ID#: 121
Abstract Title: The Failure Mechanism of Detonations Propagating in Porous Wall Tubes
Session Title: Detonation Limit/Failure
Session Date: 7/30/01
Session Start Time: 3:40 PM
Contributing Author: Radulescu, M.I.
Organization: McGill University, Dep. of Mechanical Engineering
Country: Canada
Authors: Matei I Radulescu, Marianne Huot, John H.S. Lee
Short Abstract: The gas ignition mechanism in real detonation waves is believed to be a combination of the classical ZND mechanism of ignition by leading shock compression supplemented by the intense mixing generated by transverse wave interactions. To clarify the duality between these two simultaneous mechanisms, damping experiments are carried out in tubes lined with a rigid porous material (steel mesh). On short propagation length scales (one tube diameter) the porous wall acts like an acoustic absorber only and selectively dampens the transverse wave system in propagating detonations. Global mass divergence to the permeable porous wall, which weakens the leading shock, acts on length scales larger by an order of magnitude. Experiments are performed with acetylene-oxygen mixtures with various amounts of argon dilution. The detailed study of the attenuation process of these detonations revealed two fundamentally different failure mechanisms. Stable detonations (high argon dilution), which approach the idealized ZND structure, fail by the mass divergence mechanism alone, on a length scale of approximately 10 tube diameters. The velocity dependence of stable detonations on tube diameter, hence on the amount of mass divergence, is very strong. In contrast, unstable detonations (no argon dilution) are quenched immediately upon entrance in the porous wall section. Flow visualization revealed the rapid attenuation of transverse waves and disintegration of the detonation into a turbulent flame. Under-damped unstable detonations exhibit a very weak velocity dependence on tube diameter, which reflects their strong ability to re-generate new transverse waves. The present results show conclusively the prime role played by transverse waves in the propagation mechanism of typical unstable detonations displaying an irregular cellular structure. Only for artificially stable detonations, the classical mechanism of ignition by shock compression plays the lead role.