Analysis of spontaneous longitudinal combustion instability in an O2/CH4 single-injector rocket combustor

Self-excited longitudinal combustion instabilities were analyzed in a sub-scale combustor operating with a propellant combination of O-2/CH4. Couplings between acoustics, hydrodynamics, and heat release were investigated numerically based on an experimental setup. The numerical results were in good agreement with the experimental data. Various post-processing methods have been adopted to explain the thermoacoustic behavior and reveal the underlying unstable combustion mechanisms. The classical longitudinal acoustic characteristics of the combustion chamber were identified by the pressure mode shape and dynamic mode decomposition (DMD) analysis. It was found that the incoming streams along the injector were periodically hindered by the traveling pressure wave originating from the chamber, thus, resulting in an oscillatory propellant supply and a pulsating heat release. This, in turn, feeds energy into the acoustic pressure oscillations, and maintains the combustion instability. Meanwhile, the coherent vortex structures in the reacting mixing-shear layer were found to be the principal reason for sustaining combustion instabilities. Besides, a positive feedback closed-loop system associated with periodic vortex shedding and merging is believed to sustain combustion instabilities. The plot of the Rayleigh index further confirmed that the shear layer region drives thermoacoustic instabilities. (c) 2021 Elsevier Masson SAS. All rights reserved.

Automated summary

The study analyzed self-excited longitudinal combustion instabilities in a sub-scale combustor using O-2/CH4 propellant combination. Couplings between acoustics, hydrodynamics, and heat release were numerically investigated, with results showing good agreement with experimental data. Key factors sustaining combustion instabilities were identified, including obstructed inflow, coherent vortex structures in the mixing-shear layer, and a positive feedback closed-loop system involving periodic vortex shedding and merging.