Structure, Dynamics, and Emissions of Hydrogen-Ammonia-Natural Gas Flames

<p dir="ltr">Continual efforts towards net-zero carbon emissions in gas turbine engines requires technological advancement in fuel flexible combustors. Alternative fuels like hydrogen and ammonia provide avenues towards carbon-free energy, but key challenges in operability, combustion dynamics, and emissions remain. This work investigates these challenges at relevant gas turbine operating conditions through the development and experimentation of a novel injector configuration focused on numerous jet-stabilized, premixed flames. The injector is designed to operate over a much wider range of fuel compositions than traditional combustors, and is sized to promote self-excited combustion instabilities. Several high-speed optical diagnostics are employed alongside high-frequency pressure measurements to characterize the directional sensitivity of fuel composition to prominent instability mode, and limit cycle amplitude. These findings facilitated a more detailed investigation of the coupling mechanisms and rate-controlling processes that produce significant thermoacoustic oscillations. Further experiments utilized exhaust gas sampling to study the role of blended fuel composition on key combustion product species.</p><p dir="ltr">Fuel blends of hydrogen, ammonia, and natural gas are studied with 50 kHz OH* chemiluminescence imaging. A suite of high-frequency pressure transducers located in the micromix injector and combustion chamber were sampled at 2.5 MHz simultaneously to spatiotemporally resolve the self-excited thermoacoustic modes. While mixtures of H₂-N₂-NH₃ produced negligible instability amplitudes, the addition of natural gas promoted combustion instabilities, increasing the limit cycle instability amplitude regardless of the ammonia mole fraction. In the absence of natural gas, ammonia addition produced no significant instability growth. However, instability amplitude was correlated with ammonia addition when small methane concentrations are present (as little as 5% by mole). The limit cycle amplitude was inversely correlated to the impedance across the hydrogen inlet, highlighting resonance within the micromixer tubes as a delineating contributor to instability growth.</p><p dir="ltr">With sufficient addition of either natural gas or ammonia, a transition from the full-wave (2L) mode of the combustor to the fundamental half-wave (1L) mode was observed. Growth in the axial extent of OH* distributions was observed as instability amplitude increased, with the largest change in length occurring across the transition from the 2L mode to the 1L mode. Spatially resolved heat release measurements were performed using 100 kHz OH-PLIF to elucidate the variations in flame kinematics associated with prominent instability mode. An increase in flame brush thickness near the flame base is observed as ammonia replaces hydrogen. The disparity is correlated to differences in flame curvature, which show greater negative bias as ammonia concentration increases. This is due to surface production in hydrogen flames producing high heat release rate and rapid kinematic restoration leading to flat flame surfaces. Time resolved heat release analysis revealed dissimilar timescales for unsteady reactant consumption across cases exciting the 2L or 1L mode. This was linked to flame pinch-off and the persistence of reactants into the region between jet elements in 1L cases. 2L-1L mode transition was also demonstrated through the addition of both methane and ammonia, suggesting this mode transition can be decoupled from specific fuel composition, and can instead be characterized by the underlying rate-controlling processes governing flame kinematics.</p><p dir="ltr">The effect of fuel composition on combustion efficiency was investigated. Exhaust gas samples were analyzed with a Fourier transform infrared spectrometer to enable species concentration measurements of key combustion products including NO, NO₂, CO, CO₂, H₂O, and unburnt hydrocarbons. Specific processing techniques highlight the feasibility of obtaining steady state measurements over a much shorter sampling duration than previously required. The additional complexity of blended fuel composition on output water concentration is highlighted, and considerations towards the faithful interpretation of results are discussed. The findings of this work highlight the disparate chemical timescales of ammonia, hydrogen, and methane combustion. They provide understanding towards the resulting effects on the structure, dynamics, and emissions of these flames at simulated conditions that accurately recreate key physical processes occurring in real gas turbine engines. While it is clear that significant consideration of injector geometry is necessary for fuel flexible operation, the unique properties of these fuel blends can be leveraged to produce stable flames over a wide operating space.</p>