Evaluation of a laser beam scanning method for correction of multiple scattering and attenuation effects in dense spray measurements

Abstract

When applying laser imaging to dense sprays, the acquired measurements are hindered by optical noise due to multiple scattering, attenuation of the illuminating laser beam as it interacts with the spray and attenuation of the scattered light by droplets between the probed spray region and the detector. An investigation was performed to assess the possibility of addressing these issues by scanning a laser beam across a spray instead of illuminating with a laser sheet. A high precision opto-mechanical arrangement was assembled for measurements of scattered and fluorescent light intensity across a cross section of a flat spray. These measurements were used for applying a correction procedure, described by [1], to the attenuation of the incident and collected light intensity by a spray. Application of the correction methodology on the collected scattered light profiles across a plane of a spray provided quantitative measurements of the contributions of laser intensity attenuation along the beam path, attenuation of the scattered light intensity between the probed spray region and the imaging camera and the scattering cross section of the spray droplets. From the above, the full cross section of the spray was reconstructed in terms of the scattered light intensity profiles, which are representative of the droplet surface area and liquid volume densities. The reconstructed spray shape was elliptical and symmetric along the two normal axes, as expected for a flat spray, which demonstrates that the developed approach was appropriate. I. Introductionplanar droplet sizing technique (also know as optical patternator) has been proposed for the measurement of the Sauter Mean Diameter (SMD) of spray droplets, based on combined laser-induced fluorescence and scattered light (LIF/Mie) intensity imaging on a plane of a spray determined by a laser sheet [2-10]. The LIF/Mie technique has a significant advantage over other droplet sizing methods in that it is a planar technique that can size full planes of sprays, while the other methods are limited to "point" (Phase Doppler Velocimetry, PDV), line of sight (laser diffraction), or planes of small cross-sectional areas (Interferometric Laser Imaging Droplet Sizing, ILIDS) of sprays. In addition, this technique is applicable to dense spray regions, provided that appropriate compensation of multiple scattering effects and laser intensity attenuation along the sprays is included. The fundamental hypothesis of the LIF/Mie technique for droplet sizing [2, 3] is that when a spherical droplet doped with a fluorescing dye is illuminated with a laser, the intensity of the fluorescent light from a droplet is proportional to the volume of the illuminated droplet while the intensity of the scattered light is proportional to the surface area of the illuminated droplet. However, it has been shown that the above assumption is not always valid [6-9, 11, 12] and there is a need to develop appropriate processing for droplet sizing [6]. Recently, an extensive analysis of the operation of the LIF/Mie t...