Semester

Summer

Date of Graduation

2020

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Mechanical and Aerospace Engineering

Committee Chair

V'yacheslav Akkerman

Committee Member

Hailin Li

Committee Member

Cosmin Dumitrescu

Committee Member

Derek R. Johnson

Committee Member

Ali S. Rangwala

Abstract

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.

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