Nanosecond Structured Laser Illumination Planar Imaging for Single-Shot Imaging of Dense Sprays

Abstract:
Recently, a novel laser imaging technique, structured laser illumination planar imaging (SLIPI), has demonstrated great potential for suppressing multiply scattered light in spray imaging. SLIP! uses a laser sheet whose intensity is modulated in the spatial domain. Directly scattered photons keep this structural information; whereas, photons that experienced several scattering events lose this information. Using a spatially modulated light source thus enables directly scattered photons to be distinguished from the multiple-scattering contribution. To homogeneously illuminate the spray, three images are required, where the intensity modulation is successively shifted vertically one third of a period. By adequately post processing these images, the multiple-scattering contribution is diminished. However, the time interval within which these images are recorded must be short enough to freeze the flow motion, making single-shot SLIPI of highly atomizing sprays particularly challenging. In this article, a nanosecond SLIPI system capable of freezing flow motions up to similar to 600 m/s is presented. The instrument was tested on a hollow-cone water spray, running at an injection pressure of up to 50 bars, and high-resolution single-shot images, in which multiple-scattering effects were efficiently suppressed, were obtained. Such images provide detailed information of complex dynamic flow behavior occurring in the dense spray region, e.g., primary and secondary breakups. In addition, it is demonstrated how the liquid sheet length can be estimated by extracting the rms from such single-shot SLIPI images.