Traditional methods of studying underground coal mine explosions are limited to observations and data collected during experimental explosions. These experiments are expensive, time-consuming, and require major facilities, such as the Lake Lynn Experimental Mine. The development of computational fluid dynamics (CFD) modeling of explosions can help minimize the need for large-scale testing. This thesis utilized the commercial CFD software, SC/Tetra, to examine three case studies. The first case study modeled the combustion of methane in a scaled shock tube, measuring approximately 1 foot by 1 foot, by 20.5 feet long, with a methane cloud of 2.5 feet in length, at a concentration of 9% methane. The numerical results from the CFD model were in good agreement with experimental data gathered, with all pressure peaks within 0.25 psi of the recorded pressure data. However, the model had an extensive run-time of 16 hours to reach the peak pressures. The second case study modeled the same explosion, but utilized a total pressure boundary condition at the location of the membrane, instead of the combustion of methane. A pressure-time curve was assigned to this boundary, recreating the release of pressure by the explosion. This was made possible with the knowledge of the experimental data. The numerical results from the CFD model were in excellent agreement with experimental data gathered, with all pressure peaks within 0.07 psi of the recorded pressure data. Alternatively, this model had a run-time of 40 minutes. The third case study modeled a methane explosion in a large shock tube, measuring 8 feet by 8 feet, by 40 feet long, with a methane cloud of 4 feet in length, at a concentration of 9% methane. The bursting balloon technique was employed, which did not model the combustion of methane, but instead the equivalent energy release. The numerical results from the CFD model were in good agreement with the experimental data gathered, with all pressure peaks within 0.025 psi of the recorded pressure data. Additionally, the numerical results modeled the negative pressure phenomenon observed in the experimental results, caused by suction or negative pressure created by the blast wave, immediately following the positive wave. This model had a run-time of 20 minutes. The results of this researched provided validation that there are alternative ways to successfully model methane explosion, without having to model the chemical reactions involved in the combustion of methane, providing quicker run-times and in this case, more accurate results.

Abstract not available

Currently shock effects on surface ships can be determined by full scale shock trials, Finite Element Analysis or semi empirical methods that reduce the analytical problem to a limited number of degrees of freedom and include hull configurations, construction methods and materials in an empirical way to determine any debilitating effects that an explosion may have on the ship. This research has been undertaken to better understand the effect of hull shape on surface ships' shock response to external underwater explosions (UNDEX). The study is within the semi empirical method category of computations. A set of simple closed-form equations has been developed that accurately predicts the magnitude of dynamic excitation of different 2- D rigid-hull shapes subject to far-field UNDEX events. This research was primarily focused on the affects of 2-D rigid hull shapes and their contribution to global ship motions. A section of the thesis,

La dispersion de particules solides par explosif et leur inflammation dans l'air, ainsi que des effets de souffle générés sur le milieu connexe ont été étudiés par voie expérimentale et numérique. La configuration retenue est l'explosion en champ libre de charges sphériques, constituées d'un noyau central d'explosif solide (booster), entouré de particules inertes (billes de verre) ou réactives (particules d'aluminium). Les diagrammes de marche sont tracés à partir des résultats fournis par des capteurs de pression piezo-électriques et des images obtenues par cinématographie rapide et traitées numériquement par méthode de "Background Oriented Schlieren" (BOS). Des échantillons de particules capturés dans le nuage fournissent des informations sur leur état après l'explosion. Les résultats expérimentaux sont confrontés aux simulations numériques effectuées à l'aide du code de calcul d'écoulements réactifs multiphasiques EFAE du LCD. La frontière du nuage formé présente un aspect caractéristique en forme de dendrites, due à la formation d'agglomérats de particules de quelques millimètres. Après 1m de propagation, une partie de ces agglomérats dépasse le choc incident par effet balistique. Les particules de verre sont brisées par le choc. Elles ralentissent l'onde de souffle et diminuent son amplitude. L'effet des particules d'aluminium dépend de leur taille : avec les plus fines, on observe, après une phase initiale de ralentissement de l'onde de souffle, la ré-accélération de celleci et une augmentation de l'impulsion de pression. Avec les plus grosses particules, le choc s'atténue, mais leur combustion se traduit par une augmentation de la pression dans l'écoulement en arrière du front et un faible accroissement de l'impulsion. Les agglomérats recueillis indiquent que, dans tous les cas, la combustion de l'aluminium est incomplète. Les simulations numériques sont en accord raisonnable avec les résultats expérimentaux dans le champ lointain.

<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>

This thesis describes a method for using physically based techniques to model an explosion and the resulting side effects. Explosions are some of the most visually exciting phenomena known to humankind and have become nearly ubiquitous in action films. A realistic computer simulation of this powerful event would be cheaper, quicker, and much less complicated than safely creating the real thing. The immense energy released by a detonation creates a discontinuous localized increase in pressure and temperature. Physicists and engineers have shown that the dissipation of this concentration of energy, which creates all the visible effects, adheres closely to the compressible Navier-Stokes equation. This program models the most noticeable of these results. In order to simulate the pressure and temperature changes in the environment, a three dimensional grid is placed throughout the area around the detonation and a discretized version of the Navier-Stokes equation is applied to the resulting voxels. Objects in the scene are represented as rigid bodies that are animated by the forces created by varying pressure on their hulls. Fireballs, perhaps the most awe-inspiring side effects of an explosion, are simulated using massless particles that flow out from the center of the blast and follow the currents created by the dissipating pressure. The results can then be brought into Maya for evaluation and tweaking.

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.

The present study aims to develop a mechanistic understanding of the thermal-hydraulic processes in a vapor explosion, which may occur in nuclear power plants during a hypothetical severe accident involving interactions of high-temperature corium melt and volatile coolant. Over the past several decades, a large body of literature has been accumulated on vapor explosion phenomenology and methods for assessment of the related risk. Vapor explosion is driven by a rapid fragmentation of high temperaturemelt droplets, leading to a substantial increase of heattransfer areas and subsequent explosive evaporation of the volatile coolant. Constrained by the liquid-phase coolant, the rapid vapor production in the interaction zone causes pressurization and dynamic loading on surrounding structures. While such a general understanding has been established, the triggering mechanism and subsequent dynamic fine fragmentation have yet not been clearly understood. A few mechanistic fragmentation models have been proposed, however, computational efforts to simulate the phenomena generated a large scatter of results. Dynamics of the hot liquid (melt) droplet and the volatile liquid (coolant) are investigated in the MISTEE (Micro-Interactions in Steam Explosion Experiments) facility by performing well-controlled, externally triggered, single-droplet experiments, using a high-speed visualization system with synchronized digital cinematography and continuous X-ray radiography, called SHARP (Simultaneous High-speed Acquisition of X-ray Radiography and Photography). After an elaborate image processing, the SHARP images depict the evolution of both melt material (dispersal) and coolant (bubble dynamics), and their microscale interactions, i.e. the triggering phenomenology. The images point to coolant entrainment into the droplet surface as the mechanism for direct contact/mixing ultimately responsible for energetic interactions. Most importantly, the MISTEE data reveals an inverse correlation between the coolant temperature and the molten droplet deformation/prefragmentation during the first bubble dynamics cycle. The SHARP observations followed by further analysis leads to a hypothesis about a novel phenomenon called pre-conditioning, according to which dynamics of the first bubble-dynamics cycle and the ability of the melt drop to deform/pre-fragment dictate the subsequent explosivity of the so-triggered droplet. The effect of non-condensable gases on the perceived mechanisms was investigated on the MISTEE-NCG test campaign, in which a considerable amount of non-condensable gases (NCG) are present in the film that enfolds the molten droplet. The SHARP images for the MISTEE-NCG tests were analyzed and special attention was given to the morphology (aspect ratio) and dynamics of the air/ vapor bubble, as well as the melt drop preconditioning and interaction energetics. Analysis showed twomain aspects when compared to the MISTEE test series (withoutentrapped air). First, the investigation showed that the meltpreconditioning still strongly depends on the coolant subcooling. Second,in respect to the energetics, the tests consistently showed a reducedconversion ratio compared to that of the MISTEE test series. The effect of the melt material in the steam explosion triggerability was also summoned, since it would in principle directly implicate the melt preconditioning. Since a number of the thermo-physical properties of the material would influence the triggering process, we focused on the material properties by using the same dioxide material with difference concentrations, i.e. eutectic and non-eutectic. Unfortunately, due to the high melt superheat the possible differences were not perceived. Thus, inaddition to other materials, lower melt superheat tests were schedule inthe future. / QC 20101110

Research with heterogeneous mixtures involving solid particulate in closed, constant-volume bombs is typically limited by the powder dispersion technique. This work details the development of an experimental apparatus that promotes ideal conditions, namely a quiescent atmosphere and uniform particle distribution, for measuring laminar, heterogeneous flame propagation. In this thesis, two methods of dispersing particles are investigated. In the first, heterogeneous mixtures are made in a secondary vessel that is connected to the main experiment. Particles are dispersed into the secondary vessel by adapting a piston-driven particle injector, which has been shown to produce uniform particle distributions. The heterogeneous mixture is then transferred to the main bomb facility and ignited after laminar conditions are achieved. In the second method of dispersion, particles are directly injected into the main experimental facility using a strong blast of compressed air. As with the first approach, enough time is given (~4 minutes) for the mixture to become quiescent before ignition occurs. An extinction diagnostic is also applied to the secondary mixing vessel as well as the primary experimental facility (for both dispersion methods) to provide a qualitative understanding of the dispersion technique. To perform this diagnostic a 632.8-nm, 5-mW Helium-Neon (HeNe) laser was employed. Aluminum nano-particles with an average diameter of 100 nm were used in this study. It was found that for typical dust loadings produced with both dispersion techniques, a pure dust-air system would not ignite due to the current spark ignition system. Thus, a hybrid mixture of Al/CH4/O2/N2 was employed to achieve the project goal of demonstrating a system for controlled laminar flame speed measurements in aerosol mixtures. With the hybrid mixture, the combustion characteristics were studied both with and without the presence of nano-Al particles. Based on the experimental results, the simplicity of the "direct-injection" methodology compared to that of the "side-vessel" is desirable and will be further investigated as a viable alternative, or improvement, to the side-vessel technology.

This thesis describes a method for using physically based techniques to model an explosion and the resulting side effects. Explosions are some of the most visually exciting phenomena known to humankind and have become nearly ubiquitous in action films. A realistic computer simulation of this powerful event would be cheaper, quicker, and much less complicated than safely creating the real thing. The immense energy released by a detonation creates a discontinuous localized increase in pressure and temperature. Physicists and engineers have shown that the dissipation of this concentration of energy, which creates all the visible effects, adheres closely to the compressible Navier-Stokes equation. This program models the most noticeable of these results. In order to simulate the pressure and temperature changes in the environment, a three dimensional grid is placed throughout the area around the detonation and a discretized version of the Navier-Stokes equation is applied to the resulting voxels. Objects in the scene are represented as rigid bodies that are animated by the forces created by varying pressure on their hulls. Fireballs, perhaps the most awe-inspiring side effects of an explosion, are simulated using massless particles that flow out from the center of the blast and follow the currents created by the dissipating pressure. The results can then be brought into Maya for evaluation and tweaking.