Planar Droplet Sizing in Dense Sprays

Abstract

Abstract : An investigation was undertaken to examine the application of a laser beam scanning methodology on the measurement of the time-averaged spray spatial structure. This approach can directly reduce secondary light scattering effects from the spray droplets and can take into account probe laser beam and scattered light intensity attenuation in order to reduce associated uncertainties on the spray characteristics. Evaluation of two approaches was performed, where one considered scanning the probe laser beam across the spray and the other considered traversing the spray while the probing beam and the imaging optics were fixed. It is determined that for the purposes of evaluating the methodology for correcting quantitative measurement errors due to attenuation, the second approach is more appropriate. This is because scanning the beam would require constant refocusing of the imaging system to the probed region. Another issue is the changing imaging resolution as the distance between the probe laser beam and the imaging optics is constantly changing. By traversing the spray across the probe beam, both of the aforementioned effects are eliminated and the signal correction methodology can be applied directly. The scattered light intensity correction methodology was successfully applied to a flat spray with measurements close to the nozzle and beam probing along the long dimension of the spray, so that attenuation effects are more profound. It is determined that attenuation of the laser beam along the spray can be as high as 20% and attenuation of the measured intensity signal can exceed 10%. Corrections can be made to address the attenuation effects and in the investigated spray the measurement error exceeds 10% especially at the regions of the spray further along the laser beam, where attenuation is strongest. This highlights the need for accounting for the attenuation effects of the laser beam and scattered light intensity for quantitative spray measurements.