Structured Laser Illumination Planar Imaging Research Articles

Visualization and understanding of combustion processes using spatially and temporally resolved laser diagnostic techniques

Laser diagnostic techniques have for more than 30years added very valuable input for a deepened understanding of combustion processes. The present paper will focus on techniques developed for visualization of important parameters with the ability to get detailed information in space and time. The paper is not meant to be a complete review of the entire research field but rather a survey with a majority of the contributions from the authors’ laboratory. After a short introduction and background, essentially three techniques will be highlighted. Planar Laser-Induced Fluorescence, PLIF, will be described in terms of further development directed towards the use of a high-power Alexandrite laser, in some cases yielding a substantially increased sensitivity as compared to the more conventional Nd:YAG and Excimer lasers. There will also be a description of possibilities for high-speed visualization, 3D imaging and the potential to visualize species requiring a two-photon excitation process. Regarding thermometry, the paper will describe and exemplify the use of Thermographic Phosphors, mainly for surface measurements but also in droplets and gas flows. The third area to be described is the use of Polarization Spectroscopy, PS, for spatially and temporally resolved measurements in the IR spectral region, e.g. CO2 and H2O, various hydrocarbons, HCl and HCN. The last chapter focuses on present challenges and future applications. A major challenge is diagnostics in environments with limited optical access. Two methods that are applicable under such conditions are discussed, namely endoscopic approaches and picosecond LIDAR. Another challenging area addressed is application of diagnostics in optically dense environments, e.g. sprays, where a new technique, Structured Laser Illumination Planar Imaging, SLIPI, has been developed for suppressing the influence of multiple scattering. Finally, approaches for measurements of “new” species, challenges associated with LIF quantification, as well as measurements in environments possessing strong background radiation are discussed.

Structured illumination for 3-D Mie imaging and 2-D attenuation measurements in optically dense sprays

Qualitative description of spray structure and geometry in the near-field region of atomizing sprays is particularly challenging due to the lack of visibility through such optically dense media. Similarly, quantitative determination of the optical depth can be particularly underestimated as a result of detecting photons which have experienced a number of scattering events. In this paper we demonstrate, based on average imaging, the possibility to reconstruct the “true” 3-D Mie representation of an aerated spray, where image artifacts introduced by multiple scattering have been suppressed by means of Structured Laser Illumination Planar Imaging (SLIPI). Here, six images were used, instead of three usually employed in structured illumination, to reconstruct a single SLIPI image. Based on a similar approach, a 2-D estimation of optical depth has been performed and compared with the conventional single-point attenuation measurement.

MULTIPLE SCATTERING SUPPRESSION IN PLANAR LASER IMAGING OF DENSE SPRAYS BY MEANS OF STRUCTURED ILLUMINATION

A novel method to reduce the multiply scattered light contribution to images recorded with planar laser imaging is demonstrated. The technique, structured laser illumination planar imaging (SLIPI), is based on spatially modulated excitation light and is tested here within the dense region of a hollow-cone spray. The main idea is to use a laser sheet that is spatially modulated along the vertical direction. By both shifting the spatial phase of the modulation and using adequate image post processing of the successive recorded images, it is possible to remove a significant amount of the multiply scattered light detected. In this paper, SLIPI is applied for imaging within a typical hollow-cone water spray generated in ambient air at 50 bars injection pressure from a pressure-swirl nozzle. Because this type of spray has a known inner structure, the method can be evaluated, demonstrating that 47% of the detected light arising from multiple scattering can be suppressed, resulting in an increase from 61% to 89 % in image contrast. Such an improvement allows more accurate interpretation and analysis of the near-field region of atomizing sprays. The possibility of extracting instantaneous flow motion is also demonstrated for the case of a dilute nebulizer. All these results indicate promising applications of the technique in denser turbid media, such as air-blast atomizer or diesel sprays.

Application of structured illumination for multiple scattering suppression in planar laser imaging of dense sprays

A novel approach to reduce the multiple light scattering contribution in planar laser images of atomizing sprays is reported. This new technique, named Structured Laser Illumination Planar Imaging (SLIPI), has been demonstrated in the dense region of a hollow-cone water spray generated in ambient air at 50 bars injection pressure. The idea is based on using an incident laser sheet which is spatially modulated along the vertical direction. By properly shifting the spatial phase of the modulation and using post-processing of the successive recorded images, the blurring effects from multiple light scattering can be mitigated. Since hollow-cone sprays have a known inner structure in the central region, the efficiency of the method could be evaluated. We demonstrate, for the case of averaged images, that an unwanted contribution of 44% of the detected light intensity can be removed. The suppression of this diffuse light enables an increase from 55% to 80% in image contrast. Such an improvement allows a more accurate description of the near-field region and of the spray interior. The possibility of extracting instantaneous flow motion is also shown, here, for a dilute flow of water droplets. These results indicate promising applications of the technique to denser two-phase flows such as air-blast atomizer and diesel sprays.