Two-phase SLIPI for instantaneous LIF and Mie imaging of transient fuel sprays

Abstract

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.

Comparison between two-phase and one-phase SLIPI for instantaneous imaging of transient sprays

“…This effect is induced by the highly forward scattering peak of the scattering phase function shown in Fig. 2c ( Storch et al 2016). The solution exploits the birefringence properties of calcite crystals as shown in the zoomed area of Fig.…”

“…In the experiment presented in this article, the same 2p-SLIPI setup used by (Storch et al 2016) is employed, where laser 1 is a Quanta-Ray from Spectra-Physics and Fig. 4 Schematic of the 2p-SLIPI experimental setup: two polarized light pulses are generated from two Nd:YAG laser sources and are recombined using a polarization beam splitter.…”

“…This approach requires two standard double-pulsed laser systems and either an interline transfer CCD or a dualframe scientific CMOS camera to acquire temporally frozen SLIPI images, thus circumventing the factors limiting instantaneous 3p-SLIPI. In a recently presented 2p-SLIPI setup (Storch et al 2016), the line pattern is optically shifted using the birefringence properties of a calcite crystal, which greatly simplifies the optical setup.…”

Development of limited-view tomography for measurement of Spray G plume direction and liquid volume fraction

“…implemented to reduce multiple-scattering effects on planar imaging in a GDI spray (Berrocal et al 2019), but attenuation of the incoming laser light and signal trapping remain important, and it is not clear that the technique could be applied at higher optical thicknesses. Similar concerns may be cited for a two-pulse SLIPI technique recently applied to a GDI spray (Storch et al 2016). Non-imaging optical techniques such as laser Doppler velocimetry (LDV) and phase Doppler interferometry (PDI) are also applied to understand plume direction and spray characteristics.…”

Analysis of the LIF/Mie Ratio from Individual Droplets for Planar Droplet Sizing: Application to Gasoline Fuels and Their Mixtures with Ethanol

“…For this purpose, a large number of fluorophores, mainly dissolved in water, were characterized in the past including dissolved sodium salicylate [21], rhodamine 6G in water [9], and pyranine ("solvent green 7") [20]. Several studies dealing with droplet sizing in alcohol sprays already exist in the literature [5,15,16,20,22,23]. The tracer Eosin Y, which has been used in previous studies [22][23][24][25], cannot be dissolved in alkanes or gasoline fuels.…”