Investigation of Rotating Detonation Combustion Dynamics using advanced in-situ Optical Diagnostics

Rotating detonation combustors (RDCs) provide a promising avenue for incorporating step changes in thermodynamic efficiency improvements through isochoric heat release for gas turbine and rocket propulsion systems. However, these systems have multiple challenges that need to be addressed to realize practical pressure gain systems. Chief among them is the total pressure loss, which can be caused by multiple processes including wake shock features, non-ideal detonations, heat losses, mixed-mode heat release (deflagrative and detonative) and geometry-induced flow losses. While multiple experiments in the past have used probe-based techniques and imaging systems to characterize the flow losses, very few experiments have spatio-temporally resolved the detonation structure and propagation using advanced high repetition rate optical diagnostics.

In this work, a novel high-pressure optically accessible RDC is designed and developed for implementing advanced high repetition rate qualitative and quantitative laser-based diagnostics to facilitate detailed detonation characterization in annular channels. The RDC uses axial-injection of air and radial injection of hydrogen with full optical access extending from the oxidizer injection plenum to the exit plane of the RDC. Several in-situ diagnostics are deployed to characterize the three-dimensional propagation characteristics of detonation waves and their dependence on mass flux, global equivalence ratio and geometry. In parallel to the experimental measurements, 3D Unsteady Reynolds-Averaged Navier-Stokes (URANS), simulations are performed for this geometry and were used in interpreting the experimental results.

By combining MHz-rate broadband flame chemiluminescence, MHz-rate OH planar laser-induced fluorescence (PLIF), and URANS simulations, the complex 3D features of the non-premixed detonation waves were characterized as a function of the rate of reactant mixing. Due to the large fuel/oxidizer stratification in the injection nearfield within a non-premixed RDC, the detonation in this region is relatively weak, with some unburned reactants passing through the leading detonation wave and combusting in a trailing wave stabilized behind the leading wave via an azimuthal shock-induced detonation. These results explain why the detonation in the injection near field has a characteristic dual-wave structure over a wide range of mass flow rates. Further downstream along the axial length of the RDC, the reactant mixedness improves, leading to a single-wave structure with a higher detonation strength. While the freely propagating leading detonation waves have a higher-pressure ratio across the detonation, a shock-induced trailing detonation has a relatively lower pressure ratio. The elucidation of the physical mechanism for mixed-mode combustion of freely propagating detonation and shock-induced detonation leads to a better understanding of the origins of non-ideal flow losses in non-premixed RDCs, particularly in the context of complex shock-flow interactions occurring for due the dynamics of axial reflected shocks that also affect the detonation wave structure. These imaging studies also reveal a non-ideal cellular propagation of detonation waves around the annulus with cell sizes increasing with increasing axial distance from the injection plane.

In addition to the experimental studies having reactant injection into a stepped combustor chamber, a design with the reactants expanding fully into the annular channel via a ramped cross-section was also designed and evaluated at the same operating conditions. This design eliminates product recirculation regions induced by the backward facing step (BFS) of the stepped combustor design. The detonation wave shape drastically varied in case of the ramp inlet geometry, compared to that of the stepped inlet. Additionally, multiple trailing shock induced detonation waves were observed, which leads to an overall drop in the C-J velocity for the ramp design at the same operating conditions. However, the ramp-design provided a lower lean limit with stable operation at lean conditions compared the stepped design, highlighting the sensitivity of the detonation wave to the chamber annular geometry and the need to understand its impact on RDC performance.

To evaluate the propagation of combustion waves into the exhaust, and potentially into downstream post-combustion devices, quantitative measurements of temperature were performed in the exhaust of an RDC with a custom outlet plenum by using hybrid fs/ps rotational Coherent anti-Stokes Raman (RCARS). Through the use of URANS simulations, extensive analysis was performed to understand the uncertainty associated with RCARS in a detonating environment. The experimental and numerical results show that the temperature variation in the exhaust is ~ 300 K with both the numerical and the experimental results predicting a combustor pattern factor of ~0.20 and ~0.19 respectively, a nominal value similar to modern day Brayton cycle-based gas turbines. These results show the utility of advanced in-situ optical diagnostics for understanding the dynamics and performance implications of various RDC configurations.