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In this article, structured laser illumination planar imaging and polarization ratio techniques are successfully combined to size droplets in various optically dense sprays. The polarization ratio approach is based on the acquisition of the perpendicular and parallel polarized components of Lorenz–Mie scattered light, for which the ratio is proportional to the surface mean diameter, D21. One of the main advantages of this technique, compared to some other laser imaging techniques for particle sizing, is that no fluorescent dye is required. This makes the technique suitable for characterizing sprays under evaporation conditions, such as combustion or spray drying applications. In addition, the SLIPI technique aims at suppressing the detection of multiple light scattering and at extracting the desirable single-light scattering signal. To test the reliability of this novel approach, an industrial hollow-cone nozzle is used, injecting at 50 bar water mixed with Glycerol (in the range of 0–60%). The first aim of this work is to study the experimental parameters that influence the reliability of the technique, such as the polarization orientation of the incident light, the refractive index of the injected liquid and the variation of the droplet size distribution. Using Phase Doppler Anemometry, the results show that a linear calibration is obtained for droplets ranging between 10 and 70 μm, when the incident illumination has a polarization set to 10° and 20°. In addition, this article demonstrates the feasibility of the technique for the measurement of liquids having a refractive index reaching 1.41. In the last stage of this work, after rotating the nozzle every 5°, a 3D tomographic reconstruction of D21 is performed. This demonstrates the robustness and efficiency of the technique for droplet sizing in 3D, under challenging conditions. [ABSTRACT FROM AUTHOR]

The aim of this work is to investigate the planar LIF/Mie droplet sizing technique from an experimental and theoretical point of view. This technique is a good alternative compared to point measurements (e.g., phase Doppler anemometry) or integral approaches (e.g., laser diffraction). It allows measurement of the Sauter mean diameter over a wide field, providing the spray topology and droplet size in a limited amount of time. Nevertheless, its implementation remains challenging due to the fact that the usual assumptions underlying the technique are not fully valid in practice. To overcome these limitations, an innovative experimental set-up has been developed including the use of a telecentric lens and a TwinCam beam splitter device. The benefit of the key optical elements introduced in the set-up will be discussed and quantified in the paper. In particular, it is shown that the telecentric lens removes angular dependency of the Lorenz–Mie optical signals. This is demonstrated using an ethanol hollow cone spray (seeded with rhodamine 6G) generated from a simplex injector at atmospheric pressure and ambient room temperature. A calibration procedure, based on phase Doppler anemometry measurements, is used to convert the LIF/Mie into Sauter mean diameter maps. The optical depth of the spray has been measured, exhibiting values less than 0.4 (equal to 67% light transmission), which corresponds to a dilute cloud of droplets where multiple light scattering is not an issue. These effects have been quantified using Monte Carlo simulations. Finally, a study of the injection parameters has been undertaken, showing the robustness and efficiency of the planar LIF/Mie droplet sizing method to obtain two-dimensional maps of the Sauter mean diameter. [ABSTRACT FROM AUTHOR]

In order to study spray combustion, an experimental test rig was developed at ONERA to partially characterize the flow conditions inside the combustion chamber of a gas turbine. Experimental campaigns using laser-based diagnostics were performed to provide an experimental database under reacting and non-reacting conditions. The paper first describes the Mie scattering image-processing to detect the droplets in the spray, and to calculate 2D maps of droplet number density and mean inter-droplet distance. The method is subsequently used to investigate the spray behavior under both reacting and non-reacting conditions according to global-averaging and phase-averaging methods. Experimental findings on the spatial droplet distribution in the spray are compared to the simple regular grid distribution and the Hertz–Chandrasekhar distribution. Results show that, under both conditions, there is an affine relationship between the inverse square root of the mean droplet number density and the nearest-neighbor inter-droplet distance. Moreover, observations suggest that the droplet spatial distribution fits more closely to a Hertz–Chandrasekhar distribution than a simple regular grid distribution, which may bring new insight for spray modeling. [ABSTRACT FROM AUTHOR]

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Fluorescence-based techniques are commonly implemented to characterize droplet sprays in flows at temperature above 293 K. Nevertheless, in the case of gas turbine altitude relight, aeroengines are exposed to critical operating conditions where the liquid fuel temperature can reach values as low as − 40 °C. As a result, it is necessary to address the influence of this parameter on fluorescence intensity for temperatures below room values. In this paper, an innovative saturated laser-induced fluorescence (SLIF) excitation scheme is proposed to overcome the strong temperature dependence of fluorescence in liquid kerosene seeded with a dye. The findings of this work are based on gated sequential images recorded with an intensified camera during and after the laser pulse duration so as to sample the temporal evolution of the fluorescence emission. The results show that, for low laser excitation intensity, the fluorescence intensity remains proportional to the laser power. However, a non-linear response is observed in the case of high excitation intensity. This difference can be used to limit the fluorescence dependence on temperature by carefully selecting the temporal window where fluorescence is detected. Finally, although the method presented in this paper is applied to cold liquid kerosene, its range of application can be extended to other liquids and other temperature ranges. [ABSTRACT FROM AUTHOR]