Scaling of Droplet Breakup in High-Pressure Homogenizer Orifices. Part I: Comparison of Velocity Profiles in Scaled Coaxial Orifices

Abstract

Properties of emulsions such as stability, viscosity or color can be influenced by the droplet size distribution. High-pressure homogenization (HPH) is the method of choice for emulsions with a low to medium viscosity with a target mean droplet diameter of less than 1 µm. During HPH, the droplets of the emulsion are exposed to shear and extensional stresses, which cause them to break up. Ongoing work is focused on better understanding the mechanisms of droplet breakup and relevant parameters. Since the gap dimensions of the disruption unit (e.g., flat valve or orifice) are small (usually below 500 µm) and the droplet breakup also takes place on small spatial and time scales, the resolution limit of current measuring systems is reached. In addition, the high velocities impede time resolved measurements. Therefore, a five-fold and fifty-fold magnified optically accessible coaxial orifice were used in this study while maintaining the dimensionless numbers characteristic for the droplet breakup (Reynolds and Weber number, viscosity and density ratio). Three matching material systems are presented. In order to verify their similarity, the local velocity profiles of the emerging free jet were measured using both a microparticle image velocimetry (µ-PIV) and a particle image velocimetry (PIV) system. Furthermore, the influence of the outlet geometry on the velocity profiles is investigated. Similar relationships were found on all investigated scales. The areas with the highest velocity fluctuations were identified where droplets are exposed to the highest turbulent forces. The Reynolds number had no influence on the normalized velocity fluctuation field. The confinement of the jet started to influence the velocity field if the outlet channel diameter is smaller than 10 times the diameter of the orifice. In conclusion, the scaling approach offers advantages to study very fast processes on very small spatial scales in detail. The presented scaling approach also offers chances in the optimization of the geometry of the disruption unit. However, the results also show challenges of each size scale, which can come from the respective production, measurement technology or experimental design. Depending on the problem to be investigated, we recommend conducting experimental studies at different scales.

Scaling of Droplet Breakup in High-Pressure Homogenizer Orifices. Part II: Visualization of the Turbulent Droplet Breakup

“…The influence of the Reynolds number of the flow on the droplet deformation can be shown in Figure 11. As already shown in the comparison of the flow fields [41], the turbulent free jet zone shifts towards the orifice, since the free jet becomes unstable more quickly due to the higher velocity gradients and therefore the laminar core area decays earlier. The relative velocity fluctuations are similarly large as in the case of Re = 2000, but the reference value is significantly larger, so that the absolute velocity fluctuations are also significantly larger for Re = 5700.…”

“…In order to relate the observations made during the measurement of droplet break-up to the flow conditions, single-phase flow measurements were first carried out on each experimental scale to be investigated. The comparison of these single-phase measurements to the experimental scale is investigated by Preiss et al [41].…”

“…In addition, the material systems were selected in such a way that all relevant dimensionless numbers (We, Re, λ = η d /η c , κ = ρ d /ρ c ; η d denotes the disperse phase viscosity, ρ d denotes the disperse phase density) are kept constant. Aside from certain limitations [41], the transferability of the velocity fields and the material properties of the three different homogenization units have already been shown.…”

“…This conclusion is supported by the fact that only larger or clearly recognized and deformed droplets are shown as droplets; this is necessary due to the lower spatial resolution. The comparison of the flow fields (Preiss et al [41]) shows, however, that the flow fields are spatially similar, so that a coincident droplet break-up location can be assumed.…”

“…A more detailed description of the scaling considerations and a comparison of the flow profiles in the three homogenization units of different scales is given by Preiss et al [41]. The objective of this study is to show that it is possible to transfer results from one scale to another when there is a well-developed scaling concept.…”