THE ACCELERATION OF FLAME PROPAGATION IN A TUBE BY AN OBSTACLE

A quantitative determination was made of the effect of a single baffle on the characteristics of gas explosions in a 76-mm-diameter closed vessel of large length to diameter ratio (L/D = 21.6). Mixtures of methane-air were predominantly used, but other gases were also investigated. Ignition was effected at one end of the vessel. Single hole plates were employed as baffles with varying blockage ratios (20%-80%). The flame speed and rate of pressure rise were greatly enhanced downstream of the baffle. The relative effect of the baffle increased with increasing blockage ratio. It was 8 times more severe with the baffle at 7D from the spark than at 14D. The flow rate of the unburned gas, set in motion ahead of the flame was determined by measuring the pressure drop across the baffle. From the unburned gas velocity the rms turbulent velocity (u') was determined using experimental correlations of grid-generated turbulence, and from this the turbulent burning velocity and turbulence factor beta were calculated. The turbulence factor was found to be equal to the normalized rate of pressure rise. This demonstrated that current turbulent combustion theory (for high Reynolds number flows) can explain and predict the phenomena observed in such combustion regimes. Turbulent flame extinction was predicted for high blockages and experimental evidence of localized flame quenching was found. However, no total flame extinction was observed as the turbulence generated by the baffle was nonuniform and the flame could propagate round local high turbulence regions. The turbulent burning velocity was found to be as high as 110 times the laminar value. In the current literature for vent design a turbulence factor of 10 is suggested for severe cases of turbulence. The present results show the need for reviewing these guidelines if high blockages to the explosion gases exist. A method for estimating beta-more accurately is tentatively introduced and shown to give very good agreement with the experimental results.