Effect of obstacle size and spacing on the initial stage of flame acceleration in a rough tube

Experiments were conducted to study flame acceleration in an orifice plate laden detonation tube. Orifice plate area blockage and spacing were varied to determine their affect on flame acceleration. The tube used in the study was 3.05 m long with an inner diameter of 14.0 cm. Experiments were primarily carried out with stoichiometric propane-air, however the affect of mixture reactivity was also investigated by varying the mixture equivalence ratio. The distance required for the flame to achieve a velocity equal to the speed of sound in the unburned gas mixture was measured. This run-up distance is used to characterize the early stage of the flame acceleration process. It was found that in all cases, the flame run-up distance decreased with increased blockage ratio and with increased mixture reactivity. It was found that for higher blockage ratios plates flame acceleration was greatest for a plate spacing of one tube diameter, but for lower blockage ratio plates the results obtained for one-half, one, and one and one-half tube diameter plate spacing were very similar. The most rapid flame acceleration was observed when the ratio of the orifice plate spacing and the orifice plate height (half of the difference between the tube and orifice plate diameter) is on the order of 5. It is proposed that this optimum acceleration corresponds to the condition where the plate spacing is roughly equal to the length of the unburned gas re-circulation zone downstream from the orifice plate.