Time-resolved X-ray Tomography of Gasoline Direct Injection Sprays

Quantitative measurements of direct injection fuel spray density and mixing are difficult to achieve using optical diagnostics, due to the substantial scattering of light and high optical density of the droplet field. For multi-hole sprays, the problem is even more challenging, as it is difficult to isolate a single spray plume along a single line of sight. Time resolved x-ray radiography diagnostics developed at Argonne's Advanced Photon Source have been used for some time to study diesel fuel sprays, as x-rays have high penetrating power in sprays and scatter only weakly. Traditionally, radiography measurements have been conducted along any single line of sight, and have been applied to single-hole and group-hole nozzles with few plumes. In this new work, we extend the technique to multi-hole gasoline direct injection sprays. By taking time-resolved measurements over a raster-scan pattern from multiple lines of sight, we are able to tomographically reconstruct the time-resolved ensemble mean density field in a plane intersecting the spray. Traditional Fourier back-projection methods are not well-suited for this experiment, so a model-based iterative reconstruction algorithm has been employed in this particular application. Three gasoline direct injection sprays with various 6-hole patterns were studied at injection pressures of 100 to 175 bar and atmospheric back pressure, at selected axial positions several mm downstream of the nozzle. These measurements reveal that the sprays are quite unsteady and interact with each other strongly during the early phase of injection. The spray plume cross-sections are very non-uniform, exhibiting small rich regions on the outer sides of the plumes surrounded by much leaner regions on the inner sides. We propose that this may be due to spray-spray interaction, interaction with the nozzle hole counter-bore, and inhomogeneities in the sprays due to the hole geometry and needle lift.