Date of Award

Spring 4-3-2026

Access Type

Thesis - Open Access

Degree Name

Master of Science in Aerospace Engineering

Department

Aerospace Engineering

Committee Chair

Scott Montgomery Martin

Committee Chair Email

martis38@erau.edu

First Committee Member

L.L. Narayanaswami

First Committee Member Email

swami@erau.edu

Second Committee Member

William Engblom

Second Committee Member Email

engbl7de@erau.edu

College Dean

James W. Gregory

Abstract

Combustion instability is one of the leading factors in limiting the progress and innovations of liquid rocket engines and power plants. Under certain operating conditions, unsteady heat release and acoustic wave interactions can couple and lead to instabilities, such as self-sustaining pressure oscillations, that can lead to reduced combustor performance and potentially hardware damage. In this project, the thermoacoustic effects of controlling the equivalence ratio of a single-element natural gas combustor were modeled and analyzed. The project used Fidelity Pointwise for meshing, ANSYS Fluent for modeling, ERAU VEGA HPC for computing, and MATLAB for acoustic analysis. A numerical model of the Purdue University Continuously Variable Resonance Combustor was created as a two-dimensional, axisymmetric domain and modeled in Fluent with finite rate chemistry and a hybrid RANS/LES approach. The acoustic analysis and power spectral densities were done with a Fast Fourier Transform algorithm.

The dominant frequencies, found to be between 1463 Hz and 1551 Hz, compare well with the experimental result of 1390 Hz, showing that the numerical model captured the primary acoustic modes of the combustor. However, the peak amplitudes of these frequencies were found to be around 3 to 7.5 psi2 /Hz, lower than the experimental result of 24.5 psi2 /Hz. This is potentially due to the reduced-order chemical kinetics, numerical dissipation, and lack of three-dimensional effects. Across three cases of varying equivalence ratios, the dominant frequency modes remained relatively constant, confirming that this characteristic is largely driven by system geometry. The stoichiometric case exhibited the lowest amplitudes, as stoichiometric combustion is typically the most stable and has the most complete heat release. The rich mixture exhibited the highest amplitude as it tends to have more incomplete combustion and slower reaction zones, which allows the unsteady heat release to more easily couple with these pressure oscillations.

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