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 E23: Reacting Flows II: Detonations & Supersonic Combustion |
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Chair: Elaine Oran, Naval Research Laboratory Room: 326 |
Sunday, November 20, 2011 4:40PM - 4:53PM |
E23.00001: Hydrogen-air detonation cells computed using skeletal and reduced reaction mechanisms David Kessler, Brian Taylor, Vadim Gamezo, Elaine Oran The multidimensional instability of gas-phase detonations results in a complex dynamic structure at the detonation front that leaves behind characteristic cellular patterns as it propagates. In fuel-air mixtures with high effective activation energies, such as hydrogen and air, these detonation cells can become irregular and modelling this behavior using reduced chemical reaction mechanisms can be challenging. Using complex reaction mechanisms, however, can be computationally overwhelming for problems of practical interest. We compare the reaction front structures and dynamic behavior of two-dimensional detonations in a stoichiometric hydrogen-air mixture computed using a 12-step skeletal mechanism and several reduced mechanisms, including a calibrated one-step model. We pay particular attention to how transverse instabilities that form in this high-activation-energy mixture are affected by the details of the chemistry model. We then discuss how to adjust the parameters in reduced reaction models to better describe irregular triple point behavior. [Preview Abstract] |
Sunday, November 20, 2011 4:53PM - 5:06PM |
E23.00002: Numerical Simulations of Detonation Instabilities and Magnetic Field Interactions Lord Cole, Hai Le, Ann Karagozian Numerical simulations of magneto-hydrodynamic (MHD) effects on high frequency and low frequency one-dimensional detonation wave instabilities are performed, with applications to flow control and MHD thrust augmentation in Pulse Detonation Engines and their design variations.\footnote{Zeineh, et al., {\bf J. Propusion \& Power}, to appear} The dynamics of the hydrogen-air detonation are explored via high order shock capturing schemes and complex reaction kinetics. The flame is initially strongly coupled to the shock and the wave is over-driven. As the degree of overdrive decays and the detonation approaches the CJ limit, high frequency instabilities begin to appear. Eventually the average induction length continues to increase and a second mode can be seen which directly couples the flame speed with the shock, resulting in fluctuations with lower frequency but much higher amplitude. A simple model for flame-shock coupling replicates the quantitative features of these instabilities quite well. Effects of an externally applied magnetic field on these detonation instabilities are explored. In addition, the complex chemical kinetics calculations are ported onto a GPU, and computational performance may be compared with standard CPU-based computations. [Preview Abstract] |
Sunday, November 20, 2011 5:06PM - 5:19PM |
E23.00003: Structure of detonation waves in mixtures containing chemical inhibitors Miltiadis V. Papalexandris In this talk we present numerical results of detonation transmission in mixtures containing chemical inhibitors. In our study, the chemical kinetics model consists of a three-step chain-branching mechanism for the fuel combustion and an one-step mechanism for the endothermic reaction between inhibitor and radical. Results from both one and two-dimensional simulations are discussed. It is shown that the absorption of heat and the consumption of radical in the endothermic step result in longer induction zones. This leads to a temporary detachment of the reaction zone from the precursor shock. Eventually, the material behind the shock starts to burn rapidly, thus producing pressure waves which reach the precursor shock and re-initialize the detonation. This is followed by large over-pressures and highly irregular oscillations of the shock. The talk concludes with a discussion on the necessary inhibitor concentration in the initial mixture to achieve permanent detonation suppression. [Preview Abstract] |
Sunday, November 20, 2011 5:19PM - 5:32PM |
E23.00004: Deflagration-to-Detonation Transition in Unconfined Media Alexei Poludnenko, Thomas Gardiner, Elaine Oran Deflagration-to-detonation transition (DDT) can occur in environments ranging from experimental and industrial systems on Earth to astrophysical thermonuclear supernovae explosions. In recent years, substantial progress has been made in elucidating the nature of this process in confined systems with walls, obstacles, etc. It remains unclear, however, whether a subsonic turbulent flame in an unconfined environment can undergo a DDT. We present simulations of premixed flames in stoichiometric H$_2$-air and CH$_4$-air mixtures interacting with high-intensity turbulence. These calculations demonstrate the DDT in unconfined systems unassisted by shocks or obstacles. We discuss the mechanism of this process and its implications. [Preview Abstract] |
Sunday, November 20, 2011 5:32PM - 5:45PM |
E23.00005: Richtmyer-Meshkov instability in shock-flame interactions Luca Massa Shock--flame interactions occur in supersonic mixing and detonation formation. Therefore, their analysis is important to explosion safety, internal combustion engine performance, and supersonic combustor design. The fundamental process at the basis of the interaction is the Richtmyer-Meshkov instability supported by the density difference between burnt and fresh mixtures. In the present study we analyze the effect of reactivity on the Richtmyer- Meshkov instability with particular emphasis on combustion lengths that typify the scaling between perturbation growth and induction. The results of the present linear analysis study show that reactivity changes the perturbation growth rate by developing a non-zero pressure gradient at the flame surface. The baroclinic torque based on the density gradient across the flame acts to slow down the instability growth for high wave numbers. A non-hydrodynamic flame representation leads to the definition of an additional scaling Peclet number, the effects of which are investigated. It is found that an increased flame-contact separation destabilizes the contact discontinuity by augmenting the tangential shear. [Preview Abstract] |
Sunday, November 20, 2011 5:45PM - 5:58PM |
E23.00006: Thermomechanics in an Externally Heated Reactive Gas Hot spot David R. Kassoy The response of a finite volume of inert gas to spatially resolved, transient power addition on a time scale short compared to the local acoustic time has been described by Kassoy, J. Eng. Math., \textbf{68}, 249-262. Formal parameter asymptotic methods are employed to describe the thermodynamic changes caused by a relatively brief burst of thermal energy, as well as the induced mechanical response. Related methods are used in the present work to describe similar physical processes occurring in a heated reactive gas capable of a high activation energy reaction. Localized power deposition from an external source to a spherical volume of reactive gas causes \textbf{nearly} constant volume heat addition. The spatially distributed temperature and pressure increase together, accompanied by an induced low Mach number gas expansion and a concomitant small decrease in fluid density. The resulting low Mach number velocity of gas expelled from the sphere surface is the source of acoustic disturbances, confined to a thin shell surrounding the finite volume, on the short heating time scale. A spatially distributed thermal explosion is initiated on a much longer induction time scale within the hot spot. Thermodynamic and gas dynamic consequences of the subsequent chemical heat addition are identified. Results are compared with those of Vasquez-Esp' and Linan (CTM,\textbf{5}, 485-498) found from a more intuitive formulation using coordinate expansion asymptotic methods. [Preview Abstract] |
Sunday, November 20, 2011 5:58PM - 6:11PM |
E23.00007: Direct numerical simulation of supersonic combustion with finite-rate chemistry Amirreza Saghafian, Heinz Pitsch Three-dimensional direct numerical simulations (DNS) of reacting and inert compressible turbulent mixing layers have been performed. The simulations cover convective Mach numbers from subsonic to supersonic. A detailed chemistry mechanism with 9 species and 29 reactions for hydrogen is used in the reacting simulations. Effects of different initial conditions on the structure of the mixing layer, and time required to reach self-similarity are studied. Flame/turbulence interaction is analyzed by studying turbulent kinetic energy, Reynolds stresses, and their budgets in the reacting and inert simulations. The effects of different reactions on the heat release and mixture composition especially in the regions where shocklets impinge the flame are studied. These DNS databases will provide a better understanding for the compressibility effects on the combustion, and will be used to assess the accuracy of Flamelet/Progress variable approach in supersonic regime. [Preview Abstract] |
Sunday, November 20, 2011 6:11PM - 6:24PM |
E23.00008: Including compressibility effect in combustion modeling for LES of supersonic combustion Ronan Vicquelin, Johan Larsson, Julien Bodart Predicting scramjet performance requires accurate estimations of heat losses, wall friction and heat release distribution. The combustion modeling strategy is therefore important although most numerical simulations of these configurations use simple approximations to model the reactive flow. Two tabulated chemistry models are used here to perform LES of a model scramjet combustor. A wall model is used to compute heat losses and friction at walls. The first combustion model is based on non-premixed flamelets with a progress-variable approach. The compressibility effects of the flow on chemistry are taken into account by rescaling the progress-variable source term with ad-hoc laws. The second model extends the first chemical table by adding pressure and enthalpy dimensions, which enables to give a better estimation of the compressibility effect on the flame. LES results are compared to experimental data (OH LIF, OH* and pressure measurements) to assess the importance of dealing with compressibility effects in combustion models. [Preview Abstract] |
Sunday, November 20, 2011 6:24PM - 6:37PM |
E23.00009: Flame structure in a compact inlet/scramjet combustor Mirko Gamba, Victor A. Miller, M. Godfrey Mungal, Ronald K. Hanson OH PLIF is used to investigate the flame structure in an inlet/scramjet combustor model tested in the Stanford 6-Inch expansion tube. The model is characterized by a compression inlet that generates a shock train confined within the constant area combustor. The inlet flow is maintained at $M=2.8$, $p=40 kPa$ and $T\sim1250 K$. Hydrogen is injected through a single transverse injector at equivalence ratios up to 0.44. Optical access along the combustor allows flow imaging on orthogonal planes. Global effects of fuel injection are quantified by wall-pressure measurements with oxidizing and non-oxidizing freestreams. It is found that mass addition alone does not significantly impact the flowfield. Fuel injection and combustion effects manifest well downstream even under conditions where combustion is stabilized at the injector. The general flame structure in the near-field of the jet shares many similarities with isolated transverse jets. Similarly, in spite of the shock train, the flame structure in the wake region behaves as an undisturbed shear layer where a laminar-like structure typical of low-speed nonpremixed combustion is observed. Finally, the results suggest that the shock train may favor flame stability and anchoring at the shock reflection points. [Preview Abstract] |
Sunday, November 20, 2011 6:37PM - 6:50PM |
E23.00010: LES of supersonic combustion in a realistic scramjet combustor Johan Larsson, Ronan Vicquelin, Ivan Bermejo-Moreno, Julien Bodart Large eddy simulation is used to study the reacting flow in the HyShot scramjet combustor. The H2/air chemistry is modeled using a flamelet/progress-variable approach, which is adapted to the supersonic regime through a parametrized rescaling with pressure of the progress-variable source term. An algebraic wall-model is used to alleviate the need to resolve the inner part of the boundary layers, while still capturing the large momentum and energy losses to the cooled walls. Results are shown for two geometries and compared to the limited experimental data available for each: a model combustor with OH LIF, OH* and wall pressure measurements, as well as the HyShot scramjet combustor with mainly wall pressure data. [Preview Abstract] |
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