-interval effects and allow direct comparisons with experiment. Measured versus predicted velocity and CH profile comparisons allow the validity of flow, transport, and kinetic models for methane, ethane, and ethylene flames to be assessed. [Preview Abstract] |
Sunday, November 20, 2005 9:44AM - 9:57AM |
AH.00009: Dual-Pump CARS Thermometry in Highly Luminous Flames Sean Kearney Dual pump coherent anti-Stokes Raman scattering (CARS) investigations have been conducted in highly luminous flames. While the spatial coherence of the CARS signal permits efficient rejection of many optical background sources, spatially incoherent and spectrally broad interference from highly luminous flames can still be problematic. We have compared two approaches for time-resolved gating of the CARS signal beam against the luminous background from heavily sooting flames; (1) a fast liquid crystal shutter in conjunction with a standard unintensified CCD and (2) an interline-transfer CCD with reduced dynamic range. The fidelity of the dual-pump CARS facility for temperature measurements in these extremely hostile environments is evaluated and the relative merits of both detection approaches are discussed. [Preview Abstract] |
Sunday, November 20, 2005 9:57AM - 10:10AM |
AH.00010: Modeling Decomposed Foam Dynamics using a Level Set -- Extended Finite Element Approach David Noble, Amy Sun When exposed to high temperatures, removable epoxy foam (REF) undergoes liquefaction and surface evaporation. Simulations of these dynamics require that the interfacial conditions be modeled accurately including the surface reaction and surface tension. The large deformation of the liquid-vapor interface makes Eulerian simulation methods highly desirable. Extended finite element methods (XFEM) are well suited for simulating surface-dominated physics using Eulerian finite elements. An XFEM model for modeling REF is developed. The liquid-vapor interface is described using a level set method. The energy, momentum, and continuity equations are solve throughout the foam and vapor phases. To capture the discontinuous pressure caused by surface tension, the pressure field is enriched with a heaviside function. The species equations are solved only within the elements that contain foam. The mass flux due to the surface reaction is weakly applied along the embedded interface. XFEM is shown to capture the complex interfacial physics while allowing the interface to move freely through the mesh. [Preview Abstract] |
Sunday, November 20, 2005 10:10AM - 10:23AM |
AH.00011: Experimental Study of the Spill and Vaporization of a Volatile Liquid Douglas Bohl, Gregory Jackson Assessment of potential hazards during shipment of flammable liquids (both cryogenic and otherwise) requires an understanding of the coupling between large-scale spill dynamics and formation of a flammable vapor cloud. The scale of such events prohibits detailed experiments and thus hazard will require improved spill/vaporization models validated against smaller-scale experiments. Sub-scale wind-tunnel experiments were undertaken to characterize pool and vapor cloud formation from an acetone spill issuing from a large rectangular flow obstruction. The spill event was largely governed by the temperature of the spill surface in relation to the boiling point of the spilled liquid though the free-stream velocity also impacted the spreading of the spill. Planar Laser-Induced Fluorescence (PLIF) was used to measure acetone vapor concentrations during the pool spreading and vaporization downstream of the obstruction. Because of obstruction induced recirculation region, regions of vapor within the flammability limits were localized near the flow obstruction. The highly unsteady vapor cloud was observed to grow well past the downstream edge of the measurement domain. With decreasing wind speeds, both the mass of vapor within the flammability limits and the total event time increased significantly. [Preview Abstract] |
Sunday, November 20, 2005 10:23AM - 10:36AM |
AH.00012: Reaction patterns in a blinking vortex flow Carolyn Nugent, Matt Paoletti, Tom Solomon We study the patterns formed by the excitable Belousov-Zhabotinsky reaction in a blinking vortex flow produced by magnetohydrodynamic forcing. Mixing in this flow is chaotic, as has been documented extensively in previous studies. The reaction is triggered by a silver wire, and the result is a pulse (``trigger wave'') that propagates through the system. We investigate the patterns formed by the propagating pulse and compare them with theories\footnote{T. Tel, A. de Moura, C. Grebogi and G. Karolyi, Phys. Rep. 413, 91 (2005).} that predict fractal patterns determined by the unstable manifolds of the flow. We also consider ``burn-like'' reaction fronts, and compare the results with previous experiments for patterns of oscillatory reactions in this flow. [Preview Abstract] |
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