Turbulent flame propagation in obstacle-filled tubes

Turbulent flame acceleration experiments have been carried out in steel tubes of 5 cm, 15 cm and 30 cm diameter and ranging from 11 m to 17 m in length. Circular orifice plates spaced on diameter apart were used as flow obstructions. The blockage ratios BR=1−(d/D)2 are 0.44, 0.39 and 0.28 corresponding to orifice diameters of 3.74 cm, 11.7 cm and 25.8 cm for the 5 cm, 15 cm and 30 cm diameter tubes, respectively. Mixtures of hydrogen, acetylene, ethylene, propane and methane with air were used over a range of fuel compositions. The results indicate the existence of four propagation regimes: the quenching, the choking, the quasi-detonation and the C-J detonation regimes. In the quenching regime, the flame is first found to accelerate and then extinguish itself after propagating past a certain number of orifice plates. The flame propagation process in the quenching regime can be considered as the successive ignition of a sequence of chamber separated by the orifice plates. Ignition in one chamber is due to the venting of the hot combustion products from the upstream chamber through the orifice. Quenching occurs when the jet fails to ignite the mixture due to too short a mixing time when compared to the chemical reaction time. For mixtures very near the limits, an alternative quenching mechanism due to flame stretching is proposed. When the flame is not quenched, it eventually reaches a steady state. It is found that gasdynamic choking (sonic conditions) due to friction and heat release provide the controlling mechanism for the steady state flame speed. Again, for near limit mixtures, the steady state condition may be brought about by the positive and negative effects of flame stretching in the augmentation of the burning rate. For most of the cases of interest, the choking mechanism prevails. For the more sensitive mixtures, transition to detonation is observed. The transition criterion requires that the ratio of the orifice diameter d to detonation cell size λ be in the range 1d/λ≤13. In the present study where the blockage ratios are of the order of BR≈0.4, the critical value is d/λ≈3. The detonation under these conditions is observed to travel in the obstacle-filled tube at velocities significantly below the normal C-J value in accord with the previous observations of detonation propagation in very rough tubes. Such detonations are referred to as quasi-detonations. When the mixture is sufficiently sensitive such that d/λ13, then the detonation propagation becomes insensitive to the blockage effects of the obstacles and the combustion front is observed to propagate as a normal C-J detonation wave.