Investigation on flow patterns, local stresses, and resulting droplet deformation in orifices during high-pressure homogenization

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 homogenisation (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 to shear and elongational stress, turbulence and cavitation, 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 an HPH process, which should result in a specific droplet size, is still mostly based on 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 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. However, due to the extremely small dimensions of the disruption unit, the CFD results 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 the HPH process conditions.
In this work, a micro particle image velocimetry setup was used to obtain velocity profiles in an optical accessible orifice. The obtained averaged two-dimensional velocity profiles were used to calculate local shear and elongation stresses in the laminar inlet of the orifice, and the turbulence intensity of the free jet at the exit of the orifice. In addition to the flow patterns, this experimental setup was also used to visualize the drop deformation.
Combining the results of visualized droplets with the flow measurements, the influence of the geometry on droplet break-up mechanism can be determined.