Global rainbow refractometry with a selective imaging method

Originally developed to measure the refractive index, global rainbow refractometry also permits the extraction of a size distribution. When this technique is applied to a spray with a large number of non-spherical particles, the number of small particles tends to be overestimated. A new approach in data acquisition is presented to solve this problem at least partially, resulting in a global rainbow of mainly spherical particles. Thus, measurement errors due to non-sphericity can be minimized. The global rainbow is constructed from an image series of rainbows of individual droplets, where images of the most spherical droplets are selected by use of certain criteria derived from computations of rainbows of single spheres using the Lorenz-Mie theory. The method is tested with measurements in a spray. The mean droplet temperature and the size distribution, obtained from the inversion of the global rainbow, are compared to results obtained without using the selective imaging method and to results from other measurement techniques. The comparison of the results shows a better agreement when the selective imaging method is applied, especially for size distribution measurements close to the atomizer where a high amount of non-spherical droplets can be expected.