The purpose of this investigation is to assess and improve the accuracy of Sauter Mean Diameter measurements in dense sprays using a Planar Droplet Sizing (PDS) technique, based on the intensity ratio of scattered and fluorescence light. A novel data processing method of the PDS technique is suggested, which was derived from a theoretical light scattering investigation, and reduced possible sizing errors larger than 30% to below 10%. The novel approach for droplet sizing was applied to measure in a spray generated by a pressure swirl atomiser in a liquid-fuelled burner operated with water at isothermal conditions, in order to avoid the effect of liquid evaporation on the accuracy of PDS technique. The Sauter Mean diameter results from the PDS technique were compared to Phase Doppler Anemometer (PDA) sizing measurements. Good agreement was obtained between the two techniques in dense regions of the spray. Discrepancies remained in dilute spray regions due to systematic statistical uncertainties of the PDS technique and the dynamic range of the intensity of the CCD cameras, which did not allow detection of large single droplets in the dilute spray region.
A combined theoretical and experimental study of the parameters affecting the accuracy of Planar Droplet Sizing (PDS) measurements is presented. The principle of the PDS technique relies on the assumption that the intensity emitted by a fluorescent dye added to a liquid is proportional to the volume of a resulting droplet during atomisation and that the scattered light intensity is proportional to its surface area, allowing measurement of Sauter Mean Diameter (SMD) by taking the ratio of these intensities. A geometrical optics light scattering approach was extended to calculate the fluorescence intensity emitted by a droplet, in addition to providing the scattered light intensity integrated over the collection aperture. The theoretical approach quantified the influences of scattering angle, refractive index, droplet size and dye concentration on the PDS technique. Experiments with monodisperse droplet streams confirmed the calculations in terms of dependence of the scattered and fluorescence intensities. The limitations of the technique have been established together with an appropriate calibration procedure for application in dense sprays.
The dependence of fluorescence intensity distributions within droplets on added dye concentration has been calculated by extension of the geometrical-optics approximation and verified by experimental observations. With rising dye concentration, surface plots of the equatorial fluorescence pattern show decreasing relevance of intensity enhancement at focusing points of internal light rays and increasing effects of linear absorption on the characteristic features of the distribution. For comparison with experimentally obtained images of the fluorescence intensity distribution within droplets, a method for calculating volume-integrated intensity distributions was developed in which image distortion at the fluid-air interface is included. A comparison of the calculated and the experimentally determined fluorescence intensity distributions within a droplet confirmed the accuracy of the geometrical-optics approach at high dye concentrations. However, discrepancies from experimental results are visible at low dye concentrations owing to nonlinear optical effects.
The dependence of energy distributions within droplets on
internal absorption effects has been investigated by calculations
based on Mie theory and the geometrical optics approximation and
experiments. The objective was to evaluate the accuracy of the
geometrical optics approximation in calculating droplet volume to
fluorescence intensity proportionality, required for planar
droplet sizing measurements in sprays based on Mie scattering and
fluorescence intensity from droplets. A geometrical optics
approach was used to calculate the energy-density distribution in
the meridional plane of a droplet and this was compared to the Mie
theory solution for a range of imaginary refractive indices
mi = 1×10-5 to mi = 5×10-4. Integration
of the energy density distribution over the droplet volume
provided a method to compare experimental and theoretical
results. Good agreement was found for the energy density and
volume integrated energy distribution patterns obtained from both
calculation methods and the experimental results. Quantitative
comparison of the volume integrated energy results shows that for
the investigated range of absorptivity Mie theory calculations
lead to results that are ≈30% higher than in the
geometrical optics case. This discrepancy is independent of light
absorption and droplet size. Tunnelling waves were identified as
the cause for the discrepancies between Mie theory and
geometrical optics; these contribute to high energy density in the
rim region of the droplet images.
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