Experimental investigation of near-limit gaseous detonations in small diameter spiral tubing

Abstract

The near-limit propagation of gaseous detonations in seven explosive mixtures with different reaction sensitivities is investigated. Experiments were performed in transparent tubing of four different inner diameters with relatively long tubing length (l/d > 2500 except l/d > 1000 for the largest diameter) arranged in a spiral configuration. Up to 83 fiber optics spaced at regular intervals along the tube were used to provide high resolution velocity measurement. Up to 8 cycles of the galloping mode were recorded, and the spiral boundary did not influence the persistence of galloping detonations. Results confirm that for mixtures with increasing argon dilution, making the detonation more stable with regular cellular pattern, the occurrence of galloping detonation diminishes. For stable mixtures with sufficiently large amount of argon dilution (e.g., stoichiometric C2H2/O2 with 70%Ar), the galloping mode was not observed in all tested tubing. For unstable mixtures, smaller diameters were necessary to achieve the galloping mode. The range of initial pressures, within which galloping detonations were observed decreases rapidly with increasing tubing diameter. These results suggest that both the instability and the boundary effect are essential for galloping detonations. From the velocity histogram and the probability distribution function, a bimodal behavior was also observed in all galloping regimes of different unstable mixtures, with dominant modes near half of the Chapman-Jouguet detonation velocity (DCJ) and DCJ. With decreasing pressure, the lower velocity mode became more prevalent until no more galloping detonation occurred. The normalized wavelength of the galloping cycle (L/d) ranges from 250 to 450 within experimental variation. Nevertheless, few results show a clear minor trend that the wavelength increases with decreasing initial pressure. By looking at the velocity amplitude in the galloping cycle, the lower value as well as the average is relatively constant, while the upper peak has larger fluctuations.