Experimental, Computational and Analytical Studies towards a Predictive Scenario for a Burning Accident

Historically, accidental gas and dust explosions constitute one of the major hazards to both personnel and equipment in the process industries. The current knowledgebase on such explosions does not provide an acceptable level of risk. Therefore, novel preventive mining/fire safety strategies, based on a rigorous predictive scenario for burning accidents, are critically needed. The present dissertation is devoted to such a predictive scenario, with a particular focus on the flame and pressure evolutions in explosions encountered in an enclosure with or without obstructions. The experimental component of this dissertation comprises a series of experiments on explosion venting. Specifically, the influence of the vent area on the overpressure and dynamics of the fuel-lean, stoichiometric, and fuel-rich methane-air flames was studied. First, the experiments were conducted in a transparent polycarbonate cylindrical chamber to allow for real-time visualization of the flame front. Experimental parameters included ignition location, central or rear, and three various vent areas (with negligible vent relief pressure). As expected, the highest maximum pressure was associated with the stoichiometric conditions and the smallest vent area. For a fuel-rich mixture with central ignition, a flashback phenomenon was observed after an external explosion. The experimental study was subsequently extended to a twice longer cylinder (with only rear ignition). It showed that an increase in the length of the cylinder promotes the overpressure and the acceleration rate. An engineering model to predict the pressure-time histories of stoichiometric methane-air vented deflagrations was updated and compared to the experiments. Good agreement between the experiments and the simulations was obtained in terms of pressure rise and peak pressure predictions. The future work was recommended on further development of the model in larger scales, congested volumes, and multi-compartment enclosures. For future development of the model, the mechanisms of flame propagation in the passages with or without obstructions were studied. First, the assumptions used on finger flame acceleration were reviewed. The mechanistic and thermal impacts of the passage walls on finger flame acceleration were studied by means of the fully-compressible computational simulations of the reacting flow equations. It was shown that the difference between the effects of slip and nonslip walls was generally minor during the acceleration stages of burning. After a flame skirt contacted a sidewall, wall friction played a role and promoted the flame further. As for the thermal boundaries, cold isothermal walls cool down the flame skirt. Within the theoretical component of this dissertation, the theory for a globally-spherical, self-accelerating expanding premixed flame front was combined with that of extremely fast flame acceleration in obstructed conduits to form a new analytical formulation. The coalmining geometry is imitated by two-dimensional and cylindrical passages of high aspect ratio, with a comb-shaped array of tightly-placed obstacles attached to the walls. Specifically, the key stages of premixed flame front evolution were identified and scrutinized, by quantifying their major characteristics such as the flame tip position and its velocity. Starting with an incompressible assumption, the analysis was then extended to account for gas compressibility, because the latter cannot be ignored as soon as the burning velocity starts approaching the speed of sound. It was shown that the effect of gas compressibility moderates flame acceleration, and such an impact depends strongly on various thermal-chemical properties of the combustible mixture. The theoretical investigation of the problem revealed that the influence of both the obstacles and the combustion instability on the fire scenario was substantial, and this effect grew stronger with the blockage ratio. Starting with gaseous methane-air combustion, the formulation was subsequently extended to gaseous-dusty environments. Specifically, the coal (combustible, i.e. facilitating the fire) and inert (such as sand, moderating the process) dust and their combinations were considered. The impact of the size and concentration of the dust particles on flame acceleration was quantified. Eventually, the analytical predictions were compared with the experiments and the numerical simulations from the literature, with good agreement obtained. Finally, the comparison of the theory, simulations and experiments of this dissertation was conducted in terms of the exponential acceleration rates, with qualitatively good agreement demonstrated

A numerical study of dust explosion properties of hydrogen storage alloy materials

Hydrogen as a clean fuel has gained increased attention in the recent years and considerable research is being undertaken to develop hydrogen storage technologies. Hydrogen storage using metal hydride is one such technology. Hydride materials, used in hydrogen storage technologies, in powder form can be an explosion hazard and testing these materials using standard techniques is also difficult. Research reported in this paper is an attempt to develop numerical methods to obtain explosion properties of such materials. In this work a one dimensional transport-type model is presented to simulate the dust explosion process in a closed 20-L spherical vessel. Transport equations for energy, species and particle volume fraction are solved with the finite difference method, whilst velocity distribution and pressure are updated with numerical integration of the continuity equation. The model is first validated with experimental data and then applied to simulate the explosion process of an AB2-type alloy powder used for hydrogen storage. Two kinetic models accounting for the particle burning mechanism are investigated in the current study. One is based on an Arrhenius surface reaction law, the other is based on a simplified diffusion-type d2 law. The former is found to be better in terms of prediction of the deflagration indices. This work is of great significance in safety assessment of new hydrogen storage materials in the processes of their production, storage and transportation

CFD Simulations to Study Parameters Affecting Gas Explosion Venting in Compressor Compartments

In this work, a series of vented explosions in a typical compressor compartment are simulated using FLACS code to analyze the explosion venting characteristics. The effects of relevant parameters on the pressure peaks (i.e., overpressure and negative pressure) are also numerically investigated, including vent area ratio of the compressor compartment, vent activation pressure, mass per unit area of vent panels, and volume blockage ratio of obstacles. In addition, the orthogonal experiment design and improved grey relational analysis are implemented to evaluate the impact degree of these relevant parameters. The results show that the pressure peaks decrease with the increase of vent area ratio. There is an approximately linearly increasing relationship between the pressure peaks and the vent activation pressure. The pressure peaks increase with the mass per unit area of vent panels. The pressure peaks increase with the volume blockage ratio of obstacles. Based on the grey relational grade values, the effects of these relevant parameters on the overpressure peak are ranked as follows: volume blockage ratio of obstacles > vent activation pressure > vent area ratio > mass per unit area of vent panels. These achievements provide effective guidance for the venting safety design of gas compressor compartments

Unsteady 3-D RANS simulations of dust explosion in a fan stirred explosion vessel using an open source code

Dust explosion is a constant threat to industries which deal with combustible powders such as woodworking, metal processing, food and feed, pharmaceuticals and additive industries. The current standards regarding dust explosion venting protecting systems, such as EN 14491 (2012) and NFPA 68 (2018), are based on empirical correlations and neglect effects due to complex geometry. Such a simplification may lead to failure in estimating explosion overpressure, thus, increasing risk for injuries and even fatalities at workplaces. Therefore, there is a strong need for a numerical tool for designing explosion protecting systems. This work aims at contributing to the development of such a tool by (i) implementing a premixed turbulent combustion model into OpenFOAM, (ii) verifying the implementation using benchmark analytical solutions, and (iii) validating the numerical platform against experimental data on cornflour dust explosion in a fan-stirred explosion vessel, obtained by Bradley et al. (1989a) under well-controlled laboratory conditions. For this purpose, the so-called Flame Speed Closure model of the influence of turbulence on premixed combustion is adapted and implemented into OpenFOAM. The implementation of the model is verified using exact and approximate analytical solutions for statistically one-dimensional planar and spherical turbulent flames, respectively. The developed numerical platform is applied to unsteady three-dimensional Reynolds Averaged Navier-Stokes simulations of the aforementioned experiments. The results show that the major trends, i.e. (i) a linear increase in an apparent turbulent flame speed St,b\ua0with an increase in the root mean square (rms) turbulent velocity u\u27 and (ii) and an increase in St,b\ua0with an increase in the mean flame radius, are qualitatively predicted. Furthermore, the measured and computed dependencies of St,b(u\u27) agree quantitatively under conditions of weak and moderate turbulence

Pressure Systems Energy Release Protection (Gas Pressurized Systems)

A survey of studies into hazards associated with closed or pressurized system rupture and preliminary guidelines for the performance design of primary, secondary, and protective receptors of these hazards are provided. The hazards discussed in the survey are: blast, fragments, ground motion, heat radiation, biological, and chemical. Performance guidelines for receptors are limited to pressurized systems that contain inert gas. The performance guidelines for protection against the remaining unaddressed degenerative hazards are to be covered in another study

Lithium-ion Battery Safety Analysis with Physical Sub-models

Ever-increasing explosions occurring globally at a rapid rate and in diverse situations have re-established the fact that novel, faster, and more accurate approaches must be developed to analyze and possibly curb these explosions and avert their future occurrences. Experimental endeavors and computational fluid dynamics (CFD) simulations as compared to engineering models generally require enormous time, and resources, as well as a high-level of expertise and technicalities. This might, however, delay prompt analysis and the ability to draw conclusions, thereby causing setbacks in recommending safety measures for different situations and conditions. Therefore, robust models which are accurate and fast to give an insight into an explosion, and possibly simulate various conditions easily and quickly, are critically needed. Also, this dissertation develops a comprehensive approach towards explosions, by not only considering characterizing the contemporary gaseous explosions in an enclosure at various configurations but extending the approach to prediction and characterization of the explosions of lithium-ion batteries (LIBs). As we move (paradigm shift) towards the renewable energy age, fire explosions from energy storage systems such as LIBs are on the rise. This makes computational tools a timely, robust, and versatile mechanism for predicting LIBs explosions, characterizing them quickly, and accurately, thereby giving an edge in prompt LIB explosions analysis and helping to proffer potential solutions, as well as aiding future design considerations and safety analyses. However, there are limited works and numerical models that have attempted to quantify and characterize the hazards associated with the explosion of gases ejected from LIBs during thermal runaway inside the battery pack enclosure. Therefore, in this dissertation, LIBs explosion hazards are analyzed, and a computational model is developed to study the explosion venting scenarios of hazardous gases released from LIBs during failure, as well as developing sub-models to characterize the LIB failure, explosions, and their associated hazards. The model developed showed good accuracy and present itself as an easy-to-use, fast, and reliable tool to predict explosion characteristics of both single-compound and multi-compound fuel-air mixtures including LIBs vented gases. The model was further enhanced by integrating a machine learning model capable of predicting laminar flame speed (a key parameter in explosion modelling) of both single and multiple-compound fuel-air mixtures. The effect of various parameters such as vent size, battery chemistry, CO2 concentration, and the state of charge on LIB explosions National Fire Protection Association (NFPA) reduced pressure was also scrutinized. The results of this dissertation can be integrated into LIBs battery management systems algorithm, to develop safer and more robust models for averting battery failures as well as mitigating the severity of possible explosions. Furthermore, this work will aid LIB safety analysis and can also be employed in the design of safety vents used in LIBs energy storage compartments to mitigate the effects of explosions. For practical scenarios, the NFPA standards are widely used in industry because of their simplified equations. Therefore, the numerical model developed in this dissertation will enable a field engineer, who does not have experience in numerical methods or/ and the physics of complex explosions to use this code to design a vent for a compartment, where an explosion from a LIB failure might occur. Overall, this dissertation will greatly support the fire safety research community in terms of LIB safety analysis and explosion characterization

A vented corn starch dust explosion in an 11.5 m3 vessel: Experimental and numerical study

A vented corn starch dust explosion in an 11.5 m3 vessel is studied using both experimental and numerical methods. The reduced explosion overpressure in the vessel is recorded using two pressure sensors mounted on the wall inside of the vessel. Unsteady three-dimensional Reynolds-Averaged Navier-Stokes simulations of the experiment are performed using the Flame Speed Closure (FSC) model of the influence of turbulence on premixed combustion. The model was thoroughly validated in previous studies and was earlier implemented into OpenFOAM CFD software. The self-acceleration of a large-scale flame kernel is associated with the influence of combustion-induced pressure perturbations on the flow of unburned reactants ahead of the kernel. Accordingly, the FSC model is extended by adapting the well-known experimental observations of the self-similarity of the kernel acceleration. Influence of different turbulence models on the simulated results is also explored. Thanks to the extension of the FSC model, the measured time-dependence of the pressure is well predicted when the k-omega-SST turbulence model is used

The effects of different degrees of confinement on the deformation of square plates subjected to blast loading

Includes abstract.Includes bibliographical references.This work relates to the effcts of the degree of confiement for air blasts only. The response of a structure subjected to a blast load is dependent on several factors; for instance stand off distance, geometry and mass of explosive, geometry of the structure, medium (air, water, soil) and degree of confinement. Depending on the location of the explosion relative to the surrounding structures different degrees of confinement are obtained. In addition, depending on the degree of confinement the accumulation of high temperature gas products will exert additional loads on the structure. This thesis reports the results of experimental and numerical investigations into the effct of the different degrees of confinement and target plate thickness on the response of square mild steel target plate