Autoignition of a Jet-A Fuel Spray in a High Temperature Vitiated Air Flow

This study investigates the autoignition of a Jet-A fuel spray in a preheated air flow in a 75mm inner diameter quartz tube at atmospheric pressure. Fuel was injected via a pressure-swirl atomizing injector enclosed in an aerodynamically-shaped outer body (7mm diameter) that is installed coaxially with the flow using 3 water-cooled pylons. The air temperature and oxygen content were in the range of 1000–1400 K and 9–12%, respectively by controlling the equivalence ratio in the primary zone of the pre-burner/vitiator and by adding dilution air downstream. The co-flow air velocity ranged from 25–30 m/s for this study (Re = 10,000–12,000), with a trapezoidal profile. Fuel spray was characterized using PDPA. Shape of the spray was transitional between hollow and solid cone with the corner angle of 35° to the axis near injector which reduced to 22° downstream. Spray density varied significantly over cross-section of the tube with the minimum on the axes. Droplets produced have average diameters (D₁₀) of 15–70μm on the axes and periphery, respectively, at 6 cm downstream from the injector. Character of the droplet size distribution was polydisperse. Auto-ignition time delays were captured using a time-averaged camera with CH* (432nm) filter. The measured values agree with delay times previously reported in literature. Two synchronized high-speed cameras with 432nm and 307nm filters were used to investigate dynamics of auto-ignition kernel initiation and convection by capturing of CH* and OH* chemiluminescence at 5000 f ps. This methodology allowed qualitative characterization of the equivalence ratio of kernels in process of their convection and growth. It was shown that kernels are always initiated on the axes of the spray where the average droplet size is minimum. Kernels were formed leaner and become richer as they grow down-stream as indicated by the increase of CH*/OH*intensity ratio. Additionally, kernel behavior depends greatly on air temperature with kernels transitioning from randomly appearing (i.e. single kernel), to periodic, to a constantly auto-igniting flame with the spatial scatter of ignition kernels decreasing with temperature.