A fundamental study on premixed methane-air flame propagation dynamics in straight long ducts

The propagation of flames in industrial facilities is associated with a number of adverse outcomes, including the potential loss of life, traumatic injuries and significant property damage. Flammable hydrocarbons, including premixed methane-air mixtures, which are commonly present in underground coal mines, oil and gas refineries, and across a broad range of petrochemical based industries, are often considered to be the underlying root cause of incidental fires and explosions. In the presence of an ignition source with adequate energy, a methane-air mixture within the flammable range (5% - 15%) can ignite and propagate along an enclosed space such as in a combustion chamber, pipe or similar enclosed space. The characteristics of the propagation of flames in a confined space containing such critical concentrations of combustible gas are heavily influenced by the geometry and configuration of the enclosed space. Despite ongoing efforts to prevent these types of unwanted fires, the nature of the hazard is such that the complete elimination of the risks associated with the use of combustible gases has been at best, problematic, and at worst, not practicable. The ultimate goals of the experiments described in this research study were to: (i) more fully understand the mechanisms of the propagation of flames from methane-air mixtures in see-through enclosures (‘pipes’) with various geometries and configurations (including straight, branched and elbow-shaped configurations); (ii) determine the influences of the different shapes, configurations and blockage ratios of obstacles on the properties of the flame propagations; and (iii) examine the viability of a typical deflagration flame arrestor (acrylate styrene acrylonitrile (ASA) polymer) to stop the propagation of flames for a range of experimental criteria. In these experiments, various concentrations of methane, ranging from 5% - 15% v/v, were deployed within an experimental enclosure and ignited, with the subsequent flame propagations being investigated. Chemical pyrotechnic igniters with 50 mJ energy were utilised as an ignition source in all of the experiments. The results obtained indicated that the placement of a range of obstacles in the propagation pathways of flames significantly impacted on the explosion characteristics of the methane-air mixtures, including the pressures and velocities of the flames. Obstacles with circular shaped openings were found to be more effective in suppressing the propagation of flames compared to obstacles with a square opening. The experimental results suggested that the branched-tube and elbow-shaped configurations increased the propagation characteristics of the methane-air flames more than a straight tube configuration. The presence of the obstacles in both the straight, branched and elbow-shaped configurations also resulted in increased propagation velocities, however the effects were more pronounced in the branched and elbow-shaped configurations. In addition, the experimental work associated with the testing of the performance of the flame arrestors indicated that a flame arrestor with double elements (i.e., perforated disks) was able to quench the propagation of the flames from all of the combustible concentrations of the methane gas.