Velocity fluctuation and cellular structure of near-limit detonations in rough tubes

Detonation limits are characterized by a decrease in the propagation velocity, cellular structures to lower unstable modes and an increase in the velocity fluctuation of the detonation. The increase in the average velocity deficit as the limits are approached is not a sensitive change since the failure of the detonation can occur at a relatively small velocity deficit of the order of 20%. A more sensitive indication of the onset of detonation limits is the lowering of the unstable mode (i.e., towards single-headed spin) and the large longitudinal fluctuation of the detonation velocity. In this paper, recent results are reported for the aforementioned near-limit detonation characteristics for a number of detonable mixtures and tube diameters for both smooth and rough tubes. Mixtures include H-2, C2H2, C3H8, CH4 fuels with both O-2 or N2O as oxidizers. Tube diameters were 25.4 mm, 38.1 mm, 50.8 mm and 76.2 mm. To investigate the effect of wall roughness on the limits phenomena in tubes, wire spirals with different diameters were inserted into the different diameter test tubes. Regularly spaced photodiodes (IF-950C) along the tube were used for velocity measurements and smoked mylar foils were inserted into the tube for the measurement of the cellular structure. Results confirm that the cellular structure evolution towards the lower unstable modes follows well the observed increase in velocity fluctuation; the subsequent detonation failure defined by the absence of cells occurs also at high-velocity fluctuation and an abrupt increase in the average velocity deficit.