Fuel droplets, with diameters larger than 200 times the laser’s wavelength, were detected in great quantity during our experiments on fuel injection systems equipped with heating matrixes. A large number of particles condensed on the heater grid, forming giant droplets, which were later released with the flow. Laser imaging technique was performed to visualise fuel sprays. For small droplets (<100 microns), only droplets located inside the light sheet area and within the sensor sensitivity region were detected. For large droplets, several illumination patterns and light intensity levels were identified.Large spheres are often dealt with in literature as a special type of optics, since light diffraction is not the only dominant effect in this case. When a light beam encounters an obstacle, the obstacle scatters the light energy into different angles. The term “light scattering” describes a number of mechanisms in which a droplet/particulate emits electromagnetic waves as a secondary source of energy. For particles with a certain degree of transparency, the refracted light through the particle generates secondary scatters, in addition to the primary edge scatters, due to a set of internal reflections inside that particle . Beside the light diffraction and interreflections, large particles show a big deal of light reflection off the external surface (see the figure). The latter effect becomes much more dominant in particles much larger than the wavelength, and for high refractive indices.
Two effects appear in this case ; the first is called the “edge phenomena,” where fields distribute near the edges causing an increment in the forward scattering. The second effect is called the “surface wave” phenomena where the light waves spread (or “creep”) to the dark side of the droplet. These two effects were observed and imaged in the current work. For a droplet image not completely saturated on the CCD, it was noticed that a single interreflection point appeared inside it, sending the light rays backward towards the source, and contributing to the overall backscattering. This effect is referred to in the literature as the “glory effect”, which is responsible for the rainbow phenomenon in nature. These factors together contribute in the illumination of the droplet surface and, therefore, assist in identifying the droplet size and shape. Read full article here.
Source: Rami Zakaria, “Inspection of Large Droplets in Fuel Sprays Using a Gaussian Laser-Sheet: An Experimental Approach,” ISRN Optics, vol. 2012, Article ID 691637, 8 pages, 2012.
Kerosene droplet (approx. 2.2 mm in diameter) photograph; demonstrating the light vectors behaviour inside a fluid droplet (in air).