Optimization of defocused particle sizing methods for continuous measurement of high flux and concentration conditions of the disperse phases

The characterization of dispersed multiphase flows requires measurement techniques capable of determining particle concentration, size distribution, and particle type under realistic operating conditions. This work presents an advanced implementation of the Hydrodynamic Nuclei Concentration Technique (HDNC), a defocused imaging method for the simultaneous detection of bubbles and solid particles in liquid flows. Particle sizing is performed by analyzing interference structures within defocused particle images generated by coherent laser illumination. While spherical particles are characterized using fringe frequency analysis, non-spherical particles are evaluated through their speckle structures. To enable measurements at elevated particle concentrations, a square aperture design is introduced, providing improved particle separation and computationally efficient signal processing. In combination with statistical detection-volume correction, the approach allows reliable estimation of particle concentrations and size distributions under high-throughput conditions. The performance of the method is demonstrated using measurements in the K21 cavitation tunnel at the University of Rostock. Time-resolved results reveal the dynamic evolution of bubble concentration and bubble size during controlled bubble injection, including transport delays and dissolution effects relevant for cavitation studies. The presented technique enables continuous and real-time capable characterization of dispersed multiphase flows and provides a foundation for future environmental monitoring applications, including e.g. the detection and quantification of microplastic particles in water.