The influence of initial and boundary conditions on gaseous detonation waves

Results of five experimental investigations on the initiation, propagation and transmission of detonation have shown that the wave behavior depends on the relative rates of gasdynamic expansion and chemical energy release occurring within the cellular detonation front. The former rate is controlled by the boundary conditions defined by the physical system, while the latter rate depends on the chemical and physical properties of the combustible mixture. The fractional increase zeta in the area of th post-shock stream tube, evaluated over a chemical kinetic distance equal to the cell length, has been identified as a parameter which satisfactorily characterizes the competition between these two rate processes. For zeta less than about 20%, the chemical processes survive the gasdynamic expansion and self-sustained propagation is possible. However, under these supercritical conditions, the wave propagates with a velocity deficit which appears to be a universal and theoretically predictable function of zeta. For zeta greater than 20%, the shock/reaction zone coupling breaks down, resulting in failure of the wave. The critical conditions for the propagation of detonation waves subjected to a wide range of expansion inducing mechanisms, including viscous boundary layers, compressible boundary gases and yielding walls, are all found to be consistent with the 20% criterion. However, the criterion becomes inapplicable as the cell size approaches the characteristic transverse dimension of the geometry.