A Study on Flow and Turbulent Structure around Bubbles in Bubbly Flow

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In order to clarity the microscopic flow structure around bubble on turbulent two phase flow, UVP was applied to the measurement of counter-current and co-current bubbly flows. By treating experimental data statistically, liquid velocity profile around bubbles were investigated. The following results were obtained :(1) The flow field around bubbles can be separated into three regions ; a boundary layer, a main flow region, and a transition region.(2) Velocity gradient is independent of gas flow rate conditions near bubble.(3) The flow structure surrounding a bubble can be well characterized by the bubble Reynolds number.(4) When a bubble is influenced by the wake of the leading bubbles, the flow is highly agitated by the vortex shed from the leading bubbles. When a bubble rises up in liquid without the influence of the leading bubbles, the flow in the bubble boundary layer remains laminar.A new electrode-mesh device has been applied to studies on bubble flow. Extensive data processing methods have been developed that offers the possibility to evaluate the local instantaneous void fraction, bubble rising velocity and bubble volume as well as their time and cross-sectional averaged values. These averages were used to conclude on the local distribution of the superficial gas velocity. The following results were obtained :(1) The proposed algorithms have been applied to bubble flow in a rectangular channel (<UG> = 0-5 mm/s , <UW> = 0-25 mm/s and two air injection configuration). Accuracy in the prediction of void fraction was found within 5 % by comparison with VIP supplied data.(2) The reconstructed volume flow showed uncertainties in the order of up to 20 % for true gas velocities lower 300 mm/s and up to 30 % above.