Experimental studies on interactions between a freely propagating flame and single obstacles in a rectangular confinement

Experimental investigations were performed to assess the effects of different shaped obstructions on flame propagation in a rectangular confinement, 235 mm in height, with a 1000×950 mm1000×950 mm cross section and a large top-venting area of 1000×320 mm1000×320 mm. Four different single obstacles were used: rectangular, cylindrical, triangular, and square cross-sections with blockage ratios of 5 and 10%. Temporally resolved flame front images were recorded by a high-speed video camera to investigate the interaction between a propagating flame and the obstacle. The local flame displacement speeds and their probability density functions (pdfs) were obtained for the different obstacles. Before the freely propagating flame impinges on the obstacle, the flame propagation speed remains close to the laminar burning velocity, regardless of the obstacles used. As the propagating flame impinges on the obstacle, the local propagation speed increases due to the expansion of the burnt gas and the blockage of the obstacle. This local speed increase becomes larger in going from a circular to a triangular and to a square obstacle. The averaged flame displacement speeds were not significantly different with different blockage ratios for the same obstacle investigated in this work, nor were they significantly different for different shapes at the same blockage ratio investigated in this work. However, the fastest increase in the averaged flame speed with time was observed for the rectangular plate. In order to explain why the results obtained from this work were different from those published in the literature for large L/DL/D, a discussion of both the flame speed and pressure was given.