Experimental investigation and two-fluid model large eddy simulations of the hydrodynamics of re-circulating turbulent flow in rectangular bubble columns
Experimental and numerical studies have been carried out to study the hydrodynamics of rectangular bubble columns. Wire-mesh tomography was used for the measurement of void fraction distribution and bubble size distributions in plumes with different aspect ratios. Digital image processing algorithms have been developed to track the bubbles in a Lagrangian sense and estimate bubble sizes and bubble velocities. The image processing techniques were successfully applied to estimate flow parameters in the non-oscillating region of the long bubble plume. Large Eddy Simulations (LES) of short and long bubble plumes have been performed using a commercial CFD code FLUENT. The Smagorinsky sub-grid scale stress model was programmed into FLUENT using user defined functions. Large eddy simulations were performed on a 5 mm grid (filter width of 5 mm) and with a 0.5 millisecond time step. LES was found to provide a better spatial resolution of the flow field. The large eddy simulation results were found to be in excellent qualitative and quantitative agreement with the experimental data. Previous LES studies in bubble columns were limited to superficial gas velocities of less than 2 mm/s. The current study extends this range to 10 mm/s. Previous LES studies used lift coefficient values of 0.5 which is twice the experimentally measured values lying in the range 0.25–0.28. This study used an optimal value of 0.25 for the lift coefficient and a parametric study on the effect of the lift coefficient for a flow with a superficial gas velocity of 10 mm/s has been reported. The feasibility of two fluid model LES is established through the work performed as part of this dissertation. Transient computations were also performed with the mixing length and the k−ϵ turbulence models. The cross-section of the bubble column has a large aspect ratio (width to depth ratio of 5) and the length scale (column depth of 2 cm) governing turbulence and therefore the large scale structure was resolved equally well by the three turbulence models.
Bertodano, Purdue University.
Mechanical engineering|Chemical engineering|Nuclear physics
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