With the growing interest in lean-burn combustors for aviation gas turbine engines, further understanding of the connections between combustor stability and pollutant emissions is required to achieve reliable and sustainable combustor designs. An important parameter that remains to be systematically studied in regard to lean-premixed-prevaporized (LPP) combustion is the level of fuel-air prevaporization and premixedness, henceforth referred to as reactant inhomogeneity. Reactant inhomogeneity, generally considered to be partial premixedness or stratification phenomena, has been shown to enhance both laminar and turbulent flame stability in several ways. Unfortunately, this comes at the cost of increased pollutant emissions, such as nitrogen oxides (NOx), due to poorly mixed “hot spots” which produce disproportionate amounts of pollutants compared to a homogeneous mixture. Studies of reactant inhomogeneity phenomena have been conducted on benchtop burners with canonical geometries operating with gaseous fuels at atmospheric pressures. This dissertation explores when and how reactant inhomogeneity effects influence a liquid fueled LPP combustor operating at flight relevant conditions. 

A multi-element, high pressure LPP test facility was constructed featuring a variable premixer, enabling fuel injection at one of four locations, each a different axial distance from the combustor inlet. This premixer enabled the systematic study of the effects of reactant inhomogeneity on LPP combustor performance. NOx emissions were shown to increase with increased reactant inhomogeneity, demonstrating the significant effect of hot spots on pollutant production. However, the effect of reactant inhomogeneity on lean blowoff limits showed conflicting trends. For conventional Jet A fuel, increased reactant inhomogeneity led to poorer static stability (LBO limits), whereas a synthetic aviation fuel benefited from reactant inhomogeneity. These results highlight the complex nature of such reactant inhomogeneity effects. 

Through the development and deployment of advanced laser diagnostic techniques, the mechanisms through which reactant inhomogeneity influenced combustor performance were identified. Simultaneous droplet Mie scattering and fuel planar laser induce fluorescence (PLIF) yielded semi-quantitative measurements of fuel-air mixing, while OH PLIF measurements characterized the turbulent flame structure. These measurements showed that injection closer to the combustor inlet led to an uneven mixing field, characterized by increased droplet vaporization at the inlet of the combustor. Such droplet vaporization creates fuel-rich regions near the flame holder that help anchor the flame but starve regions further downstream, ultimately leading to increased flow-induced pinching and flapping. Fuel injection further from the combustor inlet yields a more uniform mixing field which is less dominated by droplet vaporization, reducing anchoring upstream but increasing the f lames resistance to flow perturbations downstream. It is this trade-off between upstream anchoring and downstream flow induced flame perturbation which influences the static stability of the combustor, highlighting the multifaceted effects of reactant inhomogeneity on LPP combustor performance.