Comparison of µ-PIV flow pattern measurements and CFD simulation of high-pressure homogenizer orifices

Emulsions have a wide scope of applications. They can be found in the cosmetic, chemical, pharmaceutical, and food industry. Emulsion properties, such as stability, rheological behavior, and color depend on the droplet size and droplet size distribution (DSD) of the emulsion. High-pressure homogenization (HPH) is mostly used to produce emulsions with droplets smaller than one micrometer. During this process, an emulsion premix with larger droplets is pumped with a pressure of several hundred bar through a disruption unit. In this unit, the characteristic dimension through which the product flows, is significantly reduced. This results in a drastic increase in flow velocity. Thereby, droplets are exposed stresses resulting from laminar, turbulent and cavitating flow patterns, which cause droplet breakup. Nonetheless, the influence of geometrical or process parameters on resulting droplet sizes is still focus of ongoing research. Therefore, the design of a HPH process, which should result in a specific droplet size, is still mostly based on process functions and empirical knowledge.
This investigation focuses on the influence of the geometry of the disruption unit, and the process parameters on the local flow pattern and resulting stresses. ... mehrPrevious experimental works have demonstrated that even small changes in the geometry of the disruption unit result in significant changes on the DSD. In addition, CFD simulations have shown that the elongation rate in front of the smallest cross section depends on the geometry of the disruption unit. These higher elongation rates may be responsible for smaller droplets, as they induce elongation into extremely thin filaments (hypothesis of this work). However, due to the very small dimensions of the disruption unit, CFD results (already presented in this ProcessNet section) could not be corroborated up to now. Micro particle image velocimetry (µ-PIV) proposes a promising approach to verify these results. It is a non-intrusive measurement method, which allows an inside view on local HPH process conditions.
In this work, a micro particle image velocimetry setup was used to obtain velocity profiles in two optical accessible orifices with varied inlet geometry. The obtained averaged two-dimensional velocity profiles were used to calculate local shear and elongation stresses in the laminar inlet of the orifices. Furthermore, three-dimensional CFD simulations were carried out and the obtained results were compared with the µ-PIV flow patterns.