Executive Summary : | Structured Laser Illumination Planar Imaging (SLIPI), is a technique which was developed in the last few years, to enhance the ability to make measurements in dense sprays. Optical techniques are the first choice in characterizing sprays, as they are 'non-intrusive'. A typical spray may contain over a billion droplets, and the region close to the nozzle has a large number of droplets in a small volume. Due to this high number density of droplets, in conventional imaging, light which is passed through the spray interacts with multiple droplets before it reaches the detector. This multiple scattering of light makes any measurements erroneous. In the SLIPI method, the light beam is converted into a structured sheet having alternate bright and dark bands. By taking images of the spray with this sheet moved by a definite amount at three positions, it is possible to eliminate the effects of multiple scattering, and back-calculate the actual spray details. This makes the SLIPI technique attractive.
The standard implementation of the SLIPI technique calls for taking a set of three images. Since the light source is the same, the three grayscale images have to be taken one after the other so that the information in each image is available separately. However, due to the unavailability of cameras which can store three images in quick succession (of the order of a few nanoseconds apart, at high resolution), the standard SLIPI technique is typically applied only on ensemble averaged images from sprays. Consequently, only average spray data can be obtained, and transients and shot-to-shot variations in pulsed sprays cannot be resolved. This poses limitations on the use of the SLIPI technique.
The present proposal aims to overcome the above shortcoming of the SLIPI technique by using three light sources having different wavelengths. If the three images are taken with different light sources, they can be taken simultaneously. This, then, has the potential to remove the limitations of the SLIPI technique. Hence, in the present project, images of the spray illuminated by the combination of three light pulses of different colours will be taken in a single frame using a colour camera having a Bayer filter, by means of a carefully chosen setup. The image will then be decomposed to obtain the three sub-images which can then be processed to obtain the SLIPI image without multiple scattering. Thus, the aim of the technique is to enable SLIPI using just one image frame from a single camera, which can then open up the technique for measuring transients. While the principle of the proposed technique is simple, the implementation is expected to be challenging, yet achievable.
The project is expected to lead to better characterization of sprays which are widely used in process industries, automotive and aerospace engines, agriculture, and food processing, to increase the surface to volume ratio and hence enhance heat / mass transfer, or as ways of dispersing liquids. |