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Exploration of gas entrainment from surface swirls using the processing of flow visualisation and PIV

Abstract : Surface swirls appear daily in bathrooms and in any liquid container emptied by gravity through a hole. The combination of a free surface shear flow and a downward flow generates a strained two-phase (air and water) spiral vortex looking like to a tornado. These vortices may provoke gas entrainment within the flow. In sodium cooled fast nuclear reactor, gas entrainment studies address fortuitous gas entrainment phenomena from the free surface of the main sodium vessel to the bottom of the reactor. To study gas entrainment and its relation to the generation and intensity of surface swirls an experimental rig (BANGA) amenable to produce intermittent surface swirls and gas entrainment is used. To vary the shear flow responsible for these intermittent phenomena, three reference experiments with different geometrical inlets are performed i) using a flat plate, ii) and iii) adding half-cylinders at the end of the flat plate with diameters of 12.5mm and 25mm. The inlet Reynolds number varies from 3000 to 25000. Optical measurements such as flow visualisation and PIV are performed to studies these flows. The combination of pictures and PIV processing permit to estimate the gas entrainment, to track the surface swirls and the source vortex (single phase vortex before the appearance of the surface swirls within the measurement plan) so as to quantify the local circulation associated to these vortices and the correlation of this local circulation with the gas entrainment. Main results show that the geometrical inlet conditions change the quantity of gas entrained and the longitudinal position of the surface swirls. For similar flow rate conditions, with the presence and the increase of the size of the obstacles the gas entrainment increases, the surface swirls move in the upstream direction (i.e. closer to the flat plate or obstacle). A first quantification of the circulation along a tracked trajectory combining source eddy and surface swirl tracking is given. Current work include the construction of circulation criteria and its correlation to gas entrainment. They should be included in the presentation according to progress.
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B. Moudjed, L. Rossi. Exploration of gas entrainment from surface swirls using the processing of flow visualisation and PIV. LXLASER 2018 - 19th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics, Jul 2018, Lisbonne, Portugal. ⟨cea-02338602⟩

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