Structured Laser Illumination Planar Imaging SLIPI Applications for Spray Diagnostics

Abstract: Laser sheet imaging, also known as planar laser imaging, is one of the most versatile optical imaging techniques known and is frequently applied in several different domains. It furthermore constitutes the basis for a variety of other 2D methods, in turn allowing visualization of e.g. velocity, particle size, species concentration and temperature. However, when applied on turbid, light scattering media, the accuracy of laser sheet imaging (and techniques based thereon) is significantly reduced. The inaccuracy can be attributed to three phenomena known as laser extinction, signal attenuation and multiple light scattering. The work presented in this thesis is motivated by these challenges, with the overall aim of making laser sheet imaging applicable for studies of optically dense media. High-pressure, liquid atomizing spray systems, used for combustion purposes, are typical examples of such media. Here sprays are used to disintegrate liquid fuel into fine droplets, which evaporate and burn, an approach which is essential to generate the mechanical power needed for all combustion-based vehicles, such as cars and trucks. Accurate characterization of these multiphase, dense, turbulent spray systems is key in order to reduce pollutions and to improve combustion efficiency. To enable the utilization of laser sheet imaging in optically dense environments a novel 2D imaging technique named Structured Laser Illumination Planar Imaging (SLIPI) has been developed. The method is based on planar laser imaging but uses a sophisticated illumination scheme - spatial intensity modulation - to differentiate between the intensity contribution arising from directly- and multiply scattered light. By recording three images, between which this encoding is altered in a way only noticeable for the directly scattered light, the approach enables the suppression of the undesired diffuse light intensity contribution. This thesis presents convincing experimental evidence that SLIPI leads to improved and enhanced visualization up to an optical depth of ~6, thus improving the range of applicability by a factor of ~150 (in terms of light transmission). Also presented herein are solutions to acquire instantaneous SLIPI images of rapidly moving samples, a key feature in order to study dynamic transient spray behavior. It is imperative for accurate instantaneous imaging of turbulent flows that the sample remains static during the time of acquisition, which is experimentally challenging since the SLIPI technique requires three realizations. Based on the SLIPI method, several quantitative imaging techniques have also been developed within the framework of this thesis, all designed to measure the extinction coefficient. This optical property contains information related to both particle size and number density and is thus of great importance for spray characterization. Moreover, since the extinction coefficient is the origin for both laser extinction and signal attenuation, accurate readings of this parameter by means of SLIPI makes the methods almost completely unaffected by the three previously mentioned sources of error associated with laser sheet imaging. The SLIPI technique is relatively inexpensive - the cost does not exceed an ordinary laser sheet arrangement noticeably - and can be combined with several other linear imaging techniques. These characteristics can be of great benefit for the spray community, in particular by providing accurate and reliable input data for spray modelers.