Behavior of Autoignition in Polydisperse Jet-A Fuel Spray With High Temperature Co-Flow

Abstract

Liquid fueled combustors are commonly used in the gas turbine industry in situations such as high temperature fuel mixing ducts, liquid fueled reheat combustors, and other high temperature liquid fueled combustors. Modern combustors operate at high inlet temperatures, increasing the likelihood of autoignition events. Autoignition is primarily characterized using a single-step Arrhenius rate equation. Generally, this method is ideal for modeling the chemical processes involved in simplistic settings such as for analyzing ignition delays with premixed reactive mixtures in shocktubes, however it may not fully encapsulate the underlying physio-chemical processes involved in the presence of a multi-phase flow which can significantly affect the chemical processes such as autoignition. These conditions are often encountered in reality, for example, in a gas turbine combustor using fuel sprays where interactive phenomena such as fuel droplet evaporation, mixing, and chemical reactions may occur simultaneously and non-homogeneously. The results presented in this report begin to elucidate the role of droplets in determining the behavior of autoignition kernels with an attempt to improve our capability to predict autoignition phenomena in liquid fuel injector application in gas turbine industry. To investigate the autoignition phenomena in a multi-phase flow inside a gas turbine combustor, a simplified co-flow type geometry is considered at atmospheric pressure where a single Jet-A fuel spray enters the co-flowing high temperature vitiated products of a pilot burner. Fuel is injected using an aerodynamically shaped pressure-swirl atomizing injector installed co-axially with the flow inside an optically accessible quartz test section. The air temperatures and oxygen content of the flow can range from 950–1300K and 9–11%, respectively. It has previously been found that while average ignition delay times agree or nearly agree with prior theoretical and experimental studies (eg. for prevaporized fuel, electrically heated), high speed imaging experiments illustrate that the spatial location of the formed kernels can be broadly scattered. Also, this variation in autoignition kernel location is higher at lower temperatures. Simultaneous high speed CH and OH chemiluminescence also suggest that the kernels are formed at lower equivalence ratios at lower preheat temperatures and then proceed to increase in equivalence ratio. While at higher preheat temperatures, kernels form at a higher equivalence ratio and stay at the ratio as they propagate downstream. In the current study, a 5000fps, 283nm laser sheet is introduced along the center axis of the test section. Two synchronized, intensified, high-speed cameras simultaneously captured the fluorescence of Jet-A and OH chemical reaction at 308nm and the Mie scattering of droplets at 283nm. Autoignition kernels and that droplets are visualized at flow velocities ranging from 40–50 m/s and temperatures ranging from 1100–1300K. This technique allows the fuel and reaction fluorescence to be differentiated and from this image, information is obtained on the proximity of fuel droplets and autoignition kernels during their formation and subsequent propagation.