On flow-fields in a high pressure homogenizer and its implication on drop fragmentation

AbstractThe hydrodynamics of the High Pressure Homogenizer has been investigated in order to increase the understanding of the emulsification process. Fragmentation of drops in the homogenizer is generally assumed to be caused by cavitation and/or turbulence. Both processes are investigated experimentally in order to find its location in the HPH valve region. Cavitation was visualized by investigating light scattered in a HPH valve model with optical access. Detailed measurements of the one phase flow turbulence were obtained by Particle Image Velocimetry (PIV) on a carefully scaled model. The effects of dispersed phase flow volume fraction on the continuous phase turbulence was studied with refractive index matched PIV. The experiments show cavitation being focused in the first half of the gap whereas turbulence intensities are very low inside the gap. The turbulence is most effective in the outlet chamber downstream of the narrow gap. This is even more evident for the turbulent eddies of sizes comparable to the drops that are well known to be most efficient for the fragmentation. Increasing the disperse phase volume fraction does not alter the conclusion of highest turbulence downstream in the gap, however, it leads to an increase in energy of large turbulent eddies and a decrease in the energy of small sale eddies. This would imply a relative increase of the strength of the turbulent viscous mechanism compared to the turbulent inertial mechanism when increasing the volume fraction of dispersed phase. When comparing these findings on with visualizations of drop break-up; turbulence rather than cavitation seems to be the dominant mechanism of fragmentation in these geometries