Secondary waves dynamics and their impact on detonation structure in rotating detonation combustors

In this work, we experimentally investigate secondary waves in rotating detonation combustor (RDC) operation. Secondary waves are finite-strength periodic perturbations of the field, which manifest as reacting fronts and/or pressure rise rotating around the annulus superimposed to one or more detonation waves. Secondary waves interact with the detonation wave, affecting the operation of the RDC, as well as the potential of realizing pressure gain. Through analysis of high-speed end-view chemiluminescence imaging in the detonation channel, the characteristics (speed, multiplicity, and strength) of different systems of waves are identified. The analysis indicates that in addition to the main detonation wave, two secondary wave systems are present: (1) a pair of co-rotating waves moving counter to the detonation wave at a speed near the acoustic speed of the products of combustion and (2) a wave moving counter to the detonation wave at a speed near that of the detonation wave. These two types of secondary waves are consistently observed in three canonical injection schemes. We further investigated the impact of the wave pair on the structure of the main detonation wave in one inlet configuration. By constructing conditional phase-averaged distributions of the pressure and OH* emission over the interaction between the detonation and secondary waves, we reconstruct the structure of the detonation wave during the interaction. The results show that there is a nonlinear interaction between the detonation and secondary waves, which results in an augmentation of the pressure rise (increase by as much as 60%) across the detonation wave as well as partial suppression of the heat release as the two waves interact (variation up to an order of magnitude). This nonlinear interaction is supported by differences in the temporal response of the air and fuel streams subject to the propagation of secondary waves, generating fill region stratification leading to the partial suppression of heat release when the secondary wave collides with the main detonation wave.