Bridging the gap between annular and can-annular acoustic spectra

Area of Science:

• Combustion dynamics
• Acoustic instabilities
• Gas turbine engineering

Background:

• Thermoacoustic instabilities are critical in gas turbine combustors, with existing literature distinguishing between annular and can-annular architectures.
• Real-world combustors often present hybrid geometries, necessitating a more generalized understanding beyond these two extremes.
• The coupling strength between combustor elements, influenced by axial gap distance, plays a key role in instability phenomena.

Purpose of the Study:

• To investigate the acoustic spectrum of idealized can-annular combustion chambers with variable geometry.
• To analyze how adjusting the axial gap distance affects the coupling strength and acoustic modes.
• To bridge the understanding between purely annular and can-annular combustor configurations.

Main Methods:

• Development of two theoretical models based on Bloch wave theory.
• Validation of theoretical models using finite-element simulations of the Helmholtz equation.
• Analysis of acoustic spectrum across a range of axial gap sizes, from isolated cans to fully open annular systems.

Main Results:

• Demonstrated the transformation of azimuthal modes into eigenmodes of an annular chamber as the gap increases.
• Showcased that varying gap sizes allow a transition from can-annular to annular acoustic behavior.
• Established that acoustic modes below a specific frequency can be classified by axial and azimuthal mode orders across the geometry spectrum.

Conclusions:

• The study provides a unified framework for analyzing thermoacoustic instabilities in a broader range of gas turbine combustor geometries.
• Understanding the influence of axial gap distance is crucial for predicting and mitigating combustion instabilities.
• The developed models and classification scheme offer valuable tools for gas turbine design and optimization.