Characterizing acoustic modes in a hollow rotating detonation combustor

Abstract

A multifaceted approach to studying the acoustics in a center-bodiless, Rotating Detonation Combustor (RDC) is conducted. Initially, a fundamental eigenmode study of the combustor volume is performed in COMSOL across a range of gas properties. These are computed via NASA CEA Detonation models to represent varying fueling conditions. The resulting values are compared against calculated fundamental modes for closed/open cylinder based on linearized wave equations. These two findings are then compared to experimental data across a range of bulk flow rates and equivalence ratios. While some modes showed good agreement with the computed results, others were left unexplained. Further direct analysis of the experimental data is then conducted to characterize these modes. Cross-spectral phase analysis is performed on high-frequency pressure transducers spaced azimuthally around the combustion chamber to identify the structural nature of each acoustic mode in the 2D plane. Bi-spectral Mode Decomposition (BMD) is conducted on high-speed video captured looking into the exit plane of the combustor. Good agreement is observed between these tools and the image analysis allows further characterization of the structures associated with each mode. Furthermore, BMD extracts the role of triadic interactions in the formation of an acoustic mode. This shows how the coupling of a few primary acoustic modes then form many of the additional modes observed in experimental results. The combination of these results begins to shed some light on the complex processes through which classical acoustic modes in a combustor interact to set up conditions which allow for rotating detonations to form.

Novelty and significance statement

Rotating detonation combustors (RDCs) are a rapidly expanding topic in combustion research. Presently, no clear guidelines or methodologies have been developed for designing RDC geometries; and most recorded RDC experiments are derivatives of the originally studied annular RDC. Recently, various non-traditional RDC geometries have demonstrated detonative operation and are being further studied. In traditional deflagrative combustors, one of the primary design challenges is ensuring that there are no thermoacoustic modes which are readily excited during normal operation; this challenge is no different in RDCs. Furthermore, thermoacoustic oscillations play a critical role in the transition from a deflagrative mode of combustion into detonations. If a practical RDC is to be realized, it is critical that the interactions between rotating detonation waves and traditional combustion acoustics are studied further across a range of unique geometries.