Development of an optical imaging technique for particle number density

Abstract

Accurate measurements of particle number density along with particle diameters and velocities are strongly required both in academic and industrial fields. A new imaging technique, through the evaluation of the effective depth of field of a camera, is developed using standard solid particles with constant diameters. To measure the effective depth of field for a wide range of particle diameters, three optical setups, named microscale, mesoscale, and macroscale setups, are used for the diameters of 50 μm -201 μm, 201 μm -3.97 mm, and 3.97 mm -15 mm, respectively. The measured effective depth of field is further applied to measure the size dependence of the number density of entrained bubbles by breaking waves in a wind-wave tank. The results show that the slopes of the number density of the entrained bubbles in the experiments corresponded to those measured by a phase Doppler particle analyzer under 500 μm, and was -5 over 500 μm in both fresh and salt waters. This emphasizes that the present imaging technique can measure the diameters and particle number density with high precision and is important for measurements of droplets, bubbles, and solid particles with a wide range of diameters.

Novel Method for Measuring Fine Particle Distribution in Shock-Induced Atomization

“…Laser diffraction [1], phase Doppler [2], and shadow sizing [3][4][5] are the main methods used to measure the size of fine droplets. The laser diffraction method and the phase Doppler method have the advantage that the particle size can be measured with high accuracy; however, the measurement range is narrow.…”

“…One is to binarize the image by setting the particle contour threshold value in the luminance information in the image, and the other is to define the high luminance gradient value as the particle contour. In previous studies, the particle contour with the lowest measurement error at the point of focus was derived from calibration experiments [3,4]. However, the derivation of optimal particle contour brightness or measurement accuracy has not been verified at out-of-focus locations, and there is concern that large measurement errors may occur.…”