| Résumé | Intermittent and highly transient dense diesel sprays were investigated using two-dimensional laserlight scattering and transmission techniques to decipher the internal spray structure. The experiments were conducted at maximum injection pressures ranging from 19 to 32 MPa using an electronic diesel injector with a single-hole nozzle. The experiments involved the imaging of the unperturbed structure of the dense core region of a full-cone intermittent diesel spray on photographic film at high resolution and the simultaneous measurement of laser sheet transmission along the centerline of the spray by an array consisting of 2048 diodes. At all injection pressures, line-of-sight laser sheet transmission measurements showed that the dense core region is fragmented very near to the nozzle exit, about 25–30 nozzle diameters downstream, and perhaps much closer. Further downstream from this location, the transmission measurements and simultaneous 90° scattering images revealed that the structure has an intermittent appearance with pockets of dense spray separated by relatively void regions. Two-dimensional images displayed a highly atomized spray structure beyond 50 nozzle diameters downstream, with no indication of an intact liquid core for the range of injection pressures studied. After eliminating the secondary scattering present by applying a low-pass filter and subtracting the filtered image from the original image, it was demonstrated for enlarged views of the spray that the scattering is from randomly spaced point sources, which were interpreted as droplets, rather than liquid columns, ligaments, or large blobs of fuel. It seems that pressure-atomized and fully pulsed round liquid sprays have a significantly different near-field structure than their steady counterparts. These findings, along with the recent work of others, are inconsistent with the existence of a long (on the order of 100 nozzle diameters or more) intact liquid core in unsteady diesel sprays. |
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