From graph theory and geometric probabilities to a representative width for three-dimensional detonation cells

We present a model for predicting a representative width for the three-dimensional cells observed on detonation fronts in reactive gases. Its physical premise is that the dynamics of the transverse waves of irregular cells obeys a stochastic process both stationary and ergodic and produces the same burnt mass per unit of time as the average planar steady ZND process. Graph theory then defines an ideal cell whose grouping is equivalent to the actual 3D cellular front, geometric probabilities determine the mean burned fraction that parameterizes the model, and ZND calculations close the problem with the time-position relationship of a fluid element in the ZND reaction zone. The model is limited to detonation reaction zones whose sole ignition mechanism is adiabatic shock compression, such as those of the mixtures with H2, C3H8 or C2H4 as fuels considered in this work. Indeed, the comparison of their measured and calculated widths shows an agreement better than or within the accepted experimental uncertainties, depending on the quality of the chemical kinetic scheme used for the ZND calculations. However, the comparison for CH4:O2 mixtures shows high overestimates, indirectly confirming that the detonation reaction zones in these mixtures certainly include other ignition mechanisms contributing to the combustion process, such as turbulent diffusion. In these situations, the cell mean width derived from longitudinal soot recordings shows a very large scatter and may thus not be a relevant detonation characteristic length. The model is easily implementable as a post-process of ZND profiles and provides fast estimates of the cell width, length and reaction time.Comment: Extended versio

Contribution à l'étude de la Transition déflagration détonation (TDD) dans des mélanges gazeux binaires H2/C3H8/Air

Cette étude traite de la détonation et de la Transition Déflagration Détonation, TDD, en conduite dans les mélanges binaires H2/C3H8-Air. L'accent est mis sur les mécanismes d'accélération de flamme en présence d'obstacles. Les grandeurs caractéristiques de détonation autonome (célérité, pression et taille de cellule) et la distance de TDD ont été mesurées pour différentes richesses et proportions d'hydrogène dans le propane. En particulier, les effets de l'obstacle (longueur et nature) et du diamètre du tube sur la TDD ont été examinés. Les résultats montrent que les corrélations classiques Taille de cellule - Longueur d'induction chimique pour la détonation et Taille de cellule - Longueur de transition pour la TDD - bien établies pour les mélanges simples - restent valables pour ces mélanges binaires. L'addition du C3H8 à H2 diminue la détonabilité du combustible binaire. Des visualisations d'accélération de flamme par ombroscopie ont été réalisées à l'aide de caméras ultra-rapides dan le but d'identifier les mécanismes physiques contrôlant ce processus pour différentes configurations d'obstacles. Les enregistrements ont mis en évidence deux phases de propagation. Dans la première, les instabilités intrinsèques de la flamme, l'augmentation de sa surface ainsi que la combustion turbulente retardée - résultant de la zone de recirculation entre deux obstacles successifs - jouent un rôle prédominant. Dans la deuxième, l'accélération est contrôlée par l'interaction du front de flamme et des ondes de choc réfléchies sur les obstacles ou sur les parois du confinement. Il en résulte une forte accélération de la flamme avec établissement d'un régime de blocage thermique qui, dans certaines conditions, mène à l'apparition de la détonation.This study analyses the processes of detonation and transition from deflagration to detonation (DDT) in the binary gaseous mixtures H2/C3H8-Air contained in squared- or circular-section tubes. Focus is on the mechanisms of flame acceleration when obstacles are positioned in the tubes. The characteristic properties of the self-sustained detonation (velocity, pressure and cell size) and the DDT distance were measured for various equivalence ratios and hydrogen proportions in the propane. In particular, the effects on the DDT of the nature and length of the obstacle and of the tube diameter were investigated. Results show that the classical correlations as the detonation cell size - the chemical induction length, and the detonation cell size - the distance to DDT - well-established for simple mixtures - also apply to the considered H2/C3H8-Air binary fuels mixtures. The addition of C3H8 to H2 decreases the detonability of the binary fuels. Visualization of flame acceleration based on a shadow technique was performed by means of high-speed camera to identify the predominant physical mechanisms under different obstacle configurations. Shadow records have evidenced two stages. The first shows the predominant role of the flame intrinsic instabilities and area increase, as well as that of the delayed turbulent combustion resulting from the recirculation zone between two successive obstacles. The second shows that acceleration is controlled by the interaction of the flame front and the shock waves reflected on the obstacles or the tube walls. This leads to a strong flame-acceleration up to the "choking" regime which, under specific conditions, results in detonation initiation.POITIERS-ENS Mécanique Aérot (860622301) / SudocPOITIERS-BU Sciences (861942102) / SudocSudocFranceF

Influence of propane additives on the detonation characteristics of H

Hydrogen is more and more considered as a potential fuel for propulsion applications. However, due to its low ignition energy and wide flammability limits, H2-air mixtures raise a concern in terms of safety. This aspect can be partly solved by adding an alkane to these mixtures, which plays the role of an inhibitor. The present paper provides data on such binary fuel-air mixtures where various amounts of propane are added to hydrogen. The behavior of the corresponding mixtures, in terms of detonation characteristics and other fundamental properties, such as the cell size of the detonation front and induction delay, are presented and discussed for a series of equivalence ratios and propane addition. The experimental detonation velocity is in good agreement with calculated theoretical Chapman-Jouguet values. Based on soot tracks records, the cell size λ is measured, whereas the induction length L i is derived from data using a GRI-Mech kinetic mechanism. These data allow providing a value of the coefficient K = λ/L i

Effect of the initial pressure on the characteristics of the flame propagation in hydrogen-propane-air mixtures

This paper is aimed at an experimental investigation on effects of initial pressure on flame propagation characteristics of binary fuels hydrogen-propane-air mixtures at room temperature. The experiments are performed in a square channel equipped with perforated orifice obstacles. Four initial pressures are examined. Based on pressure transducers along the channel, the flame velocity, maximum pressure of the front peak and characteristic distances are measured. Successive stages are observed as flame propagates: (i) a velocity increase at the beginning, (ii) a velocity equal to the sound speed of combustion products and (iii) a decrease of the velocity. When the initial pressure is more important, the flame velocity and the maximal pressure of the front peak are higher, which yields a shorter characteristic distance of flame propagation. By means of a Schlieren photography technique, the physical mechanisms of flame propagation are identified in its initial stage. The physical mechanisms such as flame surface area increase and combustion product expansion as well as delayed combustion between two adjacent plates are responsible for flame acceleration upon its initial stage. The oscillations of the centerline flame velocity are due to the constrained-expanded structure of flow in reactants ahead of flame when it crosses the plates

Etude de la détonation continue rotative (application à la propulsion)

Le concept du moteur à détonation continue, ici appliqué à la propulsion spatiale, est basé sur la détonation entretenue dans une chambre annulaire par une arrivée continue de mélange réactif devant elle. Un dispositif expérimental a été conçu pour caractériser ses performances propulsives à pression atmosphérique ou sub-atmosphérique. Des blocages ou tuyères peuvent être adaptés à la section d'éjection du moteur pour en augmenter les performances. L'observation des phénomènes physiques liés à la détonation rotative a nécessité une métrologie et des caméras rapides. Les performances sont évaluées sur la base de mesures de poussée. La température de paroi et les vibrations sont également mesurées. Les observations expérimentales montrent l'existence d'un régime de fronts réactifs continus. Les études paramétriques sur le fonctionnement et la géométrie du moteur mettent en évidence la constance de ce régime de fronts. Leur nombre, constant en phase stationnaire, est généralement compris entre 1 et 8 selon les conditions d essai, leur célérité évolue peu entre 1000 et 1300 m/s et le rapport de pression à travers les fronts est proche de 2 ou 3. Les caractéristiques de ce régime (pression, célérité) sont nettement inférieures aux propriétés des détonations Chapman Jouguet principalement car le brassage du mélange frais avec les gaz brûlés dégrade ses propriétés réactives. La faisabilité du moteur à détonation continue a été démontrée mais ses performances devront faire l'objet d autres études pour en préciser l intérêt en propulsion.The concept of the continuous detonation engine, applied here to the space propulsion, is based on the detonation maintained in an annular chamber by a continuous supplying of reactive mixture in front of it. An experimental device was designed to characterize its propulsive performance at or under atmospheric pressure. Restrictions or nozzles can be adapted to the ejection section of the engine to increase its performance. The observation of the physical phenomena related to the rotary detonation requires fast metrology and cameras. The performances are evaluated on the basis of thrust measurements. Wall temperature and vibrations are also measured. The experimental observations show the existence of a regime of continuous reactive fronts. Parametric studies on the operation and the geometry of the engine highlight the constancy of this mode of fronts. Their number, constant in stationary phase, is generally contained between 1 and 8 according to the conditions of test, their celerity very little between 1000 and 1300 m/s and the pressure ratio through the fronts is close to 2 or 3. The characteristics of this mode (pressure, celerity) are definitely lower than the properties of the Chapman Jouguet detonations mainly because the mixing of the fresh mixture with the burned gases decreases its reactive properties. The feasibility of the continuous detonation engine was proven but its performances will have to be the subject of other studies to specify its interest in propulsion.POITIERS-BU Sciences (861942102) / SudocSudocFranceF

A study of continuous rotation modes of detonation in an annular chamber with constant or increasing section

International audienceThe detonation regime is an alternative to the conventional constant-pressure combustion mode typically used for propulsive systems because of its higher thermal efficiency and temperature and pressure of products, and shorter characteristic combustion time and length. The classic implementation is the rotating detonation engine, with the combustion chamber consisting of the annular space between a center-body and an outer cylindrical wall. This experimental study focuses on the effects of the chamber inner geometry, the total mass flow rate, and the detonation cell width on the conditions for detonation rotation. Cylindrical and conical center-bodies with several lengths and half-apex angles are considered to approach the hollow configuration of the RDE chamber. The cell width is varied by testing with mixtures of ethylene and enriched air, with several equivalence ratios and nitrogen dilutions. The combustion modes and the detonation velocities and pressures are characterized by analyzing pressure signals and high-speed camera visualizations. Three detonation regimes are identified, characterized by one or two fronts propagating in the same or opposite directions. Decreasing the center-body length and increasing the half-apex angle increases the measured detonation velocity and pressure. Velocities range between 53 and 89% of the Chapman–Jouguet value, and the pressure reaches about 11 bar. For the conditions tested, higher detonation velocity and pressure are obtained for the conical center-body configuration. Our interpretation is that center-bodies that are too long, or channels that are too narrow, hinder the exhaust of the burned gas. As a result, the proportion of products in the unburned gas mixture ahead of the detonation wave (consisting of fresh and burned gas) increases, resulting in a decrease in the magnitude of the detonation properties

Continuous rotating detonation in annular chambers: pressure, velocity and stability

International audienc

Instabilities and efficiencies of continuous rotating detonation in annular chambers with constant or linearly-increasing section

International audienc