A significant hurdle to the understanding of sprays is the link between nozzle geometry and the fluid distribution in the spray. X-ray radiography can help to clarify this link by providing quantitative measurements of the spray density in the near-nozzle region, including at the exit plane. The current work describes x-ray radiography measurements performed at Argonne National Laboratory under the “Spray A” conditions of the Engine Combustion Network. Four injector samples have been studied, and model-dependent reconstructions have been used to generate 3-D maps of the average fuel density as a function of time. These measurements reveal differences between the sprays from nominally identical injectors which can be interpreted in terms of geometric differences in the injector nozzles that have been measured previously. Introduction Due to the importance of fuel and air mixing in practical diesel combustion, diesel sprays have been the object of research interest for many years. Several optical techniques have been used to characterize spray parameters under both combusting and non-combusting conditions, including spray penetration [1], cone angle, liquid length[2], flame liftoff length[3], and the velocity field surrounding the spray[4]. While these diagnostics have provided important insights regarding spray behavior, they have provided only limited data regarding the internal structure of sprays. The numerous small spray droplets in diesel sprays create a flowfield with high optical density. Advanced optical diagnostics have been attempted on diesel sprays to overcome the optical density limitations, though with only limited success [5,6]. A significant problem in previous diesel spray research has been the difficulty of comparing results from different experiments. For example, nozzle geometry is known to strongly affect spray structure. However, due to the small size (~ 100 μm) of diesel spray nozzles, it is difficult to characterize the nozzle geometry in detail [7]. The Engine Combustion Network (ECN) was conceived to remove this impediment to diesel spray research. A series of nominally-identical injectors have been characterized under well-defined conditions by a variety of institutions. These efforts have already yielded fruit in showing both the importance of matching all pertinent experimental parameters and the ability to achieve good agreement between different institutions on parameters of interest in diesel sprays [8]. The current study uses x-ray radiography to probe a non-vaporizing version of the Spray A ECN condition. Radiography has been used for several years to characterize the near-nozzle distribution of fuel sprays under a wide variety of conditions [9-11]. The current radiography data will be used to describe the development of the spray velocity, shape, and density with downstream distance. Important differences between the nominally-identical ECN injectors will also be described. Experimental Methods The x-ray radiography experiments in this work were performed at the 7BM beamline of the Advanced Photon Source (APS) at Argonne National Laboratory. The beamline layout and performance are described elsewhere [12]. A monochromatic beam of x-rays at 8 keV photon energy was focused to a 5 x 6 μm FWHM focus size to probe the spray with high spatial resolution. The incident x-ray flux on the detector was approximately 2.6 x 10 ph/s. The radiography measurements represent a pathlength-integrated measure of the fuel density along one beam path through the spray. To measure the spatial distribution of the fuel, a two-dimensional raster-scan approach is used, with each point measured from a different set of spray events. To further improve the signal/noise in the final data, each data point is an average of 128-256 individual spray events. Measurements of 1 Corresponding Author 2 Current Affiliation Combustion and Engine Research Team, AIST, Ibaraki, Japan 12 ICLASS 2012 Time-Resolved X-Ray Radiography of ECN Spray A 2 the needle lift of these injectors [13] and of spray events from a similar injector [9] have shown that shot-to-shot variations are likely to be fairly minor. As such, the spray data shown in this work represent the persistent, ensemble-averaged behavior of the spray, rather than the precise structure of any one spray event. The coordinate system used in this work is illustrated in Fig. 1. When the data were processed, the signal was binned for each cycle of the APS (i.e., 3.68 μs). Due to the use of monochromatic x-rays, the conversion of x-ray transmission to fuel mass is relatively simple. A simple Beer’s Law approach can be used; the projected density of the spray M (in mass/area) can be calculated from the fuel absorption coefficient (μ), the x-ray intensity during the spray (I) and before the spray (I0) using Eq. 1.