Droplet breakup in turbulent flow fields such as those occurring during high-pressure homogenization is not yet fully understood and the existing models for describing the process are not adequate for process design. Therefore, in this work the droplet breakup in transitional flows and in particular in the turbulent flow field of the decaying free jet behind an orifice is investigated in more detail using optical measurement techniques. On the one hand, the flow fields behind the homogenizing orifice used are characterized by means of particle image velocimetry (PIV) and, on the other hand, the droplet breakup process is resolved spatially and temporally very closely by means of highspeed cameras. The investigations take place in a test facility geometrically scaled up by a scale factor of 50 and with material combinations adapted according to a scaling concept based on six dimensionless ratios at two Reynolds numbers (Re = 2000 and Re = 5700). The droplet deformation and breakup process is analyzed starting with the deformation as the droplet passes through the orifice unit, continuing with the deformation process behind the orifice, and ending with the high turbulent deformation and breakup process in the turbulent free jet region behind the orifice. Comparison of the velocity field measurements with the droplet deformation and breakup studies shows that the droplet breakup is significantly influenced by the interaction of the droplet with the vortex field in the turbulent decaying free jet region. The droplets are deformed to filaments or, in some cases, to sheet-like droplet films and break up into many secondary droplets under locally very strong deformation. This finding is confirmed by investigations in which undeformed droplets are injected into the free-jet region behind the orifice. Depending on the viscosity ratio of the two phases, there are clear differences, especially in the deformation process in the orifice. The deformation of the primary droplets to droplet filaments is much more pronounced at lower viscosity ratios. Increasing the Reynolds number has a similar effect. In addition, the Reynolds number increase shifts the laminar-turbulent transition point of the free jet towards the orifice, which also shifts the drop breakup in this direction. In addition to the shift, the droplet breakup occurs much faster at the higher Reynolds number because the velocity fluctuations and vortices are much more intense. The use of passive flow control structures at the trailing edge of the orifice can significantly influence the velocity field behind the orifice, but the structures must be designed to prevent flow separation.    «

Droplet breakup in turbulent flow fields such as those occurring during high-pressure homogenization is not yet fully understood and the existing models for describing the process are not adequate for process design. Therefore, in this work the droplet breakup in transitional flows and in particular in the turbulent flow field of the decaying free jet behind an orifice is investigated in more detail using optical measurement techniques. On the one hand, the flow fields behind the homogenizing or...    »