Friday, May 31, 2013
Planar Reflection of Gaseous Detonations
Jason Damazo, PhD Candidate, Graduate Aerospace Laboratories, California Institute of Technology
Pipes containing flammable gaseous mixtures may be subjected to internal detonation. When the detonation impinges on a closed end, a reflected shock wave is created to bring the flow back to rest. We built on the work of Karnesky (2010) to experimentally characterize and numerically model the elastic and plastic deformation of thin-walled stainless steel tubes subjected to internal reflected gaseous detonation loading. A novel ripple pattern was observed in the tube wall for detonations of intermediate pressure, and a criteria was developed that explains the conditions under which this ripple pattern develops. A two-dimensional finite element analysis was performed using Johnson-Cook material properties to account for strain-rate hardening, and a previously developed model to account for the pressure profile created by normally reflected gaseous detonations. Good agreement between experiments and computations was obtained, with the best agreement occurring for the high-pressure regime when the elastic oscillation (which is sensitive to small errors) is less important.
During the examination of detonation-driven material deformation, discrepancies were discovered in our understanding of how reflected gaseous detonations behave. A previously developed model for the reflected detonation did not properly describe the nature of the reflected shock wave, and this motivated further experiments in a detonation tube that allowed for optical access to the reflecting wave. An array of pressure sensors and a sequence of schlieren images were used to gather data examining the behavior of the reflected shock, and we determined that the source of the previous discrepancies lies in the finite reaction zone thickness extant behind the detonation front. In these experiments, it was noted that reflected shock bifurcation did not appear to occur, although the nature of the unfocused visualization system made certainty impossible. This prompted the construction of an extended source focused schlieren visualization system to examine the possibility of shock wave--boundary layer interaction, and heat flux gauges were used to examine the boundary layer behind the detonation front. Using data gained from these experiments with an analytical boundary layer solution, it was determined that the strong thermal boundary layer present behind the detonation front inhibits the development of reflected shock wave bifurcation.