The work presented in this thesis is an investigation of the dynamics of unconfined hydrogen-air flames in the presence of buoyant effects and the determination of an ignition criterion for flame propagation between adjacent pockets of reactive gas separated by air. The experimental work was conducted using the soap bubble technique and visualized with high speed schlieren or large scale shadowgraph systems. A study was first conducted to determine the most suitable soap solution additive among glycerol, guar and polyethylene oxide for conducting the experiments, isolating guar as the best candidate. The soap solution was then used to study the dynamics of flames in single or multiple soap bubbles filled with reactive mixtures of different compositions. The soap bubble method was also further improved by designing a soap dispenser that can maintain a bubble indefinitely and a method to burst the soap solution prior to an experiment using timed heated wires. In the experiments with single bubbles, it was found that for sufficiently lean hydrogen-air mixtures, buoyancy effects become important at small scales. The critical radius of hemispherical flames that will rise due to buoyancy was measured and estimated using a model comparing the characteristic burning speed and the rise speed of the flame kernel. Excellent agreement was found between the model predictions and the measured critical flame radii. The experiments with multiple bubbles provided the scaling rules for flame transition between neighboring pockets of hemispherical or spherical shape separated by an inert gas. The test results demonstrated that the separation distance between the bubbles is mainly determined by the expansion ratio when the buoyancy effects are negligible, corresponding to near stoichiometric mixtures. For leaner mixtures with stronger buoyant effects, the critical separation distance was no longer governed by the expansion ratio alone, as buoyancy forces render the flame propagation across the inert gas more difficult. Visualization of the ignition dynamics confirmed that buoyancy forces tend to accelerate the first kernel up before ignition of the second kernel can be achieved.

En raison de sa nature hautement inflammable, l’hydrogène constitue un risque technologique important et son utilisation nécessite un très haut niveau de sûreté. Le travail de thèse présenté dans ce mémoire a été réalisé en collaboration avec EDF et s’inscrit dans le cadre des études de sécurité liées à la libération d’hydrogène dans un des locaux de l’îlot nucléaire. Le dégagement d’une fuite peut, en effet, entraîner la formation d’une atmosphère inflammable, qui peut exploser et provoquer des graves dégâts.Cette étude vise ainsi à apporter une meilleure compréhension des phénomènes de dispersion et de déflagration à l’issue d’une fuite d’hydrogène. Les résultats expérimentaux obtenus à échelle de laboratoire sont comparés aux simulations numériques obtenues via le logiciel FLACS. Un plan détaillé a donc été élaboré, en divisant le mémoire en deux parties : dispersion et déflagration. Pour chaque partie, un point bibliographique est proposé, ainsi qu’une description des dispositifs expérimentaux utilisés. Une partie expérimentale et numérique est présentée pour chacun des deux phénomènes. Les travaux réalisés ici ont donc permis d’obtenir une analyse réelle et complète du phénomène de rejet d’hydrogène en milieu fermé et obstrué et d’explosion de mélange hétérogène hydrogène-air à petite et grande échelle. Les cas les plus pénalisants en termes de concentration maximale et gradient de concentration ont été identifiés à travers une étude paramétrique sur l’influence du débit et de la position du rejet sur la dispersion. Les conséquences lors de la déflagration de ces mélanges hydrogène-air ont ensuite été étudiées à travers l’analyse de l’onde de pression et de la propagation de la flamme. / The highly combustible nature of hydrogen poses a great hazard and its use imposes an accurate analysis of risk characterization and consequences to protect the installation and to reduce the potential risk. This thesis has been done in collaboration with EDF and it is included in the context of the explosion risks of an air hydrogen mixture in a room of the nuclear facility where there is a risk of accidental release. Indeed, the hydrogen can disperse quickly and burn easily in the presence of an ignition source causing heavy damage. The goal of this study is to provide a better understanding of the phenomena of dispersion and deflagration after an accidental release of hydrogen. Experimental results obtained at small scale are compared with numerical simulations obtained using FLACS code. A detailed plan has been drawn up, dividing the thesis into two parts: dispersal and deflagration. For each part, a bibliographic point is proposed, as well as a description of the experimental devices used. An experimental and numerical part is presented for each of the two phenomena. The studies carried out here have thus enabled us to obtain a real and complete analysis of the phenomenon of closed and clogged hydrogen discharge and small-scale and large-scale heterogeneous hydrogen-air mixture explosion. The most penalizing cases in terms of maximum concentration and concentration gradient were identified through a parametric study on the influence of the flow rate and the position of the rejection on the dispersion. The consequences during the explosion of these hydrogen-air mixtures were then studied through the analysis of the pressure wave and the propagation of the flame.

The safe and efficient propagation of the Deflagration to Detonation Transition (DDT) is a topic that has been researched for many years due to its applications in Aerospace and Mechanical Engineering. DDT is when fire caused by the burning of fuel is accelerated to the upper CJ point on the Rankine Hugoniot curve due to instabilities in the flame and the turbulence caused by these instabilities. The complex flame dynamics that go along with DDT have ensured that the process is yet to be fully understood and defined. This research will work towards observing the early stages of burning hydrogen-air mixtures in DDT conditions in order to better understand the processes that cause DDT. The research will also involve the testing of multiple different equivalence ratios of hydrogen known to undergo DDT. This research will assist in making places that store reactive gasses such as hydrogen safer by searching for the method of DDT formation and ways to prevent it. This research will also allow for safer commercial use of DDT in Detonation Based Engines. The research was tested in a secure facility and observed the first four inches of ignition and deflagration using schlieren and chemiluminescence imaging techniques. Through the research, it was found that flames at higher equivalence ratios tend to be longer, more top-biased, and have more instabilities than flames of lower equivalence ratios, better preparing them for DDT. This study will be elaborated on in future research using a variety of different fuels to solidify the findings of the research performed and to assist in the ability to innovate using DDT.

No description available.

In this thesis we cover the dynamics of the macro magnetic transformations (spin avalanches) in crystals of molecular nanomagnets, also known as magnetic deflagration and detonation. Taking a single-molecule Hamiltonian, we calculate the dependence of Zeeman energy and the activation energy as a function of an external magnetic field at different angles relative to the easy axis of the crystal. Using quantum mechanical calculations, we show that the energy levels of the molecule exhibit complex behavior in presence of a transverse component of the magnetic field. For an arbitrarily aligned magnetic field, the energy levels do not arrange in a simple "double-well" manner. We extend existing theoretical models by generalizing the Zeeman energy for a wide range of magnetic fields and its different orientations. We obtain a new type of front instability in magnetization-switching media. Due to the dipole-dipole interaction between the molecules magnetic instability results to the front banding and change in the front propagation velocity. The magnetic instability has a universal physical nature similar to the Darrieus-Landau instability. The instability growth rate and the cutoff length are calculated for the spin avalanches in the crystals of nanomagnets. Finally, we investigate the internal structure of the magnetic detonation front. We calculate the continuous shock profile using the transport processes of the crystal such as thermal conduction and volume viscosity. Such an approach can be applied to any weak shock wave in solids. Zero volume viscosity leads to an isothermal jump, i.e., the temperature changes continuously while the pressure and the density experience discontinuity. The analysis has shown that the volume viscosity plays a major role in the formation of the detonation front.

<p>In this thesis, gas explosions inside pipes are considered. Laboratory experiments and numerical simulations are the basis of the thesis. The target of the work was to develop numerical models that could predict accidental gas explosions inside pipes.</p><p>Experiments were performed in circular steel pipes, with an inner diameter of 22.3 mm, and a plexiglass pipe, with an inner diameter of 40 mm. Propane, acetylene and hydrogen at various equivalence ratios in air were used. Pressure was recorded by Kistler pressure transducers and flame propagation was captured by photodiodes, a SLR camera and a high-speed camera. The experiments showed that acoustic oscillations would occur in the pipes, and that the frequencies of these oscillations are determined by the pipe length. Several inversions of the flame front can occur during the flame propagation in a pipe. These inversions are appearing due to quenching of the flame front at the pipe wall and due to interactions of the flame front with the longitudinal pressure waves in the pipe. Transition to detonation was achieved in acetylene-air mixtures in a 5 m steel pipe with 4 small obstructions.</p><p>Simulations of the flame propagation in smooth pipes were performed with an 1D MATLAB version of the Random Choice Method (RCMLAB). Methods for estimation of quasi 1D burning velocities and of pipe outlet conditions from experimental pressure data were implemented into this code. The simulated pressure waves and flame propagation were compared to the experimental results and there are good agreements between the results.</p><p>Simulations were also performed with the commercial CFD code FLACS. They indicated that to properly handle the longitudinal pressure oscillations in pipes, at least 7 grid cells in each direction of the pipe cross-section and a Courant number of maximum 1 should be used. It was shown that the current combustion model in FLACS gave too high flame speeds initially for gas explosions in a pipe with an inner width of 40 mm.</p>

In this thesis, gas explosions inside pipes are considered. Laboratory experiments and numerical simulations are the basis of the thesis. The target of the work was to develop numerical models that could predict accidental gas explosions inside pipes. Experiments were performed in circular steel pipes, with an inner diameter of 22.3 mm, and a plexiglass pipe, with an inner diameter of 40 mm. Propane, acetylene and hydrogen at various equivalence ratios in air were used. Pressure was recorded by Kistler pressure transducers and flame propagation was captured by photodiodes, a SLR camera and a high-speed camera. The experiments showed that acoustic oscillations would occur in the pipes, and that the frequencies of these oscillations are determined by the pipe length. Several inversions of the flame front can occur during the flame propagation in a pipe. These inversions are appearing due to quenching of the flame front at the pipe wall and due to interactions of the flame front with the longitudinal pressure waves in the pipe. Transition to detonation was achieved in acetylene-air mixtures in a 5 m steel pipe with 4 small obstructions. Simulations of the flame propagation in smooth pipes were performed with an 1D MATLAB version of the Random Choice Method (RCMLAB). Methods for estimation of quasi 1D burning velocities and of pipe outlet conditions from experimental pressure data were implemented into this code. The simulated pressure waves and flame propagation were compared to the experimental results and there are good agreements between the results. Simulations were also performed with the commercial CFD code FLACS. They indicated that to properly handle the longitudinal pressure oscillations in pipes, at least 7 grid cells in each direction of the pipe cross-section and a Courant number of maximum 1 should be used. It was shown that the current combustion model in FLACS gave too high flame speeds initially for gas explosions in a pipe with an inner width of 40 mm.

A Pulse Detonation Engine (PDE) is a propulsion device that takes advantage of the pressure rise inherent to the efficient burning of fuel-air mixtures via detonations. Detonation initiation is a critical process that occurs in the cycle of a PDE. A practical method of detonation initiation is Deflagration-to-Detonation Transition (DDT), which describes the transition of a subsonic deflagration, created using low initiation energies, to a supersonic detonation. This thesis presents the effects of obstacle spacing, blockage ratio, DDT section length, and airflow on DDT behavior in hydrogen-air and ethylene-air mixtures for a repeating PDE. These experiments were performed on a 2 diameter, 40 long, continuous-flow PDE located at the General Electric Global Research Center in Niskayuna, New York. A fundamental study of experiments performed on a modular orifice plate DDT geometry revealed that all three factors tested (obstacle blockage ratio, length of DDT section, and spacing between obstacles) have a statistically significant effect on flame acceleration. All of the interactions between the factors, except for the interaction of the blockage ratio with the spacing between obstacles, were also significant. To better capture the non-linearity of the DDT process, further studies were performed using a clear detonation chamber and a high-speed digital camera to track the flame chemiluminescence as it progressed through the PDE. Results show that the presence of excess obstacles, past what is minimally required to transition the flame to detonation, hinders the length and time to transition to detonation. Other key findings show that increasing the mass flow-rate of air through the PDE significantly reduces the run-up time of DDT, while having minimal effect on run-up distance. These experimental results provided validation runs for computational studies. In some cases as little as 20% difference was seen. The minimum DDT length for 0.15 lb/s hydrogen-air studies was 8 L/D from the spark location, while for ethylene it was 16 L/D. It was also observed that increasing the airflow rate through the tube from 0.1 to 0.3 lbs/sec decreased the time required for DDT by 26%, from 3.9 ms to 2.9 ms.

Turbulent combustion has a profound effect on the way we live our lives; homes and businesses predominantly rely on power generated by burning some form of fuel, and the vast majority of transport of passengers and cargo are driven by combustion. Fossil fuels remain readily available and relatively cheap, and so will continue to power the modern world for the foreseeable future. Combustion of fossil fuels produces emissions that detrimentally affect air quality, particularly in highly-populated cities, and are also widely believed to be contributing to global climate change. Consequently, increasing attention is being focused on alternative fuels, increased efficiency and reduced emissions. One alternative fuel is hydrogen, which introduces challenges in end-usage, storage and safety that are not encountered with more conventional fuels. Advances in computational power and software technology means that numerical simulation has a growing role in the development of combustors and safety evaluation. Despite these advances, many challenges remain; the broad range of time and length scales involved are coupled with complex thermodynamics and chemistry on top of turbulent fluid mechanics, which means that detailed simulations of even relatively-simple burners are still prohibitively expensive. Engineering turbulent flame models are required to reduce computational expense, and the challenge is to retain as much of the flow physics as possible. Furthermore, the choice of numerical approach has a significant effect on the quality of simulation, and different target applications place different demands on the numerical scheme. In the case of hydrogen explosion, the approach needs to be able to capture a range of physical behaviours including turbulence, low-speed deflagration, high-speed shock waves and potentially detonations. One such numerical approach that has enjoyed widespread success is finite volumes schemes based on the Godunov method. These methods perform well at all speeds, and have positive shock-capturing capability, but recent studies have demonstrated difficulties with numerical stability for more complex thermodynamics, specifically in the case of fully-conservative methods for multi-component fluids with varying thermodynamic properties. A recent development is the so-called double-flux method, which retains many of the positive properties of the fully-conservative approaches and does not suffer from the same numerical instabilities, but is quasi-conservative and involves additional computational expense. The present work consolidates the state-of-the-art in the literature, and considers two equation sets, based on mass fraction and volume fraction, respectively, along with fully-conservative and quasiconservative schemes. Comprehensive validation and evaluation of the different approaches is presented. It was found that both quasi-conservative approaches performed well, with a better conservative behaviour for the quasi-conservative volume fraction, but a better stability for the quasi-conservative mass fraction. Finally, the numerical tool developed is applied to turbulent combustion of premixed hydrogen in the context of the semi-confined experiments from the University of Sydney. The LES results showed an good overall agreement with the experimental data, and the critical parameters such as overpressure and flame speed where globally well captured, highlighting the large potential of LES for safety analysis.

Les supernovæ de type Ia (SNe Ia) sont devenues un outil important pour retracer l'expansion de notre Univers, leur étude est donc importante pour la cosmologie. Le modèle le plus populaire est celui de l'explosion d'une naine blanche (NB) accrétante dont la contraction relance la combustion sous la forme d'une déflagration subsonique, qui transiterait ensuite en une détonation supersonique. Ce scénario de détonation retardée repose sur un mécanisme physique de Transition Déflagration-Détonation (TDD) encore très mal compris, que nous étudions dans cette thèse.Les modèles actuels de détonation retardée reproduisent les observations en se fondant sur le mécanisme des gradients de Zel'dovich. Cependant, les échelles d'ignition n'étant pas résolues, ces simulations n'expliquent pas à elles seules la TDD, phénomène mal compris, même sur Terre, lorsqu'il s'agit de milieux non-confinés. D'autre part, ce mécanisme requiert une turbulence trop intense et impose des conditions probablement trop restrictives.C'est dans ce contexte que nous avons proposé un nouveau mécanisme de TDD: le chauffage acoustique de l'enveloppe du progéniteur. Un modèle simplifié, en géométrie plane, permet de mettre en évidence l'amplification d'ondes acoustiques (générés par une flamme turbulente) dans un gradient de densité similaire à ceux d'une NB. Selon leur fréquence et leur amplitude, leur amplification peut aller jusqu'à la formation d'un choc suffisamment fort pour initier une détonation. Ensuite, ce mécanisme est analysé en géométrie sphérique dans le cadre plus réaliste d'une NB en expansion. Une étude paramétrique montre la validité de notre mécanisme sur une gamme raisonnable de fréquences et d'amplitudes acoustiques.Finalement, quelques simulations MHD 2D et 3D, où l'on recherche une source de perturbations acoustiques, sont présentées pour démontrer le caractère réaliste de notre nouveau mécanisme de TDD. / Type Ia supernovae are an important tool to determine the expansion history of our Universe. Thus, considerable attention has been given to both observations and models of these events. The most popular explosion model is the central ignition of a deflagration in the dense C+O interior of a Chandrasekhar mass white dwarf, followed by a transition to a detonation (TDD). We study in this thesis a new mechanism for this transition.The most robust and studied progenitor model and the postulated mechanism for the TDD, the so called 'Zel'dovich gradient mechanism', are presented. State of the art 3D simulations of such a delayed detonation, at the price of some adjustments, can indeed reproduce observables. But due to largely unresolved physical scales, such simulations cannot explain the TDD by themselves, and especially, the physical mechanism which triggers this transition - which is not yet understood, even on Earth, for unconfined media. It is then discussed why the current Zel'dovich mechanism might be too constraining for a SN Ia model, pointing to a new approach, which is the core result of this thesis.In the final part, our alternative model for DDT in supernovae, the acoustic heating of the pre-supernova envelope, is presented. A planar model first proves that small amplitude acoustic perturbations (generated by a turbulent flame) are actually amplified in a steep density gradient, up to a point where they turn into shocks able to trigger a detonation. Then, this mechanism is applied to more realistic models, taking into account, in spherical geometry, the expanding envelope. A parametric study demonstrates the validity of the model for a reasonable range of acoustic wave amplitudes and frequencies.To conclude, some exploratory 2D and 3D MHD simulations, seeking for realistic acoustic source compatible with our mechanism, are presented.