Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 55, Number 16
Sunday–Tuesday, November 21–23, 2010; Long Beach, California
Session MU: Reacting Flows IV |
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Chair: David Kassoy, University of Colorado at Boulder Room: Hyatt Regency Long Beach Regency A |
Tuesday, November 23, 2010 8:00AM - 8:13AM |
MU.00001: Magnetohydrodynamic Augmentation of Pulse Detonation Engines Christopher Zeineh, Lord Cole, Ann Karagozian Pulse detonation engines (PDEs) are the focus of increasing attention due to their potentially superior performance over constant pressure engines. Yet due to its unsteady chamber pressure, the PDE system will either be over- or under-expanded for the majority of the cycle, with energy being used without maximum gain. Magnetohydrodynamic (MHD) augmentation offers the opportunity to extract energy and apply it to a separate stream where the net thrust will be increased. With MHD augmentation, such as in the Pulse Detonation Rocket-Induced MHD Ejector (PDRIME) concept, energy could be extracted from the high speed portion of the system, e.g., through a generator in the nozzle, and then applied directly to another flow or portion of the flow as a body force. The present high resolution numerical simulations explore the flow evolution and potential performance of such propulsion systems. An additional magnetic piston applying energy in the PDE chamber can also act in concert with the PDRIME for separate thrust augmentation. Results show that MHD can indeed influence the flow and pressure fields in a beneficial way in these configurations, with potential performance gains under a variety of flight and operating conditions. There are some challenges associated with achieving these gains, however, suggesting further optimization is required. [Preview Abstract] |
Tuesday, November 23, 2010 8:13AM - 8:26AM |
MU.00002: Simulations of a Detonation Wave in Transverse Magnetic Fields Lord Cole, Ann Karagozian, Jean-Luc Cambier Numerical simulations of magneto-hydrodynamic (MHD) effects on detonation wave structures are performed, with applications to flow control and MHD power extraction in Pulse Detonation Engines (PDE) and their design variations. In contrast to prior studies of MHD interactions in PDEs,\footnote{Cambier, et al., AIAA-2008-4688} the effects of the finite relaxation length scale for ionization on the stability of the detonation wave are examined. Depending on the coupling parameters, the magnetic field can quench the detonation and effectively act as a barrier to its propagation. Conversely, an applied transient magnetic field can exert a force on a pre-ionized gas and accelerate it. The dynamics are subject to non-linear effects; a propagating transverse magnetic field will initially exert a small force if the gas has a low conductivity and the magnetic Reynolds number ($Re_m$) is low. Nevertheless, the gas accelerated by the ``piston'' action of the field can pre-heat the ambient gas and increase its conductivity. As the wave progresses, $Re_m$ increases and the magnetic field becomes increasingly effective. The dynamics of this process are examined in detail with a high-order shock-capturing method and full kinetics of combustion and ionization. The complex chemical kinetics calculations are ported onto a GPU using the CUDA language, and computational performance is compared with standard CPU-based computations. [Preview Abstract] |
Tuesday, November 23, 2010 8:26AM - 8:39AM |
MU.00003: RANS Simulations of Supersonic Combustion using a Flamelet-based Model Vincent Terrapon, Rene Pecnik, Frank Ham, Heinz Pitsch A flamelet-based model for supersonic combustion is introduced. Since viscous heating and compressibility effects play an important role in high-speed flows, the flamelet implementation originally based on a low Mach number assumption has been reformulated. In this new implementation temperature is not any longer given by a chemistry table but computed from the total energy and the tabulated species mass fractions. Additionally, the source term in the progress variable transport equation is rescaled by the pressure to better account for compressibility effects. This approach allows the use of complex chemistry with only 2 or 3 additional scalar transport equations. The model is applied to a RANS simulation of a hydrogen jet in a supersonic crossflow and the results are compared with experimental measurements. Finally, the model is also used in the RANS computation of the hydrogen fueled HyShot II scramjet and simulation results are compared with experimental data from a ground experiment. [Preview Abstract] |
Tuesday, November 23, 2010 8:39AM - 8:52AM |
MU.00004: The Dynamics of Unsteady Detonation with Diffusion Christopher Romick, Tariq Aslam, Joseph Powers We consider an unsteady one dimensional detonation with diffusion. The system studied is a standard one step kinetics model whose inviscid stability properties are well characterized. The introduction of diffusion creates an interaction between the length scales of reaction and diffusion thus delaying the onset of instability of the system when the length scales of diffusion and reaction overlap. This interaction is admitted in systems of complex kinetics where the finest reaction length scales are comparable to those of a viscous shock. [Preview Abstract] |
Tuesday, November 23, 2010 8:52AM - 9:05AM |
MU.00005: A multivariate quadrature based approach for LES based supersonic combustion modeling Pratik Donde, Heeseok Koo, Venkat Raman The direct quadrature method of moments (DQMOM) was developed to solve high-dimensional probability density function (PDF) equations that arise in the description of turbulent combustion. This method is particularly useful in shock-containing supersonic internal flows such as those encountered in scramjet engines. In the DQMOM approach, the PDF is described in terms of a finite number of weighted delta functions whose weights and locations in composition space are obtained by solving specific transport equations. Since this approach is fully Eulerian in nature, it is advantageous compared to conventional Lagrangian methods used for solving the PDF transport equation. However, implementation of this formulation in the context of the large eddy simulation (LES) methodology leads to large numerical errors. For instance, the high-resolution numerical schemes used in LES lead to non-realizable and diffusive evolution of the DQMOM equations. Here, we propose a novel semi-discrete quadrature method of moments (SeQMOM) that overcomes this problem. A decoupling procedure is used to extend this method to multivariate PDF descriptions. The numerical implementation in LES as well as validation exercises will be presented. [Preview Abstract] |
Tuesday, November 23, 2010 9:05AM - 9:18AM |
MU.00006: Large Eddy Simulation of Supersonic Combustion Amirreza Saghafian, Vincent Terrapon, Frank Ham, Heinz Pitsch Large eddy simulation of supersonic combustion is performed based on a flamelet/progress variable combustion model. This model was originally formulated for low Mach number, where temperature and species mass fractions are looked up from a pre-computed flamelet library. In the compressible formulation presented here, the equation for the total energy is solved to find temperature. Because total energy is a non-linear function of temperature, an iterative method like Newton-Raphson is inevitable. However, a new formulation is introduced to eliminate this expensive iterative step. Large eddy simulation of under-expanded hydrogen jet in supersonic cross-flow is performed and results are compared with experiments. For sufficiently high jet to cross-flow momentum ratio, burning of the fuel is observed in the upstream region of the jet exit and the length of this burning region is in good agreement with experimental data. In addition, reaction is also observed in a large portion of the boundary layer downstream of the jet consistent with experimental observations. [Preview Abstract] |
Tuesday, November 23, 2010 9:18AM - 9:31AM |
MU.00007: The Interaction of High-Speed Turbulence with Flames: Turbulent Flame Speed Alexei Poludnenko, Elaine Oran The interaction of flames with background turbulence occurs in systems ranging from chemical flames on Earth to thermonuclear burning fronts in supernovae. We present an analysis of a set of numerical simulations aimed at studying the dynamics and properties of turbulent flames formed under the action of high-speed turbulence in stoichiometric hydrogen-air mixture. The simulations were performed using the massively parallel reactive-flow code Athena-RFX. Previous analysis of these simulations showed that this system represents turbulent combustion in the thin reaction zone regime even in the presence of intense turbulence (Da = 0.05, U$_{rms}\sim $ 35 times the laminar flame speed). Here we discuss the processes that determine the turbulent burning velocity and show that it exceeds values that can be attributed only to the increase of the flame surface area. We suggest a possible mechanism for this excess burning rate. Finally, we discuss the implications of these results for the process of deflagration-to-detonation transition in unconfined systems. This work was supported in part by AFOSR, NRL, ONR, and by NSF through the TeraGrid resources. [Preview Abstract] |
Tuesday, November 23, 2010 9:31AM - 9:44AM |
MU.00008: Compressible, diffusive, reactive flow simulations of the double Mach reflection phenomenon J.L. Ziegler, R. Deiterding, J.E. Shepherd, D.I. Pullin We describe direct numerical simulations of the multi-component, compressible, reactive Navier-Stokes equations in two spatial dimensions. The simulations utilize a hybrid, WENO/centered-difference numerical method, with low numerical dissipation, high-order shock-capturing, and structured adaptive mesh refinement (SAMR). These features enable resolution of diffusive processes within reaction zones. A series of one- and two-dimensional test problems are used to verify the implementation, specifically the high-order accuracy of the diffusion terms, including a viscous shock wave, the decaying Lamb-Oseen vortex, laminar flame and unstable ZND detonation. High-resolution simulations are discussed of the reactive double Mach reflection phenomenon. The diffusive scales (shear/mixing/boundary layers and flame thicknesses) and weak shocks are resolved while the strong shocks emanating from the triple points are captured. Additionally, a minimally reduced chemistry and transport model for hydrocarbon detonation is used to accurately capture the induction time, chemical relaxation, and the diffusive mixing within vortical structures evolving from the triple-point shear layer. [Preview Abstract] |
Tuesday, November 23, 2010 9:44AM - 9:57AM |
MU.00009: Thermomechanical phenomena in rapidly heated inert and reactive gases David Kassoy Asymptotic methodologies are used to identify thermomechanical processes occurring in inert and chemically reactive gases subsequent to deposition of transient, spatially resolved thermal energy into a finite volume (``near-field'') of inert or chemically reactive gases. Rational models are developed for a wide range of energy depositions, heating time scales and volume dimensions. When the Mach number of the gas expelled from the heated volume boundary is small, only linear acoustic disturbances can appear in the neighboring, unheated gas (``far-field''). Larger boundary Mach numbers are associated with shock waves in the ``far-field.'' An extreme example describes a spatially resolved heated source for extremely strong blast waves associated with nuclear explosions. The classical model for strong blast waves is reformulated to provide a physically sound explanation of the singularities in the well-known similarity solutions used to describe blast wave evolution. A model for the evolution of a reaction center (hot spot) is considered to identify the characteristic gasdynamics generated by rapid heat addition. [Preview Abstract] |
Tuesday, November 23, 2010 9:57AM - 10:10AM |
MU.00010: Hot Surface Ignition and Flame Propagation of Hydrocarbon Air Mixtures Philipp Boettcher, Brian Ventura, Guillaume Blanquart, Joseph Shepherd To mitigate the risk of accidental explosions in industrial facilities and in the aviation industry, the mechanisms and parameters leading to ignition must be investigated. Of particular are isolated hot surfaces in contact with gaseous hydrocarbon fuels, and thus ignition of premixed n-hexane air and n-heptane air mixtures is examined using a high temperature glow plug. Measurements include schlieren visualization, particle streaks, pressure, and temperature measurements in the plume created by the hot surface. These measurements are performed for experiments in both air and combustion mixtures ranging in equivalence ratio from 0.5 (near the lower flammability limit) to 3.0. This allows for comparison of ignition temperature, flame speed, pressure rise, and temperature distribution with a computational flame model. For equivalence ratios above 0.7 the ignition temperature was observed to be insensitive to increasing fuel concentration and showed good agreement with the model. Three distinct combustion modes are observed that scale with the Richardson number: single flame, multiple flames, and puffing. These behaviors show the transition from flame propagation dominated to buoyancy dominated behavior, with puffing cycles of the order 10 Hz. [Preview Abstract] |
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