Investigations on the characterization of laminar and transitional flow conditions after high pressure homogenization orifices

High pressure homogenization is a well-established technique to achieve droplets in the submicron range. However, droplet breakup mechanisms are still not completely understood, since studies to characterize the flow are limited due to very small dimensions (typically several micrometers) and very large velocity ranges (from almost stagnant flow to 300 m/s and more). Furthermore, cavitation can occur resulting in multiphase flow. So far, experiments were performed only via integral measurements of, for example, the pressure drop or the droplet size distribution at the outlet. In the current study, this gap shall be closed using Particle Image Velocimetry measurements to analyze the flow field. In addition, an overall method, the characteristic correlation between the discharge coefficient ( C D ) and Re 0.5 is used to distinguish between laminar, transitional and turbulent flow conditions at Reynolds numbers based on the channel width ( d = 200 µm) between 250 and 22,500. The investigated orifices of this study had different positions of the constriction: coaxial and next to the wall. For both orifices, the C D measurement was applicable and showed different characteristic regions which can be associated with laminar, transitional and turbulent flow conditions. Mean velocity fields and fluctuations were measured quantitatively at the outlet and 50 diameters downstream using Micro Particle Image Velocimetry (µ-PIV) in an optically accessible orifice. Increased velocity fluctuations were found in the shear layers when the flow turns from laminar into unstable transitional conditions. The combination of both measurement techniques will help to optimize these systems for the future.