3D Effects of detonation re-initiation after diffraction at a back-facing step

Abstract

Detonation diffraction at a backward facing step and subsequent re-initiation at the bottom wall at near critical detonation transmission conditions is studied in a square 7.6 cm channel. The experimental techniques used for this purpose include ultra high-speed schlieren video from the side and the top views. Direct photography is also performed in the top view configuration with a soot foil placed at the bottom wall after the step. The process of detonation re-initiation is studied for 2H2+O2+2Ar and CH4+2O2 mixtures that are characterized by a very regular and irregular detonation cellular structure, respectively. For both mixtures detonation re-initiation begins at discrete points where the descending transverse reflect from the bottom wall; the spacing of re-initiation points matches the wave number, suggesting the three-dimensional interaction of descending and lateral transverse waves on the reflecting wall trigger detonation re-initiation. The subsequent growth of these re-initiation kernels differs between mixtures. For regular cell mixtures, the detonation evolution is gradual and follows the lateral transverse wave along the bottom wall; whereas for irregular cell mixtures, explosive of hot spots grow independently of the lateral transverse waves. The observed initiation processes underscore the critical role of three-dimensional effects for both regular and irregular cell mixtures. The abrupt detonation initiation observed for methane-oxygen can be attributed to the high effective activation energy, i.e., high reaction rate temperature sensitivity.