Longitudinal-mode combustion instabilities in air-breathing engines
Self-excitation of low-frequency (100-600 Hz), longitudinal acoustic modes of a rearward-facing step combustor was studied. As in combustion instabilities of air-breathing propulsion systems, the pressure oscillations are excited by a fluctuating heat release from a flame stabilized in a recirculation zone. Flow-visualization results and flame radiation-intensity data reveal that large vortex structures are responsible for this fluctuating heat release. The vortices are shed at frequencies corresponding to longitudinal acoustic modes of the system or to the first subharmonic of one of the modes. A series of parametric studies were performed to determine the dependence of the vortex shedding frequency upon the step height, mean flow speed, and fuel type, and equivalence ratio. It was discovered that the vortex shedding frequency can shift between modes as a result of changes in the chemical reaction time of the reactants or as a result of changes in the mixing process of the cold reactants with the hot products. The mechanism of sustenance of the oscillations during instability was studied for several operating conditions. A one-dimensional linearized acoustic model is used to predict the natural modes of the system and a fluctuating volumetric source is used to model the oscillatory heat release. Finally, a velocity-sensitive volumetric source with a time delay is included as feedback to determine the linear-stability characteristics of the system.
- Research Organization:
- California Inst. of Tech., Pasadena (USA)
- OSTI ID:
- 7030271
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
400800* -- Combustion
Pyrolysis
& High-Temperature Chemistry
42 ENGINEERING
420400 -- Engineering-- Heat Transfer & Fluid Flow
CHEMICAL REACTIONS
COMBUSTION
ENERGY TRANSFER
ENGINES
FREQUENCY DEPENDENCE
HEAT TRANSFER
INSTABILITY
OSCILLATION MODES
OXIDATION
PARAMETRIC ANALYSIS
PRESSURE RELEASE
THERMOCHEMICAL PROCESSES
VORTICES