An inline oil/water separator based on an axial cyclone is studied numerically and experimentally. Single phase measurements were done with LDA and compared with numerical data, obtained with Reynolds avaraged Navier-Stokes equations with the Reynolds-stress model SSG. Results show an annular region with reversed axial flow and declining rotation due to friction at the wall, giving a qualitative agreement between numerical and experimental data. A method is introduced to determine the phase distribution inside a liquid-liquid axial cyclone, based on electric conductivity. Preliminary results indicate a time-dependent oil-rich kernel. Presented results for the separation efficiency of the tested axial cyclone indicate the need for better understanding of swirling flow.

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The nucleation process on an isolated roughness element, located at the point of minimum pressure of a NACA 0015 hydrofoil was studied experimentally and computationally. The objective of this study was to investigate the working mechanism of bubble-induced sheet cavitation inception. High-speed micro-scale observations show the generation of a streak of cavitation-attached to the roughness element-in the wake of the bubble. Below its critical diameter, the bubble can detach from the streak cavity and travel on while the streak cavity remains. The solutions of a Rayleigh-Plesset equation along a streamline extracted from a RANS calculation show strong similarities with the experimental observations, but a factor 5 to 10 higher frame rate is needed to validate the calculations. 1. Introduction Sheet cavitation inception does not necessarily occur on foils and propellers when the local pressure decreases below the vapour pressure. The condition of inception is also dependent on the characteristics of the fluid, the solid surface and the local flow. Although seeding the flow with micro bubbles and applying leading edge roughness with a Reynolds-dependent grain size is adequate in most cases [1], the detailed working mechanism of the sheet cavitation inception process is not fully understood. A hypothesis formulated in [2] states that a roughness element causes a very local but significant additional pressure decrease in which a micro bubble can expand and induce a streak of cavitation. This local pressure decrease is dependent on the thickness of the boundary layer compared to the height of the roughness element and therefore also dependent on the Reynolds number [2]. High-speed micro-scale observations were made on a hydrofoil to investigate the nuclei-induced sheet cavitation inception process in more detail. The first analysis was focussed on the diameter, form and trajectories of the nuclei [3]. In the present study, a further analysis is made of the nucleation process on an isolated roughness element caused by free stream bubbles. To obtain the pressure and velocity distribution around the roughness element, the foil with the roughness element were numerically modelled and the steady wetted flow was solved using a RANS code. To study the bubble dynamics, a variant of the Rayleigh-Plesset equation has been solved along calculated streamlines passing close to the roughness element.

We report the results of experiments and numerical calculations of compression and coalescence in a monodisperse oil-in-water emulsion upon centrifugation. A custom-built setup allows in-situ monitoring of a rotating bilayer of emulsion droplets using an optical microscope. The oil volume fraction in a compressed layer of oil droplets stabilized against coalescence was measured experimentally as a function of time for different radial accelerations. The sedimentation was simulated using CFD in order to test the applicability of the computational method and the Ishii-Zuber drag law for very high dispersed phase volume fractions. Quantitative agreement of emulsion sedimentation as a function of time between the experiments and simulations is good at higher accelerations, but decreases with decreasing accelerations. Coalescence in a centrifuged emulsion, which was destabilized prior to centrifugation by adding sodium chloride, was also quantified. The growth of a pure oil phase on top of the droplet layer was measured as a function of time. From the growth rate, a characteristic time for droplet coalescence with the pure oil phase was deduced. The experimental method may serve as a tool to study the compression and coalescence kinetics of emulsions under enhanced gravity, which may be of use to assess emulsion stability for industrial applications. Possible improvements of the current experimental setup are also discussed.