Search

In most scenarios where detonations in fuel–air mixtures can present a hazard, such as in industrial accidents, or where they can be used for practical purposes, like detonative propulsion applications, the mixture is essentially non-uniform. For industrial accidents, it is important to understand what fraction of the mixture can detonate in order to limit the consequences of an explosion event by ensuring proper separation distances to critical facilities and personnel. For propulsion applications, mixture non-uniformities directly affect the performance of the device by limiting the detonable portion of the mixture. While it is known that detonation limits depend on mixture composition, geometry, and concentration gradients, these limits have only been defined for either uniform mixtures, or for detonation propagation in the direction of a smooth concentration gradient. In this study, a model is developed to generalize these classic detonation limits to consider arbitrary concentration distributions and is compared with experimental data for detonation propagation in non-uniform mixtures. The model is then applied to representative concentration distributions corresponding to two practical examples, a high-pressure hydrogen jet and a large-scale vapor cloud explosion (VCE). For both distributions, the model illustrates how a non-uniform mixture composition significantly limits the detonable portion of the flammable mixture created by a realistic release. [ABSTRACT FROM AUTHOR]

Abstract Accurate models to predict large-scale flame propagation are crucial to assessing the consequences of accidental explosions. A freely expanding spherical flame can experience significant acceleration due to the growth of the Darrieus–Landau instability, increasing the severity of the explosion hazard, and must be accounted for when modeling large-scale flames. Recent large-scale experiments have demonstrated an oscillatory rate of flame acceleration, consistent with self-similar flame propagation and the formation and growth of cells with discrete length scales. In this work, an analytical model describing the growth of multiple generations of cells on a flame surface is developed. Each generation is treated independently with a criteria for cell splitting based on a critical stretch rate. Superposition of the length scales is used to determine the global flame surface wrinkling and propagation velocity of the flame. The model is compared with experimental results and the effect of upstream flow disturbances on the development of the instability is also discussed. It is found that the model reproduces a number of features observed in the experiments, including the overall rate of flame acceleration and the frequency of oscillation. The model also captures the correct trends of flame behavior for the effect of elevated initial turbulence and the transition from positive to negative Markstein length. These results support the self-similar argument of spherical-flame acceleration and can be used in future studies to develop new models describing the behavior of large-scale flames. [ABSTRACT FROM AUTHOR]

In this work, the development of the Darrieus–Landau instability is examined at large scale using atmospheric-pressure methane-air flames in the presence of weak initial turbulence, as well as in quiescent conditions. In the absence of initial turbulence, the critical flame radius for the onset of instability is found to be on the order of 20–35 cm, increasing with methane concentration. In addition, an oscillatory flame acceleration is observed that is directly analogous to the behavior of propane-air mixtures seen in a previous study. Weakly-turbulent initial conditions lead to an earlier onset of flame instability that eliminates the rapid acceleration that occurs during global cell formation along with the oscillatory flame acceleration. It is also shown that weak initial turbulence does not significantly affect the burning velocity during the initial, laminar, flame propagation. Instead, initial turbulence triggers both an earlier onset of instability as well as an increase in the rate of flame acceleration. As a result, significant increases in overall flame velocity is observed in the presence of weak initial turbulence, and this difference increases with flame radius. [ABSTRACT FROM AUTHOR]

Abstract: Flame acceleration in tubes is studied. A tube filled with flammable mixture is closed at one end and open to the atmosphere at its second end. When ignition takes place near the closed end, it is well-known from experiments that the flame may accelerate, oscillate and eventually reach considerable speeds. A one-dimensional analysis is presented, based upon the assumption that the flame front propagates at a speed that is small compared to the speed of sound. The analysis leads to a construction of the complete unsteady solution. Results from the analysis and from a numerical simulation are compared. They are similar enough to validate the analysis. The tube acoustics are set in motion by the expansion of the fluid due to ignition at the closed end. Subsequently, both spectrum and amplitude evolve because of the motion of the temperature interface, and because of forcing by the flame front, which the analysis precisely quantifies. Oscillations in the front position are strong enough to result in flow reversal. In addition, the induced periodic acoustic acceleration of the temperature and density interface will periodically make the flame front Rayleigh–Taylor unstable, which should result in the dramatic increase in the propagation speed seen in experiments. [Copyright &y& Elsevier]

The accurate estimation of a muscle's line of action is a fundamental requirement in computational modelling. We present a novel anatomical muscle wrapping technique and demonstrate its clinical use on the evaluation of the Psoas muscle mechanics in hip arthroplasty. A volume preserving, spring model to parameterize muscle anatomy changes during motion is presented. Validation was performed by a CT scan of a cadaver model in multiple positions. The predicted psoas musculotendinous path was compared with the actual imaging findings. In a second stage, psoas kinetics were compared between a conventional versus a resurfacing hip arthroplasty during gait. Anatomy prediction error was found to be 2.12 mm on average (SD 1.34 mm). When applied to psoas mechanics during walking, the muscle was found to wrap predominantly around the femoral head providing a biomechanically efficient and nearly constant moment arm for flexion during the entire gait cycle. However, this advantage was found to be lost in small diameter hip arthroplasty designs resulting in an important mechanical disadvantage. The moment arm for flexion, was on average 36% (SD 0.03%) lower in the small diameter conventional hip arthroplasty as compared to the large diameter head of the hip resurfacing and this difference was highly significant. (p < 0.001). Despite the shortcomings of an "in silico" and cadaveric study, our findings are in accordance with previous clinical and gait studies. Furthermore, the findings are strongly in favour of large diameter implant designs, warranting their further development and optimisation. • A discrete element model for volumetric muscle wrapping of the psoas is presented. • Accuracy of anatomy predictions was 2.12 mm (SD 1.34 mm). • Small diameter total hip arthroplasty present a significant loss in psoas flexion moment. [ABSTRACT FROM AUTHOR]