Planar droplet sizing (PDS) is a technique relying on the assumption that laserinduced fluorescence (LIF) and Mie scattering optical signals from spherical droplets depend on their volume and surface area, respectively. In this article, we verify the validity of this assumption by experimentally analyzing the light intensity of the LIF and Mie optical signals from micrometric droplets as a function of their diameter. The size of the droplets is controlled using a new flow-focusing monodisperse droplet generator capable of producing droplets of the desired size in the range of 21 µm to 60 µm. Ethanol droplets doped with eosin dye and excited at 532 nm are considered in this study, and the individual droplets were imaged simultaneously at microscopic and macroscopic scale. The effects of laser power, dye concentration, and temperature variation are systematically studied as a function of LIF/Mie ratio in the whole range of droplet sizes. Finally, a calibration curve at tracer concentration of 0.5 vol% is deduced and used to extract the droplet Sauter mean diameter (SMD) from instantaneous images of a transient ethanol spray. This droplet size mapping is done using structured laser illumination planar imaging (SLIPI), in order to suppress the artifacts induced by multiple light scattering.

This paper reports on the spray structure of the biofuels, ethanol, and butanol generated by a multihole direct-injection spark-ignition injector, which is studied in a constant volume chamber. The spray shape and structure are analyzed using two-phase structured laser illumination planar imaging where both laser-induced fluorescence and Mie-scattering light are recorded simultaneously for the extraction of instantaneous laser-induced fluorescence/Mie-scattering ratio images. Quantitative planar measurements of the droplet Sauter mean diameter are conducted, using calibration data from phase-Doppler anemometry. The resulting Sauter mean diameters are presented for ethanol and butanol at various fuel temperatures at different times after the start of injection. It is found that an increase in fuel temperature results in a faster atomization and higher evaporation rate, which leads to reduced spray tip penetration and smaller droplet Sauter mean diameter. At equivalent conditions, butanol consistently showed larger spray tip penetration in comparison to ethanol. This behavior is due to the higher surface tension and viscosity of butanol resulting in the formation of larger droplets and larger Sauter mean diameters in the whole spray region. Finally, the butanol injection also shows larger cyclic variations in the spray shape from injection to injection which is explained by the internal nozzle flow that is influenced by larger fuel viscosity as well. The Sauter mean diameter distribution is also compared to phase-Doppler anemometry data showing good agreement and an uncertainty analysis of the structured laser illumination planar imaging-laser-induced fluorescence/Mie-scattering technique for planar droplet sizing in direct-injection spark-ignition sprays is presented.

We report in this Letter a two-phase structured laser illumination planar imaging [two-pulse SLIPI (2p-SLIPI)] optical setup where the "lines structure" is spatially shifted by exploiting the birefringence property of a calcite crystal. By using this optical component and two cross-polarized laser pulses, the shift of the modulated pattern is not "time-limited" anymore. Consequently, two sub-images with spatially mismatched phases can be recorded within a few hundred of nanoseconds only, freezing the motion of the illuminated transient flow. In comparison with previous setups for instantaneous imaging based on structured illumination, the current optical design presents the advantage of having a single optical path, greatly simplifying its complexity. Due to its virtue of suppressing the effects from multiple light scattering, the 2p-SLIPI technique is applied here in an optically dense multi-jet direct-injection spark-ignition (DISI) ethanol spray. The fast formation of polydispersed droplets and appearance of voids after fuel injection are investigated by simultaneous detection of Mie scattering and liquid laser-induced fluorescence. The results allow for significantly improved analysis of the spray structure.

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