Liquid volume fraction and droplet sizing in atomizing sprays using polarization ratio with dual structured laser illumination planar imaging.

<sec>Laser sheet imaging, also called planar laser imaging, is one of the most versatile optical imaging techniques and has been frequently used in several different areas. However, when applied to the limited operating space and strong light scattering media, the light originating from indirect reflections, multiple scattering and surrounding backgrounds can produce error especially in intensity-ratio based measurements.</sec><sec>This work is motivated by these challenges, with the overall aim of making laser sheet imaging technique applicable for the study of eliminating the stray light interference. Therefore a novel two-dimensional imaging technique named structured laser illumination planar imaging (SLIPI) is developed based on planar laser imaging but uses a sophisticated illumination scheme i.e. spatial intensity modulation, to differentiate between the intensity contribution arising from useful signals and that from stray light. By recording and dealing with images, the SLIPI method can suppress the diffuse light and retain the useful signals.</sec><sec>In this paper, we first use the MATLAB software to simulate the phase-shift SLIPI method, and the results show that the stray light interference can be eliminated completely. Furthermore, the phase-shift SLIPI is combined with the liquid solution (Rhodamine B solution) laser induced fluorescence (LIF) approach to imagine the concentration distribution. By recording three images, between which this encoding is changed noticeably only for the useful LIF signals, the phase-shift SLIPI method is evidenced to be able to remove the diffuse light contribution, thus improving and enhancing the visualization quality. The instantaneous SLIPI images of rapidly moving samples, a key feature to study dynamic liquid solution diffusion behavior, are also acquired. The lock-in amplifier SLIPI technique is then experimentally studied under Rhodamine B diffused solution, and the phase-shift SLIPI method can remove the unwanted background interferences and achieve the significant improvements in terms of pronounced concentration distribution within the Rhodamine B solution.</sec><sec>The SLIPI technique is relatively inexpensive: the cost does not exceed the cost of an ordinary laser sheet arrangement noticeably, and it can combine with several other linear imaging techniques, such as Rayleigh scattering, particle image velocimetry and laser-induced phosphorescence. </sec>

Structured laser illumination planar imaging (SLIPI)-based techniques have been employed during the past decade for addressing multiple light scattering issues in spray imaging. In this article, SLIPI droplet sizing based on the intensity ratio of laser-induced fluorescence (LIF) over Mie scattering (SLIPI-LIF/Mie) and SLIPI-Scan for extinction-coefficient (µe) mapping are applied simultaneously. In addition, phase Doppler anemometry (PDA) and numerical calculations based on the Lorenz–Mie theory are also employed in order to extract the droplets Sauter mean diameter (SMD), the droplets number density (N), and the liquid volume fraction (LVF) in a steady asymmetric hollow cone water spray. The SLIPI-LIF/Mie ratio is converted to droplets SMD by means of a calibration procedure based on PDA measurements. The droplet SMD for the investigated spray varies from 20 µm to 60 µm, the N values range from 5 to 60 droplets per mm3, and the LVF varies between 0.05 × 10−4 and 5.5 × 10−4 within the probed region of the spray. To generate a series of two-dimensional images at different planes, the spray scanning procedure is operated in a “bread slicing” manner by moving the spray perpendicularly to the light sheet axis. From the resulting series of images, the procedure described here shows the possibility of obtaining three-dimensional reconstructions of each scalar quantity, allowing a more complete characterization of droplet clouds forming the spray region.

Recent studies have used structured laser illumination planar imaging (SLIPI) combined with phosphor thermography to remove multiple scatter effects and near-wall reflections, which lead to biases in temperature measurements and reduced spatial resolution. We show that for the typical non-linear pump-signal range under which thermographic phosphors are used, errors may arise in the reconstruction of the temperature field using SLIPI. In this study, synthetic laser induced phosphorescence (LIP) images are generated numerically by adapting the Synthetic PIV Image Generator (SIG) for the purpose. The simulations are combined with phosphorescent signal yield functions obtained from experimental data to investigate the application of SLIPI to gas-phase phosphor thermography. We conclude that whilst SLIPI is effective in removing scattering noise for phosphors for which the two-colour signal ratio is insensitive to the laser fluence, it creates a bias in the temperature measurement otherwise. We also show that the extent of multiple scatter in LIP images is always overestimated by SLIPI, owning to the non-linear emission behaviour and particle image diffraction.

We demonstrate a technique, named dual structured laser illumination planar imaging (SLIPI), capable of acquiring depth-resolved images of the extinction coefficient. This is achieved by first suppressing the multiply scattered light intensity and then measuring the intensity reduction caused by signal attenuation between two laser sheets separated by Δz mm. Unlike other methods also able to measure this quantity, the presented approach is based solely on side-scattering detection. The main advantages of dual SLIPI is that it accounts for multiple scattering, provides two-dimensional information, and can be applied on inhomogeneous media.

Recent studies have used structured laser illumination planar imaging (SLIPI) combined with phosphor thermography to remove multiple scatter effects and near-wall reflections, which lead to biases in temperature measurements and reduced spatial resolution. We show that for the typical non-linear pump-signal range under which thermographic phosphors are used, errors may arise in the reconstruction of the temperature field using SLIPI. In this study, synthetic laser induced phosphorescence (LIP) images are generated numerically by adapting the synthetic PIV image generator for the purpose. The simulations are combined with phosphorescent signal yield functions obtained from experimental data to investigate the application of SLIPI to gas-phase phosphor thermography. We conclude that whilst SLIPI is effective in removing scattering noise for phosphors for which the two-colour signal ratio is insensitive to the laser fluence, it creates a bias in the temperature measurement otherwise. We also show that the extent of multiple scatter in LIP images is always overestimated by SLIPI, owning to the non-linear emission behaviour and particle image diffraction.

Laser-induced fluorescence (LIF) for quantitative fuel concentration measurements in a combustion engine is a challenging task. Measuring close to the walls of the combustion chamber is even more challenging as both the incident laser light and the signal are strongly reflected on the walls of the combustion chamber. By using a new technique called Structured Laser Illumination Planar Imaging (SLIPI) such background effects, as well as unwanted multiply scattered light, can be suppressed allowing for higher measurement accuracy. In this article we demonstrate, for the first time, the feasibility of the SLIPI technique for gas phase LIF and in-cylinder measurements. Results from regular LIF and SLIPI measurements are also compared. The measurements were made on a non-reacting fuel-jet with acetone as a fuel tracer in a heavy duty diesel engine (Scania D12). It is observed that the equivalence ratio measured by SLIPI in the free part of the jet is only two thirds of that measured by regular LIF during the early jet development. (Less)

In this article, Structured Laser Illumination Planar Imaging (SLIPI) is used in combination with the LIF/Mie ratio technique for extracting a reliable two-dimensional mapping of the droplets Sauter Mean Diameter (SMD). We show that even for the case of a fairly dilute spray, where single scattering events are in majority, the conventional LIF/Mie technique still remains largely affected by errors introduced by multiple light scattering. To remove this unwanted light intensity on both the LIF and Mie images SLIPI is used prior to apply the image ratio. For the first time, the SLIPI LIF/Mie results are calibrated and compared with measurement data from Phase Doppler Interferometry (PDI).

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.

Planar drop-sizing and liquid volume fraction measurements of airblast spray in cross-flow using SLIPI-based techniques

Planar droplet sizing (PDS) is a technique relying on the assumption that laser-induced 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.

The Structured Laser Illumination Planar Imaging (SLIPI) technique received significant attention in recent years due to notable improvements in the measurement of droplet size and the visualization of internal structure in both dilute and dense sprays. However, despite the improvements, the technique remains limited to these microscopic characteristics of the spray, primarily due to the complexity of the optical setup and image acquisition process. In this article, the capabilities of the SLIPI application are exploited, specifically employing the 3p-SLIPI-LIF/Mie (SLSD), and combined application of 1p-SLIPI-LIF & PIV (1p-SLIPI-PIV) for mapping of both Sauter Mean Diameter (SMD) and mean droplet velocity in combination with detailed spray structure analysis. The measurements were carried out in hollow cone sprays operated at the range of injection pressures from 25 to 107 bar. The results obtained using the SLIPI techniques were compared with the Phase Doppler Anemometry (PDA) measurements showing good agreement for both the SMD and mean droplet velocity. The significant improvements in the important microscopic characteristics (droplet size & velocity) and visualization of the internal structure in a wider flow field of hollow cone sprays allowed detailed analysis and understanding of the dispersed phase flow owing to the suppression of diffused light resulting from the multiple scattering effects.

The accuracy, precision and limitations of the imaging technique named Structured Laser Illumination Planar Imaging (SLIPI) have been investigated. SLIPI, which allows multiply scattered light to be diminished, has previously demonstrated improvements in image quality and contrast for spray imaging. In the current study the method is applied to a controlled confined environment consisting of a mixture of water and monodisperse polystyrene microspheres. Elastic scattering and fluorescence are studied and the results obtained when probing different particle concentrations and diameters conclusively show the advantages of SLIPI for imaging within moderately turbid media. Although the technique presents both good repeatability and agreement with the Beer-Lambert law, discrepancies in its performance were, however, discovered. Photons undergoing scattering without changing their incident trajectory cannot be discriminated and, owing to differences in scattering phase functions, probing larger particles reduces the suppression of multiply scattered light. However, in terms of visibility such behavior is beneficial as it allows denser media to be probed. It is further demonstrated that the suppression of diffuse light performs equally well regardless of whether photons propagate along the incident direction or towards the camera. In addition, this filtering process acts independently on the spatial distribution of the multiply scattered light but is limited by the finite dynamic range and unavoidable signal noise of the camera.

Previous research has shown that the polarization ratio technique allows the characterization of the surface mean diameter, D21, of droplets forming dilute sprays. However, its application to optically dense sprays has posed significant challenges due to the presence of multiple light scattering. Additionally, errors in measurement can arise from the angular dependence of the signal. In this Letter, we present a novel, to the best of our knowledge, method that addresses these challenges. Our approach combines the use of a telecentric objective with structured laser illumination, to both optimize light detection and suppress the unwanted intensity from multiple scattering. This approach enables the utilization of the polarization ratio technique for measuring the droplet size of challenging atomizing sprays. The method offers a promising solution for accurate and comprehensive spray characterization. It is applied, here, to a hollow-cone water spray running at 30, 50, and 70 bar injection pressure, reaching an optical depth up to three.

Previous research has shown that the polarization ratio technique allows sizing droplets generated from dilute spray systems. The polarization ratio technique is based on the acquisition of the perpendicular and parallel polarized components of Lorenz-Mie scattered light, which ratio is proportional to the surface mean diameter, D21. One of the main advantage of this technique, compared to some other laser imaging techniques (e.g. LIF/Mie droplet sizing), is that no fluorescent dye is required. This makes the technique suitable for the characterization of sprays under evaporating conditions. However, its application to optically dense sprays has posed significant challenges and restrictions due to the presence and detection of multiple light scattering. A novel approach, presented in this talk, consists in combining the polarization ratio approach with Structured Laser Illumination Planar Imaging - SLIPI - a technique that allows suppressing efficiently the unwanted multiple light scattering intensity. Thanks to this suppression of unwanted light, the method offers a promising solution for accurate and comprehensive 2D characterization of challenging atomizing sprays. Note that the SLIPI-polarization ratio technique is calibrated using Phase Doppler Anemometry and applied, here, to a hollow-cone water spray running from 20 and 100 bar injection pressure.

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.