Lateral Strain: Key to Detonation Control

Detonation waves are self-sustained supersonic combustion waves [1]. These waves areled by a shock, which compresses the fresh reactive media to a much higher temperatureand pressure for rapid reaction [1]. The tremendous reaction heat release occurring behindthe shock in return energizes the propagation process. As such, this closely coupledshock-reaction complex self-sustains. Detonation waves can be sustained in a variety ofenergetic media including reactive gases. The large overpressures generated behind gaseousdetonations make them attractive and useful for developing propulsion systems [2], suchas rotating detonation engines (RDEs) [3, 4] and pulse detonation engines (PDEs) [5, 6].These applications require reliable control of the accurate ignition and stable propagationof a detonation wave. Likewise, for safety applications [7,8], it is also desirable to have thepredictability for the eventual initiation of a detonation wave and for its propagation limitswhen different mitigation strategies are used [9]. Therefore, realizing all these practicalpurposes requires predictive capability of detonation behavior.Detonations in gases usually propagate with lateral strain. For example, in confinedgeometries of small size, such as narrow channels or tubes, detonations are subject tosignificant losses induced by boundary layers, which act as a mass sink and result in flowdivergence in reaction zones, thereby giving rise to lateral strain impacting the detonationpropagation [10]; while in geometries of varying cross-section areas or curved channels, astypically seen in PDE pre-detonator tubes and RDE combustors, detonations are curvedwith the flow also diverging after passing the leading front [11-13]. These lateral strainrates are generally known to decrease the detonation speed and its propagation limit [10,11, 13-17]. Thus, in order to achieve the practical purposes of either utilizing or avoidingdetonations, the effect of such lateral strain rates on detonation dynamics, e.g., detonationpropagation speed and its failure limit, needs to be quantified for revealing their influencingmechanism. This, however, is still poorly understood and not well treated, since the highlyunsteady nature of the well-known multi-dimensional structures of detonations greatlycomplicate the efforts.