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The effect of disperse phase volume fraction on the turbulence in a scale model of a high-pressure homogenizer was studied experimentally. Velocity fields of the continuous phase were measured using particle image velocimetry. Refractive index matching combined with digital filtering enabled measurement with disperse phase present. Geometry and physical properties were carefully scaled in order to ensure turbulent flow and disperse phase modulation comparable to that of a technical high-pressure homogenizer. The results show a widening of the jet downstream of the gap and increased Reynolds stresses in the region of high turbulence intensity. This is consistent with previous experiments under similar conditions. Furthermore, the spectra of turbulent kinetic energy were investigated, indicating that the increase in turbulent kinetic energy is mainly due to an increase in energy of eddies of large length scales. [ABSTRACT FROM PUBLISHER]

Abstract: The Oil Transfer Technique (OTT) was developed by Taisne et al. to measure coalescence during emulsification and has been applied since in several studies. One of the main drawbacks of this technique is that it only gives a qualitative measure of coalescence. This paper proposes a new evaluation method of OTT experimental results for estimating qualitative coalescence rates, e.g. for investigating the scaling of coalescence with emulsification parameters (such as homogenizing pressure, and emulsifier concentration). The method is based on comparison with simulated OTT experiments using bivariate Population Balance Equation models. Simulations have been performed under a wide variety of conditions in order to investigate the influence of assumptions on coalescence and fragmentation kernels. These investigations show that the scaling of coalescence rates could be determined accurately when the scaling of efficient residence time of drops in the active region of homogenization is known. The proposed evaluation method is also exemplified by analyzing OTT data from two previously published studies. [Copyright &y& Elsevier]

Abstract: The 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. [Copyright &y& Elsevier]

Abstract: The emulsification in a high pressure homogenizer was studied using a dynamic simulation model based on the population balance equation. The model includes fragmentation, recoalescence and adsorption of macromolecular emulsifier and uses a simple flow model in order to link the hydrodynamics in the homogenizer to the three physical processes mentioned above. A computer model offers an interesting opportunity to study the effect of model assumptions on the overall outcome of the process. The computer model is also an interesting complement to experiments in this case since internal measurements in the active region of homogenization are very hard to carry out, due to small scales and high forces, and information on the spatial position of the different processes is of great importance in design. Based on a set of assumptions, mainly that the turbulent jet responsible for break-up can be described by a one dimensional model and that the macromolecular emulsifiers hindrance of recoalescence can be described by a wall like repulsion, it is shown that the active region of homogenization can be divided into two zones; a narrow zone with fast fragmentation and nearly no recoalescence in the most intense part of the region followed by a recoalescence zone as drop–drop interactions starts to dominate with decreasing turbulence intensity. The effect of operating parameters is seen to be close to the ones found from experiment. The results are discussed in relation to a flow field obtained by a simplistic CFD and assumptions made about hydrodynamics and emulsifier behavior. [Copyright &y& Elsevier]

Abstract: The formation of oil drops from a single capillary with a diameter of 200μm into a cross-flowing continuous water phase has been studied experimentally with the particle image velocimetry (PIV) technique and numerically with the computational fluid dynamics (CFD) software Fluent. The drop formation time and the volume of the detached drop were used as validation parameters and the results from the two methods corresponded well, with a difference of less than 5% for the drop formation time and 10% for the drop volume. The cross-flow velocity has a major impact on drop size, which decreases as the cross-flow increases. An increase in cross-flow, oil viscosity and capillary pressure displace the position of necking and drop detachment away from the capillary opening, which will have a decreasing effect on the final size of the drop. [Copyright &y& Elsevier]