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

Performance analysis on Free-piston linear expander

The growing global demand for energy and environmental implications have created a need to further develop the current energy generation technologies (solar, wind, geothermal, etc.). Recovering energy from low grade energy sources such as waste heat is one of the methods for improving the performance of thermodynamic cycles. The objective of this work was to achieve long-term steady state operation of a Free-Piston Linear Expander (FPLE) and to compare the FPLE with the currently existing expander types for use in low temperature energy recovery systems. A previously designed FPLE with a single piston, two chambers, and linear alternator was studied and several modifications were applied on the sealing and over expansion. An experimental test bench was developed to measure the inlet and outlet temperatures, inlet and outlet pressures, flow rate, and voltage output. A method of thermodynamic analysis was developed by using the first and second law of thermodynamics with air as the working fluid. The experimental tests were designed to evaluate the performance of the FPLE with varying parameters of inlet air pressure, inlet air temperature, and electrical resistance. The initial and steady-state operation of the FPLE were successfully achieved. An uncertainty analysis was conducted on the measured values to determine the accuracies of the calculated parameters. The trends of several output parameters such as frequency, average root mean square (RMS) voltage, volumetric efficiency, electrical-mechanical conversion efficiency, isentropic efficiency, irreversibility, actual expander work, and electrical power were presented. Results showed that the maximum expander frequency was found to be 44.01 Hz and the frequency tended to increase as the inlet air pressure increased. The FPLE achieved the maximum isentropic efficiency of 21.5%, and produced maximum actual expander work and electrical work of 75.13 W and 3.302 W, respectively

Analysis of Gaseous and Gaseous-Dusty, Premixed Flame Propagation in Obstructed Passages with Tightly Placed Obstacles

A recent predictive scenario of premixed flame propagation in unobstructed passages is extended to account for obstructions that can be encountered in facilities dealing with explosive materials such as in coalmines. Specifically, the theory of globally-spherical, self-accelerating premixed expanding flames and that of flame acceleration in obstructed conduits are combined to form a new analytical formulation. The coalmining configuration 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. It is assumed that the spacing between the obstacles is much less or, at least, does not exceed the obstacle height. The passage has one extreme open end such that a flame is ignited at a closed end and propagates to an exit. The key stages of the flame evolution such as the velocity of the flame front and the run-up distance are scrutinized for variety of the flame and mining parameters. Starting with gaseous methane-air and propane-air flames, the analysis is 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 are considered, and the impact of the size and concentration of the dust particles on flame acceleration is quantified. Overall, the influence of both the obstacles and the combustion instability on the fire scenario is substantial, and it gets stronger with the blockage ratio

Analysis of Gaseous and Gaseous-Dusty, Premixed Flame Propagation in Obstructed Passages with Tightly Placed Obstacles

A recent predictive scenario of premixed flame propagation in unobstructed passages is extended to account for obstructions that can be encountered in facilities dealing with explosive materials such as in coalmines. Specifically, the theory of globally-spherical, self-accelerating premixed expanding flames and that of flame acceleration in obstructed conduits are combined to form a new analytical formulation. The coalmining configuration 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. It is assumed that the spacing between the obstacles is much less or, at least, does not exceed the obstacle height. The passage has one extreme open end such that a flame is ignited at a closed end and propagates to an exit. The key stages of the flame evolution such as the velocity of the flame front and the run-up distance are scrutinized for variety of the flame and mining parameters. Starting with gaseous methane-air and propane-air flames, the analysis is 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 are considered, and the impact of the size and concentration of the dust particles on flame acceleration is quantified. Overall, the influence of both the obstacles and the combustion instability on the fire scenario is substantial, and it gets stronger with the blockage ratio

Dynamics of Explosions in Cylindrical Vented Enclosures: Validation of a Computational Model by Experiments

Recent explosions with devastating consequences have re-emphasized the relevance of fire safety and explosion research. From earlier works, the severity of the explosion has been said to depend on various factors such as the ignition location, type of a combustible mixture, enclosure configuration, and equivalence ratio. Explosion venting has been proposed as a safety measure in curbing explosion impact, and the design of safety vent requires a deep understanding of the explosion phenomenon. To address this, the Explosion Venting Analyzer (EVA)—a mathematical model predicting the maximum overpressure and characterizing the explosion in an enclosure—has been recently developed and coded (Process Saf. Environ. Prot. 99 (2016) 167). The present work is devoted to methane explosions because the natural gas—a common fossil fuel used for various domestic, commercial, and industrial purposes—has methane as its major constituent. Specifically, the dynamics of methane-air explosion in vented cylindrical enclosures is scrutinized, computationally and experimentally, such that the accuracy of the EVA predictions is validated by the experiments, with the Cantera package integrated into the EVA to identify the flame speeds. The EVA results for the rear-ignited vented methane-air explosion show good agreement with the experimental results