In AWJ cutting heads an abrasive water jet is formed by mixing abrasive particles with a high-speed water jet. In order to understand the physics of this mixing process, knowledge of the velocity of the abrasive particles at the exit of the focusing tube is of major importance. In literature, different models describing the cutting process or parts of it can be found where the velocity of the abrasive particles is a key parameter. Up to now, various methods have been applied to experimentally investigate the particle velocity or the acceleration process in the focusing tube. However, none of these experiments were carried out under real conditions typical for today's AWJ cutting. In this paper we present a new technique with which we are able to determine the velocity of abrasive particles under real AWJ cutting conditions. The technique is a modification of the well known particle tracking velocimetry. It is based on two key issues: Use of abrasive particles coated with a thin layer of fluorescent dye and the application of a sophisticated, nonlinear image processing algorithm. Standard image processing algorithms fail because some of the dye detaches from the abrasive particles and this results in an extensive background noise which makes it prohibitive to detect the particle by linear algorithms.
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