COMBUSTION AND FLAME 85:363-379 (1991) 363 The Acceleration of Flame Propagation in a Tube by an Obstacle H. PHYLAKTOU and G. E. ANDREWS Department of Fuel and Energy, The University of Leeds, Leeds, LS2 9JT, UK 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 (LID = 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 ~ 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 /3 more accurately is tentatively introduced and shown to give very good agreement with the experimental results. INTRODUCTION An explosion can be characterized by two param- eters: the maximum pressure attained and the rate of pressure rise. When the venting of an explo- sion is under consideration the rate of pressure rise is crucial [1], as it determines the size of the vent required to limit the maximum overpressure to a safe level. The majority of experimental data on which vent design is based has been obtained under conditions of low turbulence and in simple ge- ometries [2]. In practical situations, many enclo- sures in which gas explosions could occur are likely to contain obstructions such as shelves, machinery, walkways, heat exchanger tubes, baffle plates, interconnecting doors, and the like. These obstructions will produce turbulence in the unburned gases set in motion by the advancing flame. When the flame encounters this turbulence it is affected in two ways: small-scale turbulence increases the local heat and mass transfer rates and large-scale turbulence distorts the flame front and increases the flame area. These factors en- hance the combustion rate and hence increase the rate of pressure rise. The majority of previous work on the influence of obstacles has been done in long tubes, gener- ally open (at either one or both ends), using methane-air mixtures, and with flame speeds be- ing the main parameter measured. A number of researchers [3-8] have shown that arrays of ob- stacles in such configurations greatly enhance the speed of flame propagation and can drastically reduce the transition distance to detonation. In view of its importance in connection with accidental explosions, other authors [9-15] have also demonstrated that when the turbulence inten- sity is maintained by placing several obstacles in the path of a propagating flame, the rate of combustion and degree of turbulence become highly coupled so as to promote a strong feed- back mechanism that in partly confined geome- tries can lead to violent explosions. For geometries other than tubes, Dorge et al. [16] demonstrated the influence of wire mesh grid plates in large volume explosions. Kumar et al. [17] discussed the effect of baffle in near spherical Copyright © 1991 by The Combustion Institute Published by Elsevier Science Publishing Co., Inc. 0010-2180/91/$3.50