The Interaction of High-Speed Turbulence with Flames: Global Properties and Internal Flame Structure

We study the dynamics and properties of a turbulent flame, formed in the presence of subsonic, high-speed, homogeneous, isotropic Kolmogorov-type turbulence in an unconfined system. Direct numerical simulations are performed with Athena-RFX, a massively parallel, fully compressible, high-order, dimensionally unsplit, reactive-flow code. A simplified reaction-diffusion model represents a stoichiometric H2-air mixture. The system being modeled represents turbulent combustion with the Damkohler number Da = 0.05 and with the turbulent velocity at the energy injection scale 30 times larger than the laminar flame speed. The simulations show that flame interaction with high-speed turbulence forms a steadily propagating turbulent flame with a flame brush width approximately twice the energy injection scale and a speed four times the laminar flame speed. A method for reconstructing the internal flame structure is described and used to show that the turbulent flame consists of tightly folded flamelets. The reaction zone structure of these is virtually identical to that of the planar laminar flame, while the preheat zone is broadened by approximately a factor of two. Consequently, the system evolution represents turbulent combustion in the thin-reaction zone regime. The turbulent cascade fails to penetrate the internal flame structure, and thus the action of small-scale turbulence is suppressed throughout most of the flame. Finally, our results suggest that for stoichiometric H2-air mixtures, any substantial flame broadening by the action of turbulence cannot be expected in all subsonic regimes.Comment: 30 pages, 9 figures; published in Combustion and Flam

The Interaction of High-Speed Turbulence with Flames: Turbulent Flame Speed

(Abridged) Direct numerical simulations of the interaction of a premixed flame with driven, subsonic, homogeneous, isotropic, Kolmogorov-type turbulence in an unconfined system are used to study the mechanisms determining the turbulent flame speed, S_T, in the thin reaction zone regime. High intensity turbulence is considered with the r.m.s. velocity 35 times the laminar flame speed, S_L, resulting in the Damkohler number Da = 0.05. Here we show that: (1) The flame brush has a complex internal structure, in which the isosurfaces of higher fuel mass fractions are folded on progressively smaller scales. (2) Global properties of the turbulent flame are best represented by the structure of the region of peak reaction rate, which defines the flame surface. (3) In the thin reaction zone regime, S_T is predominantly determined by the increase of the flame surface area, A_T, caused by turbulence. (4) The observed increase of S_T relative to S_L exceeds the corresponding increase of A_T relative to the surface area of the planar laminar flame, on average, by ~14%, varying from only a few percent to ~30%. (5) This exaggerated response is the result of tight flame packing by turbulence, which causes frequent flame collisions and formation of regions of high flame curvature, or "cusps." (6) The local flame speed in the cusps substantially exceeds its laminar value, which results in a disproportionately large contribution of cusps to S_T compared with the flame surface area in them. (7) A criterion is established for transition to the regime significantly influenced by cusp formation. In particular, at Karlovitz numbers Ka > 20, flame collisions provide an important mechanism controlling S_T, in addition to the increase of A_T by large-scale motions and the potential enhancement of diffusive transport by small-scale turbulence.Comment: 44 pages, 20 figures; published in Combustion and Flam

Effect of Initial Disturbance on The Detonation Front Structure of a Narrow Duct

The effect of an initial disturbance on the detonation front structure in a narrow duct is studied by three-dimensional numerical simulation. The numerical method used includes a high resolution fifth-order weighted essentially non-oscillatory scheme for spatial discretization, coupled with a third order total variation diminishing Runge-Kutta time stepping method. Two types of disturbances are used for the initial perturbation. One is a random disturbance which is imposed on the whole area of the detonation front, and the other is a symmetrical disturbance imposed within a band along the diagonal direction on the front. The results show that the two types of disturbances lead to different processes. For the random disturbance, the detonation front evolves into a stable spinning detonation. For the symmetrical diagonal disturbance, the detonation front displays a diagonal pattern at an early stage, but this pattern is unstable. It breaks down after a short while and it finally evolves into a spinning detonation. The spinning detonation structure ultimately formed due to the two types of disturbances is the same. This means that spinning detonation is the most stable mode for the simulated narrow duct. Therefore, in a narrow duct, triggering a spinning detonation can be an effective way to produce a stable detonation as well as to speed up the deflagration to detonation transition process.Comment: 30 pages and 11 figure

FUTURE TRENDS IN NUMERICAL SIMULATIONS OF DETONATIONS

On discute les développements en cours et à venir pour des types de problèmes résolus, la sophistication des algorithmes et les nouveaux types d'ordinateurs qui vont grandement développer notre potentialité pour la simulation numérique du comportement des détonations. Trois voies principales de progrès s'ouvrent dans : - L'application de l'architecture des nouveaux ordinateurs et du hardware associé qui font ressortir le traitement multiprocesseur, le parallélisme et le graphisme interactif, - L'utilisation d'algorithmes numériques plus sophistiqués comprenant des algorithmes pour un maillage adaptatif des méthodes monotones d'éléments finis sur des maillages non structurés, les dynamiques des particules et Lagrangienne des fluides, et, - La solution de problèmes physiques plus compliqués comprenant des espaces multidimensionnels avec des modèles chimiques plus sophistiqués et compliqués.We discuss current and future developments in scientific computation : the types of problems solved, the sophistication of algorithms, and the new types of computers that will greatly enhance our ability to simulate the behavior of detonations numerically. Three major types of advances are in : - Application of new computer architectures and associated hardware that emphasizes multiprocessing, parallelism, and interactive graphics, - Use of more sophisticated numerical algorithms, including algorithms for adaptive gridding, finite-element monotone methods on unstructured grids, and Lagrangian fluid and particle dynamics, and - Solution of more complicated physical problems, including more spatial dimensions with more sophisticated or complete chemical models

The Limits of Molecular Dynamics Applied to Condensed-Phase Energetic Materials

The limitations of various methods for computing manybody dynamics are summarized briefly in terms of the physical limits of the specific theory and generally of what can reasonably be computed. This information is then used to assess the current computational limit on using molecular dynamics to describe shocks and detonations in condensed phase energetic materials. This question is addressed by defining the computational requirements of a molecular dynamics simulation of a detonation propagating in an idealized nitromethane crystal lattice. The major questions addressed are : What is required to compute the properties of the system to obtain reasonable mesoscopic data ? and What is the size of the system we can now compute, using one of the largest computers available ? From this analysis, we discuss several directions in which future research in this field may proceed

THE STRUCTURE OF DETONATION WAVES

Des simulations numériques multidimensionnelles dépendantes du temps ont été utilisées pour étudier l'initiation, la propagation et l'extinction des détonations en phase gazeuse et liquide. A l'aide des simulations, qui calculent le comportement détaillé de l'interaction des ondes de choc avec les zones réactives formant l'onde de détonation, on étudie l'évolution de l'instabilité qui conduit à la structure cellulaire des détonations. Les simulations portent sur les solutions à deux dimensions, dépendantes du temps, des équations de transport de convection de la densité de masse, de la densité de moment et de l'énergie couplées à des modèles de libération d'énergie chimique. Les équations de transport de convection sont résolues selon l'algorithme "Flux-Corrected Transport". Les réactions chimiques et la libération de l'énergie sont habituellement modélisées par le modèle du paramètre d'induction à deux étapes. En conclusion, le comportement de la structure multidimensionnelle d'une détonation dépend des différences entre les propriétés thermodynamiques dans les zones d'induction derrière la ligne de Mach et le choc incident. La formation de poches non réactives derrière le front de détonation dépend de l'inclinaison des ondes transversales et de la courbure des fronts de choc. Des fronts fortement incurvés peuvent entraîner de grandes poches. La dépendance en température du temps d'induction est un facteur majeur pour la régularité de la structure de détonation. La structure de détonation est affectée par les paramètres de libération d'énergie. Une libération instantanée d'énergie conduit à des structures monodimensionnelles. Une libération rapide de l'énergie donne des structures moins régulières. Une libération très lente de l'énergie entraîne de grandes poches. des fronts fortement incurvés, et la détonation peut s'étouffer.Multidimensional time-dependent numerical simulations have been used to study the initiation, propagation, and extinction of detonations in gases and liquids. The simulations, which calculate the detailed behavior of the interacting shock waves and reaction zones forming the detonation wave, are used to study the evolution of the instability that leads to the cellular structure of detonations. The simulations consist of two-dimensional time-dependent solutions of the convection of mass density, momentum density and energy coupled to models for chemical energy release. The convective transport equations are solved by the Flux-Corrected Transport algorithm. The chemical reactions and energy release are usually modelled by the two-step induction parameter model. We conclude that the behavior of the multidimensional structure of a detonation depends on the differences of the thermodynamic properties in the induction zones behind the Mach stem and the incident shock. The formation of unreacted pockets behind the detonation front depends on the inclination of the transverse waves and the curvature of the shock fronts. Highly curved fronts may result in large pockets. The temperature dependence of the induction time is a major factor in the regularity of detonation structure. Detonation structure is affected by the energy release parameters. Instantaneous energy release leads to one-dimensional structures. Fast energy release results in less regular structures. Very slow energy release results in large pockets, highly curved fronts, and the detonation may die out

Investigation Of Flame Regimes For Flame Acceleration To Detonation

Expanding hydrogen-air flames are studied in a semi-confined duct with an optical access test section. The focus of the work is to characterize the regimes of flame development in a highly turbulent environment and observe the Deflagration-to-Detonation (DDT) phenomenon. DDT has been observed and thoroughly identified in obstructed channels, but further investigation to identify the key mechanism of transition for propagating flames in turbulent flowfields will reveal details of turbulence driven DDT that has only been supported by computational simulations. Schlieren captures the global details of the propagation of expanding flames which consists of turbulent flame augmentation, generation of compression waves, shock formation and flame interaction with turbulent flow field. Understanding the flame acceleration regimes in a highly turbulent environment will aid in the control and prediction of detonation onset for the development of detonation-based propulsion engines