Analysis of LIF and Mie signals from single micrometric droplets for instantaneous droplet sizing in sprays

Abstract

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.

Investigation of Five Organic Dyes in Ethanol and Butanol for Two-Color Laser-Induced Fluorescence Ratio Thermometry

“…The standard deviation in the averaged fluence was approximately 3%. All measurements were performed in the linear regime of fluorescence and the dye concentrations (see Table 1) were chosen to have sufficient fluorescence intensity required for future 2c-LIF (two-color laser-induced fluorescence) thermometry of droplets in ethanol and butanol sprays [41,42]. Note that compared to dye concentration for the thermometry of liquid solutions in a test cell, atomizing sprays or single droplet studies require a relatively higher dye concentration (for better signal to noise ratio) [43,44], especially in two-dye approaches using FL [19].…”

Application of SLIPI-Based Techniques for Droplet Size, Concentration, and Liquid Volume Fraction Mapping in Sprays

“…However, it is worth highlighting that the intensity contribution from the multiply scattered light is very difficult to predict and the observed differences between conventional and SLIPI showed in [21] can change with the collection angle, optical turbidity, droplets size, distribution of the droplets in the spray, etc. Nevertheless, the results reported in [21], have been further demonstrated for absolute droplet SMD mapping in air blast atomizer [22], hollow-cone sprays [23], and direct injection sprays [7,24]. In another study, SLIPI-scan was used for 2D and 3D mapping of the local µ e in an air-assisted spray system with a maximum optical depth OD~3 [25].…”

“…The SMD of an ensemble of the spherical droplets in sprays can be calculated from the ratio of their LIF/Mie optical signals simultaneously measured from the same probe location [11,24,31]. The method assumes that the fluorescence from the liquid phase, I LIF , is proportional to the droplet volume, d 3 , and the Mie scattered light intensity, I Mie , from the same droplet is proportional to its surface, d 2 , [32][33][34].…”

“…Apart from this assumption, three important conditions must be respected. First, the technique is only valid for spherical droplets and it does not withstand for non-spherical liquid structures and ligaments [24,31]. Second, the fluorescence from the gas phase must be excluded [35].…”

“…Here K defines the calibration curve which is equal to the ratio K LIF /K Mie . The calibration curve can be either deduced by using the calibration of LIF/Mie ratio with the absolute SMD measured by the PDA system [21,22] or calibrating the ratio by means of monodisperse droplet generated by a droplet generator [24,36]. Some residual stripes are seen on the processed SLIPI images.…”