Volumetric characterization of dispersed two-phase flows by digital image analysis

A digital-based image analysis system has been developed for the experimental determination of size, spatial distribution and two components of velocity for particles suspended in any clear fluid flow. A volumetric measurement directly provides number density and consequently particle flow. A peculiarity of the developed system is its suitability for relatively large particles (some mm in diameter), which can hardly be measured with other systems. Double exposed particle images, in the same frame, provide the starting point for image processing. The system includes a procedure for the classification of the particles in the image, the separation of the partially overlapped particles and the selection of those that are found in the control volume. These parameters serve as a basis for the velocity determination through a particle-tracking algorithm (PTV), which is based on iteratively estimating the match probability as a measure of the Likelihood of pairing. The depth of the control volume is obtained with a new technique, based on the loss of focus of the particles in the image, combined by diffuse illumination, provided by twin short pulse argon spark discharge lamps. The system has been extensively checked using calibrated micro-balls and sprays that simulate rain.