Date of Award

Spring 2026

Document Type

Open Access Thesis

Department

Mechanical Engineering

First Advisor

Sang Hee Won

Abstract

To address increasing global energy consumption, energy industries seek to implement alternative fuels and increase energy generation efficiencies to meet energy demands. However, alternative and bioderived fuels present both chemical and physical properties significantly different from traditional hydrocarbon fuels typically used for gas turbine power generation. Consequently, they present unforeseen technical challenges. To address these challenges, ammonia has been suggested for use as a hydrogen carrier fuel in gas turbines. Ammonia presents many challenges by having a low overall reactivity as well as low energy density both by mass and volume. To address ammonia’s unique combustion characteristics, two strategies have been suggested. Ammonia blending with traditional hydrocarbon fuels, and ammonia cracking to partially break down ammonia into hydrogen and nitrogen before combustion.

This study focuses on the overall impact of ammonia combustion for both ammonia blending and cracking on operability limits in a model gas turbine combustor. The lean operability limit is evaluated by taking lean blow-out measurements at varying fuel compositions to investigate overall impact of ammonia additions. Results show that ammonia blending is limited to ~50% ammonia by mole, with a large increase of global equivalence ratio at lean blow-out. Comparatively, ammonia cracking shows similar lean operability behaviors to that of pure methane, allowing up to an ammonia mole fraction of 0.75. Further analysis on experimental results shows that cracking ~30 % of ammonia results in similar hydrocarbon lean operability, requiring ~ 5% of the energy from ammonia

combustion, thus presenting the possibility to implement in-situ ammonia cracker with a low thermodynamic penalty. To investigate parameters influencing lean operability, high speed imaging is utilized to analyze flame shape and stabilization. The flow recirculation zone, which aids in flame stabilization, is analyzed using a power spectral density (PSD) approach to assess relative strength of the recirculation zone. From the PSD analysis conducted, results suggest that increasing overall mass flow rate of air through the combustor increases the overall strength of the recirculation zone in terms of both representative velocity and angular momentum. A parametric study is applied considering the strength of recirculation zone, lean blow-out behaviors, and fuel loading impact on combustion characteristics. The parametric study can characterize LBO performance of both ammonia blending and cracking with two distinct regimes for each, with ammonia cracking able to maintain combustion at much lower thermal loading conditions. This can be attributed to the fact that the robust combustion behaviors of hydrogen produced from ammonia cracking enable flame stabilization near the fuel injector, which is not seen in ammonia blending.

To further investigate parameters influencing the differences between blending and cracking flame stabilization, high-speed laser diagnostics are employed. Planar laser-induced fluorescence (PLIF) is utilized to study time dependent flame behaviors and flame stabilization locations, while particle image velocimetry (PIV) is used to obtain 2D flow fields inside the combustor to study the flame and turbulence interactions. A flame transfer function is defined, exhibiting a reasonable correlation with the turbulent burning velocity, elucidating the impact of overall flame burning velocity to the observed flame response to turbulence. Furthermore, the location of the flame stabilization is related to the burning velocity along the shear layer of the recirculation zone. The results of high-speed diagnostics show that the ammonia cracking produces hydrogen, which makes overall flame burning velocity stronger due to the unique diffusive characteristics of hydrogen represented by Markstein length. Consequently, ammonia cracking enables flame stabilization at strong turbulent intensity at the recirculation zone, thus allowing for improved lean blow-out behaviors over ammonia blending.

Rights

© 2026, Austin Thomas Shealy

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