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Flame Propagation Limits in a Rotating Tube
A polycarbonate cylindrical tube of 32mm inner diameter and 2000mm long is mounted horizontally on a lathe, and concentration limits of flame propagation are determined as a function of rotational speed up to 2000rpm. Mixtures of air and either of methane and propane, and their diluted mixtures with either of nitrogen and carbon dioxide are investigated. To prevent entrainment of ambient air, both ends are closed and the mixture is ignited at one end by an electric spark. Results show that with increasing the rotational speed, rich methane and lean propane limits remain almost constant, whereas lean methane and rich propane limits are varied away from the stoichiometry and the ranges of flame propagation are widened. It is interesting to note that the equivalence ratio at rich propane limit is increases from 2.05 at 0rpm to 2.70 at 1210rpm, which exceeds the standard flammability limit of 2.5 determined with an upward propagation flame in a tube of 5cm diameter. With increasing the rotational speed, the area of flame propagation in the φ (equivalence ratio) x (added inert gas concentration) plane is widened. The equivalence ratios at lean limit are decreased in methane and those at rich limits are increased in propane. The maximum allowable dilution ratio x is increased, and the equivalence ratio at which the maximum occur is shifted towards to lean side in methane, whereas rich side in propane. There aspects can be explained on the basis of diffusional stratification of species of different molecular weight in the field of centrifugal force of rotation.