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Turbulent mixing of oil droplets in a round water jet

Turbulent mixing of oil droplets in a round water jet

measurements such as Hot

measurements such as Hot Wire Anemometry and Laser Doppler Anemometry. The oil dropletsthemselves are used as the tracer particles for the PIV-measurements. Therefore it is necessarythat they follow the fluid motion perfectly. This is far from trivial, as the fluid mechanical propertiesof the droplets differ from those of water. Consequently, they might have a slip velocity withregard to the water phase and alter the vortical structure of the flow field. From a theoreticalapproach it will be determined whether this effect might play a role of importance. It is obviousthat at least the droplets should be very small or their density should approximate the density ofwater.The use of this liquid-liquid dispersion in experiments gives rise to typical problems on controllabilityof droplet size distribution and stability of the emulsion. For this project, a commercialmicromixing device was available to create the oil-water mixture. Initially, the outlet of this devicewas intended to act as jet generator as well. However, the resulting minimal number concentrationof oil droplets in the jet turned out to exceed the maximum value for which velocity measurementscould be performed. Therefore, the creation of the oil-water emulsion and the jet generation areapproached separately.The concentration measurements are performed in two ways. First, the intensity of the scatteredlight will be used to estimate the concentration of oil droplets. Secondly, this will be compared toan algorithm that is able to count droplets in the image field.Subsequently, the acquired experimental data are inspected on similarity properties and the combinedturbulent statistics of the velocity and concentration fields, describing the mixing process,are calculated. A comparison is made to the experimental data of Fukushima & Aanen whoinvestigated the mixing of a real passive quantity; a fluorescent dye [Aanen, 2002].1.2 OutlineThe theoretical background of the two-phase jet is given in chapter two, divided into a descriptionof a single-phase jet and a discussion of the relevant aspects of the presence of a dispersed phasein the jet. Thereupon, chapter three gives the details of the applied measurement technique:the characteristics of the set-up, the PIV-technique and the processing of the measurement data.Chapter four contains the results of the experimental work of this project. Finally, conclusionsand recommendations can be found in chapter five.2

Chapter 2TheoryThis chapter describes the relevant theoretical background concerning the dispersion of dropletsin a round turbulent jet. Section 2.1 deals with the single-phase jet. The presence of droplets ina turbulent flow field is described in section 2.2.2.1 Single-phase jetA turbulent free jet is a well-known axisymmetric flow, not influenced by the presence of sidewalls. Fluid is issued from a circular orifice to form a axisymmetric laminar shear layer. Abovea certain Reynolds number, which must be determined experimentally (order of magnitude 500),the shear layer becomes unstable and forms ring vortices. Adjacent vortices pair off and breakup which leads to a turbulent flow. The jet starts spreading outwards by engulfing ambient fluid.The instability process is known in literature as Kelvin-Helmholtz instability [Nieuwstadt, 1998].The mentioned Reynolds number of the jet is defined as:Re jet = ρu jetd jetµ(2.1)in which ρ is the fluid density, u jet the mean jet velocity at the outlet, d jet is the diameter of thejet tube and µ the dynamic viscosity of the fluid. In figure 2.1, an example of a turbulent jet isshown. The jet is visualised with fluorescein.Figure 2.1: A turbulent jet, visualised with fluorescein.3

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