The Anatomy of a Detonation

 

Joanna M. Austin

University of Illinois at Urbana-Champaign

 

In detonation, the reaction rate behind the shock front is extremely sensitive to temperature perturbations and, as a result, detonation waves in gases are always unstable. A broad spectrum of behavior has been reported for which no comprehensive theory has been developed. The problem is extremely challenging due to the nonlinearity of the chemistry-fluid mechanics coupling and extraordinary range of length and time scales exhibited in these flows.  A key unresolved issue has emerged from the past 50 years of research on this problem: What is the precise nature of the flow within the reaction zone and how do the instabilities of the shock front influence the combustion mechanism?

 

This issue has been examined through dynamic experimentation, including unique visualizations of superimposed shock and reaction fronts, as well as high-speed movies.  We examine the role of flow features such as transverse shock waves and shear layers that result from the instability of the front. In weakly unstable detonations, we find the combustion mechanism is simply shock-induced chemical-thermal explosion behind a piecewise-smooth leading shock front.  In contrast, we observe that highly unstable detonations have substantially different behavior involving large excursions in the lead shock strength, a rough leading shock front, and localized explosions within the reaction zone.