Bulletin of the American Physical Society
2006 59th Annual Meeting of the APS Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2006; Tampa Bay, Florida
Session LG: Reacting Flows II: Non-Premixed Combustion* |
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Chair: William Sirignano, University of California, Irvine Room: Tampa Marriott Waterside Hotel and Marina Florida Salon 5 |
Tuesday, November 21, 2006 8:00AM - 8:13AM |
LG.00001: Fundamental Studies of Turbulent Liquid Spray Mixing and Combustion Christopher Rutland, Yunliang Wang Fundamental simulations are used to investigate the ignition process of turbulent n-heptane liquid fuel spray jets. Full two-way coupling between the phases and a detailed chemical mechanism with 33 species and 64 reactions are used. Both time developing and spatially developing liquid spray jets are studied. In the time developing case it was found that ignition first occurs at the edges of the jets where the fuel mixture is lean, and the scalar dissipation rate and vorticity magnitude are low. For smaller droplets, higher initial droplet velocity causes the ignition to occur earlier, whereas for larger droplets, higher initial droplet velocity delays the ignition time. In the spatially developing liquid jets, ignition and flame lift-off characteristics similar to diesel sprays are observed. Near the injector, combustion development progresses very rapidly along the stoichiometric surface. In the downstream region of the spray, combustion develops with steep temperature fronts in a flamelet mode. [Preview Abstract] |
Tuesday, November 21, 2006 8:13AM - 8:26AM |
LG.00002: Instabilities in a Turning Reacting Mixing Layer Undergoing Transition William Sirignano, Felix Cheng, Feng Liu Mixing layers composed of fuel and oxidizer streams passing through curved channels are studied by performing 2-D numerical simulations. The flows are subjected to either transverse acceleration or both transverse and streamwise accelerations. In this study, we focus on the initial development of the mixing layers from laminar to transition. The full Navier- Stokes equations coupled with multiple species equations and chemical reactions are solved using a finite-difference numerical scheme. No turbulence model is employed since the flow is fully deterministic. The mixing layers going through the curved channel are subjected to three types of instability; they are the Kelvin-Helmholtz (K-H), the centrifugal, and the Rayleigh-Taylor (R-T) instabilities. Our previous study showed that the K-H instability associated with a streamwise favorable pressure gradient produced a strain field that resulted in tearing and extinction of the flame. In the present study, we focus on the combined effects of the K-H, the centrifugal, and the R-T instabilities on the flame structures and the development of the mixing layer. Grid independence is maintained to ensure the accuracy of the numerical solutions. [Preview Abstract] |
Tuesday, November 21, 2006 8:26AM - 8:39AM |
LG.00003: Large-Eddy Simulation of a Nonpremixed Turbulent Reacting Flow with Thermal Radiation and Turbulence/Radiation Interactions Ankur Gupta, Daniel Haworth, Michael Modest Large-eddy simulation (LES) has been performed for a planar turbulent channel flow between two infinite, parallel, stationary plates. The capabilities and limitations of the LES code in predicting correct turbulent velocity and passive temperature field statistics have been established through comparisons to DNS data from the literature for nonreacting cases. Mixing and chemical reaction (infinitely fast) between a fuel stream and an oxidizer stream have been simulated to generate large composition and temperature fluctuations in the flow; here the composition and temperature do not affect the hydrodynamics (one-way coupling). The radiative transfer equation is solved using a spherical harmonics (P1) method, and radiation properties correspond to a fictitious gray gas with a composition- and temperature-dependent Planck-mean absorption coefficient that mimics that of typical hydrocarbon-air combustion products. Simulations have been performed for different optical thicknesses. In the absence of chemical reaction, temperature fluctuations and turbulence/radiation interactions (TRI) are small, consistent with earlier findings. Chemical reaction enhances the composition and temperature fluctuations, and hence the importance of TRI. Contributions to emission and absorption TRI have been isolated and quantified. [Preview Abstract] |
Tuesday, November 21, 2006 8:39AM - 8:52AM |
LG.00004: Turbulence Effects on Mass Transfer with Diffusion-Controlled Reaction Dimitrios Papavassiliou, Kien Nguyen Lagrangian numerical methods are used to examine flow effects on turbulent mass transfer with a chemical reaction. A DNS is used to describe turbulent flow in a channel and the mass transfer is simulated by releasing a large number of reactant particles distributed randomly over a cross section of the channel. In each simulation time step, the reactant particles move due to convection (obtained by the DNS), and due to diffusion (simulated by Brownian motion that depends on the Schmidt number). The reaction is such that the reactant transforms into a product when it comes in contact with the wall, i.e., it is a diffusion-controlled reaction. The reaction rate is related to the probability that a particle colliding with the wall will react. Using this methodology, the concentration profile and the mass transfer coefficient to the wall are calculated as a function of the channel length. The paper will discuss the effects of the flow and of the Schmidt number on the effective reaction rate and on the mass transfer coefficient, as well as the entry effects on the rate of mass transfer. [Preview Abstract] |
Tuesday, November 21, 2006 8:52AM - 9:05AM |
LG.00005: A Reaction Progress Variable Approach for LES of Sooty Flames Amit Kumar, Paul DesJardin A reaction progress variable approach is developed for subgrid scale (SGS) modeling of non-premixed sooty turbulent diffusion flames using a mixture fraction conserved scalar approach. Two- phase state-relations are constructed using three reaction progress variables to account for the soot formulation processes and radiation heat loss. Source/sink terms for soot formulation are based on existing phenomenological models and discrete ordinate method (DOM) is used to solve the radiative transfer equation. An assumed beta PDF distribution is used for characterizing the variation of the SGS two-phase mixture fraction for Large Eddy Simulation (LES). The resulting formulation couples the combustion, soot and radiation models to provide a self-consistent methodology to close SGS turbulence- chemistry-radiation interactions. Simulations are conducted of a turbulent diffusion flame for the experimental conditions of Coppalle and Joyeux. Comparisons are conducted of mean and RMS temperature, soot volume fraction to experimental data. A sensitivity study reveals the importance of turbulence-radiation interactions and the dependence of the results on modeling approximations. [Preview Abstract] |
Tuesday, November 21, 2006 9:05AM - 9:18AM |
LG.00006: Direct Numerical Simulation of Soot Particle Dynamics using DQMOM Guillaume Blanquart, Heinz Pitsch, Rodney Fox The understanding of soot particle dynamics in combustion systems is a key issue in the development of low emission engines. Of particular importance are the processes shaping the soot particle size distribution function (PSDF). However, it is not always necessary to represent exactly the full distribution but rather some of its moments. The Direct Quadrature Method of Moments (DQMOM) allows for a very accurate prediction of the moments of the soot PSDF without the cost of expensive methods like Direct Simulation Monte-Carlo (DSMC). This method has been validated for laminar premixed and diffusion flames with detailed chemistry and is now implemented in a semi-implicit low Mach number Navier-Stokes solver. A Direct Numerical Simulation (DNS) of an ethylene jet diffusion flame is performed to study the dynamics of soot particles in a turbulent environment. Soot particles are formed in very rich regions of the flames and are then transported to lean regions where they get oxidized. The time evolution of the soot PSDF will be analyzed and compared to similar distributions from laminar simulations. [Preview Abstract] |
Tuesday, November 21, 2006 9:18AM - 9:31AM |
LG.00007: Validation of the Multi-Environment Conditional PDF Model with Detailed Kinetics in Planar Jet Flames Sean Smith, Rodney Fox, Jacqueline Chen, Evatt Hawkes The multi-environment conditional probability-density function (MECPDF) model has been generalized to account for variable density and multiple reacting scalars. An \textit{a priori} validation of the model has been completed using the direct-numerical simulations (DNS) of temporally-evolving planar jet flames with detailed CO/H2 kinetics (E. R. Hawkes \textit{et al}., Proc. of the Combust. Inst., 31, in press.) The three-dimensional reacting DNS simulations included up to 500 million grid points and Reynolds numbers up to 9,000. Special considerations for the MECPDF model with multiple scalars will be discussed including: calculation of the weights and abscissas from the moments; and the multiple-conditioning requirement. [Preview Abstract] |
Tuesday, November 21, 2006 9:31AM - 9:44AM |
LG.00008: Numerical Prediction of Combustion-induced Noise using a hybrid LES/CAA approach Matthias Ihme, Heinz Pitsch, Manfred Kaltenbacher Noise generation in technical devices is an increasingly important problem. Jet engines in particular produce sound levels that not only are a nuisance but may also impair hearing. The noise emitted by such engines is generated by different sources such as jet exhaust, fans or turbines, and combustion. Whereas the former acoustic mechanisms are reasonably well understood, combustion-generated noise is not. A methodology for the prediction of combustion-generated noise is developed. In this hybrid approach unsteady acoustic source terms are obtained from an LES and the propagation of pressure perturbations are obtained using acoustic analogies. Lighthill's acoustic analogy and a non-linear wave equation, accounting for variable speed of sound, have been employed. Both models are applied to an open diffusion flame. The effects on the far field pressure and directivity due to the variation of speed of sound are analyzed. Results for the sound pressure level will be compared with experimental data. [Preview Abstract] |
Tuesday, November 21, 2006 9:44AM - 9:57AM |
LG.00009: Characteristics of a Strongly-Pulsed Non-Premixed Jet Flame in Cross-flow Mirko Gamba, Noel T. Clemens, Ofodike A. Ezekoye The effects of large-amplitude, high-frequency harmonic forcing of turbulent nonpremixed hydrogen/methane jet flames in cross-flow (JFICF) are investigated experimentally. Flame lengths, penetration lengths, and mixing characteristics are studied using flame luminosity imaging, planar laser Mie scattering visualization and particle image velocimetry. Mean jet Reynolds numbers of 1,600 and 3,250 (peak Re $\sim $2,500--6,500) with corresponding mean momentum flux ratios, r, of 1.9 and 3.7 (peak r $\sim $2.6--8.3) are considered. Forcing frequencies of 100 Hz and 300 Hz with amplitudes of $\sim $60{\%}--300{\%} are investigated. Consistent with previous work, a drastic decrease in flame length and soot emission, an increase in flame penetration and an improved jet fuel/cross-flow air mixing are observed for the larger forcing amplitude cases. Partial pre-mixing induced by near-field reverse flow, near-field vortex/vortex interaction and large-scale stirring, rendered stronger by large forcing amplitudes and frequencies, are thought to play a key role on the observed effects. [Preview Abstract] |
Tuesday, November 21, 2006 9:57AM - 10:10AM |
LG.00010: Effect of thermal expansion on the stability of diffusion flames Moshe Matalon, Philippe Metzener In this talk we report on the effect of density variation on the stability of diffusion flames. Similar earlier studies have always invoked the constant-density approximation. Although we find that thermal expansion has a marked influence on flame instability, it does not play a crucial role as it does in premixed flames. Furthermore, thermal expansion has a different influence on the various modes of instability, affecting primarily the marginal state and the degree of the instability. If the overall effect of thermal expansion is characterized by the ratio $r = T_a/T_u$ of the stoichiometric and unburned temperatures the degree of instability for the onset of cells increases with increasing $r$, but decreases with increasing $r$ for the onset of oscillations. The range of conditions leading to cellular flames is significantly broader than that predicted by the constant-density model, widening with increasing $r$. On the other hand, the range of conditions leading to flame oscillations, whether flat pulsations or oscillatory cells, is further limited by the influence of thermal expansion. The present analysis provides a comprehensive characterization of instabilities in diffusion flames while realistically accounting for density variations. It identifies the possible patterns that are likely to be observed as a result of differential and preferential diffusion, covering a whole range of parameters including the Lewis numbers associated with the fuel and the oxidizer, the initial mixture fraction, and the flow conditions. [Preview Abstract] |
Tuesday, November 21, 2006 10:10AM - 10:23AM |
LG.00011: Flame Structures in Heated Nonpremixed Microburners David Kessler, Mark Short We investigate the structure and behavior of nonpremixed flames formed in a co-flow of fuel and oxidizer within a submillimeter- scale channel. Two configurations are considered. In the first, the channel walls are held at an elevated constant temperature. A ``tuning fork'' flame structure is developed that is controlled by the interaction of the thermal boundary layer and the chemical mixing layer. The second configuration allows heat conduction within the channel walls and finite levels of heat loss. Here, external heating is provided at the exit of the channel, and the stability of the tuning fork flame structure in the resulting axial temperature gradient is examined. Increasing levels of heat loss lead to the appearance of oscillating flame structures. [Preview Abstract] |
Tuesday, November 21, 2006 10:23AM - 10:36AM |
LG.00012: On the Transition from Smoldering to Flaming B. Matkowsky, A. Aldushin, A. Bayliss Though there have been numerous experimental studies of the transition from smoldering to flaming, it has been virtually untouched theoretically. We focus on determining the mechanism and conditions that trigger the transition. We consider a forward smolder wave in a porous sample, driven by a forced flow of gas containing oxidizer. The kinetics includes the fuel oxidation, pyrolysis, and char oxidation reactions. There have been various speculations about the trigger mechanism, including the gaseous reactions, destruction of the porous matrix, the char oxidation reaction, and others. However, no mechanism has as yet been theoretically demonstrated to be capable of acting as the trigger. We show that, due to its small reaction rate, the char oxidation reaction hardly affects the characteristics of the smolder wave as it propagates. However, under appropriate conditions, it can act as the trigger for the transition due to its ability to self-accelerate. Thus, we provide a theoretical underpinning for char oxidation as the trigger mechanism. We introduce the concept of, and determine, a quantity that we term the flaming distance $L_F$, the distance that the smolder wave travels before the char oxidation reaction spontaneously self-accelerates, resulting in a temperature eruption in the smolder front, to a level high enough to ignite the gaseous flaming reactions. Smolder waves propagating in samples of length $L$ do (do not) exhibit a transition to flaming if $L > L_F (L < L_F)$. [Preview Abstract] |
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