A digital soot foil method for the analysis of cellular structures in detonation waves

Abstract

The advent of detonation-based propulsion systems represents an opportunity for more sustainable combustion processes. Though highly unstable, some detonations carry information on their constituting waves and instabilities through a cellular structure. Such detonations observe patterns with diamond-shaped cells, delimited by Mach reflections at their vertices. These are formed by the collision of triple points, historically described as a 3-shock structure between an incident shock, Mach stem and transverse wave. The present work investigates the dynamics of detonation waves at a sub-cellular level in hydrogen–oxygen–nitrogen mixtures. The diluent content is varied experimentally while the equivalence ratio is maintained to unity. Characteristic lengths scales such as the cell width and length are reported, along with local measurements of wave velocity through shadowgraph imaging. Due to the three dimensional nature of detonations, experiments are conducted in a thin channel to minimize gradients in the third dimension and favor a quasi 2D propagation of the detonation wave. The stochastic behavior of the phenomenon is reported using a statistical approach and leverages a new methodology for the simultaneous resolution of the velocity field and cellular structure. The results first show an exponential decrease in cell sizes with the dilution content through the decreased activation energy. Furthermore, the formation of Mach reflections is seen to be associated with sudden and sharp velocity gradients at high dilution, resulting in a more irregular distribution of the wave velocity. Ultimately, the digital soot foils provide new insights into cellular structures and the wave dynamics surrounding Mach reflections.