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

Abstract

Emulsion formation is of great interest in the chemical and food industry and droplet breakup is the key process. Droplet breakup in a quiet or laminar flow is well understood, however, actual industrial processes are always in the turbulent flow regime, leading to more complex droplet breakup phenomena. Since high resolution optical measurements on microscopic scales are extremely difficult to perform, many aspects of the turbulent droplet breakup are physically unclear. To overcome this problem, scaled experimental setups (with scaling factors of 5 and 50) are used in conjunction with an original scale setup for reference. In addition to the geometric scaling, other non-dimensional numbers such as the Reynolds number, the viscosity ratio and the density ratio were kept constant. The scaling allows observation of the phenomena on macroscopic scales, whereby the objective is to show that the scaling approach makes it possible to directly transfer the findings from the macro- to the micro-/original scale. In this paper, which follows Part I where the flow fields were compared and found to be similar, it is shown by breakup visualizations that the turbulent droplet breakup process is similar on all scales. This makes it possible to transfer the results of detailed parameter variations investigated on the macro scale to the micro scale. The evaluation and analysis of the results imply that the droplet breakup is triggered and strongly influenced by the intensity and scales of the turbulent flow motion.

Influence of the droplet trajectory on the resulting droplet deformation and droplet size distribution in high‐pressure homogenizer orifices

“…Only individual droplets were found further downstream, representing droplets that were potentially pulled back in the jet by the surrounding backflow, and deformed during this process. This phenomenon was already described by Mutsch et al [15] Similar to the results with added emulsifier, a tendency of an increasing deformation ratio with an increasing distance from the orifice exit can be found for droplets on a trajectory close to the wall. In general, the presence of an emulsifier results in a stronger deformation of the droplets on a trajectory close to the wall.…”

“…As also the time between leaving the orifice and entering the turbulent shear layer is short, the deformed droplets relax only slightly. This is also described by Mutsch et al [15] Even filaments with a deformation ratio of b > 100 are still too thick to result in droplets within the size range of 1 μm with a laminar breakup mechanism. However, the lower capillary pressure in the cylindrical part of the deformed filament compared to an undeformed droplet may facilitate additional droplet deformation caused by vortices in the shear layer.…”

“…Although the droplets are exposed to supercritical stresses (Ca > Ca crit ), the droplet breakup is still dominated by the turbulent viscous and inertial forces in the turbulent shear layer of the jet, as the deformation time of the droplet is too short to achieve critical deformation for droplet breakup. This is underlined by the findings of Mutsch et al [15] in a scaled system. As also the time between leaving the orifice and entering the turbulent shear layer is short, the deformed droplets relax only slightly.…”

“…Subsequently, they are broken by inertia and viscous forces in the turbulent shear layer. Mutsch et al [15] have shown in an upscaled model that droplets on a trajectory close to the wall resulted in smaller droplets following the breakup, as the more stretched droplets can probably be broken up more easily.…”

“…Mutsch et al [ 15 ] conducted a first investigation on the influence of droplet deformation on the resulting droplet size distribution using a scaled high‐pressure homogenization setup. The analysis showed that droplets on a trajectory close to the wall resulted in slightly smaller droplets after passing through the disruption unit.…”