Impulse generated from detonation waves in non-premixed and partially premixed reactants

Abstract

Rotating detonation engines (RDEs) are the subject of research in the combustion community due to the prospects of enhanced thermal efficiency and power density when compared to current deflagrative-based aerospace combustors. Many current simulations presume premixed reactants and therefore miss the important characteristics of transient mixing, wave-induced mixing, and injector design/spacing that are known to play a pivotal role in the system performance. Very ambitious large eddy simulations are being conducted, but necessarily on a limited number of realistic and complex cases, thus limiting their utility in deriving fundamental understanding. For these reasons, a two dimensional parametric study was conducted to assess propagation of a detonation across an idealized array of mixing/injection sites, parametrically characterized by the width and axial mixing profile.

Under such non-premixed conditions, discrete energy release and interdependence are observed. The discrete energy release sites frequently create pressures that exceed idealized Chapman-Jouguet (CJ) predictions based on perfect and uniform mixtures. The local higher pressure is shown to be caused by delayed heat release behind the shock, near constant pressure combustion, and additional compression due to the non-uniformities present. The resulting compression and the near constant pressure combustion are accompanied by time scale separation of exothermic and endothermic reactions due to the mixing efficiencies in the non/poorly-premixed cases. In contrast, the better mixed cases show that the detonation wave is sustained but unburnt fuel and oxidizer exist behind the main combustion wave and impulse performance suffers. Results show that for the conditions modeled there exists an optimal injector spacing to maximize the impulse produced and that discrete injection impulses can exceed that of the premixed systems. These somewhat counter-intuitive results imply that the detailed mixing evolution, provoked by the passage of the wave can lead to an extended heat release zone that elevates the pressure over a longer distance along the wavefront. These revelations provide potential for optimizing injector configurations for real non-premixed systems in order to exploit these physics.